Building a DIY resistance soldering unit (RSU)

[Deleted]

RSU

Evening guys

A few weeks ago I promised that I would give details of my RSU when/if I got it completed and working, I have now reached that goal and so will now share what I learned to hopefully help any other fellow members wishing to go down a similar road.
I haven’t been idle since first mentioning my wish to make an RSU, I have been experimenting and learning along the way. At first, I tried things to hand, as stated early on my first go was using an old PC PSU, I have read online where people have made this work but my knowledge of such things is far too feeble and thus gave up on that idea early on. Perhaps the PSU which I had was too fancy with all its safety features and built-in fan, it clearly wasn’t going to play ball when being short-circuited without some surgery and thus was put aside.
Wanting to at least get somewhere I then tried my old gauge master controller, for the simple reason that it had variable control although I knew it wasn’t going to have enough power to do much. My thoughts proved true as it barely managed to make the flux fizz but at least it didn’t immediately cut out and the thermal cutout only kicked in with the voltage cranked up.
By this time I had assembled hand-made earth and probe straps with the probe holding a 4mm copper sheaved carbon rod. For a handle, I used some ½” clear acrylic rod which worked well.
Not wanting to waste too much time and not wanting to swallow a book on electronics I decided to take a look at the RSU made by Frost Restorations, I knew that it wasn’t suitable for my needs being too powerful from what I had read but at least it was cheap, it had a transformer in it which clearly worked and perhaps I could modify this to meet my needs.
This, as it turned out, was a good move but having now taken the unit apart I can see that as it stands it can be built very cheaply, for example, the JPR chassis-mounted transformer within according to their price list costs as little as £20 if bought directly from JPR. Perhaps of some help to anyone wishing to follow my lead, however, there is an advantage to having the frost unit and that’s that it is already wired up with resisters for two power levels and, of course, it includes the probe and leads to boot
BTW, the Frost prob handle just happens to hold the same size 4mm carbon rods as I had already bought from eBay although the rods supplied are only 2 and very short with no copper sleeving, you’ll notice in the link posted that the rods bought from eBay much better value.

Ok, so now on to the build and what changes I made to the Frost unit. First a couple of cons about this unit, it has no on/off switch, that is as soon as you plug it in it is live which is not only dangerous (IMHO) it's also not very practical for an RSU as it will arc as the probe touches as was duly proved during testing. Next the so-called ‘footswitch’? It isn’t any such thing, as already stated there is no on/off switch, the so-called foot switch just doubles up on the available power by some clever wiring and use of resisters. I haven’t really looked into this but did note when viewing the wiring diagram that’s available from RS-Components who supply their own brand of the same transformer, that it can be wired in both series and parallel having duel 115v cores, perhaps some trickery was used here?. Btw, there are a number of transformers of the same type available from JPR with different power outputs, this particular one is 6V+6V with 50VA and since I already knew and confirmed with some testing that this unit as it stands was too powerful I don’t think there’s much point in trying the others unless you’re taking up spot welding or something..

I'll begin with a picture of the transformer, as can be seen, there are a number of extra connections and a resistor that Frost has added to assumedly achieve their doubling of power when pushing the 'push' button in, the two wires covered in clear sleeving at the top of the picture are the two wires which go to the button. I decided early on that I wished to keep the option of using the extra power, I overthought this and will explain later.



A picture of the transformer mounted in its new home, you get a better view of the 'push' button wires here, of course, the button itself has already been removed. The Frost unit has the transformer lying flat in a custom plastic cage, luckily, the mounting tabs which come with the transformer had simply been pushed flat so all I needed to do was pull them out again ready for mounting, I chose to use some 10mm acetal plastic as a suitable mounting platform that would be isolated from the box.



Here I can explain my overthinking earlier, as mentioned I decided that I would keep the extra power option by adding a proper switch but had forgotten that this wasn't required as the power would be fully controllable by the AC unit.

Therefore the switch seen in this next picture is surplus to requirements, a good job really as I had mistakenly bought the wrong switch, this one being for 12 volts DC, my fault not realizing in fact, I needed AC and 230 volts. Anyway, this soon showed itself during the first test, my first trip into the workshop next door to reset the RCD..

