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Reverse engineering a circuit - some kind of controller

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I have this control board that is connected to a 12v magnetic clutch that energizes and drives a generator from a belt on the engine. It is somewhat curious and I am trying to figure out what it does but the circuit leaves me little clues.

Sorry the pic is so poor as it is so small a PCB it is tricky to photograph with the phone camera!!! Anyway it is shown so you can see how packed it is.

So far I am about half way down the board and drawn the circuit I think is there but it is really difficult.

Does anyone have an idea what the circuit so far is trying to do? The outputs from the 339 drive 6 transistors and these in turn drive 2 relays but that part is yet to be reverse engineered.

Thank for any ideas

Q
 

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hi,
Its a 'window' comparator, A and B outputs switch at preset voltage levels.

Look at this simulation.
 

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Hi Eric

Thanks for running the simulation. So its a window comparator ie a discriminator circuit but what does it discriminate between. So based on Va and Vb it looks like a delay circuit of 350ms if the U3 and U4 comparator volts are set as you show on your diagram.

I have tried to understand each part and so far see the first rectifier bridge as redundant as it seems to do nothing?

The U1 and U2 seem to give out a square wave at the same frequency as the AC line frequency

Then it appears to convert it to DC and filter it?

Do you think it could be to detect the rate of change of the 120v AC line?

I will try to get the other half posted later today.

Thanks

Q
 
OK I have tried to get the last half of the cicuit reversed engineered but it is so convoluted with wires and the small pcb it is difficult to see if I have missed any connections. There are also many diodes to supply and ground that appear to protect the relay coil on turn-off but these are omitted for clarity!!

The inputs A.B,C and D are from the engine wiring loom - A seems to be from the low oil pressure switch B is from the ignition switch, C and D are from another switch which seems to an arm and run type device.

Both the relays are DPST and one of them powers the magnetic clutch from second half of the relay.

Most of the transistors are BD135 and some TIP42's for power - maybe?

If anyone has an idea what this does please let me know so I can figure out what this thing does and how to make it work.

Thanks

Q
 

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    Last half.GIF
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Hi Eric

Thanks for running the simulation. So its a window comparator ie a discriminator circuit but what does it discriminate between. So based on Va and Vb it looks like a delay circuit of 350ms if the U3 and U4 comparator volts are set as you show on your diagram.

I have tried to understand each part and so far see the first rectifier bridge as redundant as it seems to do nothing?

The U1 and U2 seem to give out a square wave at the same frequency as the AC line frequency

Then it appears to convert it to DC and filter it?

Do you think it could be to detect the rate of change of the 120v AC line?

I will try to get the other half posted later today.

Thanks

Q

hi,
The way the first two LM339 are biassed produces a charging voltage even without a mains source.!
The lower LM339 output is always high.
Perhaps you should recheck your circuit...

Usually when driving a clutch mechanism the driving engine has to be up to speed before the clutch is engaged.
I suspect the intention is that the 'mains' input is from the remote alternator [AC] and that the circuit detects the frequency as well as the amplitude of the alternator output before the clutch is engaged.???
 
I rechecked and seems correct for 2 339's in the AC/DC circuit. The 2 comparators controlling the transistors were indeed shown wrong and a new diagram is included.

From your comment it appears the circuit is there to detect the frequency then and this might make sense as the AC alternator is connected to a normally constant RPM engine but this can change if the operator alters the throttle so increases or decreases the revs. This may explain why there is the latch circuit which keeps the clutch energized until the revs are to speed and then the circuit detects 'out of spec' rpm and hence Hz. It would drop out the drive to the transistors if it went too high or too low based on the U3 an U4 comparators as now shown.

But it seems awfully complex with latches and locks in the transistor circuit side and the relays
 

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