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Generac Generators (SEARS, etc.)

Generac Voltage Regulator Reverse Engineered...

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Old 06-30-2010, 12:30:38 AM
jimgrease jimgrease is offline
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Default Generac Voltage Regulator Reverse Engineered...

First post here at smokstak. I found this forum while doing research on my new to me but very old JC Penny 5000 watt generator. It's made by Generac and has a 10 HP Briggs side valve engine.

The voltage regulator board is identical to the MC-40 PCB posted here. There is only one part difference that I can tell, my board has an extra MOV.

Anyway, my generator runs but the voltage output was low. The 120 was at 36 volts, with the 220 at 72 V.

So I started the tear down and inspection process last weekend and finally got around to removing the potting from my PCB. Whew, what a mess

I'm about finished with the schematic drawings and I also know that at least one part on my PCB is fried. It happens to be the extra MOV, and it's shorted out depriving the Op-Amp IC of power.

If anyone is interested in my work, please reply and let me know that there is still interest in this old voltage regulator. I can post schematics and continue the discussion on this circuit.

I was looking around to see if anything newer could be easily built, but there are no complete schematics of a better circuit that I could find on the internet. If you know of any, please post a link. I'd like to compare this old op amp circuit to more modern designs.

Here is a photo of this old 1978 JC Penny generator:

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Old 06-30-2010, 10:58:58 PM
jimgrease jimgrease is offline
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Default Re: Voltage Regulator for MC-40 Generac


The schematics seem larger than I thought considering it's just a black and white jpg. So I apologize if this ends up wasting someone's bandwidth

First up is the schematic without extra comments:

Next is the same schematic with some of my comments (which may not be right, always double check)

Last, we have the previous PCB photo with my part labels added:

The PCB part number for my board is: 42323R3, and as I said before the only difference is an extra MOV on my board compared to the R2 version posted earlier.

My parts list:

Resistors (1/4 watt, 5% unless otherwise noted)
R1 39k
R2 82k
R3 47k
R4 10k
R5 3.9k 10% 5watt (power resistor)
R6 200
R7 200
R8 47
R9 2.7k
R10 220
R11 20k 1%
R12 20k

Capacitors (note that I originally thought two MOVs were caps, hence the out of sequence numbers):

C1 .22uf 100V
C2 10uf 16V polarized electrolytic (I'm going to replace with a tantalum)
C3 .0015u 100V
C6 .22uf 100V


741 Op Amp 8 pin IC (guess based upon circuit config/pinout)
UJT 2N4871
SCR1/2 6006L
D1-D3 MR501
D5/D6 1N4005
20V Zener 1N4747_A (5%)
Rectifier SD?291 M7816 Motorola

Metal Oxide Varistors MOVs

MOV3 unknown, no label
MOV4/5 labeled: 3004 GEE8 (could not find data on these, I initially thought they were capacitors)

To trace out the schematic, I printed out in large size the previous two photos of the R2 PCB. Then I taped the printouts together back to back, and traced out the circuit. Finally I compared this to my own board, they are the same except for the R2 or R3 portion of the part number.

I think I'll start a new thread titled: "Open source voltage regulator circuit"

I'd like to compare this complete circuit to other designs and try to understand which configuration would yield the best regulator for a variety of generator loads. Please post if you have a suggestion for said circuit.

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Old 06-30-2010, 11:06:08 PM
jimgrease jimgrease is offline
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Default Generac Voltage Regulator Reverse Engineered...

Originally Posted by golddredger View Post
Ok, here's the problem...I've got a 1978 generac 4kw generator, mod # 6897-3, has a bad voltage regulator I believe....found the diagnostic and repair manual online, went thru the genny, I'm getting 150 volts at the terminals out of the generator....

Based upon what I've just learned about my R3 board (with many thanks for your photos of the R2 board) I suggest you look at that potentiometer closely for your over voltage running condition.

That potentiometer sets the feedback level that the op amp compares to the set reference. The reference is controlled by the zener, and I wouldn't suspect that to be off in voltage. However, if the pot drifted somehow, then the voltage output from the generator would change.

Have you tried adjusting the pot carefully while it's running? When I put my unit back together, I'm going to make that potentiometer accessible for running adjustments.

Be careful not to get shocked if you try to adjust while running

I'm fixing up an old 1978 Generac 5000 watt, JC Penny branded 10 HP Briggs generator. I've posted photos and a schematic of the old analog circuit board in the Voltage Regulator for MC-40 Generac thread

This is an old design, and I wonder how good it is compared to more modern voltage regulator circuits. So I decided to start this thread and ask the more knowledgeable folks here to comment on the best voltage regulator designs.

It would be great to have a collection of circuit schematics to compare with the one I just posted to the MC-40 thread. But that may not be possible unless lots of folks have done what I just did and removed the potting from their boards

Not likely, I know. But I'll try asking anyway.

Let me also ask an easier question: Which generator over the years had the best reputation for starting difficult loads?

I heard that Homelite generators were good in this regard, but I admit that I don't have much actual experience to draw from when it comes to portable generators.

In my case, the worst two loads I'm likely to try running from the JC Penny gen is my shopsmith and my smaller arc welder. Don't know if my regulator board/10 HP mechanical governor Briggs will handle that

It should be fun to find out. The JC Penny 5000 watt generator lacks circuit protection. I suppose I should at least add a pair of fuses before trying a larger load.

Anyway, if you can tell me which mechanical governor generator had the best output regulation (or perhaps point out which had the worst output regulation); that would at least get me starting searching for voltage regulator schematics.

As far as I know today, the circuit schematic I just posted in the MC-40 thread is the only complete schematic for a generator voltage regulator on the net. That one can be scratch built by anyone looking to restore an older generator. Let me know if there are any others I've missed in my searching.

