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