# Watt Minutes Meter for Small Electronics

#### avlmaker

Sep 10, 2018
7
I want to measure the energy usage of small heaters as they ramp over several minutes. The meter I'm using can measure Watt-Hours but I need something more granular.

Does anyone have a recommendation for something that can measure Watt-Minutes? Thanks!

#### Bluejets

Oct 5, 2014
6,851
Divide by 60.

Is this measuring AC or DC...??
What voltage and current range does it need to cover..??

#### avlmaker

Sep 10, 2018
7
Hi Bluejets, Thank you for your reply. I don't think that dividing by 60 will help because if I run a heater for one minute, the meter may show 1Wh. I can run the same heater for 2 minutes and have it still show 1Wh. The heater is using more electricity at first but less electricity as it ramps to a higher temperature. In the second test it hasn't quite reached 2Wh. Dividing by 60 for each of those tests would provide the same result (0.0167) when in fact those two different tests are using different amounts of electricity. That is why I need finer measurements. The power meter I'm using is measuring at 120Vac in. The highest current I've seen in these tests is ~2.5A. Perhaps 4 or 5A would be safe.

#### Bluejets

Oct 5, 2014
6,851
True as the readout is (apparently) only full value, no decimal places.
But only you would know this as the meter is in front of you, not me.

Best suggestion I have is, if the unit uses a ct for ammeter(which I suspect it does), it is possible to run the load cable through the ct ....x number of times and then divide back.
Small trick we use on Tong meters to measure very small currents on say a minimum selected 2 or 10A range.

Most watthour meters are made for higher loads so might be difficult to find one specifically for your needs.

Moderator
Nov 17, 2011
13,630

Oct 5, 2014
6,851

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,875
Does this watt-minute meter have anything to do with your previous thread from two years ago, involving sixty-four silicone insulated heaters mounted on a slowly rotating (~3 revolutions per hour) wheel?

I am curious about the purpose of the rotating heater wheel and how, with just two wires for power, you determine which heater will be energized as the wheel rotates? You mentioned slip rings, but these usually apply power continuously to objects on a rotating shaft. How do you select one heater to power out of sixty-four? Did you ever get the RFID tags to work with the spiral heating elements embedded in the silicone rubber heaters without burning out the tags?

It sounds to me like you are sorely lacking in instrumentation to perform electrical/electronic experimental setups. Spend a few bux so you will learn how to do it right. A decent digital multimeter, a digital RCL meter, and an inexpensive digital storage oscilloscope would be a good start. You can add more expensive stuff later if needed/desired/wanted. I have been using the Asian test equipment just mentioned for many years without problems, but you need to be careful of which vendor you select. Google will be your best friend there, but you have to do the research and make the final choice. Ask for suggestions here and in other online forums.

@Bluejets suggestion to run a number of turns of wire carrying the heater current through the toroid core of the current transformer is not a "trick" but a simple way to increase sensitivity of the current measuring part of your metering device. It may increase it too much, causing the current measurement to be "off scale" at higher inrush (cold) heater currents. This will be especially true during the inrush current that occurs when the heater elements are first energized, leading to inaccurate power measurements.

If it were my project, I would "roll my own" power meter, designing the "front end" electronics for the worst-case initial conditions of excitation voltage to the heater and maximum transient inrush current expected, then sampling and digitizing these two analog signals at a fast rate while multiplying the digitized results in real time. The product will be in watts of course, so you need to accumulate the products over a period of time to yield watt-minutes or watt-seconds or whatever energy units are appropriate to whatever it is you are trying to DO. BTW, just WTF are you trying to DO?

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