measuring voltage and current with a multimeter

grep · 01-05-2002, 07:52 PM

to measure voltage you place the leads in parallel with the load.
to measure current you place the leads in series with the load.
doing either is inconvienent because there are no exposed points to make the measurements at. I have been thinking of making adapters to plug into the existing connectors then into the existing loads so that I can make measurements with a typical multimeter. I can find adapter plugs both male and female for the molex connectors for the disk drive power cables easily. Finding adapters for the motherboard is the problem. I need a source for the motherboard power connectors and the auxiliary power connectors. Actually I need 2 of each male and female, I will set one up for voltage measurement and the other up for current measurement, unless I can figure out a good way to do both with a single adapter.

The second problem is the 3.3 and 5 volt lines can carry up to 50 amps. I need to figure out how to measure up to 50 amps with a 2 amp maximum meter.

One more observation. The 5 volt line is not a single line on my system its actually 6 lines, combined they can carry up to 50 amps but I am going to assume that no single line will ever actually carry 50 amps. There is no way to forsee how much current any wire will carry, but I can hook up the adapter and then use the multimeter and if there is more current than the meter can measure, I can double the wire and each new wire in parallel will carry 1/2 the current I can keep doubling the wires untill the current drops to a level I can measure.

Grep

Brad · 01-08-2002, 09:07 PM

Quote:

Originally posted by grep

One more observation. The 5 volt line is not a single line on my system its actually 6 lines, combined they can carry up to 50 amps but I am going to assume that no single line will ever actually carry 50 amps. There is no way to forsee how much current any wire will carry

Grep

I'd presume that it would be total amperage divided by the number of lines. IE the amperage would be split up over each line.

as for finding out the total amperage, I'm not too sure on how you'd actually do that..

grep · 01-09-2002, 02:16 AM

yes. assuming that each wire is exactly the same resistance then each wire will carry equal current. Of course in practice thats not true. There will be fractional differences in the resistance.
eample: with 4 wires.
wire 1: 1.43 amps
wire 2: 1.40 amps
wire 3: 1.41 amps
wire 4: 1.46 amps
total amps = 5.7 amps
since my amp meter is not able to measre almost 6 amps directly, I can devide the single wire into 4 wires, measure each and add them back together and get 5.7 amps

grep

grep · 01-09-2002, 02:29 AM

your not looking for total system amperage. what you want is the total amperage at each voltage. ie. the total amperage at +3.3 volts, and the total amperage at +5 volts and the total amperage at +12 volts now you can convert each to watts and compare the results with your power supply specs. This makes it possible to find out if that water pump is ok on your system or if that bunch of fans is too much or not. Most powersupplies may be rated at 300 watts but the 3.3 + 5 volts are rated combined at a fraction of that total.
example might be
350 watts total
175 watts on 3.3 volts
150 watts on +5 volts
but when the 3.3 and the 5 are added together they can not exceed 210 watts. Far less than the 175 + 150 the specs show for those voltages.
so how can you know how much your system is using on 3.3 volts
and how much its using on 5 volts.
by actually measuring it of course.
theres the problem, how?
what I am talking about is how with commonly available equipment.

Grep

01-05-2002, 07:52 PM	#1
grep Cooling Neophyte Join Date: Mar 2001 Posts: 37	measuring voltage and current with a multimeter to measure voltage you place the leads in parallel with the load. to measure current you place the leads in series with the load. doing either is inconvienent because there are no exposed points to make the measurements at. I have been thinking of making adapters to plug into the existing connectors then into the existing loads so that I can make measurements with a typical multimeter. I can find adapter plugs both male and female for the molex connectors for the disk drive power cables easily. Finding adapters for the motherboard is the problem. I need a source for the motherboard power connectors and the auxiliary power connectors. Actually I need 2 of each male and female, I will set one up for voltage measurement and the other up for current measurement, unless I can figure out a good way to do both with a single adapter. The second problem is the 3.3 and 5 volt lines can carry up to 50 amps. I need to figure out how to measure up to 50 amps with a 2 amp maximum meter. One more observation. The 5 volt line is not a single line on my system its actually 6 lines, combined they can carry up to 50 amps but I am going to assume that no single line will ever actually carry 50 amps. There is no way to forsee how much current any wire will carry, but I can hook up the adapter and then use the multimeter and if there is more current than the meter can measure, I can double the wire and each new wire in parallel will carry 1/2 the current I can keep doubling the wires untill the current drops to a level I can measure. Grep __________________ every thing is optional

01-09-2002, 02:16 AM	#3
grep Cooling Neophyte Join Date: Mar 2001 Posts: 37	yes. assuming that each wire is exactly the same resistance then each wire will carry equal current. Of course in practice thats not true. There will be fractional differences in the resistance. eample: with 4 wires. wire 1: 1.43 amps wire 2: 1.40 amps wire 3: 1.41 amps wire 4: 1.46 amps total amps = 5.7 amps since my amp meter is not able to measre almost 6 amps directly, I can devide the single wire into 4 wires, measure each and add them back together and get 5.7 amps grep __________________ every thing is optional

01-09-2002, 02:29 AM	#4
grep Cooling Neophyte Join Date: Mar 2001 Posts: 37	your not looking for total system amperage. what you want is the total amperage at each voltage. ie. the total amperage at +3.3 volts, and the total amperage at +5 volts and the total amperage at +12 volts now you can convert each to watts and compare the results with your power supply specs. This makes it possible to find out if that water pump is ok on your system or if that bunch of fans is too much or not. Most powersupplies may be rated at 300 watts but the 3.3 + 5 volts are rated combined at a fraction of that total. example might be 350 watts total 175 watts on 3.3 volts 150 watts on +5 volts but when the 3.3 and the 5 are added together they can not exceed 210 watts. Far less than the 175 + 150 the specs show for those voltages. so how can you know how much your system is using on 3.3 volts and how much its using on 5 volts. by actually measuring it of course. theres the problem, how? what I am talking about is how with commonly available equipment. Grep __________________ every thing is optional

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