| Electronics > Projects, Designs, and Technical Stuff |
| Differential probe or isolation probe |
| (1/1) |
| ArthurDent:
Every week it seems there is another thread started about misusing isolation transformers or disconnecting the power line ground from scopes and other test equipment. The correct answer to solve the problem is usually to use a differential probe but the really good probes are generally a little pricey and way more complicated than you need if you just want proper isolation from line voltages at low frequencies. Although some of the high end differential probes go into the gigahertz region, it seems that I would be mainly interested in just power line voltage use. I’ve used the traditional ‘one hand in your pocket’ and the high voltage DMM approach but there have been times when I wished I had something better or could use my scope. Most of what I experiment with is test equipment or tube based amplifiers that are referenced to ground so just a resistive voltage divider (in an oil filled case) for D.C. like the one shown below works fine. Several months ago I parted out a strange high voltage D.C. supply and not wanting to waste any parts, saved a board that had several isolated opamps. Recently I got to thinking that I might be able to use one of these to construct an isolation probe. The opamps on the board are Analog Devices AD210BN that have 2500V rms isolation between input, power supply, and output. The board already had TVS protection diodes (SA8.5) on the input so all I’ll have to do is change the ratio/gain resistors and build it into a box and it should be ready to go. I checked it out with my function generator and the sinewave output looks good and the response is flat to about 20Khz. That frequency response may be a little low to check out the PWM frequency in a SMPS but the price was right and it looks like a useful project, at least for me. The AD210BN wouldn’t be too practical for others to use starting from scratch if you had to buy them because the new price is about $114 ! |
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