I got a surprise this morning touching one side of the microswitch power switch and chassis ground. I was under the assumption that a conventional power transformer offers isolation from shock hazard. If you measure between chassis ground and incoming ac there is 120vac potential. I would have thought the secondary of the transformer should have no reference to the primary. Will the center tap have potential in reference to the primary?
Picture attached showing the 2 points.
Thanks all
Picture attached showing the 2 points.
Thanks all
Attachments
Only one lead of the AC line is switched on this premap. The other AC lead is still connected to the transformer primary.
You AC line hot (black) lead is probably connected to the unswitched end of the primary.
You AC line hot (black) lead is probably connected to the unswitched end of the primary.
yes that’s the case but the center tap of the secondary having potential between it and the primary
Hi rickel,
Try this experiment:
Turn on the unit, and unplug from the AC. Do an ohmmeter continuity test between the power plug and the chassis. My bet is that you will find there's no continuity.
If you do find continuity, it's likely the transformer has shorted and is now dangerous.
Try this experiment:
Turn on the unit, and unplug from the AC. Do an ohmmeter continuity test between the power plug and the chassis. My bet is that you will find there's no continuity.
If you do find continuity, it's likely the transformer has shorted and is now dangerous.
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Is the preamp completely disconnected from anything else, other than the AC line, when you measure this?
Yes, nothing else connected. As bsst requested above there is no continuity between the line cord and transformer secondary. I don’t think this is due to defective transformer as I have another unit and it measures the same. The only thing I can think of is the voltage is being generated through a the flux of the transformer.
Hi , this is not very rare case , that mains transformer have some capacity between primary and secondary windings. In that case , you will see about half mains voltage potential on all secondaries . Some transformers have seperate bobin for secondary and primary, and isolation is improved . Actually you see this voltage potential only, but there's no current flowing ,if transformer's isolation is not damaged. If you will connect another equipment, like amplifier as example , which is grounded , you will no longer have these 120V on preamplifier chasis.
Also you can try to ground chasis via a 100k resistor temporary, and measure voltage on that resistor, it should be now close to zero .
Also you can try to ground chasis via a 100k resistor temporary, and measure voltage on that resistor, it should be now close to zero .