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DC-DC converter

Discussion in 'General Electronics Discussion' started by Canaldweller, Sep 29, 2017.

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  1. Canaldweller

    Canaldweller

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    Sep 29, 2017
    Hi.

    I live on a narrowboat and live off-grid. My power comes from 12v batteries.

    I am trying to power my shunt based 12v ammeter using a dc-dc converter. I bought 3 from STK on Amazon. The blurb says that they are 12v -16v input to 12v output. These would be ideal. However, when I connect one up with an input of 13.21v I get 13.56v on the ouput. This happens with all three converters. Am I doing something wrong?
     
  2. Alec_t

    Alec_t

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    Jul 7, 2015
    Welcome to EP!
    What load is the converter driving? Regulation might be poor if the load is only a few mA.
     
  3. Canaldweller

    Canaldweller

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    Sep 29, 2017
    Thanks for the reply. I don't know but have just looked on eBay and similar ones quote <20 ma.

    These are some of the comments left on the converter sellers page :-

    "Works perfectly powering a shunt based Ammeter / Voltmeter. -1 star as no instructions. Doesn't appear to get warm."

    "The pins closest together are the input, the two spaced apart at the other end are output. For both sets of pins, the one's furthest from the center are positive. So if you hold it up with the two pins closest together on the left, the pins from left to right are:
    +in, -in, -out, +out."

    "Add a couple of capacitors to the input and output. I don't have the exact specs but similar models specify a maximum of 4.7uF. I use 2.2uF on the input and on the output and that seems to work well."

    "Based on similar specs there seems to be a minimum current draw required of 9ma. I added a 1k resistor and LED to the output; makes sure the minimum is drawn and lets me know it is on"

    Hope this helps you to help me.
     
  4. Alec_t

    Alec_t

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    798
    Jul 7, 2015
    Try increasing the load current to, say, 50mA or more and see if the output voltage is closer to the spec.
     
    Arouse1973 likes this.
  5. Canaldweller

    Canaldweller

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    Sep 29, 2017
    Could you tell me how to go about that? I have already blown one ammeter by just wiring it up and switching it on.

    Why is the ouput higher than the input? Shouldn't it be the other way round? Or is it that I don't understand how these things work (I would not even describe myself as a beginner in electronics :().
     
  6. Tha fios agaibh

    Tha fios agaibh

    2,129
    721
    Aug 11, 2014
    A link so we can read the specs would help.

    I assume the current required to power your 12v ammeter is quite low. I would opt for a regulator like a lm7812 if needed.

    Even if the 12v source to power your meter varies by a few volts I doubt it would impact the accuracy of the current display by much.
     
  7. Alec_t

    Alec_t

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    Jul 7, 2015
    How did you wire it up? Connecting an ammeter directly across (i.e. in parallel with) a power supply will almost certainly destroy the ammeter (and the supply if it isn't current-limited), because the ammeter is almost a short-circuit. You wouldn't be the first person to try to "measure how much current the supply can provide".
    DC-DC converters usually have a potentiometer to adjust the output voltage, which can be above or below the input voltage depending on whether the converter is a boost type or a buck type .
     
  8. Tha fios agaibh

    Tha fios agaibh

    2,129
    721
    Aug 11, 2014
    Just a clarification. The Shunt ammeters read current by the (usually) 0 to 50ma, or 0 to 100ma range. 50ma or 100ma being the meters full scale reading from the shunt.

    Additionally, some meters (usually digital) have a separate input for powering the display which is a separate input.

    So the shunt itself is in series with the load and the feedback wires from the shunt go to the meter. When shunt based meter is mentioned it is assumed your talking about an external shunt.

    The latter mentioned power supply to the meter does go in parallel with the voltage supply.
     
  9. Canaldweller

    Canaldweller

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    Sep 29, 2017
    Thank you for this. It's exactly the information that I should have put in my first post.

    At the moment I am using two 6v torch batteries, in series, to power the display. Problem is that I keep forgetting to switch the display off so, after a couple of months, they run flat. I was hoping the converter would overcome this problem.
     
  10. Tha fios agaibh

    Tha fios agaibh

    2,129
    721
    Aug 11, 2014
    It won't overcome the problem of exhausting your torch batteries, but it will give a nice regulated 12v .
    I think I'd opt for a simple push button to check the display.
     
  11. Canaldweller

    Canaldweller

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    Sep 29, 2017
    Ha ha. I should have thought of a push button. It would have saved me a lot of torch batteries. Doh!

    Could you explain why I am getting a higher voltage (as measured by my multimeter) on the ouput than is going to the input.
     
  12. Tha fios agaibh

    Tha fios agaibh

    2,129
    721
    Aug 11, 2014
    Like Alec said if there is little or no load on it it probably wont be a solid 12v output.

    You never showed us what model it is or how your hooking it up.

    Why do you need one? Does you ammeter need a regulated source? what is its power requirements?
     
  13. Canaldweller

    Canaldweller

    6
    0
    Sep 29, 2017
    Thank you for taking the trouble, and patience, to help. Am unable to provide a link but here's a screen grab.

    I am powering it from my batteries which can be anywhere between 12.2v and 15.5v. The ammeter needs a voltage of between 9v and 12v.

    I need one to do away with the torch batteries that I am using at the moment. If I can't get this to work then I will go with your suggestion of a push button.
     

    Attached Files:

  14. Cirkit

    Cirkit

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    10
    Oct 28, 2015
    One of the reviews for that regulator mentions that the output voltage is slightly higher. I believe that regulator has a 1W output? Try connecting a 200 Ohm 1W resistor directly across the output of the regulator with nothing else connected and measuring the voltage. Is it closer to 12V? I've found that isolated DC-DC regulator modules have a higher regulation % tolerance compared to non-isolated (3 terminal) DC-DC regulators.
     
  15. Tha fios agaibh

    Tha fios agaibh

    2,129
    721
    Aug 11, 2014
    I can't explain why it's voltage output is high but i don't think it'd be a problem unless its goes above 15v.

    For automotive circuits, voltage is around 14.7 when charging system is operating.

    Are you sure about your 9-12v requirements?
     
  16. AnalogKid

    AnalogKid

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    Jun 10, 2015
    After 15 posts, the only information we have about the component being used is that it is black and has 4 legs. So do each of my dogs. Can you post links to the sellers page, a datasheet, etc?

    ak
     
    Tha fios agaibh likes this.
  17. Cirkit

    Cirkit

    132
    10
    Oct 28, 2015
    Searching other sites for similar specifications, looks like a VB1212S-1W which is a 12V 1W isolated DC-DC with 68% efficiency.
     
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