Testing AC currents - Page 1
Forum section: electronics / beginners

author: Aggressive_Doughnut
author_rank: Contributor
timestamp: April 26, 2019, 02:51:29 am
content: I have a CT rated for 30A and wanted to test it at around 10A (15A circuit breakers on a nominal 120V line). I can use a variac to make the voltage more palatable, but I'm not sure how to get the 10A safely/cheaply/easily. Should I just invest in a bunch of 50W chassis resistors and parallel them together?

author: Gregg
author_rank: Super Contributor
timestamp: April 26, 2019, 04:54:44 am
content: Current transformers only measure the current; therefore if you run the test power through a low voltage / high current transformer you won’t need a lot of power converting to heat for your experiment.Enclosed are some pictures of an old school Weller soldering gun with wire attached instead of the heating tip. These solder guns are basically a transformer with a single turn for the low voltage / high current that is short circuited at the tip which is higher resistance than the rest of the single loop. Here I have used several turns to increase the current that the CT sees and I have used two parallel wires to handle the current. The magnetics of the Weller heat gun are designed to saturate at full short which magnetically limits the current to not damage it.You could use a Variac ahead of the primary of any suitable low voltage transformer to vary the voltage which in turn varies the current (ohms law) and wrap more than one wrap of the secondary around your CT. If you want to check circuit breakers, look at what people have done to convert microwave oven transformers into spot welders.

author: TheNewLab
author_rank: Frequent Contributor
timestamp: April 26, 2019, 04:56:35 am
content: modern vacuum cleaners can use up to 12 amps. kitchen appliances also use a lot of current. a hair blow dryer use 1200 watts to 1600 watts. power tools? 7 amp draw on a circular saw..just combine enough on a multiplug outlet, and it will be easy to get up to even 15 watts.If you want to try out a 20 amp circuit. Dedicated circuits for refrigerators and garbage disposals are often 20Amp... Check the mains circuit breaker box for higher amp circuits to test.Still a good idea to start with a variac...make sure unit is OK in the first place.

author: TheNewLab
author_rank: Frequent Contributor
timestamp: April 26, 2019, 04:58:16 am
content: ay, yes, microwave ovens are great!

author: Brumby
author_rank: Unknown
timestamp: April 26, 2019, 10:11:42 am
content: A lot of those alternatives can give a qualitative result - with very limited quantitative value.What if the OP wants something to help with calibration?

author: Aggressive_Doughnut
author_rank: Contributor
timestamp: April 26, 2019, 11:58:09 am
content: Great suggestions by all, thanks! I was kind of hoping for something of known value to use as a (rough?) calibration, but maybe I have an appliance that will give some known, repeatable value.

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2019, 01:10:19 pm
content: Using a Weller soldering gun like that is ingenious. I will have to remember that.

author: soldar
author_rank: Super Contributor
timestamp: April 26, 2019, 06:44:06 pm
content: Microwaves and other non-linear loads may give false readings.You don't have incandescent lightbulbs any more? Space heaters? Hot plate /stove?In the past I have also chained step-down transformers to the point where I was getting a volt or less. A microwave oven transformer connected backwards is useful for this.There are many ways to skin this rabbit.

author: Aggressive_Doughnut
author_rank: Contributor
timestamp: April 26, 2019, 07:55:39 pm
content: Chaining transformers is a pretty amazing idea, not sure why I didn't think of that. Once you dip into the millivolts, do you find that you run into other barriers to making reliable measurements?

author: bob91343
author_rank: Super Contributor
timestamp: April 26, 2019, 10:18:44 pm
content: The problem with millivolts lies in making a galvanic connection. The least bit of oxide and it becomes nonlinear. But if you are working with ac it tends to average out as a first approximation.Thermal voltages can be a problem if the circuit uses more than one material. Various grades of copper, tinned copper, iron, and so on will create thermal voltages all over the place.Temperature variations cause resistance changes, resulting in drifting readings.Nothing is ever as simple as you want, especially if you are looking for precise results.

author: Gregg
author_rank: Super Contributor
timestamp: April 27, 2019, 12:00:29 am
content: A word of caution using CTs: don’t leave them open circuit with a load. Because of the high turns ratio, the voltage can become very high and degrade the insulation between turns, ruin test equipment and give you a nasty shock.Industrial installations of CTs often include a junction block with shorting bars positioned so that the CTs can be shorted before removing the metering devices.

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2019, 01:31:54 pm
content: Why not get another current transformer and run it in reverse?You can then get 10A out from a much lower current supplied by the variac, with a small 6V transformer on the secondary.