| Electronics > Projects, Designs, and Technical Stuff |
| Isolation Transformer and Oscilloscope |
| << < (3/3) |
| electr_peter:
This topic has been discussed multiple times before, isolation transformers are not magic safety devices - do not trust them blindly, use differential probes. Isolation transformer has one meaningful use with oscilloscopes, but it is still dangerous. As oscilloscope and DUT are both grounded, you can only clip oscilloscope ground clip to the earth and measure signals with respect to the earth. This may not be what you want. If you power DUT from isolation transformer, you can clip oscilloscope probe ground clip anywhere and measure what you want. However, by doing this you once again make DUT referenced to earth (floating becomes earth referenced) and DEFEAT RCD/GFCI PROTECTION FOR DUT. See simplified diagram. Note that GFCI/RCD is not effective in this configuration even if you add it after isolation transformer. Isolation transformer did not add safety in this case, it made things more dangerous!!! Please use real differential probes or pseudo differential probes technique( sum[1CH; 2CH inverted] ) instead if you mind safety. Isolation transformers are mainly used to provide isolated power or add some noise filtering. They do not add safety by default. Also, install GFCI/RCD, but do not trust it too much: * for one, GFCI/RCD may fail to operate (could be device failure, could be some faulty wiring, etc.) * GFCI/RCD are designed to trip at 5-30mA or higher currents in short period of time. However, GFCI/RCD do not limit instantaneous maximum current during the fault in any way, which could be 100mA-500mA or more!!! Do you really want to try that? |
| calzap:
A bit of warning on "isolation" transformers. Not all transformers carrying the label of "isolation" provide isolation in the sense discussed in this thread. Some only provide noise reduction from the incoming mains. An example is the Tripp-Lite Isolator series. Their literature does state that the secondary "neutral" is bonded to ground, but doesn't provide a warning that the receptacles on the device can't be considered electrically isolated from the incoming mains. I bought one and was suspicious when I saw the outlets were of the three contact type. Opened it and was horrified to see one of the transformer secondary wires connected to the case; then I read the fine print. I later modified and tested it to be a true isolation transformer. I'm not the first to do this; there are examples on the net. Mike in California |
| bogart219:
Hey fellows, Reviving an old thread. I have this 1 KVA control step down step down transformer. I measure no continuity between the primary and the secondary. Is it safe to assume that this is a isolation transformer? I called Dongan and asked them but the guy I talked to seemed confused whether it was or not. what do you think? When I apply 120 volts to the 480 primary setup, I get 120 volts on the secondary. |
| Ian.M:
Probably a that's what the diagram on the label indicates, but as an isolation transformer is safety critical, you need to be 100% certain of the integrity of the insulation between the primaries and the secondaries. If its a split bobbin transformer, visual inspection may be sufficient, otherwise you need to tie all the primary terminals together, and the same on the secondary side and do a Hi-Pot test, at a test voltage or 1KV or higher. Once you are certain its insulation is OK, the best way to wire it would be to parallel the nominally 240V primaries, and put the nominally 120V secondaries in series (link H1-H3, H2-H4 and X2-X30). Then its wired 1:1 and is good for 1KVA @ 240V so 4.16A. If you use it for 120V input isolation, wire the supply as-if it was 240V and the whole series secondary will give you 120V out, but you don't get any more current capability than the 4.16A its rated for. |
| bogart219:
Hi Ian, thanks for the info. I have a 500 volt megger. I will do that test. :) |
| Navigation |
| Message Index |
| Previous page |