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Old 07-01-2010, 11:16:40 AM
jimgrease jimgrease is offline
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Default Re: Open Source Voltage Regulator Circuit design

Post #11 was my attempt to start a new, generic discussion with the forum members regarding voltage regulator circuit boards. A topic that seems to interest at least some of the folks here based upon my previous searches on regulator circuits.

This head post was moved here and inserted as another reply??? The new thread I started appears to be missing? No explanation given for this move/change

I suppose I may have broken some forum rules somewhere, but it would have been nicer to have a quick explanation as to why the change?

With my post buried in another person's thread, I'm not sure we will have as good a discussion regarding regulator circuits as we could have had.

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Old 01-17-2011, 05:41:30 PM
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Talking Generac Voltage Regulator Reverse Engineered...

This is very good work. Thanks Jim... I am making it a Sticky....
Bruce in alaska
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Old 01-21-2011, 12:58:33 AM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Thanks for the sticky on this topic.

I do hope we can continue the discussion and perhaps design a more modern and useful "open source" regulator circuit.

I'm still working on my Generator. Life has taken it's usual twists and turns so that some of my projects have been put on the shelf. Or in this case, the corner of the garage, waiting for me to get back to it. Hopefully soon.

I still sneak peeks on SmokStak when I have a free moment. So if anyone has anything to say regarding voltage regulator circuits, please post.

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Old 01-24-2011, 09:28:02 AM
David K David K is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Originally Posted by jimgrease View Post
So if anyone has anything to say regarding voltage regulator circuits, please post.


Yeah I got something to say.....I wish I had just 1/10 of your electronics knowledge!! Engines I'm good at. Electrical work I'm good at. But electronics.... I'm practically lost from the word 'go'. None the less, I'm following your work as it is very useful. Thank you.

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Old 02-01-2011, 12:38:10 PM
Warren914 Warren914 is offline
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Default Generac Voltage Regulator Reverse Engineered...

I need to do something similar for a Coleman Powermate generator. The AVR boards are getting very scarce and expensive!

A simple regulator would rectify the DPE line to power the board. The AC output needs to be rectified and sent to a comparator to control a series transistor for the field winding. All relatively simple.

I built a similar circuit for the alternator regulator in my 914 Porsche. You can do anything with an op amp and handful of other parts.

Here’s my take on the circuit description. MY UJT and SCR theory are kind of weak. Both are relatively simple but I'll have to hit the books and brush up a bit.

It looks like the DPE coil is center tapped and referenced to ground. Diodes D4 and D5 form a full wave rectifier to create an unregulated DC power supply voltage. The Zener regulates the voltage for the op amp and UJT as well as the comparator reference voltage. 1N4747 is a 20V Zener. Oddly there is no capacitor across the Zener to smooth the regulated DC.

The generator AC output is sampled through R1, R2, and the 50k potentiometer. The potentiometer adjusts the voltage that is rectified and filtered by C2. R3 / C2 form a ˝ second time constant to discharge the sample voltage in the case of heavy load causing output voltage drop. This allows the op amp to compensate and bring the voltage up again. Without R3 the sample voltage would remain high for an extended period even if the AC output voltage dropped considerably.

The op amp attempts to make the inverting and non-inverting inputs equal by controlling its output voltage. R11 and R12 form a voltage divider to create a fixed 10VDC reference at the non-inverting input. The DC voltage from the sample circuit is applied to the inverting input. The op amp adjusts its output to the UJT to increase or decrease the generator AC output voltage that is then fed back to the comparator.

C3 helps prevent oscillation at the op amp output.

R9 and C6 form a very short time constant at the input of the UJT. This helps prevent oscillation in the field circuit.

The UJT is essentially a variable resistor. It forms a voltage divider between 20VDC (Zener supply), R10 and R8. When the voltage at the junction of R6, R7, and R8 reaches the SCR threshold voltage they will switch on. SCR1 / D2 and SCR2 / D3 act as pairs to conduct through the field coil. The circuit can be redrawn to show DPE as an AC input, and the field coil connects to the full wave rectified DC output. This allows the field to conduct for up to the full AC cycle as determined by SCR1 and SCR2 turning on and off. Higher levels of SCR conducting will increase the field voltage and thus the AC output voltage.

MOV are essentially voltage variable resistors, almost like a zener diode. When the threshold voltage is reached the resistance will rapidly decrease. They are spike type protection devices, and under normal operation should not conduct. MOV5 limits the DPE output voltage and MOV3 limits the unregulated DC voltage applied to the Zener regulator.

D1 is reverse biased across the field coil. When field voltage is removed there will be a reverse voltage created that is discharged through this diode. Without the diode, there could be a very high DC voltage created that could potentially damage the MOV3 or MOV5. MOV4 is protection against spike voltages going to the field.
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Old 02-01-2011, 01:14:35 PM
Lloyd H Lloyd H is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

First I want to thank Jimgrease for all the work involved with removing encapsulation and drawing out the schematic. Warren I think you have it nailed except I don't believe the DPE winding is center tapped, if it were D2 and D3 would full field the rotor irregardless of the SCR conduction. I struggled with the circuit for a while until I figured the DPE ground reference to the rest of the circuit was through the field. I too had to brush up on UJT theory. Lloyd H
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Old 02-01-2011, 02:19:44 PM
Warren914 Warren914 is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Agreed on the DPE winding. It's a single wiring with no ground reference. I went through the diagram again and redrew the SCRs and diodes. The DPE return is through diodes D2 and D3 forming a full wave rectifier to ground through the coil.

Still doesn't look right. I'm confused with the polarity of the SCRs, D2, and D3. If the position of the diodes and SCRs were changed and all polarities reversed, D4 and D5 would create a full wave bridge rectifier with D2 and D3 connecting to ground at wire F.
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