# beginner assistance with measuring AC amps

#### Johnny7

Joined Feb 5, 2022
20
The crude diagram represents a current limiting ("dim bulb") setup. The mains pass through a DPST light switch, then onto a duplex outlex, where the bridge between the Line terminals has been broken.

The box at the top (4 circles) represents a power strip which allows various bulb combinations to be plugged in.

My questions:
1. Is the location I'm showing the proper place to insert an ammeter or DMM if one wishes to know what the DUT is drawing?
2. Is a shunt needed here? - I don't really understand when one is/isn't called for

Thanks in advance to those who take the time to share their knowledge.

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Joined Jul 18, 2013
28,827
A shunt is called for when a 'proper' all-in-one ammeter is not available, and a shunt is used to measure the volt drop across the resistor. .
The shunt has to be very low compared to the load in order not to distort the reading.

#### Ian0

Joined Aug 7, 2020
10,276
Shunts are more commonly used for DC Ammeters. For AC Current, a current transformer would be more usual.
For low currents (<15A) meters tend to be direct reading.
For higher currents a current transformer would be used to produce a current level that is more convenient to measure.
Industrial current transformers tend to have 1A or 5A secondaries which are connected to a 1A or 5A meter.

#### Johnny7

Joined Feb 5, 2022
20
Shunts are more commonly used for DC Ammeters. For AC Current, a current transformer would be more usual.
For low currents (<15A) meters tend to be direct reading.
For higher currents a current transformer would be used to produce a current level that is more convenient to measure.
Industrial current transformers tend to have 1A or 5A secondaries which are connected to a 1A or 5A meter.
Thanks for that.
Are you able to answer my first question?

#### Ian0

Joined Aug 7, 2020
10,276
Thanks for that.
Are you able to answer my first question?
Presumably it places a lamp in series with the mains supply to your "device under test".
The meter goes in series with the lamp and the DUT.
They can go in any order.

However, if there is a possibility of a short to earth in the DUT, then the lamps and meter should go on the live side of the DUT.
Then the earth current would be limited by the lamps, and the meter would show the fault current.

#### Johnny7

Joined Feb 5, 2022
20
Presumably it places a lamp in series with the mains supply to your "device under test".
The meter goes in series with the lamp and the DUT.
They can go in any order.

However, if there is a possibility of a short to earth in the DUT, then the lamps and meter should go on the live side of the DUT.
Then the earth current would be limited by the lamps, and the meter would show the fault current.
Thank you!

#### LowQCab

Joined Nov 6, 2012
4,296

#### Johnny7

Joined Feb 5, 2022
20
I thought briefly about using my Kill-A-Watt device (similar to what you've shown) but it literally can't fit into my DBT setup.
More importantly, this isn't just about solving a problem -- I want to learn the broader principles of ammeter use.

Thanks for the thought.

#### neonstrobe

Joined May 15, 2009
191
Your diagram might be improved a little. You show wires going to the device under test and the upper device. Is the upper device a lamp? Then you would have the live wire going to the lamp, the return from the lamp to your device under test then your meter and back to neutral. I don't know what your standard electricity colours are but we used to have red as live (danger - don't touch) and black as neutral. Some thought it better to change live to brown (could be mistaken for more earthy?) and neutral to blue (sky high=high voltage ?).

If your meter measures AC the meter is wired directly in series. If it does not then a low value resistor is needed instead and you measure the voltage across the resistor. If the current is above 10A (most meters are 10A AC, max, if they have AC current) then you need a current transformer (or low value but high current resistor and an AC (milli) voltmeter). If you have a Hall effect current meter you can simply clamp it over the neutral wire on its way back to the mains, if it can measure the current you are trying to measure (these meters tend to measure high currents, 10 A upwards not low)