The unit to the left is the kettle lead socket which includes an on/off switch and a fuse.



Here we can see the inside of the front panel showing the banana sockets and AC controller, none of which are yet wired up here.



An overhead view of the interior after the wiring has been completed. As already pointed out the switch top right is no longer required and can be ignored. I should be able to see that the two 'push' button wires have now been joined so that the unit is set to double the basic power that Frost designed and this power is now under total control via the 2000w AC controller. I actually bought 3 AC units not being sure which to use, one of which is rated for 4000w. Having tested the unit with the 2000w unit I think that this is more than enough.



Last picture to show the assembled RSU including its footswitch and two connecting leads. I had planned to fit a volt gauge to the front of the unit to use as a guide for setting the AC controller but my son thinks this wouldn't work as it would get damaged as soon as the unit was used. this makes sense as you are basically causing a short which probably wouldn't do the volt gauge any good? So, my plan of action is plenty of testing using a voltmeter to note the voltage used for each test and then design some form of grid to go around the control knob, my youngest son can help with a laser-cut panel or perhaps even a CNC cut panel if we get his router completed soon.




Listed below are the links to the parts used including the Frost unit as it stands and the actual transformer available independently direct from JPR Electronics. Note that I have used the wiring from the Frost unit throughout, it just made sense to do so and thus eliminate possible wiring gremlins. I did have my own gremlins from miswiring to deal with which I can give details later but really it just shows my own lack of knowledge on the subject..


Chassis mounted transformer www.jprelec.co.uk/categories/electrical-and-power/transformers/chassis-transformers--single-dual-primary/chassis-transformers-50va-output/product/chassis-transformers-50va-output/835-513~835-513

Banana plug sockets (insolated) www.ebay.co.uk/itm/292286589044

Mains kettle lead socket with on/off switch and fuse www.ebay.co.uk/itm/182486577896

GX16 socket for footswitch to plug into www.ebay.co.uk/itm/283143222904?var=585738923254

4mm copper sheaved carbon rods www.ebay.co.uk/itm/133844557722?var=433293950035

Assortment of electrical terminals www.ebay.co.uk/itm/323239889362

AC controller www.ebay.co.uk/itm/284435183201

Electronics project box www.ebay.co.uk/itm/373664281780

AC footswitch www.amazon.co.uk/gp/product/B093GFYLSR/ref=ppx_yo_dt_b_asin_image_o04_s00?ie=UTF8&psc=1

4mm banana plugs and leads www.amazon.co.uk/gp/product/B01BKJK32Y/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

Frost RSU www.frost.co.uk/carbon-resistance-soldering-system/




I hope this is of use to some and very happy to answer any questions, I'll try to draw up a wiring diagram in the near future, I'll also make a short video showing the unit in action.

Kind regards

Pete

[Deleted]

evening chaps/chapesses

I promised a short video and here we have two, it was supposed to be just one but I needed to set up for soldering a heavier gauge piece of brass and so stopped to swap handles and also find a suitable piece of brass. I was going to use a much thicker piece which you can see just before I stopped the first video but this was going to push the unit beyond what I have tested so far and so decided to show how easy it is to solder a thin piece to a much thicker piece, in this case, 1/4 x 1/4 x 1/16 right angle. I'm confident that the RSU will solder larger than seen in both videos once I get used to the settings and use of it.

The test subject in the first video is some of the starfish platforms from the 'Hood' kit, all PE parts from the kit are sacrificial as I bought a much better, highly detailed, and more accurate after-market PE kit for it.

This RSU will be used mostly for the 'Hood' model and the gauge 1 GWR king when I make a start.

Ok, the first video, shows me soldering some supports to the underside of the starfish platform.



Part 2 shows the right angle being soldered to a thinner piece, in this case, the thinner piece is tinned, flux is applied between the parts and the thinner part is heated from the outside, the heat penetrates straight through it and thus bonds the two parts together, a bit like a spot weld.




