Hello so i have a microcontroller that is driving the following circuit: enter image description here

Both the MCU and the circuit are powered by a 5v power supply, when PAD_PWM and PAD_PWM_return are not shorted ore there is no load between them the circuit its Drawing 6.22 mA which is correct. But when i short PAD_PWM and PAD_PWM_return with a wire i see a voltage drop from 5V to around 0.5V and the total current consumption of 18.88 mA. the duty cycle of the PWM its 50% so in theory the difference in current I'm seeing its correct as 5*.5 its 2.5V/ 200 ohm its 12.5 mA which its almost the current difference that we got from working under no load and working under load.

But i cant know for sure if that increase in 12mA its in fact going through the R10 and the wire. because I'm seeing a voltage drop, it doesn't make sense to me.

The MCU works flawlessly and it doesn't stutter or anything at all.

  • 1
    \$\begingroup\$ " i see a voltage drop from 5V to around 0.5V" - Where? Also what frequency is the PWM, and what voltage do you measure at PAD_TREATMEN? \$\endgroup\$ Jun 12 '20 at 21:08
  • \$\begingroup\$ the voltage drop its measured from PAD_PWM to PAD_PWM_return, PWM frequency its 10.81 Htz and PAD_treatment its 5v on the ON part of the PWM \$\endgroup\$
    – ywx99
    Jun 12 '20 at 21:55
  • \$\begingroup\$ By '10.81Htz' do you mean 10.81 (ie. ~11) pulses per second? What was the measured voltage at PAD_TREATMENT with 50% PWM? \$\endgroup\$ Jun 12 '20 at 22:09

With the output shorted and 100% PWM a current of ~5 V / 200 Ω = 25 mA is expected. At 50% PWM it should be 25 / 2 = 12.5 mA on average. Your circuit draws 6.22 mA with no load, so this must be added to the load current to get total current. 12.5 + 6.22 = 18.72 mA. So far so good.

The only problem is you are not getting zero volts across your 'short', but around 0.5 V. Assuming 0.5 is the average voltage, the 'shorting' wire must be dropping ~1 V during PWM 'on' time (assuming 50% PWM). This should reduce the load current to ~4 V / 200 Ω = 20 mA peak, or 10 mA average. So you have an apparent anomaly of ~2.5 mA.

However this small 'anomaly' may vanish when more accurate measurements are made. Depending on PWM frequency, a DC meter may not measure the DC component of the PWM waveform accurately. At high PWM frequency the transistor may turn off more slowly than it turns on, increasing the effective PWM ratio. If the supply leads have significant inductance and/or the power supply has poor stability then the supply voltage may increase under load, and the flyback diode could produce a voltage offset of up to ~0.8V. Another possibility is a bad connection at the 'short', producing randomly changing load current and voltage across the 'short'.

If you use the same meter for different measurements then the current and voltage could change from one measurement to the next, and you wouldn't know. To see what is really happening (including inductive effects etc.) you should look at the waveforms with an oscilloscope.


The anomaly was on the side of the multimeter after changing the leads the issue was resolved


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