Is it possible to detect current ramp-up to DC with a coil?

Thread Starter

mkerikss

Joined Dec 29, 2015
4
Hi,

I am thinking of ways to detect when an electrical device is turned on and off without actually touching or modifying the existing circuitry. More specifically, I have a hand held device powered by 2 x 9V batteries and with 2 electrodes connected through leads and plugs to sockets on the device, and that way connected to the internal circuitry. When turned on, the current ramps up linearly from 0 mA to 2 mA. The ramp-up rate is 0.1 mA/s. When shut down, the current is ramped down correspondingly.

It's been a while since I had anything to do with electronics and the Maxwell laws, and my googling didn't give me any more clarity in whether what I was thinking of doing is possible, so my hope is that someone here could enlighten me.

So, can I use a coil around the lead/plug and detect that the device is turned on due to the changing current? Or should the coil be placed perpendicularly to the lead and plug? I would then connect the coil to a microprocessor unit and send it by GSM from a GSM module to a server and collect the data there.

Or if this is not possible at all, are there any alternative options to induction into for how to do this without this coil solution that I'm considering?

If my idea with a coil works, are there any specific considerations I should think about with regards to technical requirements on the coil. Number of turns etc. Given that the rate of change of current is 0.1 mA/s for 20 seconds, from 0 to 2 mA.

I tried to be as clear as I could. Hope my message makes sense although I'm not very well-versed in this field yet, as you can probably tell.

Best regards,

Mikael
 

Alec_t

Joined Sep 17, 2013
11,397
Welcome to AAC!
I doubt you'll have much/any success with an external coil as detector. If you have access to the internal circuitry of the mystery device you could perhaps pass the 0-2mA current through the base-emitter junction of a transistor and then monitor the collector current?
 

Thread Starter

mkerikss

Joined Dec 29, 2015
4
Welcome to AAC!
I doubt you'll have much/any success with an external coil as detector. If you have access to the internal circuitry of the mystery device you could perhaps pass the 0-2mA current through the base-emitter junction of a transistor and then monitor the collector current?
Thanks for your answer Alec! Unfortunately that is not an option as although I do have access to the internal circuitry, modifying it would trigger all sorts of changes in already existing descriptions, documentation and regulatory constraints of the device :/

The idea was therefore to have the new circuit part of a cover or frame for the device, not touching the existing circuitry, then that would be a completely new product and the original device would be left as-is.

Do you think there's anything that can be done to accomplish this?
 

Kermit2

Joined Feb 5, 2010
4,162
A coil around the AC cord will work IF it only has the one HOT wire inside the coil. The cord has the hot and neutral wires inside a protective jacket, so some modifications would be required. If you can't modify the cord then you will have to put another socket and cord on your new device to allow you to plug the un changed equipment into your new device and monitor the current drawn from inside the new device.
 

Thread Starter

mkerikss

Joined Dec 29, 2015
4
A coil around the AC cord will work IF it only has the one HOT wire inside the coil. The cord has the hot and neutral wires inside a protective jacket, so some modifications would be required. If you can't modify the cord then you will have to put another socket and cord on your new device to allow you to plug the un changed equipment into your new device and monitor the current drawn from inside the new device.
Ok, I see! Thank you for the information :)
 

Thread Starter

mkerikss

Joined Dec 29, 2015
4
A higher sensitivity hall effect current transducer would have no problems picking up the 2 ma signal from one of the leads.
Yes, after another day of research it seems that a hall effect device would be my best bet.

At least according to this article: http://www.allegromicro.com/en/Design-Center/Technical-Documents/Hall-Effect-Sensor-IC-Publications/Non-Intrusive-Hall-Effect-Current-Sensing-Techniques-for-Power-Electronics.aspx

It clearly states that all current sensing is either 1) resistive or 2) Hall effect based or 3) based on current transformers.

1) is not possible because that would intrude on the original circuit, and 3) is only possible with AC.

So now I've got a direction for where to go and what to investigate which is awesome! :) Got some hall effect researching and experimenting to do!
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Thank you again to everybody who replied! And if anybody has anything to add I would be very grateful to hear about it of course.
 
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