Other things to note is the flat piece of steel used as a working platform, the earth is clamped to this and the brass parts to be worked on are held down by small earth magnets, in this case, I was using very small 3 x 2 mm magnets which were bought especially for working on 'Hood' These magnets and those much bigger are readily available on eBay for a reasonable price.

I will try to do the wiring diagram soon, what I can't draw is how Frost wired up the transformer for among other reasons it's not easy to see. This setup may have more to do than just doubling the power, I'm not sure how important the resistor used is, however, I suspect that it may allow longer than normal contact of the probe snd thus the ability to solder larger pieces. Frost shows in its instruction pamphlet a picture of a 15mm copper pipe with a number of spokes soldered to it like a wheel, this is certainly a much heavier application than anything that I've tried so far so I think that this unit can take some serious heat and perhaps the resistor has something to do with this? I have no idea knowing very little on the subject.

hope this is of interest?

Pete

[Jo]

That works well!

Now I am tempted to make myself one.

Jo

[David]

Very interesting! Looking forward to the circuit diagram ;)

[Deleted]

RSU wiring
Here is my very basic wiring diagram using the Frost unit, it really is that simple, below this I have included close-ups of the main components involved. I have also added the RS schematic for their own version of the same transformer which I assume should be the same as JPR or very close to it. Perhaps helpful for those with more knowledge in this field not wanting to use the Frost unit. All of the parts shown I have already posted the links for above. All of the wiring used has been taken from the Frost kit and cut to suit.






Transformer A3071 (as wired by Frost)



RS A3071 schematic ( included for those not wishing to use the Frost unit, Cost saving is significant, Frost costs £99 and the transformer alone costs £21 although the Frost kit does have everything else required in the kit, earth lead, probe handle/lead and carbon rods.)





Kettle lead socket/switch/fuse



A/C Controller





I have tried to lay this out as clearly as possible, this works very well if using the Frost unit, if not then the only thing in my mind which isn’t clear is the resistor fitted by Frost? It may be some form of protection in the circuit or it may be and probably more likely just as simple as halving the power output, in which case me joining the two wires that went to the push button may actually have set the unit at half power? If so, I can easily add a more suitable switch into the circuit as originally planned to give more power at a later date although the present setup is more than powerful enough for now.

Hope this helps, happy to answer any questions, just remember that my knowledge on such things is just above 'zero' although I have gained a little experience in making this RSU......

Kind regards

Pete

[weary]

Hello Pete,

Thank you very much for taking the time to give us the results of your enterprise and experimentation. Much appreciated!

I have a couple of questions for you, or indeed anyone. My knowledge of 'sparks' is nil, as you may be able to tell from the questions! Anyway, here goes:

1) The transformer input. I see that you (and Frost) used a 115 + 115 Volt input transformer, presumably wired as in the middle of the three 'Schematic/connection options' diagrams. Does this have any significant advantage over a 230 Volt input? (I am in UK, straight 230V seems 'easier' to my mind).

2) The transformer output. You and Frost used 6 Volt output, with, I assume the Frost 'boost button' lifting that to 12 volts using the wiring shown right-hand side of the last of their 'Schematic/connection options' diagram in your post above.

I realise that in the end you didn't bother with the 'boost button', but should one want to retain that facility does anyone have suggestions as to what kind of switch would allow that kind of 'change of connection'? A sliding or lever switch would be more practical than a press-button as these designs avoid the need for a third hand!

The demonstrations in your videos clearly showed you found 6 volt output (and perhaps even less) to be more than adequate. However, would there be any advantage to a 12 volt output transformer, regulated by the variable AC controller you fitted to supply 6 Volt as the 'usual' output which could be 'turned-up' on the (rare?) occasions 12 volt was required or would the AC Controller overheat under such sustained use?

I could see from your helpful video's showing the thicker material joined to the angle section that 6 Volt appears to be more than adequate for the kind of thin brass sheet-work that I would do, however, the 'boost' facility may be an occasionally useful facility to retain. If not too troublesome to do-so.

