Simplistic MosFET HV Shunt Regs

[merlin el mago]

Scope set 5mV/cm 0,5mS

 

[merlin el mago]

I listen low frecuency hum.

 

[Salas]

If there is 56mV RMS somewhere as the DMM "reads" you are looking for a sinewave about 160mV peak to peak somewhere across the timescale.

 

[Salas]

I listen low frecuency hum.

Sweep low like 5ms 10ms 20mS to try catch it

 

[AndrewT]

Are your DMM probe leads a twisted pair?
If you have a big loop area in your DMM probe leads, they will pick up interference !
The DMM will read that as an AC voltage measurement.

 

[merlin el mago]

Sweep low like 5ms 10ms 20mS to try catch it

 

[Salas]

Nothingness

 

[merlin el mago]

Are your DMM probe leads a twisted pair? No
If you have a big loop area in your DMM probe leads, they will pick up interference !
The DMM will read that as an AC voltage measurement. Used other short probe leads and same measurement

 

[merlin el mago]

Twisting the DMM red with black short probe leads same measurement

 

[AndrewT]

Can anyone explain why his DMM is reading 0.056Vac, yet the scope is down in the very few mVpp?
Broken scope, or broken DMM?

 

[merlin el mago]

Or broken ears.

 

[Salas]

Twisting the DMM red with black short probe leads same measurement

The scope is a time machine the AC voltmeter will rectify and average out everything happening in its bandwidth capability. What the AC voltmeter hints you about then the scope must really see where and how. If it can't see it, most probably is erroneous.

 

[merlin el mago]

Real load 5mV/cm 5mS

 

[AndrewT]

Could the DMM be reading a combined AC + DC signal and reading the power average and showing it as Vac?

 

[Salas]

Can anyone explain why his DMM is reading 0.056Vac, yet the scope is down in the very few mVpp?
Broken scope, or broken DMM?

Or broken ears.

Or the leads inductance pick hum field. The scope is grounded, the DMM floats.

 

[AndrewT]

Real load 5mV/cm 5mS

Mains frequency !

17mVpp is very approximately 6mVac.
I wonder where the other 50mVac is coming from?
Check the DC voltage.

 

[Salas]

Real load 5mV/cm 5mS

Ground loop?

 

[merlin el mago]

Real load to DMM

 

[Salas]

Could the DMM be reading a combined AC + DC signal and reading the power average and showing it as Vac?

Some few like the MASTECH MS8218 have AC+DC true RMS button so you can see the real value of any one way only going pulse for instance.

 

[Salas]

Real load to DMM

Better stick to the scope and see about wiring and grounding until the 6mV RMS hum gets better in the build. Press the frequency button on the DMM to see if it also reads 50Hz. About the value it shows don't be very worried if its not like the scope's because rectifies and averages everything it can so it could be wide band interference added and/or its own ADC chip noises.