Good cheap multimeter?

[samgab]

Since we are discuss adjusting cheap meters. I dont think I remember seeing A pot on my cheap harbor frieght DMM.
So has anybody attempted to try to adjust A HF DMM by changing resistors or installing a pot?
Or maybe even know which resistor affects the voltage reading. I have one out of three thats about .03v off.
I liked to see if I could correct it if possible.

I have opened up all of my cheap DMMs, and they all have trim pots. I don't have your Harbour Freight one, but I'd say that it almost certainly has at least 1 trim pot. Look very closely at the circuit board, sometimes they are quite small, just a little metal circle with a + cutout... Or post a link to a hi res photo of the circuit board. I'd be suprised if there was no trim pot at all. There should be some way for the factory to calibrate them... The high end meters have a software calibration function, but most low end meters just have potentiometers.

 

[xul]

I have opened up all of my cheap DMMs, and they all have trim pots. I don't have your Harbour Freight one, but I'd say that it almost certainly has at least 1 trim pot. Look very closely at the circuit board, sometimes they are quite small, just a little metal circle with a + cutout... Or post a link to a hi res photo of the circuit board. I'd be suprised if there was no trim pot at all. There should be some way for the factory to calibrate them... The high end meters have a software calibration function, but most low end meters just have potentiometers.

They might use PC track jumpers which are cut at the factory to get close enough. With cheap labor anything goes.

 

[DFiorentino]

Since we are discuss adjusting cheap meters. I dont think I remember seeing A pot on my cheap harbor frieght DMM.
So has anybody attempted to try to adjust A HF DMM by changing resistors or installing a pot?
Or maybe even know which resistor affects the voltage reading. I have one out of three thats about .03v off.
I liked to see if I could correct it if possible.

My CEN-TECH HF cheapy does indeed have a pot. And I did indeed adjust it by about 5 (or less) degrees to regain 0.01VDC of accuracy. Adjustment seemed linear in the range of voltages I use it to measure with.

-DF

 

[Mr Happy]

Since we are discuss adjusting cheap meters. I dont think I remember seeing A pot on my cheap harbor frieght DMM.
So has anybody attempted to try to adjust A HF DMM by changing resistors or installing a pot?
Or maybe even know which resistor affects the voltage reading. I have one out of three thats about .03v off.
I liked to see if I could correct it if possible.

On the Cen-Tech 90899 meter from Harbor Freight (the "$2.99 special"), the adjustment pot is on the right hand side about 1/3 way down from the top. It is raised off the circuit board on three legs. You can adjust it by making very tiny movements with a small flat blade screwdriver to match any DC voltage references you have. I just adjusted two of mine to align closely with some more accurate references. I only needed to turn the pot by half a gnat's whisker.

Be careful with this meter as the rotary range selection dial can be a bit temperamental. Give it a good few turns back and forth to clean the contacts before you try adjusting the meter. You really don't get gold plated quality with a $2.99 device.

 

[moderator007]

Thanks for all the replys.
I had the cover off one time for something, But I didn't ever see a trim pot. I wasn't really looking for it though.
I will go back and examine it a little closer tonight. Will post back what i find. Mr happy that is the meter I have the bad reading on.
I do know exactly what a trim pot looks like and does. No flame intended. Thanks again!

 

[moderator007]

Well, feel kind of silly. Opened it up and there it was as plain as day. Eactly where Mr Happy said it was.
I was expecting a small pot. But this one is quite large for a surface mount pot.
Its location on the board is kind of in the top right hand side. Its kind of stuck there all by itself away from all the main componets on the board. This is probably why I didn't see it the first time.
I did remember why I had the cover off the HF DMM, to change the battery.:thinking:
I did notice that the temperature played a part in what the reading was. I had one out in the cold garage. When I brought it in and started adjusting it would start to drift. So I let them all stabilize in temp then started adjusting.
Got all my HF DMM's tuned up last night. Thanks to all! :thumbsup:

 

[pobox1475]

Without reading closely through thread, does it mean my Harbor Freight meter will not be accurate enough for the Eneloops and AW's I use ?

 

[moderator007]

The HF DMM will be fine for those type measurments. Mine was reading .03 volts to low.
Not that big of deal for batteries.
But it is best to adjust it with a voltage reference or agaisnt a known good meter.

 

[Mr Happy]

All of my HF cheapo meters were reading close enough out of the package and didn't really need adjusting. For example they typically showed 5.01 V against a 5.000 V reference, but there is no guarantee that they will always be that way.

Over the weekend I discovered an interesting cheap voltage reference in a brand new super heavy duty 9 V battery. It was very close to 10.0 V, at one point reading 10.000 V on my accurate meter. I need to try a few other samples to see how common and consistent this is, or whether it was just a one off thing. (To clarify, the battery seems to be somewhere between 9.95 V and 10.05 V, which is not precise by any means but it might a good basis for a rough and ready test.)

 

[moderator007]

I have three HF DMM's that all were good but the one. The rest were within .01v and sometimes they were dead on. Temps may have played a part in the deference. But that one never read in better than .03v accuracy.

Any way I thought I upload a pic for all to see what and were the adjustable potentiometer is on the HF DMM.
When I google searched I never come across any info on this. So here it is. Clock wise reads more voltage, counter clock wise reads less voltage.

 

[Mr Happy]

Well that's interesting. Apparently they are not all the same inside. This is what one of my meters looks like:

Does your meter have the transistor tester socket?

 

[moderator007]

Yes it does Mr Happy. Its just to the right of the fuse slighly under the red and black wires. You can make out the pins in the pic above. And on the HF coupon that I believe ran out today for a free one.
They list two different model numbers on the coupon.
I think one is new design and one old design. I do have both but neither look like yours.
One has a blue on and off switch while the other is yellow.
Update : checked the coupon says model# 90899 or 98025. And coupon still good until 11-17-2011.