Thanks again for the time you put into updating us. Hopefully not too-many have been offended by my simple questions!

Regards,

Phil.

[Deleted]

Hi Phil

I'll try to answer what I can but as stated, my knowledge of these things is also very limited.

1: I can not answer this question, there must be a reason that Frost chose this particular transformer, perhaps due to them wanting the ability for more power if needed. remember, Frost is a car restoration supply company and therefore may need more power for certain things like old battery terminals etc. when researching into how to make an RSU there are a number of DIY builds detailed out there, transformers used are generally from old items, microwaves, battery chargers and PSU's, most of these seem to state that a new second winding is required although I did read one article that stated a 6volt car battery charger can be used without modification. Perhaps the wiring done by Frost is similar to the mods required on these old transformers, however this is above my current knowledge level.

2: I'm not sure which wiring Frost has followed, it's partially covered with the sheaving and I didn't want to prod around in there too much. As noted the RS wiring diagram shows either double voltage or double power? Not being an electrician I'm not clear on what they mean by double power, is that watts or amps? When reading about the various RSU's available out there the main difference between them is the wattage, so I assume the doubling up is in watts?

I probably will add the switch to increase power later, again, assuming that I have read things right and that currently, I am only using just half the available power with the wires that were connected to the button currently being hotwired. I'm thinking this way as resistors are usually used to reduce the voltage, however, they are also used to prevent a short circuit so perhaps, in this case, this is its main function here considering the role of the RSU is to create a short circuit? In regards to the actual switch, I'm thinking of a simple AC rocker switch that will retain the same position on the back of the unit, I don't see it as something that needs to be activated during normal operation.

From what I have read 12 volts although possible is not the normal voltage used in an RSU, more likely to be under 6, perhaps only 3 to 4 required as a norm, at least certainly in as far as the model railway hobby requires.

Regarding the AC controller, I was advised early on by my son to fit this before the transformer, not after as was my original plan had been. Also in regards to the footswitch placement, this also needs to be fitted before the transformer. At first I tried this after the transformer along the probe circuit thinking all I needed was a method to cut the power but this reduced the power of the unit significantly, ie it wouldn't give enough to create much heat at all. I put this down to the length of wiring to the footswitch (1.5 meters) and that the output, being so low at only 6 volts, was draining all the power. Once wired up on the AC 240v input circuit it worked perfectly.

To summarise, I'm pretty impressed with what I have managed to create, even if all that I've done is improve on a commercial design. An RSU is only supposed to have a short duration in operation, a sec or two to avoid too much heat buildup and possible damage caused to the transformer. Well, I have had much longer than that with no signs of overheating or power drop/cut during the operation.

I'm fairly confident that this unit will do much more than I require and am very much looking forward to getting stuck in with my gauge 1 kit... There is still a role for the standard soldering iron, both methods are needed but where the RSU really comes to play is in neater work and easier placement of parts. It's also perfect for soldering parts that are close to other previously soldered joints.

hope this is helpful

Pete

[Neale]

The reason for the transformer having multiple windings as shown isn't to make it difficult for people - just seems that way...

Actually, it means that one transformer can meet several different requirements. There are two primary (that is, "input") windings so that you can put them in series to run off nominal 240V input or in parallel for 120V input. You should not connect just one winding to 240V. Similarly, assuming a 6V+6V 48VA transformer, the secondary windings could be used separately for two independent supplies giving two 6V 4A outputs (6x4A, twice = 48VA) or (series connection of secondaries) 12V 4A (again, that's 48VA) or (parallel connection of secondaries) 6V 8A. Note that what you do with the secondaries is entirely independent of what you do with the primary windings. You choose the primary connections based on mains voltage, and the secondary connections based on the output required.

In this case, based on the comment that you don't need more than 6V, I would be looking at series primary connection (in UK) and parallel secondary connection for max current output when required. I'm really curious about how the "boost" switch works, though!

The use of a mains speed controller is interesting in conjunction with a transformer. Voltage readings of the output aren't going to be particularly reliable as the controller messes with the AC waveform which will confuse a voltmeter but it will at least vary the overall output power available - as we see here.

