Resistance-No Resistance wire welder

[awsum140]

One of the problems with a schematic is the variations in designs being used. A lot of folks are using "boost" style supplies, others are using camera flash units, actually another "boost" circuit and I'm using a brute force transformer with a bridge rectifier. The biggest thing is a surge resistor in series, ahead of, the capacitors to limit the current surge. How the voltage actually gets developed isn't all that critical as long as it can supply enough current.

I am thinking about using a "boost" supply that is computer controlled. Compared to the LM2755 it's very expensive, but the precision level may be well worth it. So far from what I have experienced, slight variations in voltage, Joules of energy, can make a significant difference in the quality of a weld. The programmable supply would allow tweaking down to tenths or even hundredths of a volt.

 

[Ta2ooz]

And here it is, with the little charge led left in

 

[awsum140]

OK, here you go, Yo Han. It ain't much, but it should act as a guide for you. I couldn't find my electronics template so it's all by "eye", my apologies. All resistance values are approximate and can vary to your own preference and the current draw of your DVM module. I used this DVM module - Digital DC 0 100V LED Voltmeter Voltage Meter Green Digital Panel Meter | eBay . Both push buttons are normally open. The transformer and bridge rectifier can be replaced by any "boost" style power supply you might want to use, like the LM2577 mentioned in this thread.

Incidentally, for those interested here's a link to a supply similar to the LM2577 but supposedly more accurate and efficient AND in lots of ten -
http://www.ebay.com/itm/380697435204?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649 - making their cost actually 1.98 each.

 

[tobarger]

I notice that the DVM has three connections in the schematic but there are only two connections on the part
How did you connect the DVM?
What is your value for C1?
What is the value of the Fuse?
Is the Transformer/Rectifier output connected to the DC Input?

 

[awsum140]

I notice that the DVM has three connections in the schematic but there are only two connections on the part
How did you connect the DVM?
The DVM actually is a three wire device, the illustration on eBay is incorrect. It has three wires, black for negative supply voltage, red for positive supply voltage and yellow for sensing voltage. This DVM seems to have a high enough input impedance so that it doesn't "load" the welding capacitors, dragging voltage down, very much at all.
What is your value for C1?
The value of C1 can be any electrolytic you have handy as long as it has rating of at least 50VDC. I used another 1000mfd since I had that handy.
What is the value of the Fuse?
Fuse F1 is a 3amp fuse I had hanging around and fuse F2 is another 3 amp fuse I had hanging around. Both are "slo blow" due to surge current in both locations.
Is the Transformer/Rectifier output connected to the DC Input?
The output of the rectifier is connected to the DC input to the welding capacitor bank. I didn't show a connection since DC voltage could also be supplied by am LM2577 style "boost" power supply. Of course, some sort of DC power is also required for that and the transformer/bridge circuit will work for that as well.

A pilot light could also be added to let you know the thing is on. I used an NE2 and 1meg resistor on the AC supply side, but it really isn't needed if the DVM is used since it acts as a pilot light.

I also just ordered a panel mount ohm meter and will add that and an LM7812 regulator to power it. I usually weld wires with the spools attached for the first weld, then measure out the resistance I want/need, cut and do the second weld. Having an ohm meter built in will save me from having to keep my DVM on my desk as well.

I'm also adding switches to let me switch capacitors in or out to better control welding. I'm having a problem getting 34ga to weld to nickel, copper or wire wrap (silver plated copper) and hope that raising voltage/lowering capacitance will help solve that problem.

This is certainly not the cheapest way to go, but it works for me.

Hope that helps.

 

[yo han]

Thanks for the schematic awsum!

[edit] Never mind the comment below. I'd overlooked your answers in the prvious post [/edit]

Just not sure about the DVM module though. You've got one with 3 wires?
And also, it seems to me as the DVM would drain the capacitors. Or does it use so little power that it's negligible?
I've seen someone posting a model somewhere that's using a DVM that needs a separate power source to operate (9V battery).

 

[awsum140]

yo han, the DVM is actually a three wire device with the negative lead being "common" to supply voltage and sensing voltage. It seems to have a high enough input impedance, at least 100K ohms, to not "load" or drain the capacitors much at all and the voltage remains stable enough to get good welds at the right voltages. The voltage does slowly drop on the capacitor with it attached, but they also lose voltage naturally due to "leakage". Just charge them a little higher and make the weld when it drops to what you want it to be.

