04-07-2023, 06:23 PM
Any test instrument has limitations and can give misleading results.
DMMs vary widely in performance and how they work. Some can't measure wiring resistance of a large value inductance or higher voltage winding on a transformer as well as the cheapest analogue meter if they are not using static DC energising.
I've a couple that take too long to respond to shorts (continuity tests) or low resistance. You have to hold the probes on far too long. IMO those are junk even though nearly new. Also I have one that not just has 10A on a separate socket but irritatingly has Ohms and mA separate from volts.
I had one with a switch and a 750V DC range that died with a blue flash, set to that and connected to a 450V HT. I keep an old AVO to measure more than 250V or resistance of massive inductuctor/transformers.
I've a VVM* that unlike the DMM (which are 1M, 10M or 20M on volts depending on model) can measure AC up to about 150 MHz ( though not very accurately at some frequencies). Due to the 100M Ohm approx input impedance it can be a bit mental. It also can measure huge resistances, though my 400V via 2M leakage tester can roughly measure 20M Ohms to maybe several Gigaohms by looking at flash rate. 1M ohm looks like solid on and 100 M Ohms is a slow flash.
[*A VVM unlike a FET based one is maybe more robust a really high input impedance one is really an Electrometer. A cheap one is a strip of aluminium foil folded over a plastic cocktail stick across top of a glass jar]
DMMs vary widely in performance and how they work. Some can't measure wiring resistance of a large value inductance or higher voltage winding on a transformer as well as the cheapest analogue meter if they are not using static DC energising.
I've a couple that take too long to respond to shorts (continuity tests) or low resistance. You have to hold the probes on far too long. IMO those are junk even though nearly new. Also I have one that not just has 10A on a separate socket but irritatingly has Ohms and mA separate from volts.
I had one with a switch and a 750V DC range that died with a blue flash, set to that and connected to a 450V HT. I keep an old AVO to measure more than 250V or resistance of massive inductuctor/transformers.
I've a VVM* that unlike the DMM (which are 1M, 10M or 20M on volts depending on model) can measure AC up to about 150 MHz ( though not very accurately at some frequencies). Due to the 100M Ohm approx input impedance it can be a bit mental. It also can measure huge resistances, though my 400V via 2M leakage tester can roughly measure 20M Ohms to maybe several Gigaohms by looking at flash rate. 1M ohm looks like solid on and 100 M Ohms is a slow flash.
[*A VVM unlike a FET based one is maybe more robust a really high input impedance one is really an Electrometer. A cheap one is a strip of aluminium foil folded over a plastic cocktail stick across top of a glass jar]
