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Charging circuit drawing too much current

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Charging circuit drawing too much current

Postby rms59 » Sun May 19, 2019 10:20 am

Hi,
Craftsman Lawn Tractor
Model 917.271022 with Kohler 15.5 hp engine
The wiring schematic is here: https://i.imgur.com/08Cj187.png

PROBLEM -The 3 Amp Diode coming from the 28VAC alternator/stator winding burned out and was replaced with the exact type as the old one. After a minute of run time the new Diode gets hot enough to melt it's solder connections.

As you can see in the schematic, the stator has only two wires coming from it and with the engine running I get 14VAC and 28-30VAC at their respective stator windings. When I attach a common 12 volt 3 Amp tail-light bulb to the 14VAC it lights up as bright as it would be expected. I quickly tapped the same bulb to the 28VAC and it also (but brightly) lit up so I know the stator is working since it has the correct voltage and has no problem supplying current on demand.

Without going into detail explaining all the tests, fixing bad grounds, and frustration I've gone through I finally decided to isolate the 28VAC circuit entirely by disconnecting the positive battery terminal so nothing in the tractor would be drawing current. I had to use a small battery to keep the fuel shutoff valve energized and also removed the stator brake circuit, both relays and jumped the seat switch so the engine would run after the start sequence.

So, the "charge" circuit now goes like this.... The 28VAC alternator lead is connected to the anode of a 3 Amp Diode. The cathode connects to one wire of a current meter and the other wire of the meter connects to the positive terminal of the battery.

I momentarily connect the +battery cable to the +battery terminal to start the engine, then remove it so only the cathode of the diode remains connected to the +battery terminal. My current meter shows a 6 Amp current draw at idle and 9 Amp at mid range continuous. Understand there is nothing else connected to that battery. Suspecting a battery that may be shorted or a positive terminal having some current leakage from its case to ground, I replaced it with a working fully charged battery from another tractor and got the exact same result. That diode could boil water and the battery is the only load.

I'm using a silicon diode and observe a forward voltage drop close to .7V. (Vr is 50 and Io is 3.0)

What am I missing? Is there some current limiting wire that I'm unaware of on this tractor? If so, it would have to be enormous.
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Re: Charging circuit drawing too much current

Postby bgsengine » Sun May 19, 2019 11:21 am

while running that test,observe voltage at the battery,as well as engine RPM. Function of the diode is to rectify half the AC voltage wave into DC output , only. Current draw would be a function of the resistance of the circuit - higher load/resistance anywhere along the line would cause higher current draw, so I'd want to check the resistance of the whole circuit. Might be a poor ground of the stator, weak connection under load (only 1-2 strands of wire making actual electrical contact for example at the solder connection to the diode?) might increase resistance. fully charged battery would be around 12.8 volts , unregulated charge output would top out at 14.8 volts DC.
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Re: Charging circuit drawing too much current

Postby KE4AVB » Sun May 19, 2019 3:35 pm

These diodes should be crimped in place and not soldered. You are trying to dissipate 6-7 watts @ 9 amps.
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Re: Charging circuit drawing too much current

Postby Arkie » Mon May 20, 2019 8:36 am

Couple tests:

Check the front to back resistance ratio of your existing diode. (if the front to back ratio is low the diode is passing AC current and the amps will be high and some ammeters will not differentiate between AC and DC current. You are reading AC current instead of DC and your stator is going to eventually burn.
Diode front to back Should be low one direction and very high the opposite direction when tested with a ohmmeter.

If that is ok check the 28 v AC to the diode under load (while the diode is connected to the bat.
It should drop down to around 14-16 volts at 3000 rpm or so. It should not remain high. The 28 v AC output (on the wiring diagram) from the stator is a no load voltage.
You can connect the or a diode to the 14vAC light wire as a temporary test of charging current also. (do the tests like you are doing now but use the 14vAC stator supply) Disconnect the loght so as they will not get turned on and overload the stator output.

Of course the reason your silcon diode is getting hot is the charge current is too high which you have determined (3 amp diode at 6-9 amps is going to have a melt down)

For testing so as I would not be stressing the existing EXACT replacement diode I would probably get or use a 10-12 amp diode with a PIV of at least 100 volts.

Do you know if the existing flywheel and stator are the original OEM for that engine? (have you always owned this rig)
Reason I ask is we are assuming that the flywheel or stator has not been changed out in past to the larger charging system and the 3 amp diode is not correct for the parts used?

Let us know what you find?
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Re: Charging circuit drawing too much current

Postby rms59 » Mon May 20, 2019 2:28 pm

All good stuff.... Thanks.

@bgsengine... My connections are minimum. I only have a solder joint from the anode of the diode to the 28VAC wire from the stator. Then a strong VOM meter lead from the cathode to the battery terminal. Ohms Law I=E/R would dictate that any extra 'R' in that line would lower 'I' instead of causing more current draw assuming 'E' stays constant. I did check the stator (which still looks like new) and reinstalled it using lockwashers to get a good ground. Funny thing is that the diode charging circuit looks to be going through several connection points (ammeter, fuse, a connector, ignition switch, and starter solenoid jumper, before getting to the battery as if resistance was low on their totem pole.

@KE4AVB... I checked my Husqvarna which has a similar 2 wire system and I see a very small diode that only draws 1 amp at idle and 2 amps at WOT. The diode looks soldered or could be welded but I will try the crimped leads anyway tomorrow when I get back to that tractor.

