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Generac Generators (SEARS, etc.)

Generac Voltage Regulator Reverse Engineered...


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  #21  
Old 02-04-2011, 11:57:56 AM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Kevin K, thank you very much for the link. That is exactly the kind of info I was searching for last year when I started this.........and never found.

I'll study that manual to see if I can learn something about these regulator circuits. I see that the IC in the Onan board is also an op-amp with the same pinout.

I did not think that these regulator circuits were complicated with regards to phasing between the DPE output and the DC provided to the field. Would that really be the case for a single phase 220V generator like this generac?



Regarding the open spot on the R2 board, let me make sure we are clear about that, it's not open on MY BOARD (version R3). My board has a MOV in that spot.

Further, if I was to put a filter cap in the board, it would go across the 20V Zener which I drew with the dotted lines. I would not put a filter cap across R5 and the Zener, which is where those two unused connections go. The full pulsing DPE DC voltage would require a high voltage cap.

The photos we see here of the PCB are from Golddreger's R2 board. I checked for differences between his board and my R3 board and concluded that the presence or absence of this one MOV was the only real difference.

The source for the PCB pics is post #6 here:
http://www.smokstak.com/forum/showpo...66&postcount=6

I tried to be very careful when I did this reverse engineering last summer, but now I wonder if the component values were the really the same.

My notes say its the same board except for the R2/3 label change. But this discussion has caused a new problem for me that I can't explain

Here is my new problem:
My board (the R3), had a failed component. It just so happens that it is this extra MOV. The board showed some burning just below this component as the MOV had shorted out. This allowed current to bypass R5 and the Zener. And of course the Op-Amp was deprived of power.

Now I realize from our discussion here that if this MOV shorted out, then wouldn't the field receive power through the shorted MOV? I had better go back and check my diodes again. My generator ran but the voltage output was low, which I assume means the field was not getting any current.

Well, for now, I'm going to follow through on my earlier proposed experiments and breadboard part of this circuit. But now I realize I also need to double (make that quadruple) check my R3 board and it's components.
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  #22  
Old 02-04-2011, 03:33:03 PM
Kevin K Kevin K is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Adding a capacitor here should place it across the Zener diode. I'm not sure how much effect it would have on the circuit.
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  #23  
Old 02-05-2011, 01:43:17 AM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Kevin, yes that location works fine

I thought you meant the other edge of the board

Did you happen to verify my schematic by tracing out the board for yourself?

As far as whether or not we could use a cap across the Zener, I intend to find out. Perhaps tomorrow. I don't think having a cap here would hurt anything. It was my intention to buy a tantalum cap to replace the 16V electrolytic anyway. I'll get a better potentiometer as well.
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  #24  
Old 02-06-2011, 01:37:08 AM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

I managed to get some bench time in today on the Generac Regulator board. Things didn't go as planned. There were more bad parts on my board than I expected.

-As previously mentioned MOV3 is bad, it measured 25 ohms
-R5 is bad, it was open
-my UJT is fried, I need to find another

Because the UJT was fried, I couldn't test it. I was able to verify that the Op-Amp was good. I made a voltage follower with a 741 first and used my O-scope calibration signal to test it. Then I substituted the unlabeled Op-Amp from the Generac board and it worked the same as the 741. This is good news since I wanted to measure the power between a 741 and the unlabeled original to see if there are any differences. I'll still be able to do that test.

I used a product called Quik Chip to help remove the IC and UJT since they were still on the board. It was still difficult due to the excess potting around the parts. Here is a photo of my R3 regulator board before removing the IC (the other small parts were removed last summer, notice the burn mark in the upper right from the MOV):


After testing the Op-Amp, I built a simulator circuit to check whether or not the Zener needed a bypass/filter cap. I took a transformer from my junk box that had multiple outputs, and combined two of the secondaries to achieve 56 Volts RMS. Then I found a 6 Volt Zener in the junk box and started building the DC regulator portion of the circuit. Since R5 was bad, I used a 1k 10 Watt resistor instead. The load was 1K (which may not be enough).

Here are the traces without the field winding in series (top is Zener, bottom is Bridge Rectifier output):

No filter cap here, and the waveform shows the pulsing on the Zener regulator output.

Next I just added in the field winding in series with the zener and 1K load resistor. Here is the result:

This seems promising, but I'm going to withhold judgment until I get a better handle on the actual load value. So that will have to wait until I get a replacement R5, UJT, and 20V Zener.

Here is a photo of the field winding, (that is a big inductor!)
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  #25  
Old 02-06-2011, 10:33:37 PM
Lloyd H Lloyd H is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

For you folks out there that could use a 120/120 Volt isolation transformer try this:
Take two microwave oven transformers (as close to the same as you can find) and cut the cores apart per transformer-core.jpg (see post 7 here, http://www.smokstak.com/forum/showthread.php?t=86359) and remove the coils and the magnetic shunts (small laminated blocks between windings) from the cores. Save the two primary coils and reassemble them on one core. Clamp the removed core piece and reweld. One primary coil is the new primary, the other is the new secondary. Can be used as a 120/240 autotransformer as well. Lloyd H P.S. hook the two coils in series, leave the core piece off and you have a powerful tool and part degausser/demagnetizer that doubles as a bulk tape and floppy disk eraser. PM me if you have any questions.

Last edited by Lloyd H; 02-07-2011 at 11:52:40 AM.
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  #26  
Old 02-07-2011, 10:15:24 PM
Kevin K Kevin K is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

I breadboarded most of the circuit from parts I had in the junk box, substituting 1/4 of a LM324 op amp for the 741 and a 2N2646 UJT for the 2N4871. Everything worked pretty much as expected. The waveforms in the pictures below are the output of the UJT when it is turned on. Approximately 3V P-P, with a pulse width of 5us and a pulse repetition rate of 0.7 Ms. These pulses turn on the SCR's.

