Multimeter Voltages............

Stu Diddly · December 17, 2007, 06:02:20 PM

I'm having multimeter issues. I just bought a digital multimeter from Radio Shack and I'm having some issues measuring my DC Voltages on a Boost Circuit. I'm setting my meter to DC and I'm alligator clipping the black lead to the ground on my output jack and I'm touching the red lead to emitter lead of my transistor and the meter goes crazy. It'll start at like 3v and then rapidly decrease down to 0v. Am I doing something wrong here? I can't seem to get a normal reading out of my circuit.

HELP...........................

This is the meter I bought..

http://www.radioshack.com/product/index.jsp?productId=2103176&cp=&sr=1&origkw=digital+multimeter&kw=digital+multimeter&parentPage=search

PerroGrande · December 17, 2007, 06:15:50 PM

A couple of things to check --

Make sure the selector switch is solidly in the correct position. Sometimes the selectors don't make good contact until you rock the switch a bit. Connect it to a known voltage (like a 9V battery) and make sure it gives the correct reading.

Secondly, make sure the unit has fresh batteries. I had one go haywire when the batteries were dying.

Lastly, a number of comments on the associated thread (on the RS site) indicated that the meter gives odd readings when one of the fuses is blown.

Stu Diddly · December 17, 2007, 07:10:08 PM

Quote from: PerroGrande on December 17, 2007, 06:15:50 PM
A couple of things to check --

Make sure the selector switch is solidly in the correct position. Sometimes the selectors don't make good contact until you rock the switch a bit. Connect it to a known voltage (like a 9V battery) and make sure it gives the correct reading.

Secondly, make sure the unit has fresh batteries. I had one go haywire when the batteries were dying.

Lastly, a number of comments on the associated thread (on the RS site) indicated that the meter gives odd readings when one of the fuses is blown.

Thanks John. This is a brand new multimeter and the batteries are brand new. The multimeter gets a mv reading even when the leads aren't plugged in. When I set it to the "check 9 volt" setting, I do get a good read on my battery. I don't know if this is normal activity for a multimeter or not.

I'm still clueless...............Should I take this one back and buy a different type of meter?

PerroGrande · December 17, 2007, 07:12:22 PM

What happens if you set the meter to DC then use the leads to test a 9v battery?

Stu Diddly · December 17, 2007, 07:23:47 PM

Quote from: PerroGrande on December 17, 2007, 07:12:22 PM
What happens if you set the meter to DC then use the leads to test a 9v battery?

I just tried that and I got the same reading on DC as I did on the "9 volt test" setting. I assuming the meter is good. Am I testing my circuit wrong? I'm trying to get voltage reading for my transistor. I've recently build the Gus Smalley Boost and I can't get it to work, so everyone tells me to post voltages, but I can't even do that right. I've found myself in a frustrated state.

slacker · December 17, 2007, 07:41:29 PM

Are you using a stereo jack socket to switch the battery? If so then have you plugged a jack in? If you don't then you'll get weird readings.
What reading do you get if you measure between the positive battery lead and ground with the circuit connected?

Stu Diddly · December 17, 2007, 08:25:32 PM

Quote from: slacker on December 17, 2007, 07:41:29 PM
Are you using a stereo jack socket to switch the battery? If so then have you plugged a jack in? If you don't then you'll get weird readings.
What reading do you get if you measure between the positive battery lead and ground with the circuit connected?

I am using a stereo jack to switch the battery and I have plugged a jack in. When I measure the black to ground and the red to the positive battery lead, the meter is on mv and jump up to 0.2v and then back down to 0v. I don't understand this.

frankclarke · December 17, 2007, 09:03:57 PM

Test your meter on known voltages, resistances etc. If you get reading other than zero or infinity when it is unconnected, take it back.

PerroGrande · December 17, 2007, 09:18:56 PM

Short the leads of the meter when it is on voltage -- it should settle at 0v. If not -- I agree -- take it back.

Now -- your circuit isn't working and when you test between ground and the 9V lead with the circuit on you get zilch. Does your battery get hot or anything?

Stu Diddly · December 17, 2007, 09:37:41 PM

Quote from: PerroGrande on December 17, 2007, 09:18:56 PM
Short the leads of the meter when it is on voltage -- it should settle at 0v. If not -- I agree -- take it back.

How do I short the leads on the meter?

Quote from: PerroGrande on December 17, 2007, 09:18:56 PM
Now -- your circuit isn't working and when you test between ground and the 9V lead with the circuit on you get zilch. Does your battery get hot or anything?

The battery is brand new. I've tried several brand new 9 volts.

frankclarke · December 17, 2007, 09:42:37 PM

Shorting means touching the leads together. You should get 0V, 0 ohm there, if not get a new one.

Stu Diddly · December 17, 2007, 09:51:25 PM

Quote from: frankclarke on December 17, 2007, 09:42:37 PM
Shorting means touching the leads together. You should get 0V, 0 ohm there, if not get a new one.

I have just shorted the meter and the "mv" slowly decline down to 0V. When I release the leads the meter slowly climbs back up to 49 mv or so. I don't understand why the meter will register anything if it's not connected to a power source.

PerroGrande · December 17, 2007, 11:32:19 PM

The meter autoscales -- this means that it attempts to automatically determine the correct range setting. When there is no input, it scales up to its most sensitive option and is picking up various interferences and registering them. It doesn't strike me as a very good meter, nevertheless it seems to be measuring your batteries correctly. It isn't too much of a stretch to believe that your circuit, or a portion thereof isn't getting power -- which would explain why it isn't working.

petemoore · December 18, 2007, 02:11:03 AM

Meter leads, battery level and setting..also which holes you put the leads in..
Whenever my meter acts funny I get some voltages I can know, and measure them.
Starting with a tasty 9v battery and getting a reading of about >8v- about 9.5vdc is a promising start.
Using two equal value resistors to make a voltage divider can test meters ability to read about 1/2 of what it says battery is reading. Which means at least it's getting 'percentages' in that range pretty close.
Not knowing how 'spot on' my meters are, and not having any 'spot on' DC voltages I know of here to measure...so can't really comment on how close to 9vdc meters generally read when measuring 9vdc.
A 9vdc regulator would make a decent comparator to measure DMM's by...lol. Two DMM's can be reassuring sometimes.

Stu Diddly · December 18, 2007, 12:59:30 PM

Ok, here's an update on my multimeter situation. I just took back my multimeter that I got from Radio Shack and I went and bought an Extech 410 multimeter, which seems to be a lot nicer.

http://www.grainger.com/Grainger/items/1GUG7

I just put it on the setting of 200v DC and I again measured my circuit by touching the black test lead to the ground on the input jack (stereo jack) and the red test lead to the postive that connects to my perfboard for my circuit and I'm not getting a reading at all.

I now feel more confident with this multimeter and hopefully I'll figure out what's going on with my circuit. Thanks to everybody for all of your advice and help.

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Multimeter Voltages............