weird continuity with multimeter - any ideas?

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regularjohn

Well-known member
Joined
Jan 23, 2009
Messages
321
Location
Raleigh, NC
So I'm finishing up a build, and I was going through my normal pre-power up tests.  I went around the circuit checking continuity - to make sure it is where it should be and isn't where it shouldn't be - and I encountered something bizarre and I wanted to see if anyone might know what the deal is here.

When testing continuity between the DC voltage rails, I get a beep when the red probe is on the positive rail and the black probe is on the negative rail, indicating continuity.  BUT, if I switch the probes so that the red probe is on the negative rail and the black probe is on the positive rail then there's no beep, indicating no continuity.  I tried it everywhere I could access the voltage rails always making sure the probes weren't simply making bad contact, and it's the same everywhere.  Also, with the red probe on either voltage rail and the black probe on ground I get continuity, but not vice versa.  I thought continuity was non-directional.  Anyone know what's going on here?  Is my multi-meter f'd up?

Suffice to say, I haven't yet powered up. 
 
You are just experimenting with semi-conduction, which is the property that some weird chemical substances have that make them conduct electricity in one direction and not in the other. What does your continuity checker indicates?
Some multimeters, in ciontinuity-check mode, will indicate the actual residual voltage across the two probes. In presence of a real continuity (short-circuit) it will indicate zero-nada, but with a resistive or semi-conductive connection, it will indicate ...something...
If you check a single junction (such as a diode or a B-E or C-B junction in a transistor), it will indicate 0.5-0.7V
Not all continuity testers are equal, some will say yes to as much as 1k, some will reject anything over 10ohms...
 
My MM only indicates continuity for 30 ohms or less.  It doesn't indicate voltages.
But what happens is that when I get continuity between the rails, it isn't constant.  It beeps for a second or two - showing 0 ohms resistance - and then the resistance begins to rise until there is no longer any continuity indicated. 

I'm just baffled because I've never had this happen before on any other build.  So if the MM is simply displaying the semiconductive properties of some of the components, then it's ok to power up, right?
 
regularjohn said:
But what happens is that when I get continuity between the rails, it isn't constant.  It beeps for a second or two - showing 0 ohms resistance - and then the resistance begins to rise until there is no longer any continuity indicated. 

Sounds like a capacitor charging to me?
 
I have been building circuits for over fifty years, now.  I have never used a continuity tester to check-out active circuits (with tubes, transistors, diodes or capacitors).  I have used the continuity tester to puzzle out a maze of wires or switches. 
 
it could be the other way around, also.

when you connect neg to pos on a lytic, you have major leakage.
depends on meter.
9v or AA bats?


you want weird, ill tell you weird,

a DMM reading open when you have 100 ohms of copper wire wrapped around a toroid.

you think you broke a wire, so you wind another.

same thing.

so you wind half the turns to see wtf is up.

now the meter digits jump wildly between *** and %%%

then you realize that the inductor you are checking happens to match up with the clock speed of the dmm or some weird shit.

so watch out for that one,
they might have to pick you up in the looney wagon.
:D


god save the hp vac meters!
long live triplett analog!


can we not get forum code to change shit to sh*t automatically,
so i do not have to do it my self?
that way, i can cuss all i want and nobody
will think i'm an a**hole.  ;D
i'm jus sayin...
 
Speedskater said:
I have been building circuits for over fifty years, now.  I have never used a continuity tester to check-out active circuits (with tubes, transistors, diodes or capacitors).  I have used the continuity tester to puzzle out a maze of wires or switches.  

Ok.  I just like to double check all my connections before I power up.  That way I'm absolutely sure all the active components are oriented correctly when I flick that power switch.  I don't see how it's any different than ohming out resistors before soldering them in, or checking the capacitance of caps before soldering them in.  I like to be thorough so that if a problem does arise, then I can immediately rule out some possibilities.
 
I agree with regularjohn on this one.

I once had the wrong resistors in a voltage divider, and ended up with about 180 Ohm between the +/- 18 V rails. quarter watt resistors do leave a nasty stench.....was lucky it didn't take the PSU down, and could have been avoided by stickin an ohm meter across the rails before powering up. Most circuits I (now) do that to read 1k 'and rising' (due to all the caps charging), and I know I am good to go.

Important: make sure current actually flows from the red to the black probe when in resistance mode - some DMM's don't work that way!
 
Speedskater said:
But, Ohm meters are one thing and continuity checkers are another!
Different test voltages and different answers.

Sure they're different, because you're testing different things.  Using an ohm meter to check the value of resistors prior to soldering them in will give you piece of mind that any potential faults or issues are not due to a bad resistor.  Likewise, checking continuity from each voltage rail to the proper opamp pins will tell you that the opamps are oriented correctly and that they are indeed going to be receiving power from both rails.  It's just a "piece of mind" test I do before I power up.  I don't like spending $8-10 a piece on high speed opamps just to fry them by accidentally putting them in backwards.
 
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