multimeters - am I doing it wrong?

troypiggo (Troy) · #1 23-10-2009, 02:17 PM

I'm a bit of a DIY electronics gumby. I bought a multimeter so I could test stuff, but not sure if there's something wrong with the meter or it's my zero experience with it that's causing weird readings.

It's kinda like this one, but doesn't do temperature.

I set the meter to check DC voltage, V not mV, and clicked the range button so there were 2 zeros in front of decimal because I was checking for around 12-13.8V values.

There are 3 holes for the probes to plug into. I put the black one in the "com" socket, and tried the red one in both the "10A" and the "V ohm mA" one. No matter the combo, I could not get readings in the right ballpark of 13.8V when I connected the probes to the terminals on a 13.* regulated power supply turned on.

Any ideas on what I'm doing wrong?

bojan · #2 23-10-2009, 02:24 PM

10A socket is for current measurement. It has a very low internal impedance.
If you tried this one first, two things may have happened:
- If your power supply can deliver much more than 10A, the meter internal fuse may be blown by now
- If your power supply is not short-circuit protected, by connecting it to 10A socket you effectively shorted the power supply output, and eventually blew it.
And, there is a chance that meter was already faulty..

acropolite (Phil) · #3 23-10-2009, 02:25 PM

What bojan said is correct but you may not have blown a fuse if the meter wasn't switched to read current. The 10A hole is for current measurement and the meter goes in series with the power supply when reading current.

To measure voltage you should have the black lead in the common socket and the red lead in the V ohm ma socket.

I'd guess that the multimeter you have is autoranging so you shouldn't need to press the range button, doing so may lock the meter in a given range and could possibly cause it not to read (read the manual). The meter will read both AC and DC, you will most likely need to press one of the buttons to set it to read DC (MODE prehaps, check the manual)

If all else fails read the manual...

erick (Eric) · #4 23-10-2009, 02:30 PM

Well, don't connect a battery or power supply to the 10A input again. You might have upset the power supply and/or the meter.

Let's go for a simple test. Put Black lead in "COM" and Red lead in "V ohm mA". Set the scale to 4V but any DC Volts range will do. Find a good AA or AAA battery - check it has power by testing in a torch or device. Then, touch the probes to the terminals at either end. Do you get either +ve or -ve somewhere around 1.4 Volts?

troypiggo (Troy) · #5 23-10-2009, 02:38 PM

Thanks for the very quick replies, guys. I'll post results when I get home tonight and test.

kinetic (Steve) · #6 23-10-2009, 04:54 PM

Troy, with the greatest of respect mate, if you're mucking around
with a multimeter and aren't sure which jack to put the plugs
into, maybe don't do it?

I'd hate you to get hurt measuring 240v for example.

Steve

troypiggo (Troy) · #7 23-10-2009, 05:05 PM

Thanks Steve, and understand what you're saying. I'm not going to measure 240V, that's for sure! I'm off to read the manual, again - last time was a quick read late at night and must've not absorbed enough.

troypiggo (Troy) · #8 09-11-2009, 10:50 PM

Sorry for the thread resurrection, but finally got around to doing some testing tonight. I think the multimeter might be faulty unless my expectations are way off base.

I took a fully charged AA battery that I know works and tried to test it's voltage. Touching the probes on each end of the battery with the MM's range set to volts, it just showed zeros. I changed the range down to mV and I could see a reading that was slowly increasing. I mean really slow. Took maybe a minute for the values to climb to around 100mV. When I took the probes off the MM, the numbers started really slowly dropping again. If I reversed the probes, the numbers dropped and kept dropping below zero into the negatives.

This tells me it is kinda reading something, and the signs of its readings seem correct, but why is it so slow? I was expecting the reading to be instantaneous. If I waited for the reading to get to around the 1.2V I was expecting for the AA battery, it would have taken a good 15 minutes I reckon.

Am I right? Should I return it? Or am I misunderstanding and that's how it should work?

DavidU (Dave) · #9 09-11-2009, 11:00 PM

Ether you have an open circuit probe or it is FUBAR.
Send it back.

erick (Eric) · #10 09-11-2009, 11:24 PM

Yes, stuffed somehow. Some time ago I went to repair a turntable. Owner had just invested in a $10 multimeter so I used that and found no volts in (It took DC from the main amplifier). Went home to bring back a power supply to get the turntable running and brought back my meter. Checked again - volts were there! Tested his new meter on a 1.5V battery - zilch reading!

