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Generac Smart Management Module Dead

Stamey

Apr 28, 2023
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I have a "Smart Management Module", made by Generac, that has died, and I am trying to see if I can fix it. This unit's purpose is to monitor, I believe, the frequency of the power coming into it and if the frequency falls below a certain level, telling the module that the generator is being overtaxed, it cuts out the load that is connected to the SMM by opening a relay/contractor. In this case the load is my upstairs air handler.


The PCB has the a short in it. The PCB is fed by two inputs, at 240V, and has one output, to a relay that cuts the power going though it, if necessary.


Initially I found that a resistor and a PTC were cooked on the board, as I observed black soot around these components. I did manage to identify these components and replaced them, with the hope that I had just experienced a power surge at some point and those components sacrificed themselves to protect the rest of the board. This was not the case. I reinstalled the board and as as I powered it up I heard a pop and the resistor and PTC seemed to get fried.

I have attempted troubleshooting with my limited knowledge, under the impression that logic chips seldom go bad and other components such as capacitors are likely to fail first. I have not been able to identify all of the components on the board so that limits my ability to research diagnosis, hence I am here.
Looking at the overwall board, I see some brown areas on the underside, and they seem to be under resistor mounts and what I am guessing is a transformer. On the top, none of these compents have obvious signs of failure.

The large capacitor on the board, relative to the other possible caps, is a 10mf, 450v unit that looks like some soot beside it, so I replaced it even though it tested the same as the new one. It was not bloated or showing any other signs of failure. This was after replacing and popping the new resistor and PTC.

Since I do not have an AC power supply that I can limit the current on, I replaced the resistor and PTC again then connected my DC power supply feeding 30v, max 2A, into the board. The toridoral inductor, the PTC, and the resistor got quite warm. I noticed nothing else getting warm, and the power LED did not light up. I do not know if this tells me anything useful since feeding DC into an AC circuit may make components act differently than with AC.

I would like some hand holding on what to check and how to check the individual components.

Thanks,
Chris
20230423_170216.jpg
20230423_170114.jpg20230423_170237.jpg20230423_170225.jpg
 

kellys_eye

Jun 25, 2010
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Assuming the fuse (brown block at the AC input terminals) is ok the DC or AC will pass through to the bridge rectifier located to the left of L1. The DC should also appear on the +/- terminals on the other side of the bridge rectifier. I suspect, because of the heat you're noticing, that this bridge rectifier is shorted.

DC is not the best way to check operation though - practically any AC source from a small transformer would be better.
 

Bluejets

Oct 5, 2014
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Can you re-do the last photo, in focus would be good.

Really had a large blow around the fuse and diverter area(RTC), but also see a small blow mark on the negative leg of the bridge rectifier, almost like someone slipped with a screwdriver/meter probe whilst live. Bad joint on RT1 may have started the sequence of events. Relay has heat stress signs on one terminal.
 

danadak

Feb 19, 2021
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Possible electrolytic cap failure. If its bulged at top a clue :

1682798712503.png

Regards, Dana.
 

debe

Oct 15, 2011
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I would suspect this bridge rectifier as short circuit.CIRC BOARD..jpg
 

Stamey

Apr 28, 2023
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I have an AC power supply, but it is ancient and does not have any way to limit current. I can limit voltage only.
I will get a better pic. Did not realize the camera was focusing elsewhere when I took the pic.
I will test the rectifier today.

Thanks,
Chris
 

Stamey

Apr 28, 2023
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Here are new pics.
I tested the rectifier and it is bad. No voltage drop across anything, in any combination. Total short.
Now I have to identify it so I can see about getting a replacement.

20230506_151159.jpg20230506_151332.jpg20230506_151406.jpg20230506_151411.jpg20230506_151415.jpg20230506_151427.jpg20230506_151440.jpg20230506_151601.jpg

Thanks,
Chris
 

kellys_eye

Jun 25, 2010
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We now have to consider WHY the bridge rectifier shorted. Whilst random failure (potentially as a result of high in-rush current) is possible, other parts downstream of the bridge rectifier could have gone short and 'taken it out'. Test Q1. If that passes then the bridge may indeed be the only failure....
 

Stamey

Apr 28, 2023
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In diode test mode:
Neg on lead 1, pos on lead 2 or 3: .67 V
Neg on lead 2, positive on lead 3: short (.003 V)

Pos on lead 1, neg on lead 2 or 3: 2.2V
Pos on lead 2, neg on lead 3: short (.003 V)

Does that tell me that the transistor is bad?

