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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: Ohmmeter using Digital Voltage Panel Meter
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Author Topic: Ohmmeter using Digital Voltage Panel Meter  (Read 10444 times)

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Online Breaktru

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Ohmmeter using Digital Voltage Panel Meter
« on: September 13, 2013, 04:48:06 PM »
I found a circuit by Gary Lecomte for a "Low Ohm Meter" using a Digital Panel Meter.
I'm not happy with the results though. Reading for 2.0 ohm to 3.0 ohm seemed okay but lower or higher resistances where not stable.
The LM317LZ got hot w/ the 18 ohm resistor recommended. About 198 degrees. Got it down to 170 degrees with a 240 ohm and was still able to adjust to 100ma current as suggested.
I'll try some more testing but I think this circuit may not work. Or perhaps it is the type of display I used. There is no part number on the schematic.

Update: It's the panel meter that I'm using that is causing the unstable and inaccurate readings. Waiting on a new panel meter.

I used a panel meter that I had on hand. 0-200vdc, 4 1/2 digits. It was difficult to catch the last zero on the display in the photos. It showed fine in real life but thru the camera lens there is much flickering.
I did not have a display meter where I can move the decimal point. The meter shows 2.0 ohms as 0.20

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« Last Edit: December 17, 2014, 05:05:53 PM by Breaktru »

Offline CraigHB

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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #1 on: September 13, 2013, 08:11:42 PM »
You don't need to use that particular regulator.  Just about any adjustable linear regulator will work.  If you do a search on LM317 at mouser, you'll see some 1.5A regulators that will do the job without overheating. 

Your measured voltage is always going to be one tenth the resistance since you are biasing with one tenth of an Amp.  You need a panel meter with good accuracy to at least two decimal places.  For testing, you can just measure voltage with your DMM if you have doubts about your panel meter.

I don't know why the designer states two separate power supplies are required.  The power supplies share a common ground through the negative test connection.  A single power supply should work assuming the voltmeter has a high impedance between its test connections as expected.

You don't have to use that resistor network between the output and adjust pins.  You could just use a pot there and get rid of a resistor.  A 100 Ohm pot should provide the range you need.  A 20 Ohm pot would provide the finest adjustment.  The less parts the better for an e-cig mod.

« Last Edit: September 13, 2013, 08:45:04 PM by CraigHB »

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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #2 on: September 13, 2013, 08:53:24 PM »
Thanks Craig. Before buying the LM317LZ I looked through my parts bin for regulators and couldn't find and adjustable ones. I had some 1.5A LM317s but used them up.
I saw a video on a guy using a LM350 which is 3A and he said not to fire up the ohmmeter too long because that also got hot.
I was about to swap out the 50 ohm pot for a 200 ohm trimmer earlier but was called away. The 200 ohm is a multi-turn which will help because the adjustment on the 50 ohm single turn was hard to fine tune. Thanks for the info about not having to use the fixed resistor got to try that tomorrow.
The meter is not a high impedance. It's a 100k so for now I'll use the 2 batteries until I get at least a 10m input impedance one.
« Last Edit: September 13, 2013, 08:57:27 PM by Breaktru »

Offline CraigHB

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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #3 on: September 13, 2013, 09:00:08 PM »
Welcome, hopefully you'll get it working well with a minimal part count.

One other thing I noticed.  For stability, linear regultors typically need a 10uF tantalum capacitor on the output pin to ground.  I would remove the 1uF capacitor from the adjustment pin and add one to the output pin.  You have to check with the part's data sheet on the output capacitor.  Sometimes other types are fine or even none at all.  That would be ideal.  Also you can probably remove the capacitor on the input pin.  When running on battery power, the circuit should be quiet enough not to need it.





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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #6 on: September 15, 2013, 09:34:57 AM »
Thanks "T" and "X". Nice find guys. I would rather like to use the voltmeter so I can switch between Ohms and Volts for my Spark-O-Matic. I ordered a high input impedance voltmeter that I had linked to in my Spark-O-Matic topic.

