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Author Topic: Stupid question about measuring current from Power Supply OOC  (Read 534 times)
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Sunnyvale, CA
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I apologize in advance as this is probably a dumb question..but I've had a brain fart and am not sure if my thinking is on track.    I built a 5V 6A power supply (30W max) for a LED sign, and while I've measured the voltage using a multimeter, I want to test the current delivery ability to see if it can really generate up to 6A (and how warm the board will get to test thermal dissipation).  Since I haven't finished with the sign yet,  I can't test with a multimeter in series with the supply and the sign, so I was thinking of using a resistor across one of the supply's terminals, and my meter.


Supply (+)---Resistor---Meter +
Supply (-)----------------Meter -

I was looking at a resistor like this http://www.bourns.com/data/global/pdfs/PWR221T-30.pdf that can handle 30 watts.  Is this correct in a way to test with no load present?  I'm trying to measure if it can accurately produce a 6A (or even 4-5A) supply.

PS: OOC = Out of Circuit
« Last Edit: May 24, 2013, 06:11:42 pm by evanrich » Logged

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Left Coast, CA (USA)
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Sure, a 1 ohm power resistor wired as you have shown will draw 5 amps of current from the supply. A 25 watt resistor or higher would be required for continuous operation, but for just a few seconds of testing to obtain a current reading you could probably get by with a lower wattage resistor. It would be useful to have 2 meters so you could see if the voltage stays steady while it is drawing that current.

Lefty
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a 5v incan bulb could be used as a resistor instead of a 25watt resistor
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a 5v incan bulb could be used as a resistor instead of a 25watt resistor

good try, but start up inrush current might trigger over current protection circuit of power supply.

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Quote
  I built a 5V 6A power supply (30W max) for a LED sign

The typical LED is drived by constant current power supply, but you try do constant voltage one. Could you show us detail of your LED sign?
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Quote
  I built a 5V 6A power supply (30W max) for a LED sign

The typical LED is drived by constant current power supply, but you try do constant voltage one. Could you show us detail of your LED sign?

Sure.  I forgot to mention 6A max, the power supply design does UP to 6A (chip is capable of 8A), sorry for not making this clear earlier.  In testing with one module and a mockup board, I was seeing no more than 0.5A max for one board, I just want to build in some room in case more LEDs are lit at once, or the brightness is turned up.  The LED boards are 5V, 1.37A max, average ~0.3-0.5A requirements / board.

(and thanks for the replies)

I'm using 4 of these: http://www.sureelectronics.net/goods.php?id=1095


The datasheet states a max load of 1.37A with all LEDS on and 100% pwm.  While I am not intending to run at 100% brightnesss, or with every LED on at the same time, 4x 1.37 = 5.48.   Add an additional 200-300mA (generous) for the controlller + bluetooth module, and I figure at least 6A is enough (although the PS I'm using is capable of 8A.  The power source I am using is capable of 15A @ 12V.   

My project is being posted to a github page, so far only a few datasheets/BOMS are posted, however I have posted my Power Supply, as I've tested it to deliver 5V so far, but have not tested the current capabilities yet.  If you are interested, please see: https://github.com/evanrich/92x16LED-Display

The power supply centers around a TI LMZ12008 Simple Switcher power module (http://www.ti.com/product/lmz12008)  This is the first power supply I've designed, so it may not be perfect, but I'd like to see if it can at least deliver the 6A @ 5V that I need.

Thanks again!
« Last Edit: May 28, 2013, 06:45:28 pm by evanrich » Logged

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Ok, let me give you stupid answer;-   smiley-kitty

What you asked is dummy load for test PSU. The Lefty solution is worked, since you target current is 6A, the resistor should be 0.83 ohm, you do not need measure current, all you need to do is measuring voltage cross the resistor.  0.83 ohm/25 w resistor might not be easy to obtain. People some time use active dummy load.



Please seach net, you will find some design you prefer.

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Ok, let me give you stupid answer;-   smiley-kitty

What you asked is dummy load for test PSU. The Lefty solution is worked, since you target current is 6A, the resistor should be 0.83 ohm, you do not need measure current, all you need to do is measuring voltage cross the resistor.  0.83 ohm/25 w resistor might not be easy to obtain. People some time use active dummy load.



Please seach net, you will find some design you prefer.



Sonny,

Thanks for the answer.  I was looking for confirmation on the easiest way to test if the PSU can deliver the maximum current that I may require.  While an Active load may be a better long term option, I was looking for a quick way to test performance of the PSU, which in this case would be a power resistor.   for what I need, the 1 Ohm 30W resistor should be enough, as that will be able to sink 5A out of the power supply, thus validating or not if it can deliver that much current.  Once I get my hands on an Oscilloscope I can test other performance characteristics such as ripple, but as I've already validated 5V out, need the current measurement too.

Thanks all for confirming!
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