Measure maximum current draw from home appliances - Page 1
Forum section: electronics / beginners

author: Tman
author_rank: Newbie
timestamp: July 26, 2018, 02:07:59 am
content: Hi Guys! (I'm new here)Just starting to play with batteries, grid tie inverters and I need to do some planning around the max power consumption of certain appliances.I don't have an oscilloscope so can someone help me the best (cost effective) ways to see what the initial current (power) draw is when an appliance starts.Thanks!

author: sleemanj
author_rank: Super Contributor
timestamp: July 26, 2018, 02:11:09 am
content: plug in watt meterhttps://www.jaycar.co.nz/mains-power-meter/p/MS6115https://www.mitre10.co.nz/shop/elto-power-meter/p/103559https://www.bunnings.co.nz/arlec-energy-cost-electrical-meter_p04410270

author: Old Printer
author_rank: Frequent Contributor
timestamp: July 26, 2018, 02:18:20 am
content: An oscilloscope would not be the tool to make that measurement. A multi meter setup to measure current (amps) would do it if it either has data logging or has a high low function which would save the highest reading. The running power consumption is usually listed in watts on the produce lable. Watts divided by voltage gives you the current draw in amps. Two DMMs, one setup to measure volts and the other to measure current would be what I would use. Welcome to the forum.

author: Muttley Snickers
author_rank: Unknown
timestamp: July 26, 2018, 02:50:46 am
content: I noticed one of those power meters yesterday at Aldi for $15, I don't know if they are any good or if they are able to capture and display the initial inrush or startup current, I doubt they have this ability. I would keep the receipt and return the device if found to be unsuitable for your requirements.For testing mains powered devices and equipment I wired up an inline box so as to split and separate the active from the neutral, I can then conduct measurements with either a clamp meter or multimeter and the minimum/ maximum feature on these meters is ideal for this type of testing.Aldi Watt Meter.www.bauhn.com.au/product-listing/power-meter-1

author: MarkF
author_rank: Super Contributor
timestamp: July 26, 2018, 02:56:05 am
content: I would recommend a clamp on meter that has the capability to measure in-rush current. You will need to measure motor start current in addition to the motor run current. The start-up (inrush) current can be many times larger than the steady state run current.The Fluke 374 or similar.

author: mtdoc
author_rank: Super Contributor
timestamp: July 26, 2018, 05:33:07 am
content: Most quality inverters will have no problem handling the typical inrush current seen in normal home appliances - so ability to measure inrush current is not critical IME. However, larger inductive loads - large power tools, septic or well pumps, etc - could be an issue if you're really pushing the boundary with your inverter choice.You mention batteries which makes me think you are looking at a grid tie with battery back-up (aka "Grid Interactive") setup. In that case measuring your 24 hour power draw from appliances or other household loads will be critical for correctly sizing your battery bank and PV array (or other RE sources) and one of the plug in outlet meters as mentioned are excellent as long as they can log KWh.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 26, 2018, 12:21:43 pm
content: As others have said, inverters can handle some inrush current but maybe not anywhere near enough to handle a motor starting that would momentarily cause lights to dim if it started on the A.C. line. If you need to measure the inrush current safely and you have some experience with circuits you can make a current transformer from a line inductor salvaged from an old switching mode power supply. The photo below shows how to do it. You may have to experiment a little to get the turns ratio right to give you enough voltage output to measure on a scope but a few turns of insulated wire wound as the primary would probably work. The original winding will be used as the secondary and MUST be terminated by a fairly low value resistor, 10 to 100 ohms would be a good starting point. Without a load (burden) the output voltage could be high.Wire the primary into a circuit with a incandescent bulb and an A.C. multimeter in series, set to the proper current range, you can view the output on a scope and calibrate the circuit against the DMM for steady state current by adjusting the burden resistor/scope attenuation. Once you have it correct try connecting a motor load and trigger off the current transformer signal and adjust the timebase to display the inrush to steady state waveform on the scope. A meter will probably not react fast enough to display what you are looking for, even if it can capture max readings. A scope will tell you so much more. Either way, for safety, use a current transformer for isolation.I didn't just try this but I have done a similar thing in the past so it should work.