BTW the circuit above works with a Single 9v battery. It's the panel meter that is causing the unstable and inaccurate readings.
I put my DMM on it and the ohm readings are exact for all the resistance scales. Now waiting on the new panel meter. I will update the schematic and maybe incorporate the voltage reading circuit.

Update: Using the  PCMCML meter, you need a separate battery for the meter because this meter can not use a common ground.
« Last Edit: October 08, 2013, 08:13:18 PM by Breaktru »

Offline Phestr

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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #7 on: September 17, 2013, 10:51:16 PM »
 :popcorn:

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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #8 on: September 22, 2013, 08:56:20 AM »
I received the new panel meter 100Meg input impedance but it was D.O.A. I could not get anything to read on the display.
I'll have to wait on completing this project until I find a decent high input impedance meter that is reasonably priced.
To sum it up again: the LM317LZ, 100ma works great. It's finding the right display meter that is a problem.

Online Breaktru

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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #9 on: October 02, 2013, 11:27:45 AM »
I contacted Jamesco and they sent me a replacement PCMCML meter. It works. The panel meter is connected to the resistance that you are testing which goes from negative to the ADJ pin. You are measuring a small voltage which corresponds with the resistor value under test. In other words, a 2.0 ohm resistor would be 2.0 volts on the panel meter. With this panel meter a wide resistance reading range was impossible. Fine tuning the trimmer was accurate for 1 ohm to 2 ohms or 2 ohms to 3 ohms but not accurate for 1 ohm to 3 ohms. Connecting my DMM alone I accurately measured 1 to 5 ohms.

Also this meter requires optional resistors for voltage ranges. It comes set for <200mv and w/ the optional resistors you can make it either for 20v, 200v or 500v. Which will be a problem if you wanted this meter to be used for reading resistance (20v scale) and for use as a voltmeter in the Spark-0-Matic (200v scale). An option would be to use a 2 position switch to select the right resistor for the scale you want.
I think finding a better panel meter would be a better choice.


Update:
Went back to bench and played around some more.
Connecting the Panel meter again registered "-1   ." When I removed the negative on the PM for reading voltage I was able to see zeros. I had to supply two battery sources in order to isolate the PM battery source from the circuit battery source. Now the voltage reading accuracy is dead on. I tested with 1.1 ohms, 2.1 ohms and 3.1 ohms and it was exact.
The bad news is that I would need two 9v batteries for this now. Using only one 9v battery with my prior testing gave faulty results. Now I know the note on the schematic that says: "YOU MUST USE TWO SEPARATE BATTERIES" was not a lot of bologna. Although you can use one 9v with a decent DMM.
« Last Edit: October 02, 2013, 03:31:08 PM by Breaktru »

Offline Cajunsteaming

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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #10 on: October 02, 2013, 08:53:38 PM »
That is more suited for higher resistances. This would be better suited for testing coils:-
http://www.amazon.com/DROK-Digital-Resistance-Ohmmeter-Impedance/dp/B00D4251RI/ref=pd_sim_sbs_hi_2


I built one of these using the DROK ohm meter panel, works super and you can zero out the meter with the pot onthe back.

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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #11 on: October 02, 2013, 09:29:01 PM »

I built one of these using the DROK ohm meter panel, works super and you can zero out the meter with the pot onthe back.


The DROK is actually an ohm meter. The idea here is to do both volts and ohms. I drew up a schematic and when I get around to it I'll post it

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Re: Ohmmeter using Digital Voltage Panel Meter
« Reply #12 on: October 12, 2013, 06:08:02 PM »
Here is the completed project...



See more here:
http://breaktru.com/smf/index.php/topic,733.msg9175.html#msg9175
« Last Edit: October 27, 2013, 03:46:29 PM by Breaktru »

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