[weary]

Thanks Pete and Neale for the very helpful responses.

I'm guessing that the (Frost) 'boost button' simply wires the second 6Volt output in either series or parallel to the first, 'principal', power output. I assumed only one of the 6Volt output coils would be used under 'normal' conditions. From the Frost circuit diagrams and Pete's comments & videos I had supposed that 'boost' added the second 6Volt output in series thus giving 12Volt, however after considering Neale's response immediately above, I am thinking that perhaps it wires it in parallel to give a higher amperage, but still at 6Volt. The wiring/switch for that second option would be simpler.
The output, 6Volt side of the (Frost) transformer is clearly visible in Pete's initial post, first pic - seems to show separate output connections 0 on left hand side and 0 and 6 on right hand side with both 0 bridged together and both 6 bridged together. No output connection to left-hand '6' output.
(Maybe series or parallel output makes no difference for this use and/or simply needs some experimentation.)

Anyway, more 'stuff' for me to mull-over and experiment with once unit is built.

Regards,
Phil

[Deleted]

Thanks, Neale for your very informative post, very interesting, I hear what you say re the AC controller and it would explain why it's not easy to get a solid 2,3,4 or 6 volt reading, it takes very fine-tuning to get a solid figure with no extra digits. However, as you rightly observe, it does control the voltage very well and thus the power output which makes the unit easy to use.
I would be interested to know what the actual wattage is?, from what I understand you multiply volts by amps which if taken from the output of 6 volts doesn't seem very much and certainly not what I'd expect from how well this RSU works, or is it taken from the 115 volts primary winding, in which case it's pretty high? Afraid that I have no idea how the boost works and can only point to the addition of the resistor which I haven't seen on other designs?

Here's the info sheet from JPR, there is also a PDF file with more details, the transformer we are looking at is A3071

www.jprelec.co.uk/categories/electrical-and-power/transformers/chassis-transformers--single-dual-primary/chassis-transformers-50va-output/product/chassis-transformers-50va-output/835-513~835-513


Kind regards

Pete

[timb]

I assume the Voltage/Current will be the same as applied to welding (although welding is about maintaining constant current) :-

Increasing the Voltage will give a wider arc at the end of the probe - effectively increasing the heated area. Increasing the Current will increase the 'heat' at the arc point - effectively giving better 'penetration' in the welding world but basically a hotter 'flame'.

So..

As Phil rightly states, using the output in parallel - connecting the two 0's and two 6's together will give 0-6 V at 0-8A between the probe and the earth connection depending on the overall impedence - local heat area, deeper penetration.

(Impedence is like resistance but will vary with a changing waveform as in this application, it is basically the resistance of the transformer, wiring, probe and the items being soldered etc with the change in voltage waveform factored in)

Using in series - 0 to earth lead 6 to 0 then 6 to the electrode - will give 0-12V at 4A between the probe and the earth connection, again depending on impedence - larger area of heat, less heat available.

Again as Phil stated using a speed controller effects the waveform of the input voltage. As a transformer is used this is just repeated at the output. These are cheap and cheerful items but to maintain the input waveform you could use a variac on the input (or output) of the transformer.

This would make a difference in the quality of the arc - important for welding but I do not think it would make much difference in generating heat as in this application, maybe a bit of inefficiency.

Hope this helps!

Tim

[timb]

Jan 2, 2022 10:02:50 GMT @greenglade said:

I would be interested to know what the actual wattage is?

Watts only apply in DC circuits.  The transformer tells you how much power('wattage') is available - 48VA where V = Voltage - A = Amperage

2x6Vx4A = 48VA

It works the same for the input so you have two windings at 0-115V so a total of 230V at 48VA

48/230 = 0.21A

Tim

[Deleted]

Thanks, Tim... I wish that I could say, 'yep that all makes sense'.. alas my head isn't up to much just now and whatever I learned about electronics in the past ( which wasn't much) is hidden deep within today.
For now, I'll have to be content with a unit that seems to work, and perhaps when/if I'm off some of these meds will try to understand it better in the future.