One thing I missed when I looked at the circuit was that R1, R2, C1, and the 50K pot form a low pass filter that rolls off the generator voltage output rather sharply as the frequency drops below 60Hz. This is good because it gives the motor a chance to recover when a load is suddenly applied by dropping the voltage output and thus the load on the motor. Unfortunately it also works the other way around by increasing the voltage output as the motor speed rises and the frequency goes above 60Hz, say when a large load is suddenly removed. A worthwhile addition might be another voltage sampling circuit to limit the output voltage to say 125V. I could see this regulator producing a damaging over voltage if the throttle plate were to stick in the open position.
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  #27  
Old 02-08-2011, 12:23:44 AM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Kevin,

Thanks for the testing. Your numbers match the calculations for the UJT as given in my textbook. I show anywhere between 550us to 850us for the period/pulse repetition rate using these two UJTs.

Looking at the datasheets, the 2N2646 appears to be a good substitute for the 2N4871. The slight difference in intrinsic standoff ratio could be adjusted for with R9/C6. But it seems like there is a fair amount of slop in this spec, so it probably doesn't matter much. Perhaps the amplitude of the UJT pulse is different with the 2N4871? I'll find out soon.

Questions for you:
I don't quite follow the second part of your post. Let's say that the 20V supplied by the Zener regulator is constant (with or without a filter cap); this would mean that the 10V DC level set by the voltage divider is also constant.

In that case, any voltage from the feedback network (at R3) below 10V would cause the Op-Amp to ramp up its output and start firing the UJT, turning on the SCRs.

Also, any voltage at R3 that is above 10V would cause the Op-Amp to ramp down its output and deprive the UJT of its trigger, turning off the SCRs at the next DPE zero crossing.

As far as I can tell, the response time of the regulator is determined by C3/R4 regardless of engine speed/generator output frequency. If my Briggs engine governor was set low(high), the output should still be 120V, but at a lower(higher) frequency.

Does that make sense? Or did I misinterpret your post again

I'm curious to know how well this regulator circuit can provide a constant 120V output with a steady load.

I ordered some replacement parts, and I have this funny feeling I'm going to have to run the generator with the original design to understand it fully. Probably should do that before making any electrical mods
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Old 02-08-2011, 12:08:21 PM
jimgrease jimgrease is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

My brain worked on this in my sleep

I think what Kevin is saying is the following:

If a large load is applied to the generator suddenly, the engine RPMs will drop. If the RPMs were to drop to say 2400 from the additional load, then that would be 40Hz from the generator output.

If I now understand Kevin's point, the feedback network will provide 10 V at R3 for an input of 40Hz with an amplitude LESS than 120V. This would be a good thing as it will allow the engine to catch back up to the right RPMs without the field being energized to a higher level during this transient condition.

On the other side of a transient, if a large load was removed suddenly from the generator output, then the RPMs would speed up. 70 Hz would be at the generator output if the engine sped up to 4200 RPM. Again, if I understand Kevin's point properly, under this condition the feedback circuit will provide 10V at R3 when the 70Hz output is at an amplitude HIGHER than 120V. During this transient, the regulator would be attempting to energize the field to a higher output level while the engine is slowing down due to the frequency response of the feedback network.

That over voltage on the output would not be desirable

Well, I think I need to see a bode plot of the feedback network to see how much the DC value at R3 changes for input frequency changes. I'll see if I can setup the circuit simulator to run that experiment.

C3 is still playing a part here as it slows down the Op-Amp output to the UJT. I'm sure the Generac engineers in the 70s had to play these tradeoff games between performance and cost. Maybe we can do better

Anybody have some data on what changes to expect for sudden load varying transient conditions? How much RPM change could I expect for a typical furnace on/off cycling?
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  #29  
Old 02-08-2011, 01:09:16 PM
Fred M. Fred M. is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Jim-

Your analysis of voltage under/overshoot due to engine speed may be correct. There could be a spike in voltage if regulator response is slow, but would engine speed actually change that much?

1. Mass of the rotor tends to resist sudden changes in speed (flywheel effect).

2. The engine's governor attempts to keep speed constant.


Fred
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Old 02-08-2011, 09:02:53 PM
Kevin K Kevin K is offline
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Default Re: Generac Voltage Regulator Reverse Engineered...

Jim,

That's what I meant. The input voltage divider consists of R1, R2, C1, and the 50K pot. The voltage sample is being taken across C1 which has a reactive impedance of 12,000 ohms at 60Hz, 10,340 ohms @ 70Hz, and 14,475 ohms @50Hz. So you can see how the voltage roll off takes place. For a flat response replace C1 with a 12,000 ohm resistor. I meant to set up a variable frequency source of 120 volts to test the roll off, but I haven't had time. If you can simulate it please do so. The last time I used a SPICE program was years ago when I was doing microwave design, and I don't have any programs that can simulate this.

If we use a quad op-amp, maybe we can create a circuit that rolls off below 60Hz but is flat above 60Hz.

Take a look at the Stamford SX460 voltage regulator being sold on the web. It would be nice to create a circuit like this that anyone can build to replace a NLA regulator or a $200 encapsulated "magical mystery module". You have to hand it to the Onan people when they used a repairable regulator with the YD generator heads. If it fails during a protracted power outage you can easily fix it and get the lights back on in a short time. With the new encapsulated modules you can be sure you're out for the duration if the regulator fails.

The mechanical governors usually hold the frequency within + or - 3 Hz. The typical adjustment is to set the governor to output 62 Hz or so no load, and it shouldn't drop below 57 Hz at full load. It may momentarily drop below that if a large load is suddenly applied, but it should recover within a couple of seconds.
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