How do I properly measure the current of power supply?

Hi,

I just converted cheap 450W ATX power supply to 12V one for RAMPS 1.4 controller.
The current output of this unit is total +12V@30A (+12V1@14A, +12V2@16A).

I merged all +12V wires into single wire, almost same thickness of car battery cable, and attached two AWG 16 wires for
two +12V output. I installed 15A blade fuse on the 12V side.




The voltage output of the 12V wires were around 12.11V.


It hasn't blown the fuse yet.

I want to make sure the power supply is okay before I hook it up to the RAMPS board.
Is there a good way of measuring current of the power supply without risk of blowing the fuse of multimeter?
My multimeter can measure up to 20A.

Thanks in advance,
Hughe

PS: A portable circuit breaker unit I made.

If the PSU has two separate 12V outputs, connecting them together is a bad idea. One of the outputs is likely to regulate to a slightly higher voltage than the other, and will end up taking nearly all the load. If you are using RAMPS, then I suggest you use one output to power the heated bed (the 11A circuit), and the other output to power everything else (the 5A circuit).

You can't measure the "current of a power supply", all you can measure is how much current some load takes when you connect it to the power supply.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

... measuring higher currents, than your amperemeter can suffer, is made by shunt resistors - you simply measure the voltage drop of a defined resistor and calculate the drawn current with Ohm's law.

E.g. you use a resistor of 0.1 Ohm and measure a voltage across this resistor of 3 Volts -- then the drawn current is simply I = 3 [V] / 0.1 [Ohm] => 30 Amps ... so simply use the voltmeter

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Viktor
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I hooked up 10W 10 ohm resistor, 55W 12V headlight bulb, and two 55W 12V headlight bulbs in parallel to check voltage and current.
Measurement of voltage and current of each load:
10W 10 ohm resistor, 11.58V, 1.17A
55W 12V headlight bulb, 10.09V, 3.75A
2x 55W 12V headlight bulb in parallel, the PSU went dead without blowing fuse. When I turned on the PSU, the fan gave a jerk motion then nothing happened.
I was going to use it for RAMPS 1.4 controller if it passed the 110W load test, but I'm planning to convert another ATX PSU.

Quote
dc42
If the PSU has two separate 12V outputs, connecting them together is a bad idea. One of the outputs is likely to regulate to a slightly higher voltage than the other, and will end up taking nearly all the load. If you are using RAMPS, then I suggest you use one output to power the heated bed (the 11A circuit), and the other output to power everything else (the 5A circuit).

How do I identify two positive 12V rails?

1. You can identify two 12V positive rails by a label on the PSU that gives two separate 12v load limits for output 1 and output 2, and by observing that the +12V wires coming out of the PSU are connected to one of two distribution points on the PCB instead of just 1.

2. The trouble with using auto bulbs as loads is that they take a very high surge current when you first turn them on. It may be this surge current that is tripping the PSU.

3. Another reason for the PSU tripping may be that you don't have any load (or too little load) on the 5V output. Loading the 12V rail on many ATX PSUs will make the voltage on the 5V rail rise unless it is properly loaded, and this may trip protection circuitry.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

There was little progress.
I had an advice from few members in the Reprap irc chat which was to install a load in positive 5V rail.
So I installed two 10 ohm 10W resisters in parallel to the positive 5V rail.
The PSU was providing two volts higher than previous test: 10W resistor gave 12.18V, 55W light bulb gave 12.01V (previous value was 10.09V).
It lighted up two the 55W light bulb in series but again it failed to run the two light bulbs in parallel.

I read through Choosing a Power Supply for your RepRap to figure out what I have been missing.
( [reprap.org])

According to the first method, I skipped few steps:
installation of 47 ohm on PS_ON (green wire) and ground one
Soldering 3.3V sense wires

[reprap.org]

Quote

Another personal opinion: reusing an old AT or ATX PSU (instead of spending $30 to $45 for an inexpensive but new ATX PSU) is a most unwise decision - a lesson I have learned the hard way! ATX PSUs are dust magnets, sleeve-type fans (the most common type in old PSUs) become noisy and ultimately fail (and when they do so during a print, you'll definitely smell smoke coming out of the PSU), and electrolytic capacitors fail catastrophically (with smoke and a bang), sometimes killing whatever electronics you have connected to the PSU. Not only that, but ATX PSU circuits have very much improved in performance, reliability and efficiency in the last couple of years (2013-2014), so the few dollars saved when reusing old ATX PSUs in RepRap projects are just not worth the risks, hassle and time wasted.

Basically the wiki page wouldn't much help in my case because I'm using an used ATX PSU made in 2010.
What was I thinking.

PS:
+12V1 and +12V2 connectors


Edited 1 time(s). Last edit at 02/02/2015 11:33AM by janpenguin.

Quote
dc42
2. The trouble with using auto bulbs as loads is that they take a very high surge current when you first turn them on. It may be this surge current that is tripping the PSU.

I had to agree with you. I did another load test on the ATX Bench power supply I converted two years ago. It has +3.3V@13A, +5V@15, +12V1@20A, [email protected], +5VSB@2A, power rating is 450W. When I connected 55W light bulb, it blew 2A of fuse. After replacing it 10A fuse, I connected 55W light bulb and the light bulb turned on. Out of curiosity as soon as I connected two 55W light bulbs, the PSU stopped running.

I modified the Bench power supply I built before instead working on the previous one.




Four modifications I made which mentioned on the wiki document()
- 50 ohm resistor between Green wire and ground wire that acts as power switch*
- Shorted one pair of +3.3V wires*
- external fuse holder (5x20)
- 1A load on +5V rail
(*: [reprap.org])



What a mess! It was the level of my workmanship in electronics two years ago.


external fuse holder (5x20)


50 ohm resistor between Green wire and ground wire that acts as power switch
(The wiki document recommends 47 ohm.)



1A load on +5V rail
I built the 1A load using two 10W 10 ohm resistors in parallel. And I connected red and black wires to it.


Shorted one pair of +3.3V wires


Checking voltage
+12.31V


+5.15V