New wheels for the Schumacher battery charger

[cathead]

Another scrapyard treasure that needed some help followed me home a while ago. There isn't anything wrong with it except that the ammeter is gone and a few loose wires, the fan needed oil, and the wheels were all but shot. I removed the old plastic wheels in favor of a pair of ball bearing
wheelchair wheels that I had left over from a previous project. The circuit for this Schumacher 4020SE charger is somewhat interesting in that
the ammeter seems to be connected to the primary side of the transformer somehow. Apparently it used an AC ammeter although I am not positive
on that point. At any rate, if I can come up with an appropriate meter and wire it up, it should perform as intended. Meanwhile the unit has
varying voltage outputs for the different charging rates so basically it is working.

It turned out that the wheel chair wheels were a perfect fit to replace the plastic excuses for wheels that were on there. The new wheels have ball bearings and work really smoothly around the shop now.

Schumacher seems to have a lot of circuit diagrams on the web for this basic style of manual machine. Basically it has a transformer with a
tapped primary giving varying voltage outputs to the secondary of the transformer. It uses a full wave rectifier circuit using two diodes.
Interestingly, the large box is at least half empty with the transformer and fan in the bottom of the box, impressive until you look inside.
I have another Schumacher with the same 200 AMP boost in a small box with a carrying handle, probably the same basic circuit. Maybe I
will rip into that and see how that one is configured. To it's credit it is an analog charger that will last forever if taken care of in comparison
to the SMPS(switch more power supply) chargers that are currently on store shelves.



I still need to do a little more cleaning on this unit as the pigeons didn't have much respect for it.

 

[markba633csi]

Probably cheaper for them to put an ammeter in the primary and possibly gives a more accurate reading overall with stock meters
The output is of course rippling dc and might give a lower reading unless the shunt was adjusted to compensate
Normally you don't need a lot of accuracy for charging batteries though
Nice find!

 

[cathead]

I wasn't satisfied to have a battery charger with no ammeter so I decided to use an old ammeter
from a scrapped Miller 3 phase arc welder. The original meter in the charger was measuring
the primary side of the transformer using an AC ammeter. The DC side is easier to deal with so
made a shunt on the meter I had and will install the meter in the secondary direct current side
of the transformer. Installing my Simpson 260 VOM in series with a 12 volt power supply,
an automobile headlight and the new shunted meter, I determined that 25 amperes on the
meter was actually 5 amperes according to the Simpson. That would mean that full scale on
the shunted meter would be about 60 amperes, good enough for a relative reading on a battery
charger.
This is the face of the meter. I may recalibrate the face of the meter to read 60 amperes if I can disassemble the
meter face.



The copper strap is the shunt, a very thin piece of copper flat material. The full current goes through the copper
shunt and a very small portion passes through the meter. A shorter shunt copper would make the meter less
sensitive if that proves to be needed. Of course the battery charger output is connected to the left pair of
terminals and can be installed in either the positive or negative lead. The black insulating material I used is
real Bakelite one of the first plastic materials invented. It's a mixture of phenol and formaldehyde and has a
peculiar smell when drilling on it. The Bakelite came from a very old refrigerator I found out in the back woods.

Next I have to carve a hole for the meter in the front of the Schumacher battery charger and wire it up. One other
thing one could do would be to add a SPST switch to short out the meter if I need to use the charger on the 200 AMP
boost setting. I will do a little experimenting and see...