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Generac Generators (SEARS, etc.)

Generac Voltage Regulator Reverse Engineered...


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  #11  
Old 02-01-2011, 03:15:09 PM
Kevin K Kevin K is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

My take on the circuit:

D1 is used as a snubber diode for the Field. D2, D3, SCR1, and SCR2 form a full wave bridge rectifier with the DPE winding as the source and the field as the load. The bridge can be controlled, turned on or off, by the voltage on the gates of SCR1 and SCR2. I think this is called a thyristor regulator, since it uses SCR's instead of a transistor running in the linear mode. The advantage is it does not generate a lot of heat since the SCR's are either on or off. You can see this kind of circuit in the "Regulator/Rectifier" modules used to charge the battery on a lot of small engine equipment like motorcycles or snowmobiles.

One of the 120 volt output windings is sampled by a divider string consisting of R1, R2, a 50K pot, and C1. The capacitive reactance of C1 is approximately 12,000 ohms at 60 Hz, so the circuit is dividing the input voltage between 6.5% and 9%, depending on the setting of the pot. That sample is rectified by a full wave bridge, filtered by C2/R3, and sent to an op amp set up as a comparator. The 10 Volt reference voltage to the comparator is set by R11 and R12. If the sample voltage is below 10 Volts, the output of the comparator is V Zener, or 20 volts. This turns on the UJT, which turns on the SCR's, allowing current to flow from the DPE winding to the field. When the output voltage comes up to the set point, the sample voltage rises above the reference voltage, and the output of the comparator goes to 0, shutting down the UJT and the SCR's.

I think this circuit could be adapted to a lot of small generators that use a DPE winding and voltage regulator. Due to liability, you could not commercially repair any generators with this, but for your own stuff it could work when a replacement part is NLA or just priced out of sight. As soon as one falls into my hands, I am going to give it a try.

I think Onan used this type of circuit in a lot of their earlier generators. It might be worth it to ask a dealer if he has any defective regulators he would be willing to give me. I would like to reverse engineer one or two.
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  #12  
Old 02-01-2011, 03:18:10 PM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Gentlemen, thank you for the feedback.

It would not surprise me if there is a mistake in my schematic, its always good to have these things double checked.

Also, I'm glad we are focusing on the lower half of the circuit, from the UJT to the SCRs. Since this is my first generator rebuild, that is where the confusion is for me too

Let me also say: don't assume anything I labeled is correct. You might try erasing my wiring labels to see if the connections then make more sense.

The photos of the Printed Circuit Board (both sides) are in the other thread if you want to try tracing some connections visually.

My schematics were drawn in LinearTech's free switcher cad software, and I did attempt to simulate this circuit last year. I'll dig up that project and start studying this again.

To be continued.......................
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  #13  
Old 02-01-2011, 07:20:59 PM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

I took a moment this afternoon to review my notes on this circuit. I haven't found any mistakes so far, but I did redraw the lower half of the circuit based upon the descriptions posted here.

Lets see if this helps:


I did not include the following in the above hand drawn schematic:
- the feedback circuit to the negative input of the Op-Amp.
- MOVs
- protection diode D1

There are two bridge rectifiers shown in this portion of the circuit (there are 3 total in the entire regulator). Diodes D2 and D3 are shared between the two bridges.

All the current in this portion of the regulator returns through the field winding.

So I would say that as the voltage changes across the field when operating normally, the voltage across R5 also changes. It's weird, but wire F is the reference point for most of the circuit elements.

This gen set of mine appears to have thermal switches in the output windings. If the switch disconnects the load, and if the regulator is still being powered; then once the load is reconnected the voltage will be at maximum. There is a warning to disconnect anything attached to the generator output if the thermal cutoffs are activated. I was exploring ideas on how to prevent this overshoot for instances when the feedback signal drops to 0.

The easiest thing I could think of would be to add a relay that switches in the Zener only when there is power on the feedback circuit. Any thoughts?

