How to measure current to not burn out supply or components

Discussion in 'General Electronics Chat' started by booboo, Sep 5, 2015.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Hi everybody
    I have a DMM it's VC9805. For measuring the output current of this Buck module, I used it this way:

    [​IMG]
    Then I connected the red test lead to 20A and set it to DC mode(it is in DC mode itself) and connected the test leads to the output of the module. and when I turned the supply on, BOOOOM!

    [​IMG]

    Burnt out.:(
    Why? (I think there is a big load in DMM to draw current)

    I want to measure the current on my board. I want to know Is my MCU(STM32F103) drawing current or not(want to know it's functioning/working or not). but I'm worry that maybe it would burnt out my debugger(it's J-Link) or my MCU. Should I be worry?
     
    Last edited: Sep 5, 2015
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    Can you draw a diagram of exactly how you connected the meter?

    You know the meter should be connected in series with the load if you want to measure current?
     
  3. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    I just connected the red test lead to +output and the black to the -output.
    Is it incorrect?
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    Nope.
     
  5. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,810
    834
    ?
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    On what range setting was the DMM?
    What is damaged?
    The powersupply or the DMM?

    Bertus
     
  7. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    But probably it is!;)
    if it's not, then why it's burnt out?
     
  8. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    1- 20A
    2- LM2596
    3- LM2596 on the Buck module
     
  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    This is not how you measure current; you shorted-circuited the output of the buck converter.

    The meter should be in series with the load.
     
    booboo likes this.
  10. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    In the link is given that the Buck module can deliver 3 Amps.
    The 20 A connection will act as a short.
    The buck converter is likely NOT short circuit protected.

    Bertus
     
    booboo likes this.
  11. Dave Garcia

    New Member

    Mar 2, 2012
    3
    1
    First you need to check for short circuit condition.
    This is accomplished by putting a light bulb in series with your circuit.
    The light bulb should be the same wattage you expect from your circuit.
    If light is about half brightness, then you do not have a dead short.
    If light is on full brightness, then you have a dead short.
    If light is off, then you have a open circuit.

    Best Regards
     
    booboo likes this.
  12. PeterCoxSmith

    Member

    Feb 23, 2015
    148
    38
    sounds like the DVM was connected directly across the supply in which case it is a short circuit. If the supply blew up then it did not have a short circuit protection capability.
     
    booboo likes this.
  13. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    I just wanted to know can it give me 3A really.
    This is the schematic of my DMM:

    [​IMG]

    How could I measure the max current at the output of my Buck module?
     
  14. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Did you read the specs of the buck converter in the link you provided?
    It states a maximum output current of 3 A.

    Bertus
     
  15. PeterCoxSmith

    Member

    Feb 23, 2015
    148
    38
    You need a circuit like this:

    upload_2015-9-5_19-11-57.png

    The resistor sets the current and must have a value of

    R = (Expected Voltage) / (Expected Current)

    And this resistor must have a power rating of (Expected Voltage) x (Expected Current) x 5
     
    booboo likes this.
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
    1,251
    By design, a voltmeter has a very high input impedance or resistance. It tries to look like an infinitely large resistor that drawn no current.

    However, a current meter is the opposite. It tries to look like zero ohms. If you are measuring the output current of a power supply that has built-in current limiting, the supply will protect itself and the meter will show the short-circuit current. But most buck regulators do not have this kind of protection built in.

    When the manufacturer says the regulator is good for 3 A, he means that you can draw more than 3 A if you want, but it might damage the regulator. That is what happened in your case. If you want to confirm that the device can supply 3 A, you need to connect a load resistor that is calculated (with Ohm's Law) to draw 3 A at whatever the output voltage is, and put your meter in series with it.

    ak
     
    booboo likes this.
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
    1,105
    If you fried your DMM then R20 (10mΩ) and pcb track around it have probably been destroyed :(.
     
    booboo likes this.
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    The safest. Use a clamp on ammeter add-on to your meter.;)
    Max.
     
    booboo likes this.
  19. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Thanks guys
    Yeah, sure! but I would see it actually.
     
  20. Dave Garcia

    New Member

    Mar 2, 2012
    3
    1
    I would use a fuse for the maximum amperage you would expect in your circuit.

    Best Regards Again
     
Loading...