Hi all, I have a 1950's Radiation detection meter that I am using in an Arduino project to simulate the operation (electronics in original meter are totally dead). There are no readings on the meter but I notice a MASSIVE old yellow capacitor of 500 microfarads and maximum DC working voltage of 6 volts across the meter inputs.

I have the meter working through PWM, and the question is should I replace the 500 mf capacitor and put in a current limiting resister (and if so what value). I assume the capacitor just does some smoothing.

Get rid of the capacitor, especially if you don't have any series resistance. If the capacitor is in parallel with the meter and PWM output it will draw excess current.

The meter movement itself has enough inertial to "smooth" the PWM.

Do have a multimeter to measure the resistance of the meter? Do you know the full-scale voltage?

If the meter maxes-out at less than 100% PWM you can add a resistor.

If the meter resistance is too low you'll draw too much current from the Arduino and possibly damage it. The "standard Arduino" is rated for 40mA "absolute maximum" which means the load resistance shouldn't be less than 125-Ohms at 5V. And, 20mA is the "maximum recommend" (250-Ohms or more).

Thanks so much @DVDdoug - capacitor tossed! I measured the resistance across the meter and 2K Ohms. I'm unsure of the full scale voltage (no markings at all), but the capacitor was rated at 6V DC max.

I guess from what you said I should be OK, and only add a resister if it maxes out at 100% PWM?

Easy to calculate. According to the schematic the meter current is 50 uA full scale, and if R=2K, the voltage across the meter at full scale is 50uA*2K = 100 millivolts.

For 5V full scale reading, add a 98K series resistor.

Thank you so much for all the help here, and what I appreciate is that I have learnt something. I couldn't actually figure out what the schematic said - I now realize it's "50 uA" .

OK, so a follow up question - do I NEED to use a resistor if I limit the PWM output - in the program I find a value of 8 almost maxes out the meter. I guess my question is do I need the resistor to do current limiting?

Yes, you need the resistor. PWM is full on, full off, with variable on/off widths. Coil inductance will limit the meter current to some extent, but that effect depends on the PWM frequency.