Weak/No output from car battery charger

[GuySmily]

Hi guys,

I'm trying to figure out what's wrong with my battery charger.  It's an old (70s/80s?) Schumacher SE-1275A.  Where should I start looking to fix this thing?  I can solder and touch stuff with my DMM probes, but I'm still taking entry-level engineering classes (Physics of Electricity, etc) and haven't taken circuit theory yet.



I want to use this charger to power an electrolysis tank (for cleaning rusted metal), so even if I have to disable/bypass the PCB (more on this later) and break any "smart" charging capability, that's okay with me.  It would be nice to charge/jumpstart cars with it too, but that's optional at this point.

First of all, here are the voltages I'm seeing at the clamps under various situations:


Note that the electrolysis tank had been energized recently - maybe it holds a charge for a short time after being disconnected, and that caused the voltage reading even with the charger unplugged.

Anyway, the charger seems to work at least partially at 75A.  I get a ton of bubbles out of my electrolysis bath at that setting, but I know I can't leave the charger going at that level.

[GuySmily]

It sounds like a common weakpoint on these old chargers is the SCR/Thyristor.  (I don't know understand semiconductors/etc, but it sounds like this is basically a switchable diode?)

This is a Littelfuse Teccor brand Thyristor, 200V, 55A "Standard SCR."  The big terminal is the cathode, the small terminal is the gate, and the anode is attached to the heatsink.





The SCR package looks okay, even though the terminals are a little bit toasty - I'm guessing that's burned flux on the pads, but I don't know for sure.



Anyway, I tested the SCR with the procedure here (using an ohmmeter since that's all I had at the time), and it seemed to pass all the tests:
http://www.androiderode.com/how-to-check-a-scr-with-digital-multimeter/
Nevermind, I did this test and it didn't work.  Might be that my multimeter doesn't put out enough power to activate the gate.

[poot36]

If you cant find the service manual or a schematic of the charger it looks fairly easy to reverse engineer if that is the only board in the unit.  From there you could then safely connect your meter to the gate and the ground of that side of the circuit and see if there is the correct voltage there to turn on the chips gate based on the chips datasheet if you can find one for it.  It may also be senseing that there is not enough load on the charger and refusing to start up with the electrolysis tank.  I would recommend trying a old ATX PC power supply instead because it usually will not care about no load and has many other safety features built in.

[Joule Thief]

Some chargers will not apply charge voltage to the battery unless the battery supplies at least 7 - 9 volts to be sensed at the charger's output leads. Marine chargers are known for this characteristic (so as not to spark in the bilge if the charger leads are shorted together or when leads are applied to the battery terminal while the charger is powered on).

Some chargers will not begin to function until a few hundred milliamps are sensed being supplied into the load.

You may want to try placing a 12 volt car tail light bulb across the chargers leads in parallel with the electrolysis cathode / anode connections.

[bobcat]

It looks like normal constant current mode operation. The voltage is limited, and the voltage varies according to the current. That is the first stage of battery charging.
for an electrolysis tank, you need constant voltage but current limited operation. - Different kind of power supply.

[GuySmily]

Thanks so much for all the responses!

If you cant find the service manual or a schematic of the charger it looks fairly easy to reverse engineer if that is the only board in the unit.  From there you could then safely connect your meter to the gate and the ground of that side of the circuit and see if there is the correct voltage there to turn on the chips gate based on the chips datasheet if you can find one for it.  It may also be senseing that there is not enough load on the charger and refusing to start up with the electrolysis tank.  I would recommend trying a old ATX PC power supply instead because it usually will not care about no load and has many other safety features built in.

Yep, that's the one and only board in the charger.  I checked the specs for my meter (innova 3320) and the SCR.  My multimeter's diode test (1.6mA max) isn't enough to trigger the gate (5mA min).  So I plugged the charger in and poked around that way instead.

I see .6V between G (gate) and A (anode/ground/heatshield).  I assume this is good enough to trigger the gate, with a Max of 1.5V and no Min specified, but I can't check the amperage without desoldering.

I'm going to assume that it's not safe to check resistance between K->A while this thing is plugged in.   Voltage test should be useless since half of a sine wave is passing through it if it's working, right?  But I see 2.26V in DC mode and 0.44V in AC mode.  Shouldn't this be showing half of a 12V sine wave?  What would that look like on a multimeter in DC or AC mode?

I read about folks blowing out computer PSUs using them for electrolysis, but I'm willing to give it a try if I can find one in my parents' garage.  It's sounding more and more like that's gonna be the best route.

Ideally I'd love to do a setup like this, but it'll take a bit more money (and time) than I have right now.

Some chargers will not apply charge voltage to the battery unless the battery supplies at least 7 - 9 volts to be sensed at the charger's output leads. Marine chargers are known for this characteristic (so as not to spark in the bilge if the charger leads are shorted together or when leads are applied to the battery terminal while the charger is powered on).

Some chargers will not begin to function until a few hundred milliamps are sensed being supplied into the load.

You may want to try placing a 12 volt car tail light bulb across the chargers leads in parallel with the electrolysis cathode / anode connections.

I tried adding a battery in parallel (and killed the battery), but not a bulb.  I'll give that a shot.

It looks like normal constant current mode operation. The voltage is limited, and the voltage varies according to the current. That is the first stage of battery charging.
for an electrolysis tank, you need constant voltage but current limited operation. - Different kind of power supply.

That's one of my concerns.  I was wondering if I could hack this thing to skip all the circuitry on the board (except for the DC filtering) and just hook the transformer straight to my bath... But it's sounding more and more like a PC PSU will be easier.

[poot36]

It looks like that PC power supply setup that you posted is just using the input filtering and rectifier to provide 150 to 170V into the electrolysis tank.  That could be very dangerous if you do not know what your are doing!  I would stick to 12V until you feel that you need to use more power.  This may be a bad idea but have you tried jumpering the K and A terminals on the SCR and seeing what happens?

[GuySmily]

Oh, lol, I just posted that PSU pic as an example.  That one isn't really a PC PSU - it just looks that way.  It's actually a "grid charger" for the my Honda Insight hybrid  (like a plugin conversion).  I just love the case and the slick panel-mount meter.

Anyway, jumping K and A would be like bypassing the SCR (or ignoring the gate) altogether, right?  I'll play with it tomorrow.

I picked up a PC PSU today, so I might just switch efforts that way if this isn't easy to fix.

[poot36]

Yes jumping K to A would be like bypassing the SCR switch function.  Just make sure to measure the output to make sure that it does not exceed the voltage rating of any caps that are on the output side.  As far as I know a SCR passes the full AC waveform through it.

[Negative-Bias]

Left hand side as you look at it 3rd track down has the track lifted

[Seekonk]

Not sure you would really want to use this board if it could be fixed. There are many simple ways to regulate this voltage for your application.  Why don't you define what you need.  Volts, amps, constant, current, or constant voltage.  What do you have around? An old UPS would be ideal.

[BurningTantalum]

I agree with Seekonk - It is a shame to butcher this unit. It seems to be a rather nice battery charger and was probably not cheap in its day. I have a model that probably works on the same principle and it has followed me around since my youth, rescuing batteries on everything from old motorcycles, mowers and cheap cars to light aircraft.
BT