meauring resistance of a bridge rectifier

[micky]

I have a bridge rectifier that shows infinite resitance in all ranges
except the 2000K range

In the 2000K range, it shows infinite in one direction for each of the
four pairs of connectors and either 660, 770, or 880K ohms for the other
direction.

This seems high to me! It's a digital meter with iirc a 1.5 volt
battery.

The rectifier is used in a 12 amp 110VAC electric lawn mower,

Should I get out another meter? I have almost alll kinds.

I usually measure diodes that have under 1000 ohms in the conductive
direction.

AIUI the resistance drops under load, but from 880K to 10?

 

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.

 

[Michael Black]

On Sat, 21 Jun 2014, micky wrote:

I have a bridge rectifier that shows infinite resitance in all ranges
except the 2000K range

In the 2000K range, it shows infinite in one direction for each of the
four pairs of connectors and either 660, 770, or 880K ohms for the other
direction.

This seems high to me! It's a digital meter with iirc a 1.5 volt
battery.

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

The rectifier is used in a 12 amp 110VAC electric lawn mower,

Should I get out another meter? I have almost alll kinds.

I usually measure diodes that have under 1000 ohms in the conductive
direction.

AIUI the resistance drops under load, but from 880K to 10?

 

[Maynard A. Philbrook Jr.]

In article <o8lbq99nk4mpqs97lj1adoa31rug6cr2lo@4ax.com>, NONONOmisc07
@bigfoot.com says...

I have a bridge rectifier that shows infinite resitance in all ranges
except the 2000K range

In the 2000K range, it shows infinite in one direction for each of the
four pairs of connectors and either 660, 770, or 880K ohms for the other
direction.

This seems high to me! It's a digital meter with iirc a 1.5 volt
battery.

The rectifier is used in a 12 amp 110VAC electric lawn mower,

Should I get out another meter? I have almost alll kinds.

I usually measure diodes that have under 1000 ohms in the conductive
direction.

AIUI the resistance drops under load, but from 880K to 10?

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

 

[micky]

On Sat, 21 Jun 2014 15:49:08 -0400, Michael Black <et472@ncf.ca> wrote:

On Sat, 21 Jun 2014, micky wrote:

I have a bridge rectifier that shows infinite resitance in all ranges
except the 2000K range

In the 2000K range, it shows infinite in one direction for each of the
four pairs of connectors and either 660, 770, or 880K ohms for the other
direction.

This seems high to me! It's a digital meter with iirc a 1.5 volt
battery.

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.

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.

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.

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.

Michael

The rectifier is used in a 12 amp 110VAC electric lawn mower,

Should I get out another meter? I have almost alll kinds.

I usually measure diodes that have under 1000 ohms in the conductive
direction.

AIUI the resistance drops under load, but from 880K to 10?

 

[Allodoxaphobia]

On Sat, 21 Jun 2014 21:28:27 -0400, micky wrote:

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.

 

"So I
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.