meauring resistance of a bridge rectifier

On your (digital) meter there shoud be a little diode symbol. ( -|>|- )

This puts more voltage across the device so it will turn the diode on, in one direction of course. It then reads the voltage drop at whatever current it uses, probably 1 mA.

The other ranges keep the voltage low so you can measure resistances in circuit without the reading getting all screwed up be transistor and diod junctions in the circuit.

Sometimes this iformation is in the manual, others times not.

I haven't had an analog meter in so long that I can't remember.

Generally, you use the diode function on a DMM to check semiconductor juntions. That function exists for good reason.

The diode probably isn't getting enough voltage to actually conduct.

With the diode fuction on a DMM, you can actually see the difference between germanium, silicon and schottky diodes.

Michael

digital meters have a DIODE mode scale and what this does is inject around 2 volts into it.

WHen you see this, the reading is actually showing you the forward voltage, not resistance.

The reason for doing this is, DMM's have very low voltages and for good reason. Todays electronics can be active,destroyed or give you bad readings using a meter in ohms mode in circuit, because the meter is generating enough voltage to forward bias PN junctions which can then influence your readings with other components attached etc. DMM's have this option and should be off when you no longer need it while you're probing around on a circuit board.

Most of the time you don't need to worry about component damage while doing this in circuit but there has been cases using meters with voltages exceeding 1.5 volt on sensitive type front ends.

Jamie

Thanks, Jurb and Michael.

I got out a more expensive meter and it had the diode function all right!! So went upstairs and by golly the cheap on had it too. So I used the cheap one and got 49 ohms in one direction on every consecutive pair of contacts, and the out of range indication on the other direction.

So I tried the better meter and had a hellluva time. Finally I noticed that the Hold button was down from the last time I used it I think it still changed by 1 in the right-most number out of 3.5 or I'd have figured it out more quickly.

Without hold, same readings, but there seemed to be a decimal point with the label 2, meaning 0.049 ohms. Which seems more likely?

The ohms section uses the 2 three times, for this, for 2K, and for

2MegOhms. It uses 20 three times and 200 three times too. I went all around the control and if the setting wasn't for 2, 20, or 200 u###nits, there was no value displayed of the 2, 20, and 200. Just blank. Meaning I think that they didn't make a mistake when they put in 2 for the diode switch position.

That's for another time, I guess.

In the past, all the diodes I checked showed an adequate front to back ratio to convince me they were good (if they were.)

This bridge rectifier was a good part from a broken B&Decker lawn mower. So I went out to the lawnmower and measured the bridge rect. in my second bad one, hoping that was the problem, but it read the same way, except for 51's in place of the 49's.

Then I noticed that when I turned the blade manally, it squeaked, if it was on its side or even almost flat with my hand underneath.

Then I ran it and it runs slow. Then I noticed that top of the armature was no longer in the center of the hole in the bracket screwed to the permanent magnet that is the housing of the armature. I could push it in any direction.

So that's theproblem. Tomorrow I'll see what crumbled and lets the armature move around. Maybe it's a cheap part.

Well, obviously. :-) Maybe it'll be inexspenive, as well.

used the cheap one and got 49 ohms in one direction on every

Not quite. You got the voltage drop reasding at the current supplied to the DUT (device under test) that is really the same thing as an ohmeter, but just calibrated a different way.