battery spot welder problem

[SentinelAeon]

I got a cheap spot welding board and wanted to test it out. Since i got no LiPo handy, i thought i could use a microwave transformer and a 100A 1600V rectifier since board needs DC. I tried with both 7.5V and 12V coming out of the transformer. I basicaly get nothing. It barely shows on the 0.1mm strip, doesn't go through. If i had to guess i would say im not even getting 1 amp out. Transformer should be more than capable to supply enough amps to at least make a spark. Is there something im missing here ?

 

[Galgso]

I don't see any smoothing capacitors. If there are no smoothing capacitors after the bridge rectifier, the power going into the board is pulsating AC, not DC.

 

[SentinelAeon]

I don't see any smoothing capacitors. If there are no smoothing capacitors after the bridge rectifier, the power going into the board is pulsating AC, not DC.

I used a capacitor bridging the terminals on the input to the board. The problem is that all i have are small capacity caps and the ripple equation is giving me numbers in mF

 

[Galgso]

If you can't put a lot of low ESR capacitors in parallel to make a capacitor bank then a car battery may work if you can't get a LiPo.

Also, I suspect that the spot welder may work to some extent if you can get a stable DC voltage to the control electronics even if your high current supply is poorly regulated. The issue with a poorly regulated supply driving both the control electronics and the high current output is that if whenever you try to weld the voltage drops then the control electronics will also stop working. I see two DC inputs on the board. Are these tied together internally?

 

[SentinelAeon]

If you can't put a lot of low ESR capacitors in parallel to make a capacitor bank then a car battery may work if you can't get a LiPo.

Also, I suspect that the spot welder may work to some extent if you can get a stable DC voltage to the control electronics even if your high current supply is poorly regulated. The issue with a poorly regulated supply driving both the control electronics and the high current output is that if whenever you try to weld the voltage drops then the control electronics will also stop working. I see two DC inputs on the board. Are these tied together internally?

Sadly, they are. It's just 2 different inputs so you can use cables like i do, or plug in the lipo directly if you add the female plug.

I have a bunch of 18650 20A cells. I wouldn't use them for the spot weld cause its inpractical compared to lipo (would need many many cells). But a 2S5P bank should at least be enough to test the board and see if i get a spark, right ?

edit: i have misread the equation. It would appear that in my case, a 20.000-50.000uF should be enough to smooth the ripple. I will give it a go, connect 10-20 caps in parallel and see if it makes a difference

 

[Galgso]

Capacitors are the safer option so I'd definitely try that before the 18650s.

Connecting 18650s in parallel may work. The 20A rating is for continuous discharge, right? If so, the pulse current rating will be higher and you may be able to get several hundred amps for a brief pulse. If you only run the welder intermittently the 18650s should not get hot enough to trigger the built-in protection devices. The cells will have a max pulsed current rating though and I'm not sure what exceeding that will do, so proceed with extreme caution if you are not sure that you are within the limits on the datasheet.

Edit: you may want to initially test the welder outside if you're using 18650s just in case

 

[SentinelAeon]

I tried the setup today by adding 23500 uF cap bank (35V handling). Input into spot welder was about 7.5V. I was certanly able to make a dent in the thin battery tabs now, not enough to spot weld to batteries though.

So now i am still stuck between either rectifier is blocking to much current or the board is not working right. Rectifier i can test by just shorting it through a 0.2mm tab and see if it makes a hole, but for the board, i will have to find someone with a proven lipo or car battery.

I really didnt think i would have a problem with to little juice when using a transformer. Rather i thought i would spend lots of time fine tuning it to prevent holes in batteries. And i find it mind bogling that a phone size lipo in that pocket spot welder puts out a lot more juice

 

[Galgso]

A 23.5mF capacitor bank charged to 7.5v will discharge to 0v in less than a millisecond (587.5uS) assuming a constant current draw of 300A (I = C * dv/dt). Since you're discharging through a resistive load (not constant current), the time will be slightly larger and dictated by the time constant formula. The issue is that at 7.5v the caps will almost immediately go below 6.5v which is the minimum working voltage of the board according to your image. This means that you will not be using the full energy of the caps because the board will stop working almost immediately when the voltage drops. If you can, try charging them up to ~15v since they are rated for 35v. You may get better results with that.

 

[SentinelAeon]

In the meanwhile i did some more testing and it would appear that rectifier certanly holds part of the blame. I tried shorting the outputs over 0.2mm battery tab and it barely left a mark. If i did this with transformer output, the tab would instantly vaporise.

I also did current testing and shorting the outputs, i get 10 amps. So, i dont know if the problem is that it's a cheap chinese rectifier, or that its rated for 1600V 100A and basicaly 10 amps is all i can get out at such low voltage.

 

[Galgso]

I also did current testing and shorting the outputs, i get 10 amps. So, i dont know if the problem is that it's a cheap chinese rectifier, or that its rated for 1600V 100A and basicaly 10 amps is all i can get out at such low voltage.

Did you short the output of the bridge rectifer and get 10 amps AC? Are you measuring with a true RMS meter? Normal ammeters can't measure non-sinusoidal waveforms accurately.

Also, if you measure the forward voltage of one of the diodes using the diode check function on a multimeter (you can measure between either of the AC inputs and one of the DC outputs) what do you get? For a high current bridge rectifier like the one you have you should get less than the normal diode drop of 0.6v-0.7v when measuring with the low current produced by the multimeter.

 

[SentinelAeon]

I will describe the problem, just to be sure. So, my transformer can easily supply 100A at 7V AC. I connect this transformer to rectifier to convert to DC. The problem is that i expect close to 100A coming out of the rectifier. In reality, i don't even get enough to kill a 10A fuse. And since i want transformer to be used for the actualy battery spot weld (and not just to charge the capacitors), this setup becomes useless. Since the transformer itself will vaporise a 10A fuse instantly, the only other explanation is that somehow rectifier is blocking most of the current, putting out under 10A, which for my case is useless.

I will measure the forward voltage and get back to you. Tnx

 

[SentinelAeon]

Forward voltage measuring is 0.51V

 

[Galgso]

That's pretty odd. Since a rectified sine wave delivers the same amount of power to a resistive load as an actual sine wave minus the diode losses (wouldn't be too significant in your case) I would have expected the fuse to blow. The only thing I can think of is that the bridge rectifier may be damaged but if it is reading 0.51v then it seems to be working fine.

Usually diodes that are rated for less current than you are passing through them will fail short but in your case that doesn't seem to be happening as you can still read the forward voltage with a meter and it is not zero (which would be a short). I don't really know why this is happening.

The only thing I can think of at this point is if you run 10A through the diodes with a current-limited DC supply what the forward voltage would be. If you still get a normal forward voltage (should be around 0.8v @ 10A according to the datasheet of the bridge rectifier in your picture) then I don't really know what the issue would be. Maybe one of the more experienced members here can help you.