I have a couple of these cheapies spread around office, car, home but haven't seen such a gross fault.

MrB (Simon) · #11 10-11-2009, 01:25 AM

If you got it from Jaycar and still have the receipt, just ask someone there to test it for you, they will replace it.
Jaycar multimeters have a minimum 2 year warranty... even the $9.95 one.

troypiggo (Troy) · #12 10-11-2009, 05:54 AM

Thanks guys. Not sure if I still have the receipt, but will see how I go. Bit disappointed, it certainly wasn't the cheapest one they had.

Barrykgerdes · #13 10-11-2009, 07:08 AM

Many of those digital multimeters have an internal safety device like a fuse or diode to guard against connecting a 13v power source to the Amps terminal. Others just vapourise a section of the track. You have apparently done something like this.

I worked in electronics for 50 years and 50% of our income was derived from DIY "experts" who exceded their skills with their repairs. One of my best stories involved a transmitter that was given to me to repair in a thousand pieces. After a day and a half puting it back together the original fault was still evident. I went to the mains supply panel, checked the supply and changed the "C" phase fuse and the transmitter was back in operation.

Barry

troypiggo (Troy) · #14 10-11-2009, 08:01 AM

Barry, I would have thought that if I'd blown the fuse I wouldn't be getting readings like I did?

I don't profess to be an "expert", in fact read my opening line in post #1.

Barrykgerdes · #15 10-11-2009, 08:06 AM

Quote:

Originally Posted by troypiggo

Barry, I would have thought that if I'd blown the fuse I wouldn't be getting readings like I did?

I don't profess to be an "expert", in fact read my opening line in post #1.

Burning out the input circuit of these digital multimeters will not necessarily stop them from being turned on. It will just stop them reading anything from the input probes. I would expect that a quick internal inspection will quickly reveal the problem to a trained tecnician.

Barry

troypiggo (Troy) · #16 10-11-2009, 08:18 AM

Yes, I'll be taking it to a trained technician.

My question was that if the fuse was blown, or the input circuit blown, or whatever, how can it be getting readings in the right direction, just really slow?

Quote:

I took a fully charged AA battery that I know works and tried to test it's voltage. Touching the probes on each end of the battery with the MM's range set to volts, it just showed zeros. I changed the range down to mV and I could see a reading that was slowly increasing. I mean really slow. Took maybe a minute for the values to climb to around 100mV. When I took the probes off the MM, the numbers started really slowly dropping again. If I reversed the probes, the numbers dropped and kept dropping below zero into the negatives.

This tells me it is kinda reading something, and the signs of its readings seem correct, but why is it so slow? I was expecting the reading to be instantaneous. If I waited for the reading to get to around the 1.2V I was expecting for the AA battery, it would have taken a good 15 minutes I reckon.

Barrykgerdes · #17 10-11-2009, 09:12 AM

Quote:

Originally Posted by troypiggo

Yes, I'll be taking it to a trained technician.

My question was that if the fuse was blown, or the input circuit blown, or whatever, how can it be getting readings in the right direction, just really slow?

That is quite an interesting effect. The input impedance of a digital multimeter is very high. The effect certainly points to something burnt out in the input circuit. What is most likely happening is that the testing voltage is slowly charging up the input capacitance of the isolated input components through leakage across the damaged component. This then slowly discharges when the test voltage is removed and the reversing the test voltagest the readings head off in the other direction for the same reason.

Without carrying out extensive testing on the multimeter it is difficult to surmise any other reason.

Barry

troypiggo (Troy) · #18 10-11-2009, 09:22 AM

Ah, I see. Thank Barry.

MrB (Simon) · #19 11-11-2009, 02:44 AM

Troy, unfortunately every Jaycar store has recently had their POS system "Upgraded" (

) so you must have the receipt for a refund or warranty repair... the 'puter will just flat out refuse to allow it otherwise.
For an exchange you might be ok, depending on who serves you, you might get a straight swap.
Don't tell them you measured voltage with the lead in the amps socket

It honestly wouldn't surprise me if you got a bad one... I have had customers exchange a couple for various reasons. (most recent had a backlight that wouldn't turn off)
Don't let that discourage you though, in the last year we've had 4 maybe 5 meters returned.