Question on Multimeter...
idleup
Posts: 46
1. Voltage question - When using a multimeter to test the output voltage of a 9v outputting wall wart I am getting between 9 - 13 volts. I did the same thing with a 5v wall wart and got 7-10 swing on the output voltage. Is that normal or does my multimeter suck? I have an MPI.
2. Resistance Question - when I put the multimeter in resistance mode and place the probes between ground and power pins on my LCD I seem to get a slightly different resistance reading every time. I also noticed that if I switch the probes so the black and red are on opposite pins (still the same two pins though) the reading is different still yet. I thought when checking ohms (which I use to look for a short) it doesnt matter which probe is on which side. I havent tried this simple experiment but would you get different readings put the probes on each side of a resister and then switching them around?
- Matt
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2. Resistance Question - when I put the multimeter in resistance mode and place the probes between ground and power pins on my LCD I seem to get a slightly different resistance reading every time. I also noticed that if I switch the probes so the black and red are on opposite pins (still the same two pins though) the reading is different still yet. I thought when checking ohms (which I use to look for a short) it doesnt matter which probe is on which side. I havent tried this simple experiment but would you get different readings put the probes on each side of a resister and then switching them around?
- Matt
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Comments
Your meter is probably OK.
Unless your wall wart is sold as a "regulated" unit, the voltage will be above nameplate when no load is on it, gradually coming down to the nameplate value when at rated current. Its particularlly noticeable with a large ( say 800mA or 1A unit delivering only 25 or 30 mA.
You have to be careful with electronic devices that don't have their own regulators on board as you can easily subject them to over-voltage.
Resistance is subject to some variation as well. The resistance your meter displays for you is really a current reading thru your device under test, driven by your meter's internal battery. Depending on the range you've selected and the conditon of the battery and whether you've zeroed the meter, the reading can vary. For the same reason, flipping the leads flips the direction of the test current, so depending what you're checking, different components can cause a varying reading.
If you need to be able to check your meter for accuracy, just purchase a few 1% resistors and keep them check your meter from time to time.
To check your voltage readings, check the 5v bus on a stamp board. It is a very stable 4.99 to 5.01 volts, in my experience.
Cheers
Some multimeters put more than 5 volts across a load when measuring its resistance. And also, some multimeters in resistance-measuring mode put a negative voltage on the red lead.
A sensitive device could be damaged by either of these things.
If you want to know the current draw of the LCD, you'd be better to run it using its normal 5v with an ammeter in the circuit.
David
Just to see what the voltage is in resistance mode, I hooked up a voltmeter to my multimeter while in resistance testing mode. It was outputing .25 volts.
Do you think I·shorted or·screwed up·my replacement LCD screen? Learning this stuff can get expensive as there are now electronics supplies in Fresno and I have to place an order online every time I need a part.
- Matt
My ohmmeter uses different voltages on different ranges- it wouldn't hurt to test your meters to understand just what they're applying to your circuits. I was surprised to find my meter applies a negative voltage to the red lead on resistance. For most resistors, it doesn't matter, but it just seemed opposite to what I was expecting.
David