Can I measure 150V discharge current with basic scope probe.

[bobledoux]

I need to view the current draw curve of a capacitor charged to 150 volts as it discharges into an inductor.

My scope is a Tek TDS1012. I'd like to view the voltage drop across a resistor in series between the capacitor and inductor. The scope ground would go to the resistor input and test lead to the resistor output.

I'm concerned because the ground becomes 150 volts above the scope ground. The scope is rated for inputs up to 300 volts.

Is there a procedure for doing this without cooking the scope?

 

[ronsimpson]

Can you place your current sense resistor in the ground leg of the capacitor?
The ground clip on your probe is at ground.

 

[squishy36]

My method of choice for such measurements is to use a properly-rated differential amplifier; this is the right tool for the job, but many people balk at the cost -- for a hobbyist, the differential amp can cost more than their scope. Here's one example.

The other option is to use the poor man's differential amplifier: use two scope probes, one on each side of the current shunt resistor, then subtract the two signals; the difference waveform is proportional to the current waveform. The disadvantage, of course, is that it uses up two channels of the scope.

You can also float the scope, but I don't recommend this technique. I've used it successfully, but I felt the risks outweigh the benefits -- some day, an accident will occur and you only get one fatal accident in your life. That differential amp is a lot cheaper than an emergency room visit or a funeral.

 

[bobledoux]

Thanks for the comments. I'm a hobby person who rarely needs a differential amplifier.

 

[cowboybob]

Essentially this is a tank circuit, which is going to resonate, but to be sure of some things, can you provide a schematic of the circuit with power source(s) and grounds?

I think I know of a simple way for you to get the reading you want, but it depends on circuit design issues we can't know of without the schematic.

 

[bobledoux]

I've attached the schematic. The load is a solenoid with a DC resistance of 4.7 ohms.

I've added a resistor to the ground lead for current measurement using voltage drop, as suggested above.

 

[cowboybob]

OK. So not a resonant circuit. just a controlled relay.

Looking at the schematic, C1 looks like it's serving as a filter cap. When Q1 fires, the 150 VDC will (should) supply the power to energize the relay coil, i.e., the cap won't discharge much of a discernible current. Of course, when the 150 VDC is removed (at power down, or whatever), then C1 will discharge eventually, but since we don't know what sort of bleeder resistors may be in the power supply circuitry, the time constant isn't known.

That said, if the 0.5 ohm resistor is connected to an actual ground (not floating) then your ground test lead hookup looks fine.

I might add, though, that it's unlikely that you'll see much of a voltage presence across R1 except at the initial application of power as the cap charges up to the 150 VDC limit.