Transformer recommendation

Thread Starter


Joined Jul 7, 2009
When I am performing experiments, I sometimes worry that they can be affected by line voltage variations or noise. For example, I'm currently making some measurements of the UV radiation power of a consumer device (apparently, it's some device women use in nail salons to dry the crud they put on their fingernails). Power line variations that I don't know about will lead to increased variance in the measurements I make. I'd like to a) know about the variations and b) if they're bad enough, take some action to correct them. I want to start with a) first.

I would like to build a small box that could be used to supply power to experimental equipment. While the feed-through of the power is simple, I'd also like the box to have two BNC outlets for monitoring the voltage and current being delivered to the load that is plugged into the box. Both digital multimeters and a scope might be plugged into these BNC connectors for monitoring current and power. I will have both conductors of the BNC connectors isolated, but it will be fine to plug e.g. a coax cable in from a scope where the outside conductor is grounded. This will let me also monitor the instantaneous power waveform (but I don't know whether that will be useful or not for the experiments I do).

Of course, I want both the current and voltage outputs isolated from the line voltage, so I'll use transformers. For the current, I'll use a current transformer; I'll set the maximum current spec that I need to 10 A, as that matches the output of a Variac that I have.

The thing that has me slightly worried is that I would like to make sure the transformer I use has the ability to also show power line harmonics up to around 20 or so (i.e., not attenuate harmonics up to around 1 kHz or so). I feel this will let me see any nastiness on the line power when an experiment is being run (once I have this device, I'd leave a scope running that shows the current and voltage waveforms).

Thus, my questions for the experienced EEs:

1. What transformer would you use for isolating and stepping down the voltage? I don't care what the output voltage is as long as it's at or under around 10-12 volts RMS. I'd prefer it to be 1 to 2 volts. The input voltage will be 120 VAC in the US.

2. What current transformer would you use? I would like a current transformer that accurate reflects the current waveform and doesn't saturate (ideally with the same bandwidth as the voltage transformer). A voltage output of around 1 volt at 10 A would be fine, although an order of magnitude in either direction is fine too.

I'm looking for specific industrial part numbers, preferably ones I can buy from Digikey/Mouser.

Also, any other thoughts you have about things I might want to worry about would be appreciated (well, they should be related to the thread at hand, as I've got lots of other things to worry about :p).