 

[yo han]

Sorry, I see you'd already replied to those same questions above . It looked like a quote so I overlooked it...
So would a DVM with 2 wires work as well?

 

[awsum140]

I don't think that would work well. The power needed to run an LED display, say 10ma or so, would drain the capacitors very quickly. When I ordered that meter I didn't consider that so I ordered another one with three wires. That meter got here first and does have three wires and a high enough input impedance to work well, but it does draw about 10ma for power.

 

[yo han]

Thanks.
I've just ordered this one. Seems to be the same but with red LEDs.

 

[yo han]

I also just ordered a panel mount ohm meter and will add that and an LM7812 regulator to power it. I usually weld wires with the spools attached for the first weld, then measure out the resistance I want/need, cut and do the second weld. Having an ohm meter built in will save me from having to keep my DVM on my desk as well.

Great idea!
You could use a 3 position switch. Center for off, left for welding, right for ohms. That way you could switch from welding to measuring right after you've made the weld.
Did you order this one?

 

[awsum140]

No, I ordered this one - Digital Resistance Measurement LCD Impedance 0 200 Ohm Meter Electronic Ohmmeter | eBay

A switch wouldn't work the way I will be using it. The negative lead is conveniently common for power and sensing for the ohm meter. I'll just add a pin jack for the positive sense lead and clip onto the resistance wire after the first weld to get the length "just right". I'm forever forgetting to shut of my DMM and this will save those batteries as well, LOL.

 

[twgbonehead]

Great thread everyone. It was a long read and didn't quite 'get' it until I got parts and built it myself . I used the instructable found here Small Welder for joining Nichrome and Nickel Wires.

I got it to work somewhat well with 35v and 1000uF. The welds are a bit weak using 28g nickel and 28g kanthal. The problem I'm having is the unit will weld a couple wires just fine then it stops working. After letting it rest for a while and trying again it works as before, but again only for a couple wires. I checked all the solder joints as they were made and I *think* there's no cold joints and everything is routed properly. The multimeter says it goes up to 35v but its a tiny little pop sometimes. Or pressing the switch won't weld anything at all no little pop nothing. Based on these observations does anyone know where I could start to troubleshoot?

Because the weld for 35v and 1000uF isn't very strong for me I'm ordering a couple 4700uF caps and going down to 27v as was suggested in another post. Before I do that I want to know where I went wrong with the current set up! Any help would be appreciated.

My guess is that the capacitor is getting "tired". Electrolytic caps use a liquid dialectric, and many of them aren't designed to be discharged this way. Letting the cap "rest" allows the dialectric to recover. (Kind of like the difference between a regular car battery, and a "Deep cycle" battery, then again not really the same at all...). Photoflash caps are designed to better tolerate this; also look for "low esr" caps. (ESR = Effective Series Resistance", lower is better).

 

[yo han]

Will these do?

ESR (Equivalent Series Resistance) -Impedance34 mOhm

 

[awsum140]

They look fine, yo han. Just remember not to exceed 50 volts with them and keeping it under 40 volts is probably an even better idea.

 

[yo han]

Yes, that was my intention. I'm going to use an LM2577 and the DC source comes from a 9V wall wart. The 9V will also be used to directly power the Volt and Ohm meters.

 

[awsum140]

That'll work! Maybe add a diode between the power leads of the volt and ohm meters to keep the totally isolated from each other and the capacitors.

 

[yo han]

Good idea!
I'm still crunching my brain on how I can use 1 toggle switch and use the same terminals for welding and measuring though

 

[awsum140]

I wouldn't do that. Any residual charge in the caps will throw off the resistance reading and if it's too much of a charge, bye bye ohm meter. Much safer to use a separate lead for the ohm meter, no mistakes that way.

 

[yo han]

I wouldn't do that. Any residual charge in the caps will throw off the resistance reading and if it's too much of a charge, bye bye ohm meter. Much safer to use a separate lead for the ohm meter, no mistakes that way.

Not if I can find me a switch which can separate both circuits.