EEVblog #373 – Multimeter Input Protection TutorialPosted on October 19th, 2012 8 comments
Everything you ever wanted to know about how Multimeter input protection works.
Using the Fluke 27 as a baseline example.
Forum Topic HERE
first, this is another very interesting tutorial. Now, I know exactly, for what the “magic” bridge rectifier, I often saw in multimeters, is good for. And… now, HRC Fuses aren’t anymore only some special and therefore expensive Fuses for me.
Is it possible, that the 1 Meg. ceramic (precision) resistor is needed for measuring resistors with the multimeter. I think you need at least one exactly stable resistor in one of the measurement-chains to “calculate” an unknown resistor by using current and voltage.
Great tutorial Dave.
What will happen, if a (dumb) person plugs the test leads between the Voltage/Ohms connector and the Amps/µAmps connectors leaving the ground connector open? This alternative you didn’t mention…
Anyway, another good recitation from you.
Not much will happen
you have in series the I and V paths.
The I path is a short circuit with not much current => no problem
The V path then takes all voltage overflows with adequate protection.
Is there a benefit of using the diode bridge over back-to-back zeners or is this just how multi-meters like to do it?
I think there are three main reasons for preferring the diode string + bridge rectifier to say a high-power TVS, which is effectively a monolithic back-to-back zener diode. Let’s take the 1.5KE6.8 as an example. This is rated for a peak current of 139A so it certainly is rugged enough. And let’s assume its clamping voltage is low enough (it may not be but presumably there’s some equivalent part with a lower voltage).
The problems are leakage current, capacitance and fault tolerance. Leakage is specified as 1mA @ 5.5V. They don’t give a figure at low voltages but low voltage zeners (ie, true zeners) tend to have a pretty soft “knee” and so the leakage will remain relatively high down to voltages well below the rated breakdown voltage. Compare this to a 1N4004 which has a reverse leakage current of 5uA at 25 degrees at its full rated reverse voltage of 400V DC. I suspect it will be significantly lower at a fraction of a volt – probably well under 1uA. So it won’t affect readings too badly.
Capacitance – the 1.5KE6.8 has a capacitance of around 10nF at 1V (extrapolating from the curve in the data sheet). Not sure if that’s an issue in this case but it might be depending on the source impedance. Capacitance of a bunch of series 1N4004 will be very small – each one is around 15pF and that is divided by the number which are in series and reverse-biased. So just a few pF effectively.
Ruggedness – if one of the 1N4004s does go short circuit (unlikely…) then the protection circuit will still work. Not the case with back-to-back zeners or a TVS. This is a pretty minor feature considering that the circuit is designed to protect them anyway.
But the knobs don’t go to 11!
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