Hi,
I am designing an inductance bridge for iron cored inductors of around 10H 100mA

I need a constant DC current of upto 100mA, to do this I have a stabilized 100V PSU, that feeds the top of the inductor.

The circuit is a standard current sink using a high voltage Mosfet.

Looking at the drain of the Mosfet, I can see quite large voltage spikes, that seem random, but quite large.

If I replace the inductor with a resistor, all is calm and there is no noise to speak of.

So it does seem to be the fact that this is an inductor. I have tried putting the inductor in a diecast box, to see if it was some form of pick up, but this made no difference.

Any ideas?

John

 

The standard diecast box is made of non-magnetic material (aluminium) and will not block magnetic fields.

 

I always thought a diecast box was a very good electro-magnetic screen

 

Hi,
I am designing an inductance bridge for iron cored inductors of around 10H 100mA

I need a constant DC current of upto 100mA, to do this I have a stabilized 100V PSU, that feeds the top of the inductor.

The circuit is a standard current sink using a high voltage Mosfet.

Looking at the drain of the Mosfet, I can see quite large voltage spikes, that seem random, but quite large.

If I replace the inductor with a resistor, all is calm and there is no noise to speak of.

So it does seem to be the fact that this is an inductor. I have tried putting the inductor in a diecast box, to see if it was some form of pick up, but this made no difference.

Any ideas?

John

It's the inductor. Try moving it to the bottom side of the FET.

 

I always thought a diecast box was a very good electro-magnetic screen

It makes a good Faraday shield. It will keep out electric fields and radio waves but will make no difference to magnetic fields.

 

Looking at the drain of the Mosfet, I can see quite large voltage spikes, that seem random, but quite large.

It would help if you could give us a bit more information.

How large is "quite large"? Millivolts? Volts? Tens of volts?

How random is "random"? Are you seeing a pulse once every couple of hours? Several a minute? Several a second? Thousands or tens of thousands a second?

What do the pulses look like on the scope? How narrow are they? What is their shape? Can you post a scope screencap?

Is anything about the pulses-- their frequency, amplitude or shape-- at all dependent on the DC current being put through the inductor?

What does your circuit layout look like? Is it spread out over a large area, or is it nice and compact? Again, a picture would help.

Without more detailed information, any diagnosis here would be a wild-assed guess.