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Will this be a problem?

  1. Feb 6, 2004 #1
    If I have a 0-1 volt voltmeter (10 led bargraph display) and I want to measure a 12 volt bat, would it be safe to connect a Pot. between the voltmeter and the 12 volt bat to reduce the voltage down to 1 v for a full display? Thus my 0-1 volt voltmeter will show full 10 led bars if the voltage is 12 and 5 led bars if the voltage is 6. Is this safe to do? Will I burn anything since the voltmeter is designed to measure 0-1 volt?
  2. jcsd
  3. Feb 6, 2004 #2


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    What you are suggesting is a voltage divider. I would use two fixed resistors. For your 12 volt battery I would use a 11kohm and 1kohm. Remember, they need to be large enough to handle the current through them (in this case 1 amp).
  4. Feb 6, 2004 #3


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    Pretty sure Don meant 1mA.

    A 12V battery is a little more than 12V as well, like 12.6 or 12.8 when fully charged. If you are trying to measure voltage in a car, when running the alternator will hold the voltage higher as well between something like 13.5-14.5 depending on how the voltage regulator is setup.

    So you may want to use a 12Kohm in series and a 1Kohm resistor in parallel to get a 13:1 divider instead.

    For your other thread below, here's an easy to solder up prototyping board that is similar to a breadboard. This place takes a long time to ship orders, but the prices are reasonable.

  5. Feb 7, 2004 #4


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    And yes.
  6. Feb 9, 2004 #5
    Can u elaborate on how to divide the voltage? I'm not clear on how a pot really works. Oh yeah, thanks for the link. I found the strip board that i was looking for. Does the strip board holes have cooper ring around it? I can't see from the pic.
  7. Feb 9, 2004 #6


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    Ok, here's a simple way to think of a voltage divider, its just a little play on semantics but should make visualization easy.

    You have a 13V source. Ok, circuit theory says that all the voltage drops in a simple series circuit will add to equal the source.

    So across our 13V source we place a 12Kohm and a 1Kohm resistor in series with each other for a total of 13Kohm. Ohm's law says 13V/13Kohm = 1mA of current.

    We can then use current * resistance = voltage to get the drop across each resistor. And you see the 12Kohm resistor drops 12V and the 1Kohm resistor drops 1V.

    So if you hook your meter across the ends of the 1Kohm resistor (in parallel with it) it will see 1V when you have a 13V source.

    Regarding the PC board, dunno what you're asking. I've used the 777 board, it arrives as is pictured. You can drop an IC socket on one of two rows and then each pin has a pad with 4 attachment points for it. Then you have some long pads to use for common connections, and some 2 hole pads on the perimeter to use as tie points. What you see is what you get. The two rows spaced to handle an IC socket work like a breadboard, makes it real easy...

  8. Feb 13, 2004 #7
    You might want to tell us what you're hoping to accomplish with the measurement. If you want to know if your car battery is any good then it's probably a waste of time. By the time a car battery can't put out about 12V for 1 mA current you would already know it was no good!

    Car batteries need to be tested under loaded conditions. Any place that sells them usually has a free testing service to determine if it is the battery, the voltage regulator or the alternator that is bad.

    If it's for some interesting scientific purpose then please share that with us.
  9. Feb 15, 2004 #8
    A car battery that is 90% discharged can still develop 13.5 volts of electricity, no real amperage to speak of, but it will still fool a cars voltage regulator into acting as if the battery is "fully" charged.....designed that way to prevent the cars charging system from OVERcharging the battery, and burning it out that way....

    Why don't you just purchase a volt meter that handles the voltage you want to test for?
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