@Arkie... Yep. Same stator and flywheel with all the magnets that ran for years with no problems but suddenly stopped charging. Ohmmeter inspection showed diode open both directions. Added new diode (same number) and it got extremely hot. Checked everything and finally isolated it like I already stated and it still gets hot. Running out of diodes, I grabbed an encapsolated 12 amp 50V Full Wave Bridge rectifier and wired it to use one of it's diodes. It's warm but not grossly hot. Still drawing 9 amps when it should be a lot less.

I did try the 14V light circuit during my testing and it got warm but wouldn't charge the battery. I'll have to make another temporary test on the 14 and take some current readings.

This is the MR506 diode that was in there for years. http://www.surplus-electronics-sales.com/index.php?main_page=product_info&products_id=802

I definately will get back when I find the culprit.
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Re: Charging circuit drawing too much current

Postby Arkie » Mon May 20, 2019 7:01 pm

You need to check the AC voltage supply when the bat is charging a 6-9 amps.

You say you have a extra 12v stand alone donor battery that you subbed before.

Consider doing the following.

charge up your spare 12v bat. (one you know that is good and takes a full charge from a automatic battery charger)
Isolate your mowers charging output circuit. (disconnect the output of the diode from the existing mower battery charge circuit so it will be going to the stand alone battery ONLY for testing.
Set the stand alone charged battery on the floor by the mower and connect a wire from the stand alone bat's NEG post to the frame of the mower, start the mower using only the mowers own battery, and let the mower idle and then connect the diode output charge wire to the POS post of the stand alone battery and at same time monitoring the charge DC amps. If the DC charge amps are too high (more than 2-3 amps) going to the stand alone bat that is already charged up the AC output coming out of the stator will be too high. You cannot get too high DC ouput charge amps unless the AC output out of the stator is too high when doing this test with a good battery because the already fully charged stand alone battery is completely isolated with no additional loads being involved and only the mower charging circuit involved..

Make sure your ammeter is reading correctly. Compare it too a battery charger ammeter as a battery is charging.

Let us know what ya see?
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Re: Charging circuit drawing too much current

Postby KE4AVB » Tue May 21, 2019 4:31 pm

Here is something to try to verify current load. Install a 1156 incandescent bulb with the battery out of the circuit you should have a current of 2.1 amps @12.8 volts so bulb would have about 6 ohms of resistance. Of course if the applied voltage is higher so will be the current so you need to recalculate what the amps would at the new voltage.. But at least this will give a known resistance load to work from so you can verify your amp meter reading.
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Re: Charging circuit drawing too much current

Postby rms59 » Tue May 21, 2019 6:31 pm

Back again.. Still a no go...

I disconnected both AC Stator wires so I could measure the resistance to Gnd for the tenth time. The lighting coil (black) measures around .1 ohm barely moving my meter's needle and is repeatable. The charging coil (green) measures around .5 ohm. This is from my old trusted Simpson 260 analog VOM with a smooth needle.

As suggested, I used a separate fully charged "test" battery that had a beginning voltage of 13.8 for the charge test and another for starting. With the "test" battery grounded to the engine block, and the diode connected from the green wire to the battery terminal, I get a current draw of 6 Amps at idle and 9 Amps at WOT with a voltage across the battery terminals between 17 to 18 VDC. This seems high in my book.

With everything still connected and running, the AC voltage at the diode anode (coil side) is 28VAC at idle and 40VAC at WOT. Not giving up on the possibility of having two bad batteries, I grabbed a battery out of a jeep and connected that up. Now the current draw was 10 amps at WOT.

P.S. I will check the 1156 bulb (at the cathode) and see what I get but I will have to wait untill tommorrow to check that since we sheared the flywheel key. May not have tightened the nut tight when checking the stator gnds. (which I cleaned and installed back with lockwashers to ensure a good ground). That stator still looks new with no burn marks, discoloration, etc.

Hope I left you a clue....
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Re: Charging circuit drawing too much current

Postby bgsengine » Tue May 21, 2019 6:42 pm

rms59 wrote:
With everything still connected and running, the AC voltage at the diode anode (coil side) is 28VAC at idle and 40VAC at WOT. Not giving up on the possibility of having two bad batteries, I grabbed a battery out of a jeep and connected that up. Now the current draw was 10 amps at WOT.
.

Hook up a tach and tell us what exactly is WOT RPM... 40 VAC seems too high for an unregulated alternator using a single diode. and assuming nothing different from original only way I can think to increase volts is to increase RPM... and if for some reason the governor went out and engine runs too fast that might be what happened in the first place? in fact, your charge voltage should be topping out at 14.8 volts if everything is correct... So 17-18 VDC at battery is overcharging
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Re: Charging circuit drawing too much current

Postby KE4AVB » Tue May 21, 2019 7:50 pm

rms59 wrote:Back again.. Still a no go...

I disconnected both AC Stator wires so I could measure the resistance to Gnd for the tenth time. The lighting coil (black) measures around .1 ohm barely moving my meter's needle and is repeatable. The charging coil (green) measures around .5 ohm. This is from my old trusted Simpson 260 analog VOM with a smooth needle.

Hope I left you a clue....

Double check me BGS but I think I see the problem here. RMS59, you're connecting up the stator wrong. The green lead is the stator brake (not the charge) and the black is the B+ (charge). You should also have a Yellow wire for lighting.

See the diagram from the Kolher SM below.
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