Building a DIY resistance soldering unit (RSU)

[David 1/2d]

The Variac lets me wind the Frost RSU right down from its maximum output of 7V (at 250V mains) to zero.

I measured the Solder Current at each of a range of Open Circuit voltages across the output terminals.
7V     24A   /  Red Button  31A
6V     19A   /  25A
5v     15A   /  20A
4V     9.5A  /  12.5A
3V     7.5A  /    9.2A
2V     4.5A  /    5.3A
1V     2.0A  /    6.6A
0.5V  1.5A   /   2.0A 

It's a somewhat straggly graph, because the 'joint' resistance is slightly different each time, but it's a useful comparison with the LMR RSUs range from 4A to 40A under similar conditions.

[David 1/2d]

Here's a trace from the Frost electrode when it's powered through the Variac:


6V Variac Output by David Halfpenny, on Flickr

but the SCR controller is another matter.
Instead of reducing the size of the voltage wave, it's supposed to 'chops' neat bits off it, like this:


TRIAC Waveforms by David Halfpenny, on Flickr

Well, through no fault of the Frost, the output voltage is more spiky:


4V SCR Scope by David Halfpenny, on Flickr

the underlying sine-wave is still there underneath:


Sine Trace by David Halfpenny, on Flickrthough



6V Trace by David Halfpenny, on Flickr



5V Trace by David Halfpenny, on Flickr


4V Trace by David Halfpenny, on Flickr


2.2V Trace by David Halfpenny, on Flickr

Here they all are, overlaid on the sine-wave:

SCR Waveforms by David Halfpenny, on Flickr

So, with that interesting set of voltages on the carbon tip, what will the current be like?

[David 1/2d]

We certainly don't want that kind of filthy talk transmitting round our house, down the street, or across the airwaves, so I set up a late 20thC 'FM transistor radio' on the bench, to detect interference, before drawing serious current.

Here's the spiky voltage trace across the Frost output terminals equivalent to 4V Open Circuit:

4V SCR Scope by David Halfpenny, on Flickr

but here's the calmer corresponding voltage trace once I put carbon tip to earthed brass:

5V SCR Output c by David Halfpenny, on Flickr

It's now nice and smooth, and the vile 20thC symphony on the Tranny had no added Snap Crackle and Pop.
Incidentally, this is delivering 15A (Red Button 19A)

So I withdraw my earlier questioning of Pete's current regulation by Silicon Controlled-Rectifier.

Here's the Table of Results:
7V     25A / Red Button 32A
6V     20A / 27A
5v     17A / 21A
4V     14A / 19A
3V     10A / 12A
2V     5.5A / 6.5A
1V     4.0A / 5.0A
0.5V  3.0A / 3.5A

I did find it more fiddly to get the lowest currents repeatably - maybe because the circuit has a linear wire-wound variable resistance. 
But the overall essence is that Pete's £5 controller will do as good a job * as my Variac (even though it's neither Steampunk nor Red). 

For those who don't want to get into mains wiring, look for a controller ready-wired with plug and socket, and a rating of at least 500W.
Check online Reviews! I found a really appealing little lamp dimmer that I'd have bought, had it not had appalling reviews.

* why the Star?
Well, Pete's new SCR ought to be better, in almost all respects, than mine, put together half a century ago by a spotty teenager as a first venture away from Thermionic Valves. On the other hand, mine includes both a sodding-great hand-wound Inductor and a stout metal case that might be advantages. Or not.

20220131_154023 by David Halfpenny, on Flickr

(Incidentally, as the TRIAC alone cost £5 in 1968 (£100 in today's), I greatly pee'd-off the University by burning one out on my first attempt.)

[Deleted]

Hi David

Thank you for the in-depth analysis of the Frost unit and also the mods that I have undertaken. I'll be the first to admit that such things are way above my abilities and understandings which were all undertaken after some reading on google. I'm very happy with how things have turned out even if for the most part it was a case of me just falling on my feet. I have a habit of doing this, sometimes I even manage to land while running....

Kind regards

Pete

[David 1/2d]

Thanks for providing both the stimulus and the platform, Pete and all.

My next step is to wire-up a foot-switch lead, so I can do some actual soldering with the still-unmodified unit.
Only after that will I feel comfortable about taking it apart to look at the Build - and the Mystery of the Red Button.

[Deleted]

Hi David

I can share my experience when fitting the footswitch although you no doubt already know this with your superior knowledge of such things.
I saw the footswitch as a simple make/break device for the probe and at first duly fitted it in the probe line not really giving any thought to A/C vs D/C, The result being, yes it worked but the uniy produced very little power, barely made the flux fiz. My thoughts were two fold, first the switch was an A/C unit whereas for where I connected it after the transformer it perhaps needed to be a D/C and due to fitting it after the transformer perhaps it had too much resistance in its wiring.

