Measuring PSU Amps

[Big_Dog]

Which 2 wires would I connect a multimeter or ampmeter to, to see what kind of amps my PSU is putting out? TIA
Donny

[TwoRails]

Don't know if that would be realistically possible. First, a typical MM wouldn't cut it as they don't measure that high. Second, you'd need to get inside the PS for readings since there are so many "outs" that would need to be measured at each independent connection. Interesting thought, though.

[theyosh]

I don't know if you can do this but what about measuring the current that is going into your power supply instead? I figure the more your power supply puts out the more it will draw from your power plug.

theyosh

[flanzig1]

To measure currant/amps, you have to put the meter in series with the load line of a supply, which means cutting wires. Also, most DVMs have only a 10 Amp rating. To measure higher currant you need a clamp-on ampmeter(for AC).

[bailey]

you did not state what you really wanted to measure,
1. each supply source
2. each supply wire
3. total amps for each voltage
4. total amps for the entire supply

the max power rating for each voltage is on the lable of the power supply
the total watts can be added up for each devive connected to the supply.

[faulkner132]

Amps = Watts / Voltage

[bitTRL1000]

In response to the second post:

It doesn't work like that unfortunately. Typically, a power supply is rated in Watts or Amps. The rating is determined by finding out how much power or current the supply can put out while staying above a certain voltage. For a 12V PS it's probably around 10V. So to see how many amp's you PS could put out you'd have to have a load tester (a variable resistor) that you hook up in series with an ammeter. Increasing the load (decreasing resistance) while measuring the current and making sure the voltage is above the cutoff would tell you how many amps it can do. I don't know of a device available for consumer purchase that is meant to allow you to vary a load for a 12V computer PS.

Now, if the wattage of a PS is measured on the output side, then there should be a singular relation between that and the amperage. Power is definded as Voltage X Current, so since they are all 12V PS's then you should just be able to divide the wattage of the PS by 12 to figure out the amp rating. However, looking at the Watt and Amp rating of PS's shows this to not be the case. I've seen a 450W PS that's only rated at 18A, and then I've seen a 350W PS that's rated at 20A. This obviously doesn't jive with my understanding of the equations for electrical power.

My guess is that the Wattage rating is how much power the PS sucks in, so measured on the AC side, and since the Amp rating is from the output (12V) side the discrepancy could be due to the efficencies of the various PS's. I've tried to find an answer to this, but have been unable to. If anybody on here has ideas about this I'd love to hear them.

[KCD]

Woof !

6/12 Volt Load Tester $19.99

http://www.harborfreight.com/cpi/cta...emnumber=90636

CLAMP ON DIGITAL MULTIMETER $14.99

http://www.harborfreight.com/cpi/cta...emnumber=42397

KCD

[bitTRL1000]

I actually use that exact load tester at work. However that is meant to test battery output power and charging system voltage. It is not meant to load test a charging system or AC to DC converter, which is what a PC PS is. I said that I am not aware of a load tester meant SPECIFICALLY for a computer PS, that is available to the end-consumer. I wouldn't be surprised if one was though.

[KCD]

Quote: I actually use that exact load tester at work. However that is meant to test battery output power and charging system voltage. It is not meant to load test a charging system or AC to DC converter, which is what a PC PS is.

Agreed, but it will do the job.

If I remember correctly the old analog 260 Simpson had a 10 amp position for such testing.

I'd only use the milli-amp scale on the 260 to bench test long line equipment for the telephone company. The idea of paying for a 260 because I pegged the De'arsonval (spelling?) movement was a no brainer.

Quote: I said that I am not aware of a load tester meant SPECIFICALLY for a computer PS, that is available to the end-consumer. I wouldn't be surprised if one was though.

There are several mfgr's, here's one

EZ POWER SUPPLY TESTER, Model No.EZPST-2

http://www.casebuy.com.tw/tester/ezpst-2.htm

While my post was tongue in cheek the clamp on amp meter is a good tool to have for those in electronics or electrical wiring.

