# How to measure current

Discussion in 'General Electronics Chat' started by chrischrischris, Jun 28, 2013.

1. ### chrischrischris Thread Starter Active Member

Feb 18, 2012
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10

I'm currently using a 24V 15A 360W regulated switching supply driver to drive a bunch of LED lights. I both calculated and measured the current draw on the 24V active as around 13A (with a meter).

I'm now wanting to calculate the supply current required to the switching supply when connected to 240V. I bought an el cheapo clamp meter to try and measure the current input on the active of the supply driver, however it seemed to be around 0.6Amps. Does that sound right? How can I calculate this to check it.

I'm about to have an electrician supply a 100 meter long extension chord to supply power to these LEDs, hence I'd like to give him a hint on the current required.

2. ### John P AAC Fanatic!

Oct 14, 2008
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If your switching unit is fairly efficient, power in is equal to power out plus a little (10%? 20%?) for losses. So 24V * 13A = 312W, say 350W after allowing for losses. So at 240V, you'd be drawing about 1.5A.

That isn't a large current for a power outlet, so you don't need very heavy cable. However, there's a tradeoff to make there. If you use large wire, you save on the electric bill because the losses along the way will be smaller. But 100 meters, doubled (or tripled, if you have a ground wire) of large copper wire will cost you some money. So do you pay for extra copper, or extra electricity? You might want to think about whether this is a permanent setup where it's worth investing in something that will run efficiently, or if it's going to be used for a short time and then torn down, so it makes sense to build it cheaply.

Dave Garcia likes this.
3. ### chrischrischris Thread Starter Active Member

Feb 18, 2012
295
10
1.5A, yes that's what I was "guessing" based on V=IR. So I'm guessing that my reading of 0.6A only may have been due to he PWM circuit set low. This setup will be temporary and only run on weekends. In a year or 2 I expect it will be relocated, so I'll probably opt for the lighter cable. Thanks for the quick response.