Billy Mayo
Member
well a DVM meter is high impedance VS a analog meter is low impedance , maybe a low impedance meter is better at measuring OPENs?
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well a DVM meter is high impedance VS a analog meter is low impedance , maybe a low impedance meter is better at measuring OPENs?
Ok so for example your measuring a blown fuse (open circuit), which of the meters do think will give the most accurate reading?
How can a DVM measure in the MEGS on an open circuit ?Analog , DVM might say OL or it might measure in the MEGS
thanks, his are mainly model 7's (i think), and one or two are way older. I am not sure of the faults, but all work in some way, so they are not dead. Fist job is to try and sort the batteries out. I think most of his take 2 batteries, one of which may be 1.5V, the large 15V rings a bell with me, when i first looked into it for him, someone gave me an address to get replacement batteries, they were not cheap but i will look it out for youMr ghostman,
In post #109 you said your little one was looking into a dud AVO?
I have a model 8 and have most of the drawings, if its of any use.
Two problems I had with it is:
1. The 15 volt battery is hard to get so I use a 12 volt flash unit battery in series with a couple of hearing aid batterys. Pain in the neck BUT!!
2. There is a 7361 ohm resistor as part of the current shunt which went o/c on me a couple of times.
Apart from that it works beautifully. I think its a 1958 purchase. I got it from Philips when it got to its 10 year write-off point in 1968.
How can a DVM measure in the MEGS on an open circuit ?
Why would a DVM say "OL" while an Analog meter may not move ? Why would this make either meter any different for measuring a fuse ?
Except we were talking open circuit, and the example i gave was a blown fuse. Your a really nice guy and obviously see the best in everyone. The world could do with more like you, if i was younger i would probably like to be more like that, but i am old and tired. and more and more i become cynical!Hello,
He could have been talking about digital meters that dont check diodes or LEDs very well unless they can output a high enough voltage at a high enough current to check either of those. This is why digital meters started appearing on the market that have a "diode" check function. It's a basic check that works better than the meter set to the Ohms range.
Many meters still can not check LEDs very well. Some LEDs require over 3v to test properly. For these and other tests it is usually better to apply your own current with a power supply and resistor and check the voltage using the voltage scale on the meter, or something similar to that. The only caution here is that LEDs often have only a 5v reverse voltage rating, so if you go to test one with a 9v battery and 2k resistor for example and you connect the LED backwards, it can very well blow out before you get to test it properly. So using a power supply of 5v or less when possible is the best idea here.
In the past i have used a three AA cell battery holder and three fresh AA batteries which develops about 4.5 volts when they are in series and that's just enough voltage to test many LEDs using a series resistor to limit current to a safe level. If you dont have a three AA cell holder you can use a four AA cell holder and jumper one cell position with a clip lead jumper, or use a two AA cell holder wired in series with a one AA cell holder.
If you really want to use a 9v battery (these are handy for many tests) then you can use a resistive voltage divider with equal value resistors to knock the voltage down to about half, and the upper resistor will limit the current to the LED. The three AA cell tester is still a safer method however because there is no way the voltage can get higher than about 4.8 volts with three fresh alkaline cells.
or else maybe someone else will be able to use that information in the future for something else
How can a DVM measure in the MEGS on an open circuit ?
Why would a DVM say "OL" while an Analog meter may not move ? Why would this make either meter any different for measuring a fuse ?
an LED or diode will look like an open circuit without the proper test voltage
We need to be careful, and try not to get over technical!Explaining the meaning of OL is probably getting us into far more technical aspects, than the OP could be expected to grasp, seeing as he only has a few years experience, also try and remember he isnt the one they let"OL" is "OverLoad:.
This Means it is a Higher Resistance than the DMM is capable of testing.
On a VOM, the Needle Doesn't Move because this Overload (INFINITY RESISTANCE) is the LEFT SIDE of the Scale.