Of course I need to get back to this project and start testing things out. Unfortunately, I'm still dealing with too many other more pressing chores at this time.

Feel free to take it and run with it
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Old 02-02-2011, 12:47:14 PM
Warren914 Warren914 is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...



This diagram would provide continuous DC to the zener diode. The SCRs would turn on as needed to power the field. Thoughts?

In your generator, when the thermal switches open the quickest way to disable the regulator is to use a transistor either in series with R5, in parallel with the zener, or across R12 at the op amp. In either case the UJT can be disabled and the output to the field turned off. Mechanical relays are slow and bulky.
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Old 02-03-2011, 12:02:34 AM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Warren, what you drew is how I thought the circuit was setup when I started this. I don't yet understand the electrical trade offs being made between a parallel connected field (which you drew) and a series connected field (which is in the old Generac regulator board).

I suppose it could have something to do with noise and spikes in the system. Would a series connected field be less of a spike problem to the parts in the zener regulated portion of this circuit?

Another theory is that Generac did this just to confuse those who try to reverse engineer the circuit

I don't know. I was hoping someone more knowledgeable would offer an explanation.

I mentioned the relay earlier because I was concerned about the control signal return path. The relay provides the best isolation between control signal and switched outputs; and I thought it would be safest for this application.

Another idea would be to add a triac to control the AC going to the Zener regulator using a completely separate bridge rectifier. That triac would then need to be controlled by the feedback network, possibly by using the DC value at C2/R3, but again, what about the return path for the triac control signal? I need to think about that idea some more and understand better what is actually happening in this Generac regulator circuit. Triacs and SCRs are not my main area of study.

I did look up UJTs in my old Industrial Electronics textbook. Here is the relevant page:

You'll notice that this UJT oscillator circuit is just like the configuration in the regulator circuit. When the Op-Amp provides enough voltage, I think the UJT starts providing triggering spikes to the SCRs. Feedback capacitor C3 must trade off regulator response time with output stability; giving the system the necessary damping. My limited simulations in Switcher CAD showed these oscillating spikes going to the SCRs.

Regarding the DC power for the Zener regulator, resistor R5 is rated for 5 watts. I think that implies about 90 Volts average DC across R5 if we assume the worst case power dissipation is 2 watts. This could explain the lack of a bypass capacitor on the Zener output; it just isn't needed here. There appears to be more than enough head room to operate the Zener regulator and I don't think this circuit would work well unless the 10 Volts at the voltage divider R11/R12 was constant.

Which brings me to a question: what is a typical DPE output level? This old 70s Generac seems to me to have a very high DPE output voltage (and dangerous), but I'm not familiar enough with backup generator design to know. How does this compare to more modern gensets? Perhaps my labeling this winding as 'DPE' is incorrect?

If I get any free time I'll attempt to breadboard the Op-Amp and UJT portions of this circuit and try to observe the SCR triggering spikes. I'll also try substituting a 741 Op-Amp for my unlabeled original to see if the circuit still behaves the same way.
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  #16  
Old 02-03-2011, 07:58:52 PM
Warren914 Warren914 is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

I measured the DPE voltage while repairing / testing my 7000EXL. It gets surprisingly high!
http://members.rennlist.org/warren/7000exl.html

I'm expecting a new AVR board for a Coleman Powermate generator to arrive in about a week. Before it gets installed I'll try to trace it out. It uses five wires from the alternator plus another two for the brushes. Could be interesting to see what approach is taken.

It seems very strange that the diodes would be connected as in the original diagram. Unless the field current is flowing there will be no zener voltage. And with no zener voltage the SCRs can't turn on the field. Somewhat of a Catch-22.

Even with 90V applied to R5, the rectified DC will still go to zero at each half cycle. That's where a capacitor across the zener would really help.

