dmm vs vom
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Jim,
Using an old-type VOM is not a good idea because
the instrument's input is fairly low-impedance and can thereby compromise
accuracy, especially when measuring at a high-impedance node. DMMs are
ubiquitous and can be purchased at any Lowe's or Home Depot for around $20 or
less. Throwing a little more money at it gets you more bells and whistles, but
it won't be significantly more accurate.
Bill W5WVO
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In reply to this post by Jim Hoge
> Is there any problem using a VOM instead of a DMM to perform the
> resistance checks while building the K3? As long as the negative side of the internal battery that is feeding the resistance measurement circuit is applied to the ground side of whatever is being measured in the K3 during the check, and the open circuit voltage of the ohmmeter is not more than 3 volts, it should be safe. Some VOMs used a 9V battery in the resistance circuit and/or tied the positive side to the "black lead." At least, some of my old VOMs did. 73, Lyle KK7P _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Oops, I missed "resistance checks". Getting senile. Lyle is
correct, of course. I'd probably want to check the calibration of the ohmmeter function before using it, especially if the VOM is fairly old. Just don't try to use the VOM for voltage measurements. Bill W5WVO ----- Original Message ----- From: "Lyle Johnson" <[hidden email]> To: <[hidden email]> Sent: Wednesday, October 08, 2008 12:20 PM Subject: Re: [Elecraft] dmm vs vom > > Is there any problem using a VOM instead of a DMM to perform the > > resistance checks while building the K3? > > As long as the negative side of the internal battery that is feeding the > resistance measurement circuit is applied to the ground side of whatever > is being measured in the K3 during the check, and the open circuit > voltage of the ohmmeter is not more than 3 volts, it should be safe. > > Some VOMs used a 9V battery in the resistance circuit and/or tied the > positive side to the "black lead." At least, some of my old VOMs did. > > 73, > > Lyle KK7P > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Jim Hoge
In a message dated 10/8/2008 11:03:32 A.M. Pacific Daylight Time,
[hidden email] writes: Hi all, Is there any problem using a VOM instead of a DMM to perform the resistance checks while building the K3? I have yet to make it fully into the modern era and to that end, do not have a DMM. Tnx es 73, Jim W5QM _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: _http://www.elecraft.com_ (http://www.elecraft.com) Generally it is not a problem, if you are checking resistors. The area you can get into trouble is when you have semiconductors connected to the circuit - Elecraft Rigs are loaded with them. The problem can come from most VOMs not being able to limit the current applied to the circuit under test. Even at low voltage you could apply a voltage with enough current to bias a transistor to a destructive level. Most DMMs use a high impedance amplifier on input to reduce current flow to/from the meter. The resistance circuitry can also be made to sample E/I data at low enough levels to provide resistance measurement at much lower current and voltage. IMO, with many DVM/DMMs on the market at low cost, it's probably not worth the risk. Al WA6VNN **************New MapQuest Local shows what's happening at your destination. Dining, Movies, Events, News & more. Try it out! (http://local.mapquest.com/?ncid=emlcntnew00000001) _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Be careful with some of the DMM's too. I did a study on the voltage produced at the leads for several brands of industry-standard meters at work. Here is one of them:
Resistance Range Voltage on the leads 0.2 ohms 12 volts 2 ohms 12 volts 20 ohms 12 volts 200 ohms 6 volts 2M ohms 6 volts 20M ohms 1 volt We found out why certain semiconductor devices were being destroyed after "meter checking". 73, John W2XS Generally it is not a problem, if you are checking resistors. The area you can get into trouble is when you have semiconductors connected to the circuit - Elecraft Rigs are loaded with them. The problem can come from most VOMs not being able to limit the current applied to the circuit under test. Even at low voltage you could apply a voltage with enough current to bias a transistor to a destructive level. Most DMMs use a high impedance amplifier on input to reduce current flow to/from the meter. The resistance circuitry can also be made to sample E/I data at low enough levels to provide resistance measurement at much lower current and voltage. IMO, with many DVM/DMMs on the market at low cost, it's probably not worth the risk. Al WA6VNN **************New MapQuest Local shows what's happening at your destination. Dining, Movies, Events, News & more. Try it out! (http://local.mapquest.com/?ncid=emlcntnew00000001) _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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