Measuring low current with clamp-on ammeter

[Blars]

I wanted to know how much power my 22" LED TV/monitor actually uses. The inline brick power supply is rated at 1.7 A 120v in, 12v 5A out. If it realy was that bad I'm sure it would be a fire hazard from the amount of heat it puts out, but it only gets a little warm.

I've got a clamp on AC ammeter, so by modifying an IEC power cord I can get it around the hot leg only. Unfortunatly, the 200 A full scale range only reads to tenths of amps, and the 0.2 A reading has pretty wide error margins.

Thinking about it a bit, I wound 5 turns of the hot wire around the clamp. Now I get a reading of 1.7 A, which divided by 5 means the TV is actually using 0.34A of wall power.

Is there any reason this trick would not give accurate readings?

 

[KMoffett]

What makes you think that the reading isn't accurate? "1.7 A 120v in, 12v 5A out" are "maximum" ratings for the power supply, not what is being drawn by the TV.

ken

 

[peufeu]

> Thinking about it a bit, I wound 5 turns of the hot wire around the clamp.

That's the right thing to do.

Note that your brick may draw a sine current (if it has power factor correction) or spikes (if it doesn't). What your ammeter will display depends on the setting (RMS, no RMS) and the actual implementation of RMS (true, or cheap) and its reactions to spiky current.

Your values seem realistic though.

 

[crutschow]

I bought an inexpensive Kill a Watt meter just for measuring the power draw of various appliances. It measures line frequency, line voltage, load PF, load current draw, load power draw, load VA draw, and load energy used over time (KWH), of anything plugged into it. Measures down to 1W and seems to work well.

 

[MrAl]

I wanted to know how much power my 22" LED TV/monitor actually uses. The inline brick power supply is rated at 1.7 A 120v in, 12v 5A out. If it realy was that bad I'm sure it would be a fire hazard from the amount of heat it puts out, but it only gets a little warm.

I've got a clamp on AC ammeter, so by modifying an IEC power cord I can get it around the hot leg only. Unfortunatly, the 200 A full scale range only reads to tenths of amps, and the 0.2 A reading has pretty wide error margins.

Thinking about it a bit, I wound 5 turns of the hot wire around the clamp. Now I get a reading of 1.7 A, which divided by 5 means the TV is actually using 0.34A of wall power.

Is there any reason this trick would not give accurate readings?

Hello there,


The only thing you have to be careful about is that some clamps require the wire to be directly in the center of the clamp hole while making a measurement. That would make turns wound around the core inaccurate unless they were large enough to go through the center of the core hole.
You could easily check yours by using a single wire to test something and move it from the center to the edge near the core metal. If you see a change then you have to make sure your coil turns go through the center. If they dont you should be able to check the overall calibration anyway.

To check the overall calibration, you can easily run 5 to 10 turns through the core and make a measurement with a current meter in series with the clamp meter and compare readings. If the more accurate meter reads say 1 amp and the clamp reads 1.1 amps then you know it's reading 10 percent high (for example).

 

[rossw]

Current Multiplier

Blars, I made a similar "current multiplier" many years ago for measuring low-consumption devices (some as low as 3-5 watts off a 240V AC supply).

I started with a cheap ($3) Power-board.
I carefully cut the outer insulation and peeled it off, wrapped the active into 5 loops, wrapped the neutral into 5 loops the opposite direction, and wrapped the earth around the end to keep it out of the way. Bound the whole lot with several layers of electrical tape. The Loops look like this.

I can plug stuff in, clip the clampmeter on when I want to measure, and being two times 5 turns, gives me exactly 10* the readings (easy to do in your head) Here is the amplifier in use

Cheap, simple, effective.