All I know is once it was placed inline before the transformer the RSU worked perfectly.

Just me sharing my mistakes due to the little knowledge that I have on the subject.

Kind regards

Pete

[David 1/2d]

Pete, I only know about RSU cos I bought one - it's you that's got the hard-won knowledge and a Frost that does modelling!
While I've been let down today, by relying on 'book learning'.


Yes as you've found, with these high DC currents it's the input that needs switching, using a switch designed for Mains AC.

The output side of a traditional RSU transformer uses great fat wires and a minimum of sturdy electrical connections as possible. Inserting a switch drags down the already low voltage. Even had it worked, DC arcing would be chewing up the switch contacts on hight currents. 

Just an introduction to my disappointment:

I've bought a controller that seemed ideal for a No Mains Wiring outfit.
- ready-wired with three pin plug and socket,
- slightly smaller than the Frost box,
- on-off switch and calibrated control knob handily place on the top
- bright built-in LED voltmeter.

It looks so cool beside the Frost - two little black boxes with red details - but pride comes before . . .

[David 1/2d]

Fost Accessorised by David Halfpenny, on Flickr

Here's the little beauty accessorised with ceramic soldering slab, foot-switch on the carpet, and voltage controller beside the RSU, set up for electrode in right hand and controls under left-hand,

specifically, from left to right:
-  Red Button
-  Input Volts
-  On-Off Switch
-  Power Dial

Next job: trip to electrical shop(s) to make the controller safe to use.

[David 1/2d]

Here follows a Deviation

Irrelevant to Resistance Soldering in general, and Frost in particular, yet important to anyone trying to control electricity at modest cost.


To fix the Amazon-supplied 4000W AC Power Controller, half a dozen things need changing.

While the cost of the necessary parts is a trivial £2 to £3, it's all annoyance that cuts right across my objective of using the Frost RSU with minimal Wiring.



1) The "UK 3 Pin Plug" is neither safe nor legal in the UK:


20220211_164833 by David Halfpenny, on Flickr

because it's unfused.

Which means a short-circuit inside the box would melt the cable, which could start a fire.
(Remember a UK ring-main is nominally only 30A, but will deliver a lot more than that before tripping its Breaker.)

While substituting a proper UK plug will provide a fuse, I spotted that the cable is only rated 10A:



20220211_165306 by David Halfpenny, on Flickr

H05VV-F 3G1.0mm2 Cable

which means the actual rating of the product is only 2200VA at 220V.


2) The plug needs a 10A fuse (or lower)

(3A will be enough for this application, but I do have a packet of 10A fuses if I need the controller for a meatier purpose.)



Looking inside the box, there's a simple circuit (very similar to my antique one, and none the worse for that).

20220211_165024 by David Halfpenny, on Flickr

All the connections have been made with solder, which means that a stray live wire could flail around inside if the under-rated cable were either thermally or mechanically over-stressed.
 
While a proper fuse should deal with Thermal, look at this Mechanical blooper:

 20220211_164920 by David Halfpenny, on Flickr

there is no cable-restraint - just a slack grommet, so:

3) The grommet needs replacing with a restraining cord-grip

suitable for nominally 7mm diameter PVC cable.


Looking again at the insides, the metal case is completely un-earthed so:

4) The metal case desperately needs Earthing

which, to give the manufacturer due credit, is spelt out clearly right at the start of the Amazon description,

Screenshot 2022-02-12 at 17.08.18 by David Halfpenny, on Flickr

but has been ignored by somebody along the supply chain.

The Description also rightly says that the nominal rating only applies to a Resistive load.
Its actual rating with an Inductive load like the Frost could be well below 2000W.
I'll try to measure the Power Factor but, suffice it to say for now:

5) The dangerously misleading 4000W label on the box needs editing or obliterating



There's another shortcoming to fix if you have a mixed-sophistication household:

6) The 3-pin output socket needs a blanking plate when out of use

because there is no automatic safety shutter on the Line or Neutral pins.


Needless to say, there is no CE mark.

What does need saying is that similar devices, some of identical appearance, are available under other brand names with other Descriptions. They may be better, but could be worse, so all that stand between ourselves and danger are scrutiny of the website before buying, followed by scrutiny of the device before plugging in.

END of Deviation

Sorry if it's put you off, but that could be a Good thing.

None of this is Frost's fault.

[timb]

I would think Amazon need to know about this unit, it is breaking the law, as are they by allowing it to be sold in the UK. Having said that I doubt they would do anythig about it, but the HSE might be interested! You are quite right, very dangerous to those that do not know the difference!

Excellent continuation to the thread well done indeed sir!

Tim

[David 1/2d]

Thanks, Tim - yes, it's all reported in the Amazon feedback and Star Rating for that specific Vendor. No, Amazon don't generally interfere in such matters.
Been a bit busy, but still intend to use the frost for model soldering. D