Electronics > Beginners
Measure high Voltage 500V DC and Current in a floating system
mjoelner:
Hi guys and girls.
I have a power supply of 480V DC that powers some equipment (quite steadily and boring, no signals or exciting stuff) that pulls 0.8 amps. I wanted to do some measurements on this and I thought it was going to be very simple, but oh no, not so much. I would like to measure the voltage and the current the gear is consuming using picoscope, because its nice to see what is going on.
Since 500 volts is quite high I started with reading up on people that have tried the same, and I quickly found that first of all there was a good chance of frying the gear and a differential probe was the way to go, so I bought two of the chinese ones that everyone loves because of the low price.
My idea was to just use the differential probe directly across the 500VDC source, and that worked like a charm. My next idea was to put two resistors in series with the equipment, one for each line, thinking both of them would consume some volts. I would then be able to measure the voltage, and that voltage across one of the 4 ohm resistors should also give me a pretty good idea what the current in the system is doing.
When measuring a handheld scope using one 1x probe, it works pretty good I have to say. But, seeing that I want to save this and use as reference later on, the picoscope is what I really wanted to use.
Using a differential probe actually worked pretty good, but I thought it was a bit to noisy at 50x. One solution then would be to up the resistance a bit, but I cant steal to much of the voltage either. So, I thought about using the a-b differential method using two probes connected together and then measure across the resistor. A calculation in Picoscpoe would then give me the DC voltage across the resistor, and I was hoping to see the same as the handheld oscilloscope. Unfortunately that did not happen. When I connected the probes across the 4 ohm resistor, they both gave me a channel overrange alarm and the numbers made no sense whatsoever. The laptop and the picoscope is not grounded or connected to power anywhere.
Any idea what is causing this, and better, is this what is expected or can I adjust my experiment to give me better values.
I have a picoscope with 4 inputs. Two differential probes 50x/500x. 2x 4 ohm resistors, or even 2x 10 ohm resistors. The goal is just to be able to read the amps in my circuit, and I dont have much fancy equipment to be able to do that.
Thanks for any inputs on this.
fcb:
If the differential probes are properly designed/built I can't see why this wouldn't work. Might be worth checking their isolation to scope ground.
The noise thing is an issue with the way diff. probes work and their need for high attenuation. Is this a one off measurement or something you need to do on a regular basis?
mjoelner:
Hi yes, the differential probes are the chinese 150 USD ones that everyone on a low budget buys nowadays from micsig.
If the measurements give me some usefull results, then I may do this on a regular basis.
I see now that I was a bit unclear. When I used the "A-B differential method" I used the standard probes on the picoscope. It was the standard probes that got overloaded.
Thanks
Ice-Tea:
So, eh, you have two regular probes hooked up to 500VDC power supply? Maybe you should draw exactly what/where you're measuring. Seems to me like you're not taking into account the grounding of the probe or the max voltage of the inputs..
mjoelner:
hehe, yes, you are right, there might be some issues with this approach.
I have two probes, channel A and Channel B connected to each side of a 4 ohms resistor. This resistor has 0.8 amps going through it. I would expect to measure a bit less than 4 volts in this setup, but I get channel overrange on both probes. This resistor is in series with the equipment I want to measure. The power supply is almost 500 volts DC. The ground on these two probes are connected to themselves only, nothing else. The picoscope is powered by a laptop, and the laptop is only running on battery. There is no metal parts touching any other metal parts anywhere. As far as I can understand nothing is grounded. Even if it was grounded, from what I understand there is a 1 Mohm resistor in series in both the two probes, so there would not flow much dangerous current through in any case. If I did connect the ground cables anywhere, I would fry the equipment, yes.
https://www.picotech.com/images/uploads/library/topics/_med/pseudo-differential.png
This picture resembles pretty much what I am trying to do with two standard probes.
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