Have a good one, KCD

[bailey]

Originally Posted by bitTRL1000 I actually use that exact load tester at work. However that is meant to test battery output power and charging system voltage. It is not meant to load test a charging system or AC to DC converter, which is what a PC PS is. I said that I am not aware of a load tester meant SPECIFICALLY for a computer PS, that is available to the end-consumer. I wouldn't be surprised if one was though.

maybe I can help you understand this :

the lable on the side of the power supply is what it is rated at.
for example the 12 volt 20 rating means that the 12 volt portion of the power supply can deliver up to 20 amps and still be withen the rated voltage, if more amps were drawn from the 12 volt supply the voltage would drop below the rated output and will then be what is called overloaded. but it will still work up to a point set in the design of the power supply to shut it down.

the amps that are actually drawn from the supply will depend on the loads applied to it, the rateings are what the design of the voltage regulators are set for.
it is the same for all the other voltages that the power supply has for its output.

[bitTRL1000]

Originally Posted by KCD Agreed, but it will do the job.

Actually it won't. That is not a variable load tester. There is only a "on/off" load switch.

Originally Posted by KCD There are several mfgr's, here's one EZ POWER SUPPLY TESTER, Model No.EZPST-2

Cool, I'll check it out. Although at first glance it doesn't seem to be able to load test the individual rails, only determine if there is voltage.

Originally Posted by bailey maybe I can help you understand this : the lable on the side of the power supply is what it is rated at. for example the 12 volt 20 rating means that the 12 volt portion of the power supply can deliver up to 20 amps and still be withen the rated voltage, if more amps were drawn from the 12 volt supply the voltage would drop below the rated output and will then be what is called overloaded. but it will still work up to a point set in the design of the power supply to shut it down. the amps that are actually drawn from the supply will depend on the loads applied to it, the rateings are what the design of the voltage regulators are set for. it is the same for all the other voltages that the power supply has for its output.

Thanks for trying to help me out, but you didn't really address the question I had in my post. I understand, and indicated so in my first post, what the rating of a PS means on the label. I'm trying to answer the question of the original poster about how to actually test the output power or current of a PS, as well as answer my own question about the discrepencies between Wattage ratings and what their associated Amperage ratings should be.

[bailey]

ok, maybe I just don't understand what it is that he actually wants to do, and why ?

[bitTRL1000]

Well, you're right. He could mean either of two things (hopefully he'll answer back). Either he just wants to measure how many amps the PS is supplying to the motherboard on a specific rail, which would be doable, but wouldn't be advisable IMO because you'd have to splice an ammeter in series from the PS to the motherboard. Or he means what I think he means: how does one measure what the PS is actually capable of putting out. In other words, how do you verify the rating that is on the side of the PS. To do that you'd need the variable load test that we're talking about.

Anybody on here know why the amp rating of a PS isn't just the wattage rating divided by 12? -That's my main question.

[bailey]

mainly because that will just tell you what the maxium could be with out any voltage drop, and not what it is actually delivering at any one moment of the said load,
example, the flopy drive would only draw a load while it was running
and the cpu will only draw the amount of power for what it is doing at any one period of time, it won't really tell you anything about the power supply.

if you placed a computer on a actual watt meter and monitored the wattage used during a normal operation, you would see the watt meter vary from about 75 watts when it was doing nothing all the way to 200 watts when it was doing something very intensive and the cpu and hard drive and ram were all working very hard, so placing a amp meter in the line of any one of the supply's output voltage would also be such a varring reading that it would mean nothing to you.

now, it you were to connect a known load to each rail, and you knew it would not vary at all, then you would end up with the very same reading that are printed on the lable of the power supply.

so my question is, what are we wanting to actually measure here?

[bitTRL1000]

I'm sorry. Are you trying to answer my "main question" from post #14? If so, I know that the ratings (both watts and amps) are a maximum and not what the PS is doing all the time. That's not my question.

[bitTRL1000]

Originally Posted by bailey so my question is, what are we wanting to actually measure here?

Like I said, there are two possible things to measure. A) You measure how much power the PS is putting out at a given time, just like you described. Of course the power varys as the PC doesn't always use peak power. It uses more when playing a game and uses less when idle. Or B) You measure the peak power that the PS is capable of putting out.