Thanks,
Chris
 

Stamey

Apr 28, 2023
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No one has any advice on the latest step in my troubleshooting?

Thanks,
Chris
 

kellys_eye

Jun 25, 2010
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No one has any advice on the latest step in my troubleshooting?

Thanks,
Chris
We need to know which leads are 1, 2 and 3 to be sure but the 'zero' measurements across the two/three sounds totally wrong. Remove that part and retest to make sure there are no other components in-circuit causing false readings.
 

Stamey

Apr 28, 2023
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Well, 1, 2, and 3 are left to right, as per the picture above. I am having difficulty reading the numbers on the unit, so I don't know which lead is which, besides what I already said.
But, that brings up another question, can anyone help identify the components in a manner that would allow me to find an order new parts, or even find data sheets on them so I know which leads are which?

Thanks,
Chris
 

kellys_eye

Jun 25, 2010
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I am having difficulty reading the numbers on the unit, so I don't know which lead is which, besides what I already said.
The middle pin (connected internally to the larger 'tab' heatsink) is invariably the Drain. The Gate is most often 'driven' so will be connected via a resistor in the 100's or 1,000's ohms range (usually the lower value) or directly from another driving device - it is also a very low current signal so the PCB tracks will be thinner than the others. The Source also carries high current and will have a thicker PCB track connection.

You device would therefore have Gate, Drain, Source respectively. As to the device type - again invariably an N-type (not always but 90%+ times) but the voltage and current ratings (and particularly the Rds(on) value) will be 'unknown' unless you can get some identification from it.

If you have to replace it 'blind' I'd look for one with a 600V rating and the highest Amperage (and lowest Rds(on)) values available in that package size. This is where sellers (like Mouser, Farnell, RS etc) come in as they have parametric search engines that allow you to select the parameters you KNOW and the list pops up all the possible choice remaining to you - subject to the correct pin-outs to match the one you're replacing.

But first, have you removed it and retested it?
 

Stamey

Apr 28, 2023
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I am working on it. Got to learn techniques and tools for removing surface mount components.

Chris
 

Stamey

Apr 28, 2023
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OK, finally got it off the board. Unfortunately, I ripped off the left pad it was connected to, by accident. It was tough to get the heat sink desoldered. I hope I can fix that pad.
Now, the test of the unit off the board.
Positive to the right leg and negative to the center leg: .575 volts, in diode mode.
All other combinations of leads to legs: OL
New pics:
This has th elegs in the order I am talking about them, from left to right, 1, 2, 3
20230517_172746.jpg


Other pics for aid in possibly reading the numbers on it.
20230517_172758.jpg20230517_172812.jpg20230517_172733.jpg

Thanks,
Chris
 

kellys_eye

Jun 25, 2010
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Oooo....shame you caused such damage in removing it - more so since the readings you give indicate it may well be ok. The 'short' you measured previously is likely to have been due to the switching inductor as part of the remaining circuitry. Such are the tribulations of trying to repair this type of equipment.

Not easy to read any potential number as moving the device under light (at various angles) is required to make out any real detail although I think I see the beginning of something ...N60.... The 'N60' bit refers to the 600V rating as I made mention to above (for fitting a spare) so we're nearly there!

The best bit about it - if you can see that far - is that the destruction of the bridge rectifier doesn't, at this stage, look as if it propagated further into the unit so it's replacement (and a new MOSFET) is potentially the 'cure'.

Can you post a pic of the 'damaged' board?
 

Bluejets

Oct 5, 2014
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I have an AC power supply, but it is ancient and does not have any way to limit current.
Best way , or at least one way , when repairing any switchmode supply, is use a 150w floodlight bulb in series with the input.
If it's bright, you have a short, if it flickers in and out at low light level, you might be in with a good chance that it's working.

I have one permanently connected into a power point just for this purpose.
Plug in the suspect power unit and turn on, follow above.

From the latest photos, appears immaterial now anyhow.
 

sambeet

Dec 30, 2023
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Great discussion. So, which component was damaged ? I already have 2 failures and wondering if these can be repaired. Also, I see now they have a new board. These are Blue in color and have been modified to address the above resistors over heating issue. Attached is a sample picture that I found.
 

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