05-04-2017, 08:11 AM
Indeed. It's a good idea to learn why an AVO 7 (and probably laso an AVO 8) will give sensible readings under these conditions while a DVM won't. It's also a good idea to learn why taking irrelevant and potentially misleading measurements isn't a good idea. The emphasis in both cases needs to be on "why". So here goes. The explanation is for the benefit of those who are less experienced and I'm sure it could be improved.
There will always be stray capacitance between the different windings of a transformer. AC can pass through a capacitor hence a sensitive meter connected to different windings may well observe a current. As an example let's say there's 100pF stray capacitance between 2 windings. Using X = 1/(2*PI * f * C) this is a reactance of about 33M. If there's 100V trying to drive a current through this you'll get about 3uA. You won't see this on an AVO 7 which takes 1mA from the circuit under test to give full scale defelction. You'll just about see it on an AVO 8 which is 50uA FSD. You'll certainly see it on a DVM, whether it's set to voltage or current ranges.
Incidentally I've talked about currents in this explanation. All moving coil meters such as AVOs are current meters. They can measure voltage because the maker puts a resistor in series with the meter so that (by Ohms law) the full scale current equals the required full scale voltage divided by the resistor.
There will always be stray capacitance between the different windings of a transformer. AC can pass through a capacitor hence a sensitive meter connected to different windings may well observe a current. As an example let's say there's 100pF stray capacitance between 2 windings. Using X = 1/(2*PI * f * C) this is a reactance of about 33M. If there's 100V trying to drive a current through this you'll get about 3uA. You won't see this on an AVO 7 which takes 1mA from the circuit under test to give full scale defelction. You'll just about see it on an AVO 8 which is 50uA FSD. You'll certainly see it on a DVM, whether it's set to voltage or current ranges.
Incidentally I've talked about currents in this explanation. All moving coil meters such as AVOs are current meters. They can measure voltage because the maker puts a resistor in series with the meter so that (by Ohms law) the full scale current equals the required full scale voltage divided by the resistor.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
