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Pulse Width Modulation no output voltage

FrankT75

Jun 11, 2011
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I have a 24 volt electric motor powered by two 12 volt batteries and the speed is controlled by a Pulse Width Modulation (PWM) circuit with a 0 - 10Kohm potentiometer.

The controller has stopped working. It shows no voltage across the output terminals. It does show about 23 volts between one output terminal and the input terminal of the opposite polarity. How do you troubleshoot a PWM circuit?

I'm also considering replacing the PWM circuit and found one on eBay that is rated as 12 - 40 V, 30 amp and 360 watts. According to the manufacturer, my motor draws between 2 and 24 amps. I am unfamiliar with the voltage / current relationship for a PWM controlling a DC motor. Can anyone help me with determining if the 360 watt rating is adequate for this application?
 

(*steve*)

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Firstly, if you look at the max power as defined by your figures it is 24 (volts) x 24 (amps) which is 576 watts.

Even though neither 24 volts nor 24 amps exceeds the alternate controllers capability, the power rating does (so presumably the controller does not permit the maximum current at the maximum voltage and so your motor falls outside the safe operating parameters for the controlled.

Getting back to the fault, the problem is most likely that either a fuse has blown, or that the power transistors that turn the motor on have failed. Have you opened it up?

Can you photograph it?

Do you have a circuit diagram?
 

FrankT75

Jun 11, 2011
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Yes, I have opened it up and took some pictures of it. I'll see if I can post them.

I have tried to get a circuit diagram from the manufacturer, but they are only interested in selling me a replacement.
 

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(*steve*)

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If you note the three devices attached to the heatsink. They are Q1, Q2, and D1.

I would suspect Q1 or Q2 have died. However that's not the only possibility.

Undo the bolts attaching these 3 devices to the heatsink, then remove the heatsink. Be very careful about any washers or other mounting hardware that may be attached to those bolts (I suspect the devices may have a plastic case though -- so no special mounting hardware).

Then see if there are any markings on the cases (you may have to bend them up to see). If there is a white goo between the heatsink and the devices you'll have to replace it when you reattach the heatsink (but don't do that yet).
 

FrankT75

Jun 11, 2011
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OK. I removed the board from the heat sink. They used some sort of double back tape to attach the components to the heat sink.

Both Q1 and Q2 have the followingon them:

IRFP044N
I*R 045N
78 30

Where I have an * is what I think is the symbol for a transistor.

I attached a picture of Q1 and Q2
 

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davenn

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Yeah they are both International Rectifier devices

google brought up datasheet for it

IRFP044N = VDSS 55V; ID 53A; Single N-Channel HEXFET Power MOSFET

cheers
Dave
 

FrankT75

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Is there a way to test the transistor or do I just replace both of them and see if that fixes it?
 

(*steve*)

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It's relatively difficult to test a mosfet in circuit with a multimeter.

You can do some basic tests. Use a multimeter to measure the resistance (diode test would be better) across the drain and source both ways (i.e. red on drain, blank on source, then black on drain, red on source.) You should see a diode junction.

From gate to either drain or source you should see an open circuit, however it's likely that components in the circuit will give you some measurable resistance.

With power applied, you should see a voltage appear between the gate and the source. Be careful when measuring this that you don't short anything, and use something like a 100 ohm resistor in the place of the motor because you're running without a heatsink.
 

FrankT75

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Which is the drain, the source and the gate? When I'm looking at the side with the part number, which is left, center and right? I checked the data sheets for the transistor and none gave this info. I take it that there is an industry standard.

My digital multimeter has a diode test function, so I will use that.
 

rob_croxford

Aug 3, 2010
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looking directly at the front of device (from the data sheet): Pin 1 (left) is the Gate, Pin 2 (central) is the drain and pin 3 (right) is the source.
 
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FrankT75

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I have three boards that control three motors. I tested Q1 and Q2 on all three boards and got the results shown in the attachment.
 

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davenn

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Which is the drain, the source and the gate? When I'm looking at the side with the part number, which is left, center and right? I checked the data sheets for the transistor and none gave this info. I take it that there is an industry standard.

My digital multimeter has a diode test function, so I will use that.

data sheets always give the info on pinouts its a primary function ;)

http://pdf1.alldatasheet.com/datasheet-pdf/view/68486/IRF/IRFP044N.html

thats a link to the sheet you will need to download it to view it

scroll right down to the case and pin ID section near the end

cheers :)
Dave
 

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Are all three boards faulty?

If so, do you have access to one that isn't?

I'll take a closer look at your measurements later alongside the data sheet for the devices, but nothing sticks out for me at this point.
 

FrankT75

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I'm just a Mechanical Enfineer, but it doesn't seem right to me that all six transistors would fail in a way that gives roughly the same readings.

I'm thinking that it is more likely has to do with the forward -off-reverse switch that is common to all three boards.
 
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(*steve*)

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Although the mosfets are well worth looking at, they don't appear to have failed. I would expect to see either a dead short across D-S (and consequent failure of the fuse) or some obvious physical damage.

Can you tell me a little more about the three boards? Are they replacement boards, or are they for different motors, or are they from different devices?

It sounds like they are for different motors on the same device, and knowing this now, I would be looking at something in common.

It might be a good idea to check that power is getting to the boards.

Your initial request about replacements for a board led me to think that you were certain the board itself had failed. Perhaps I should have asked "why you thought the board had failed" :(
 

FrankT75

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Sorry if I caused any confusion. Thank you for the help you've given. It has allowed me to understand the circuit alot better and to eliminate one of the likely causes of my problem. You also kept me from wasting my money by buying a controller that is too small.

I have a trolling motor unit for a boat. The unit consists of 3 motors, 3 control circuits and one wired remote control. The wired remote control contains a forward-off-reverse switch and a 0-10 kohm potentiometer that you turn to change the speed of all three motors in unison. The 3 controllers are interconnected, but only the 3 wires for the potentiometer go between the controllers.

I do not know if one or more board has failed, I just know that none of the motors will run and when I checked the output terminals, each pair reads 0 volts, but it shows about 23 volts between one output terminal and the input terminal of the opposite polarity.

I have found a few places on eBay that sell PWM controllers that are big enough for my application but they are in China and I haven't convinced myself that I'm willing to take the risk. With the cost of shipping, if the PWM is faulty, it's probably not worth sending it back. So for now I'll continue troubleshooting what I can.
 

(*steve*)

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OK, it seems fairly unlikely that all three PWM controllers would failed simultaneously.

It is far more likely that it is a fault in something common to the three, or a fault in one that is affecting the others.

If it is possible to connect just one controller at a time, see if any of the controllers work that way. It is possible that the input to one controller has died in a way that loads down the others.

The other, and in my opinion more likely option is that something is affecting the input.

I would be expecting a voltage that varies smoothly with the position of the potentiometer. It it possible that the potentiometer is faulty, or that one of the wires leading to it has broken.

Were there any symptoms prior to the motors failing? or was it in storage for a while, went in good and came out bad?

Was it used in salty conditions?
 
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