I'm most interested in measuring the peak power the PS is capable of putting out and comparing that with the rating of the PS. In a related matter, theoretically the equation Power = Amps X Volts should allow us to take the Power rating of the PS and just divide it by 12 Volts to get the Amp rating of the PS. But that's not the way it works when you look at the ratings. Why? I gave my hypothesis in my original post.

[bailey]

ok, lets try this.
the power supply has several sources12volts, 5 volts + 5 volts - , 3.3 volts. and so on, each one is able to deliver x amount of amps as stated on the lable. the total of all the sources will be the total that the supply can deliver.
and still be within the rated plus and minus rateing of the regulator.
each source is able to deliver the amount of power that is stated on the lable.

if you want to measure each load from the power supply one at a time, then you must place a amp meter in series with each one of the supply wires, true a MM will not be able to do this so you must use a bench meter that can do this or place a shunt across the meter and that will give the results your looking for, any electronics book can show you how to pick the proper shunt for your meter, itd the same standard ohms law use.
a good source for a 12 volt dc load would be to use some 100 watt 12dc lamps connected in parrlal to provide the desired load, then just measure the ampe or add up the 100 watt bulbs.

[bitTRL1000]

See, that's what I'm talking about. I didn't think that maybe the PS could still put out power on the 5 or 3V rails while at its peak on the 12V. Which means that if I add (12 X '12V-rail amp rating') + (5 X '5V-rail amp rating') + (3 X '3V-rail amp rating') that it should be equal, or at least close to the wattage rating of the PS. Also, hopefully it will account for the fact that some PS' have a higher 12V amp rating than another PS while having a higher wattage rating than that same PS. I'll look at some PS ratings on Newegg and see.

I'm well aware of how to use just a resistor across a voltage source to measure internal resistance of that voltage source and the amp draw. The problem is that unless the internal resistance is truly linear then you need a variable resistor with fine resistance control to pick out the load which puts the PS at the cutoff voltage.

[bailey]

I am very interested in why you want to do that, there is methods for doing that, and any load on the 12 volt supply will not affect the other voltages as they are seperate sources and have there own regulators,
why do you want to backward engeneer the power supply ?

as I stated in a earlear post, just place a few 100 watt 12 volt light bulbs on the 12 volt line and when you add one more than it can handel try a 50 watt and so on. you will soon find out where the limit is, this is the least expensive way to do that.
the other method is to buy a lab load simulator, it will be as variable as you want but is very expensive to go that route.
to use resistors you will need some high wattage precision units and they are also very expensive, unless you had a need to do this in a lab enviroment, I would not recommend that you even try messing around with it.
I am lucky enough to have this type of equipment to do this type of work, and I ain't cheap eather.

[Big_Dog]

Regarding amps on the 12 volt rails in relation to PCI-E video cards on A64 systems. I simply wanted to measure the amps while the system is running. I have a 12 volt amp meter, and several PSUs, (evidently, the Antec Neo Power 480 doesn't supply enough). I thought I could simply scrape the insulation off two wires, If I knew WHICH TWO WIRES, connect the red and black leads to these wires, and get some sort of reading. All this being done on the bench. I'm sorry I'm sure I could have been much clearer originally. I'm surely showing my ignorance regarding electricity, but I don't think this should be that hard.
Thanks all.
Donny

[glc]

No, an ammeter must be connected in series unless you use a clampon meter. You can measure voltage in parallel like you describe.

[Big_Dog]

GLC, so I would have to find the correct wire, cut it and connect both leads to the respective ends of the cut wire? Putting the amp meter "inline" so to speak? Thanks

[bailey]

Originally Posted by Big_Dog GLC, so I would have to find the correct wire, cut it and connect both leads to the respective ends of the cut wire? Putting the amp meter "inline" so to speak? Thanks

that is exactly what you would need to do if you used a standard amp meter, but you can do it with a digital clamp-on amp meter. it will just clamp around the one wire and read the currant through it with no physical connection at all.

[Big_Dog]

This would be done to the main 12volt wire (green I believe) on the 24 or 20 pin PSU cable?

[glc]

No, the green wire is the 5 volt sense wire that turns the PSU on.