I have ths CT clamp (with a 3.5 mm jack on the end):


How do I connect the clamp output to the multimeter probes to get a current measurement?

Also as a novice, what is the multimeter setup (wirings and modes). I am expecting to clamp this around an electrical heater appliance (the live wire).


2 Answers 2


This is a transformer - a CT with just a clamp (probably a bidirectional TVS) and no load resistor so you will need to supply a burden resistor in order to get a usable reading.

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In the article you linked, 22\$\Omega\$ was used, and you can use something similar, such as 20.0\$\Omega\$ 1%. The output AC voltage will be proportional to the burden resistor up to some value. 20 ohms should be safe. You could also use the AC amperes range of your meter, provided you know that the input resistance is low enough and thus avoid using the resistor. Often AC voltage ranges are a bit more accurate.

The transformer will supply an output current which is related to the input current by the transformer turns ratio, provided the output voltage is not too high.

  • \$\begingroup\$ Hm, I'm not clear whether the resistor is built into the device – the article mentions things are directly plugged into a PC soundcard's mic port, and that should have a high-resistance (virtually open) termination \$\endgroup\$ Nov 5, 2016 at 14:34
  • \$\begingroup\$ @MarcusMüller It's specifically stated as not built in. There is, however, a clamp in case the burden resistor gets disconnected, to keep from shocking the user. That makes the CT a bit more flexible because the user can apply various load resistors. \$\endgroup\$ Nov 5, 2016 at 14:34
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    \$\begingroup\$ ohhh! I seem to have missed that! \$\endgroup\$ Nov 5, 2016 at 14:35
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    \$\begingroup\$ I'll edit the data in for posterity in case the link disappears from the OP's question. \$\endgroup\$ Nov 5, 2016 at 14:41
  • \$\begingroup\$ Ah I see. So the burden resistor connects to the tip and sleeve of the jack? Then I can connect probes in parallel with the burden resistor to obtain an alternating voltage if I understand correctly. From thag how can you deduce current flowing through the wire? Thanks for the help! \$\endgroup\$ Nov 5, 2016 at 15:34

It's a current transformer: it converts a current to a voltage.

So just set your multimeter to AC voltage mode, and measure the voltage between the tip and the sleeve of your 3.5mm connector.

@Spehro pointed out that you'll need a load resistor (which I wrongly thought was included in the device) over which the voltage develops. So follow his answer!

You will probably need to calibrate using a known current, so that you know the factor between current flowing and voltage displayed.

  • \$\begingroup\$ Thanks! I will test this and let you know! I was getting zero readings I guess beacuse I was on the AC current setting. \$\endgroup\$ Nov 5, 2016 at 14:32

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