Kind regards

Pete

[Neale]

I think that comparisons with welding are a bit misleading as this is a "resistance" technique - it's the relatively high resistance at the contact point of the carbon electrode with the work that generates the heat locally. And after all, for soft-soldering you are only talking 200-300C, not melting steel heat levels! Hence the low current levels compared with that needed to maintain an arc.

It's also important to remember what the "8A" rating means on the transformer (with parallel secondary windings). This means that the transformer can be asked to deliver up to this current without overheating. It is not a "limit" in the sense that that is all it could deliver. I bet that if you short-circuited the output, you would get a lot more than 8A - probably until the mains fuse blew or the transformer seriously overheated! After all, a 13A mains socket is capable of delivering 13A via a suitably fused plug, but if you plug in a table light it's actually going to take a much lower current. In the RSU case, we don't really know what current is being drawn at any given moment, just that if you keep the average down below 8A or so, the transformer will not get too hot.

I'm watching this thread as I could do with some soft soldering on my Black 5 tender that does not involve using a torch and possibly affecting existing joints...

[timb]

Hi Neale, please explain what arc welding is if it is not a "resistance" technique.

You cannot change the power of the transformer. This is set by the number of windings, the gauge of the wire used and the type of core. The 'VA' is effectively set at manufacture, you cannot buy a 500VA transformer and get 1000VA out of it by doubling the voltage. VA is Volt-Ampares, as the voltage increases the current decreases by the same ratio and vice versa.

Incidentally, the primary of the transformer is connected one side to live and the other side to neutral - with a length of wire inbetween - not quite a short in reality, the overall impedence protects it (Ohms Law) when the secondary is open circuit. When the secondary is under load, the voltage drops to keep the VA correct for the drawn current. The transformer is rated in this case that at say 6V you can draw up to 8A. More than 8A and the voltage will reduce accordingly.

The 8A on the secondary IS a limit for that voltage, you cannot get something for nothing, HOWEVER when you short out the secondary, the changing magnetic field in the transformer induces an opposing current in the secondary coil. The opposing current through the secondary produces an opposing magnetic field, which cancels out the magnetic field from the primary. This will reduce the inductance in the primary and cause the primary current to rise. Two things will happen, the transformer will heat up and the current will quickly get to the point where something has to give.

As I do not know the circuit for the speed controller, it is not clear if there is a current limiter involved. You have a fuse in the plug for the primary - if this is at 13A then it is too high. I would however recommend a fuse in the secondary circuit to prevent burning the transformer out.

Sorry for the technical stuff GG but I find that you need to be precise or taken to task!

Tim

[Neale]

Arc welding involves generating an electrical arc between two separated electrodes. In effect, a continuing "spark". The temperatures involved in ionising the atoms in the spark region so that they become conductive is very high, hence generating enough heat to melt steel. The resistance technique involves the creation of a region of high resistance (low resistance in absolute terms but higher than elsewhere in the circuit, hence the need for chunky low-resistance connections elsewhere) in which most of the power is dissipated, leading to ohmic heating in that region. Of course, in both cases, Ohm's law applies but for arc welding, the heat comes from the big, continuous, arc and for resistance soldering, from the heat dissipated in the "resistor" formed by contact between electrode and work. Resistance welding (e.g. spot welding) also uses heat generated at a "resistor" formed where the metals are forced into contact, but currents are higher so temperatures can locally reach melting point.

I don't particularly want to go through all the other points, much as I would like to, as it is straying too far from the thread topic and gets boring for everyone else. Kind of conversation best held over a pint in the pub with the back of an envelope in front of us! However, I would be very interested (academic curiosity only - the engineer says, "It works - so what?") to know what the actual current during soldering is. My guess is rather more than the nominal transformer rating, but because it is for short periods only, the average heating effect on the transformer is well within its working envelope.

