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VDO Voltmeter

Discussion in 'Sensors and Actuators' started by Evileye1000, Feb 6, 2021.

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  1. Evileye1000

    Evileye1000

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    Feb 6, 2021
    I have a 6v vintage car. I want to add a voltmeter under the dash but 6v voltmeters are scarce. So I thought to add a voltage doubler inline before the gauge and adjust the gauge for any voltage loss. I’ve built a voltage doubler with a 555 chip and some bits and pieces. The doubler seems to work perfect on the bench with my multimeter. With a 6v battery and the doubler, the multimeter reads 11.5v. With a 9v battery and the doubler, the multimeter reads 17v. The car has a 6v generator and puts out around 7v which measures 12v with the doubler and using the multimeter.
    Using the VDO gauge I get unexpected results. Not using the doubler, VDO reads the voltage correctly, 12v battery reads 12v. 9v battery reads 9v. However when using the doubler I get very low results. 6v battery and doubler read 8. 9v battery and doubler reads 11v. So my 7v car generator doesn’t barely register on the VDO which starts at 8v. So... why does the doubler show different results between the multimeter and the VDO gauge?
     
  2. Kiwi

    Kiwi

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    89
    Jan 28, 2013
    Quick guess would be extra current required by the analogue VDO voltmeter compared to digital multimeter?
    Connect both the voltmeter and multimeter to the doubler, and then check the voltage reading on the multimeter.
     
  3. Evileye1000

    Evileye1000

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    Feb 6, 2021
    When I do that, the multimeter reads about 8v. When I disconnect the VDO, the multimeter jumps up to 11v. Is this result suggesting that I can’t use an analog gauge with this doubler? Do you have a suggestion?
     
  4. Bluejets

    Bluejets

    4,772
    1,000
    Oct 5, 2014

    Attached Files:

  5. Bluejets

    Bluejets

    4,772
    1,000
    Oct 5, 2014
    Ah..ok....m
    Ah...ok....misread on my part.
    Will think a bit more on it.

    If it is basically a current issue then boost the handling ability with say a mosfet for example.
    Show us a circuit what you presently have.
     
    Last edited: Feb 6, 2021
  6. WHONOES

    WHONOES

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    May 20, 2017
    Can't you use a 100μA meter that looks the part and put a resistor in series with so that it reads the correct voltage. It would save all the messing about with voltage doublers.
     
  7. Evileye1000

    Evileye1000

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    Feb 6, 2021
    upload_2021-2-6_8-26-43.png
     
  8. Evileye1000

    Evileye1000

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    Feb 6, 2021
    My goal is to use a gauge that looks proper and that would be a vintage VDO gauge. With that said, there are only 12v gauges available. I need to have a 12v gauge work with 6v input voltage, so I assume I need to double it. If you have another way to meet these requirements, I’d love to hear it. Thanks.
     
  9. WHONOES

    WHONOES

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    May 20, 2017
    All voltmeters start life as a current meter. They just have a resistor in series with the armature coil so that it takes a certain voltage to develop the required current through armature coil for full deflection.
    If you could get inside whatever meter you choose to use, you should find a resistor of some description in there. That resistor value could be adjusted to give the correct sensitivity that you require.
    If you can't gain access, measuring the resistance of the meter would be a start.
     
  10. WHONOES

    WHONOES

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    May 20, 2017
    Further. All IC's require a minimum voltage to function correctly below which they will either not work at all or produce very erroneous results.
    A 6V battery could easily drop some way below it's nominal voltage meaning any circuit designed may not function at all or at best incorrectly.
     
  11. Evileye1000

    Evileye1000

    12
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    Feb 6, 2021
    Looks like this inside. One big resistor like you said and there’s also a diode too. Can I replace that resistor with some sort of potentiometer so I can adjust the resistance?
     

    Attached Files:

  12. WHONOES

    WHONOES

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    May 20, 2017
    68.1Ω @ ±2% seems a bit odd. It could be anywhere in the range of 66.8Ω to 69.2Ω so the 0.1Ω bit would be a bit superfluous not only that but 68.1Ω seems quite low as well.
    If you have a DMM, could you measure the actual value of the resistor?
     
  13. Evileye1000

    Evileye1000

    12
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    Feb 6, 2021
    I can’t measure the resistance without taking it apart. I will do that but it’s a process. Meanwhile I tested the voltage drop from the resistor, 12v in and 2v after the resistor. Assuming 68ohm R, I think I’ll need a 27ohm R to drop 6v to 2v. I should know more after I take it apart. But thinking that a 27ohm 1W resistor is the answer???
     
  14. bertus

    bertus Moderator

    1,761
    677
    Nov 8, 2019
    Hello,

    I would give an other value.
    680 Ohms 5% 50 ppm

    5_ring_resistor_code.png
    Bertus
     
  15. Evileye1000

    Evileye1000

    12
    0
    Feb 6, 2021
    I know absolutely nothing about resistors except what I’m looking up on the web so I’m likely wrong. This resistor has 5 bands so the first 3 blue grey brown = 681 and then the gold multiplier = .1, so 68.1ohm total. Please tell me what you see. I’ll know for sure when I get the resistor out. Thanks.
     
  16. bertus

    bertus Moderator

    1,761
    677
    Nov 8, 2019
    Hello,

    Look at the posted table.
    It shows how to read the resistors colors.

    Bertus
     
  17. Evileye1000

    Evileye1000

    12
    0
    Feb 6, 2021
    Every table I look at for a 5 band resistor has the 4th column as the multiplier except your table which has the 3rd band as the multiplier. what am I doing wrong?
    https://images.app.goo.gl/qPBtRR5fbZpyZzDo6
     
  18. bertus

    bertus Moderator

    1,761
    677
    Nov 8, 2019
  19. Evileye1000

    Evileye1000

    12
    0
    Feb 6, 2021
    Ok. Don’t understand why the differing color code tables. This resistor is circa 1975. And like I said, I should be able to test it when I get it out. Thanks.
     
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