help with se-1010 battery charger wiring diagram

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I have an old Schumacher SE-1010 10 amp battery charger that takes good car batteries greater than 16 volts if left connected.
I posted about it here last week: http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2484741#Post2484741

below is the wiring diagram.
what would cause this, and how can i fix it?

the gray bar at the top with the red lead going to the battery I'm guessing is a big diode which rectifies AC to DC ?
I don't understand the 5-ohm resistor in parallel with the on/off switch, how is thing not on all the time when connected?
My wiring diagram is correct.

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[Stu_Rock]

The gray bar is the rectifier. The transformer is center-tapped, so this design only requires two diodes in the rectifier for it to be full-wave.

The resistor is odd. I guess in OFF mode, the charger would maintain the charge on a battery. I don't know how well it would work, though.

 

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just figured it out with a meter.
not connected to battery with charger switch off but plugged into wall the battery leads are 13.5 volts. When charger on they are 14.x volts.
with battery connected and switch on, the battery goes right up to 15.6 volts and current was measured around 3.4A. If i leave it on the voltage will climb over 16 volts. When charger off battery connected the 5 ohm resistor limits current and it's maintaining the output leads right at 13.2 volts with 132mA current, so i guess it's a float charger.
What I think I might try is buying some 10w 10-ohm resistors and wiring things up to drop the 5-ohm resistance down to 3 ohms to get voltage around 14.5 volts.
What would really be nice is a 25w variable resistor, then I can just tune the thing over time and would have a nifty charger with a manual absorption mode.

 

[JimPghPA]

Re "What I think I might try is buying some 10w 10-ohm resistors and wiring things up to drop the 5-ohm resistance down to 3 ohms to get voltage around 14.5 volts."

I think one problem you have is that 40 or 50 years ago the AC RMS plug voltage was 110 Volts RMS AC.

Now days most plugs in most homes are 125 Volts RMS AC, and some are a few volts higher than that.

The charger you have is a simple transformer and full wave rectifier (center tap). There is no regulator, and therefore an increase in the AC RMS in causes a proportional increase in the DC output that this set up provides. Unfortunately the voltage that 12 wet cell batteries should be charged to has not increased over the last 40 or 50 years.

If you add some parallel resistors to the series resistor used for trickle charging, you still have the problem of the DC full wave voltage being too high because the AC line voltage in is now too high.

What you require is a bucking transformer or a variac to drop the incoming 125 RMS AC down to 110 RMS AC, or a bucking transformer to drop the output of the units transformer 16 Volt Peak (+ diode drop voltage) down to 14 Volts Peak (+ diode drop voltage).

If you put three silicon diodes in series, they would drop about 2 volts, but those would have to be diodes big enough to handle the forward current, and they would require heat sinks that were each electrically isolated.

 

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thanks,
i started thinking last night about dropping the input AC voltage to modify the output DC voltage, hadn't thought of the 120VAC vs 110VAC as the cause.

would a rotary dimmer switch allow for dropping input AC voltage down to 110 and 100 VAC with acceptable control?

was also thinking of finding a rheostat that I could modify the output DC voltage with charger switch off.

 

[JimPghPA]

Re: "would a rotary dimmer switch allow for dropping input AC voltage down to 110 and 100 VAC with acceptable control?"

No.

Re: "was also thinking of finding a rheostat that I could modify the output DC voltage with charger switch off."

Any rheostat that could do the job would be huge, and expensive.



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Probably the best solution would be to put three silicon diodes in series and then put them in series with the output. Of course those diodes would have to have a forward current rating greater than the amount of current that they would see, and each would require an electrically isolated heat sink.

 

[JimPghPA]

Also, many transformers have the input winding as the inside winding. If the outside winding is the thick wire that goes to the diodes, then another fix would be to remove some windings from the secondary to reduce the output voltage.

This would be a low cost / no cost fix.

 

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i picked up a few 8-ohm 20watt resistors at radioshack. of course they didn't have 10A 5x20mm fuses for my meter which i blew measuring current. but with two 8-ohms in parallel battery voltage on a napa group24 that went dead last weekend is holding nicely at 14.55 volts and at 0.3A. with the charger switch off and current going through the original 5-ohm resistor the battery voltage yesterday was at 13.2 volts and this evening when i got home had climbed up to around 13.8 volts.

i scored and found rheostats at alltronics for $25 or less that are rated 50 and 100 watts. going to grab one along with some voltage and current panel meters and rig this thing up. thanks for the help.

 

[JimPghPA]

Because the output is a center tap, if you reduce the windings on the secondary you would have to do it in a balanced way, or you could change to a single diode output.