SSR problem

[otchu]

Hello .... here a very light version of my circuit (sorry for the bad drawing).

So on one pole of the SSR I have a potential of 450v DC. My goal is to measure the resistance of the 'charge' (0 to 500 ohm) with a uprocessor when I don't have the 450v (SSR close). When I start to have the 450v, I do open the SSR. My problem is every single SSR who can handle 450v have a Ron resistance who change with temperature (30 ohm at 20 degrees to 60ohm at 85 degrees) and I need less than 5% tolerance of my 'charge'reading ....

[MODERATOR ACTION]

• Replaced original picture by another one (enhanced) minimally inteligible

 

[barry]

Maybe use a mechanical relay? A MOSFET with better temperature characteristics?

 

[KlausST]

Hi,

For accurate measurements you need to use a measuring method called "kelvin sensing".

Klaus

 

[danadakk]

https://pdfserv.maximintegrated.com/en/an/AN5761.pdf

How are you going to isolate the measuring circuit from the HV when applied ?

https://www.analog.com/media/en/training-seminars/tutorials/MT-071.pdf

 

[otchu]

I will take a look at all your suggestions ... forget to mention, the current for reading the value can NOT be more than 3mA

 

[FvM]

O.k., following the Kelvin sensing suggestion you are feeding 3 mA through "uP" pin and diode and measure load voltage drop through sufficient high ohmic, 450 V rated resistors and differential amplifier. Required voltage resolution for 5% of 500 ohm is 75 mV. How much time available for measurement?