For the IC, check pinouts of comparators such as the LM201 or LM301. The 741 op amp can be used as a comparator, but its output will not snap to each rail in the same manner.
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Old 02-03-2011, 10:28:23 PM
Lloyd H Lloyd H is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

quote Warren "It seems very strange that the diodes would be connected as in the original diagram. Unless the field current is flowing there will be no zener voltage. And with no zener voltage the SCRs can't turn on the field. Somewhat of a Catch-22." I struggled (a lot) with that as well but finally got it. D2 and D3 alternately conduct through the field coil to ground, through the zener diode and R5 then alternately through D4 and D5. I believe the zener current flashes the field for dependable startup. I too would be more comfortable with this theory if there were a filter cap across the zener. D1 will limit the inductive effect of the field coil to its forward bias of about a half volt, would that have an effect? Could the zener voltage vary from D1 forward bias to 20 Volts each half cycle to time the UJT firing? Inquiring minds want to know. Lloyd H
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Old 02-04-2011, 12:28:10 AM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Let me offer another hand drawn partial schematic to help others follow along and to further discuss this Zener regulator/missing filter cap portion of the circuit:

I left off the SCRs this time, but D1 is in the diagram. The missing filter cap is something I would have included in the design had I been building this circuit, and it is indeed curious that it's not there.

If the DPE peak voltage is high, and I think it is given my estimate of 90 Volts RMS based upon the power rating of R5, then the amount of time that the DPE waveform spends below 20 volts is very short. So the question is: does this short time below 20 volts matter in this circuit? Since Generac didn't include the filter cap, I would guess that not installing it does not affect the performance of the regulator. But I also want to know

I suppose I should add another experiment to perform with my spare parts. I think I can simulate the DPE with a transformer from my junk bin. Then I can hook up some other parts to simulate this section of the circuit and probe for a stable voltage across the Zener. I unfortunately broke my Zener when removing my potting compound and have yet to replace it, but I'll bet I have another lower value Zener suitable for this experiment in my junk bin.

The Op-Amp guess was based upon the circuit configuration and the pinout of the 8 pin IC. Op-Amps have power supplied on pin 7, which is where the 20 Volts is connected to the IC. The standard 311 comparator has power on pin 8 if I recall correctly. Also, the comparator is open-collector output and needs a passive pullup, which wouldn't work in this circuit. I'm pretty sure we are dealing with an Op-Amp and not a standard comparator. Any of the 40 Volt rated Op-Amps should work considering the frequencies we have in a backup generator. I think the 741 will work fine, but the 301 and 201 Op-Amps would probably also work. Electrically, they all appear to have very similar characteristics.

Won't know for sure until after some testing. So, I'll just have to find some bench time
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Old 02-04-2011, 09:20:50 AM
Kevin K Kevin K is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

If you look at the circuit board, there is an unused space available for a filter capacitor for the zener voltage. The capacitor negative lead would go to a hole just above and to the left of R5, the 3900 ohm 5W resistor. The positive lead would go to a hole straight above near the edge of the circuit board.

Onan used a circuit very similar to this in their YD series of generators. The service manual number is 900-0184. You can get it here:

http://www.cumminsonan.com/www/pdf/manuals/900-0184.pdf

Look at pages 19 and 33 for schematics of two voltage regulators. The circuits are very similar, with the Onan's not skimping on components. The one on page 33 has an option to allow the voltage to roll off with frequency by placing a jumper to select a filter network.

One would ASSUME the DPE winding produces a sine wave in phase with the output voltage, but this may not be the case. I have a couple of generators using the "harmonic winding" method of regulation with no voltage regulator, and it is not a sine wave produced by that winding, more like a voltage spike. I realize we are talking about different methods here, but it would be interesting to use a scope to check the output of the DPE winding for waveform and phase relationship.
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Old 02-04-2011, 11:23:16 AM
Jack Hottel Jack Hottel is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Quote:
One would ASSUME the DPE winding produces a sine wave in phase with the output voltage, but this may not be the case.
I would assume that the Displaced Phase Excitation(DPE) winding produces a sine wave at some angle to the output voltage other than 0 or 180 degrees.
Keep up the good work.
Jack Hottel
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