Multimeter question

[steinberg]

Just checked my RD07a's reg/rec after the battery decided to go flat after commuting home last night, and sure enough one of the diodes is shot.
I first checked the reg/rec with an old analogue multimeter which gave the correct resistance reading to 5 of the diodes (800 ohms forwards, open circuit backwards). The 6th diode gave almost nil resistance both ways confirming it's shot.
I then double checked using a new digital meter. Using the diode test function the good diodes gave a half volt drop (as they should) and the duff one caused the meter to 'beep' which indicates very low resistance. So, the two meters gave a consistant result albeit by different methods.
Now the question - I then decided to check diode resistance using the digital meter (using the ohms function, not the diode function). It gave some strange readings, nothing like the 800 ohms which the analogue meter gave. Are digital meters not able to read the resistance of diodes if they have a separate diode function? Or is the meter possibly faulty?
Thanks.

[XR Mad]

Depending on the meter you might have to select the range of the meter manually for example 0 - 20 ohms or 0 - 200 ohms. it will also have to be the correct polarity on the ohms setting (swap the leads around). Reading what you have done so far you know what your doing so you may already have tried these sugestions. You could try it against some resistors with known values or 800 ohms might be outside the operating range of the meter.
Ps
something i've had with mine is a blown internal fuse, the meter appears to work ok except on certain settings, but i can't remember if it affects the ohms range i was using m/amp when i found it.

Good luck

[tizzi]

If you leave the diodes 'in circuit' you will get the meter measuring the general resistance of the circuit. Disconnect one end of the diode and the results should be consistant.

[Lutin]

Are digital meters not able to read the resistance of diodes if they have a separate diode function? Or is the meter possibly faulty?
Thanks.

I think that you'll find that on the diode test function, your digital multimeter uses a voltage of 2 Volts or so to ensure that any diode is properly "forward biased". As you have found, the good diodes gave a reading of 0.5 Volts when forward biased.

Most, if not all, digital multimeters use a much lower test voltage when on the "Ohms" ranges. This is so that when testing "in circuit" it is possible to get correct readings of resistances without any false readings from "forward biased" diode junctions.

I've just checked my own multimeters and they give a voltage of 2.7 volts when on the "Diode test" function but only 0.3 Volts when on the "Ohms" range.

Don't know about your analogue multimeter though, I'm assuming it uses a higher voltage on the "Ohms" ranges.