voltage drop on PWM circuits?

Clark Technician Springfield, Massachusetts Posted   Latest   Bounty  

can voltage drop not measure a PWM circuit? i have tried on some AC blowers and fuel pumps but it reads wrong

Jim Curriculum Developer
Frederick, Maryland
Jim Default

I would not use a voltage drop to test a circuit like that for the reasons you have found. It is typically a low power circuit, first of all. Second, since it will typically have a control or regulated voltage that is then pulse modulated. The duty cycle and voltage level combined can be read with a meter as an AVERAGE voltage and this is taught by some manufacturers. For example, If i have a…

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Eric Owner/Technician
Edgerton, Wisconsin
Eric Default

You can measure a pulse width modulated circuit just like you would measure any other circuit. Disconnect the components from the circuit, apply voltage and a load across the circuit and measure the voltage drop from one end of the circuit to the other. If you are trying to measure the voltage drop on a working PWM circuit, why? Would you try to measure the voltage drop on a working…

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Marlin Technician
Estacada, Oregon
Marlin Default

Yes, it does work, I don't know what you tried. PWM circuits are controlled by transistors, and transistors always have some resistance. Wherever there is resistance, the will be voltage drop when the circuit is loaded. Using a DVOM will never work; even if it was capable of reading quickly enough, you would not be able to read and use the results. Use a DSO. Connect separate channels to each…

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Albin Diagnostician
Leavenworth, Washington
Albin Default

What do you mean it reads wrong? How have you tested, what tools have you used for the testing? Use a labscope for your testing, then put up the waveforms for discussion.

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Andrew Technician
Commack, New York
Andrew Default

This can be a little complicated. The problem is the way a volt meter works - it takes a ton of samples of the voltage and then does some kind of interpretation, which will be influenced by the settings you've made. If the voltage is steady, different meters will basically all give almost exactly equivalent readings, but when the voltage fluctuates rapidly they can each come up with different…

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Caleb Technician
Mishawaka, Indiana
Caleb Default

Andrews response is way better than mine will be but ill throw in my 2 cents. Not to be harsh but your over thinking it. I did the same when I first started. Just use a scope. It will allow you too see how high or low the voltage is being pulled. Most of the time its only one side of the circuit that is PWM. The other side is constant wether power or ground. If you dont have a scope just…

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Mike Mobile Technician
Mount Pleasant, South Carolina
Mike Default

Some great responses here. I think we typically think of a voltage drop test as something we do to check for a failing wire or connection. If I were looking for a voltage drop in a PWM circuit for this reason, I think I would try to command the circuit to 100% or isolate it as someone else mentioned. Think of it this way, you may only see voltage drop on a failing starter wire while cranking…

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Caleb Technician
Mishawaka, Indiana
Caleb Default

You are exactly correct Mike. I scope as much as I can because it always tells the truth assuming my equipment is functioning correctly.

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