[timb]

You cannot draw current and create an arc without resistance. The continuous arc you describe is created as the welding controller provides constant current given the changing resistance as the electrode moves. The resistance is dependant on the air gap through which the electrons flow. If the resistance reduced the heat would drop which is why the electrode sticks to the work of you get too close(no resistance, no arc, no current flow), I did allude to this earlier. The physics behind it all is quite interesting particularly to electrical engineers, any contact that opens and closes is subject to the same principals, but as you say not necessarily so for all. It’s a shame we are not closer as it would be an excellent topic over a pint!

Tim

[weary]

Fuses & circuit and component protection for 50VA 6V + 6V output transformer (UK scenario):

Following-on from Tim's and Neal's previous comments:

On Mains input side the maximum current current drawn by the transformer under normal usage is 0.21Amp (50 VA/ 230V), so a 3 Amp plug-fitted cartridge fuse would be appropriate???
(Sizes for this specific fitting are limited to 3A, 5A, 13A in UK)

Or, alternatively a separate 1Amp fuse or circuit-breaker???


However, on DC output side in order to 'protect the transformer':

If/When a single tapping is used giving 6Volts the normal current would be (up to) 4Amps (50VA/2 x 6V ??) Thus a 6Amp fuse/circuit breaker would be appropriate???


The same applies to the two output tapings wired in series = 12V output at 4A???


However, when the two output tapings are wired in parallel the output will be (up to) 8Amps necessitating a 12Amp fuse/circuit-breaker???

So, potentially, depending on wiring, two separate protection devices one for each circuit would be best to protect the transformer???

Asides:
I assume that Pete's wiring of the voltage regulator on the input (mains) side of the transformer means that the output is varied proportionally when voltage is reduced ........ otherwise the output Amperage goes up as the Voltage decreases!
i.e. if not then when regulated to 2Volt output gives 12.5Amp (50VA/ 2 x 2Volt). This would make the calculation of any 'protection device' far more complex.

I have in mind a panel-mounted circuit breaker of this kind, typical ebay link, which seem to be available in 1Amp increments


In no way criticising other's thought provoking contributions, but on a rather 'philosophical' point, and at the risk of causing apoplexy in those with knowledge:
I can see the advantage(s) of a correctly rated fuse/circuit-breaker on the AC input side.
However, given the likely use and demand cycle during modelling use is a transformer protection device really necessary on the output side, or just gilding the lily? Maybe simplicity wins over the probability of 'frying' the transformer? Perhaps for 'our' kind of usage if experience shows the transformer gets 'hot' a cooling fan would be more appropriate??

Regards,

Phil

[Deleted]

Hi Phil

Thanks for the info, I haven't checked which size fuse is fitted to the AC switch, I'll check it and follow what you have suggested. On the DC side I did buy an inline fuse holder to fit a car spade type fuse, a selection of fuses came with it, I'll try the 5amp and see how it goes.

Thanks for the help...

Kind regards

Pete

[timb]

Hi Phil/Pete

At the risk of further confusion.....

Mostly right, there is no DC side though, transformers are AC only. Most of these types of 'speed controller' use a triac to alter the trigger point or 'phasing' of the input AC waveform to change the voltage and therefore the 'Speed' (I may be wrong in this case, there are different types).

Easily confused with an inverter type speed controller that substitutes a string of DC pulses that mimic the shape of an AC waveform using burst fired thyristors.

I would recommend a 3A fuse in the live supply. The ratio of the transformer is 40:1 (240V - 6V) there will be a ratio of 40:1 windings, more on the 240V side, it is a step down transformer. The voltage is stated as 2 x 115 or 230Vrms and is an expected average. Industry now uses a range of 'averages' 115, 200V, 400V, 600V, 1100V etc.

As stated earlier the Power of the transformer is fixed and is a rating, it will only get too hot if the power is exceeded, as stated earlier, this will only happen if there is a direct short on the secondary, hence the suggested secondary protection.

To take it to its ultimate, applying 40VAC at the primary will give 1VAC at the secondary. The primary current will be around 1.2A, the secondary current will be at 48A!

Incidently, inverter drives only have protection on the live side, this is because the power side is current limited within the drive.

Tim