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I am using a 5V pulse train generated from a UC3845 PWM controller to trigger a MOSFET (K2645) which is biased with 317VDC in series with Rd= 120ohms, Rs=20ohms and primary of the PWM transformer. I am able to see the pulse train of UC3845 but since I don't have an isolation transformer, I can't use it to see the PWMT waveform, which is also not detected on a DMM(AC mode). I have varied the pulse width to see the effect on the output. My DMM is only measuring in millivolt AC (both primary & secondary.

Q1. Can these high voltage pulse train be measured using a DMM in AC mode? Why not?

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How to measure high frequency pulses on 1M DMM?

Since rise time and implied frequency is greater than DMM BW, it cannot measure.

One can measure peak voltage and average DC to obtain some meaning of the pulse duty cycle. This requires a series 1N400x diode and a plastic shunt cap of impedance about 1% of 1MOhm at f (PWM) to hold the voltage without excessive wire loop area to prevent stray induction and resonance. This also minimizes the steady state load.

If can compute this value , then you can measure Vdc peak then without this circuit for Vdc using a 10k series R and 10x bigger cap.

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If I understand you correctly, you're looking for the change in duty cycle. For this, you're better off using the DCV setting. It might be less noisy if you put a low pass filter in front of the DMM.

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  • \$\begingroup\$ Sorry , may be my question was not so clear. I am able to change the duty cycle of the pwm through UC3845 and the MOSFET is also triggered, but I want to check the ac voltage on the SMPT through a DDM, oscilloscope would have been nicer but I don't have an isolation Xmer to do so. \$\endgroup\$ – seccpur Jul 25 '18 at 15:06

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