Applying PWM to already modulated DC signal

I had a plan to use an arduino and mosfet circuit to modulate 90V DC coming out of one of these

https://www.ebay.co.uk/itm/New-DC-51-Switch-Gear-Motor-Speed-Controller-AC-220V-High-Quality-DC-90V-/142332015962

Now I'm thinking that the controller probably uses pwm already and at that price the signal is probably not filtered?

So will I have problems trying to further control the voltage using PWM?

I don't know much about it but it seems as if I would need to get the timing/phase correct or I'd end up with problems.

So will I have problems trying to further control the voltage using PWM?

Yes I would think so.

Thanks, any thoughts on whether it would be best to

a) try and overcome these problems
b) find another source of DC
c) try and find a way of replacing the pot on the unit with Arduino control

Well any of those three is a legitimate solution.
What I would try is to see if the full power out from the controller gives you a DC signal, if so then simply wind it up to full and apply PWM through a FET to the output.

The main problem with a digital pot to simulate the analogue pot is that digital pots are very restricted in the range of voltage they can tolerate, normally being not outside the power supply to apply to them and having a limited supply voltage like 5 to 15V. Also their current rating is normally a few tens of mA.

Ah ok thanks.
I’ll try it when the controller turns up.
I’ve no scope though so it might be a case of trial and error.

I've no scope though so it might be a case of trial and error.

In which case I would implement one of the many simple "make your Arduino into an oscilloscope" projects and use a potential divider to see the waveform this unit produces.

Good Tip. I’ll look into it. Thanks for your help.