# Using Current Transformer

Discussion in 'The Projects Forum' started by stmijaar, May 9, 2009.

1. ### stmijaar Thread Starter New Member

Mar 18, 2009
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I want to power an LED using a current transformer in a 120vac lamp circuit.
I need to seperate the lamp load circuit from the pilot light circuit so as not to produce heat. I want the pilot light to be on only when current passes through the torroidal current transformer primary. I don't want to use a neon bulb which lights when the switch is "off". Am I asking for the impossible?
I have tested the circuit with a 100 w lamp. I used a TRIAD CST206-1A current transformer with various loads and the voltage drop across the secondary is at most about .3 vac. The actual circuit will have 3-40 watt lamps; 1.67Amps. Can anyone lead me to a CT which will give me about 3 volts on the secondary. Or 24v? (voltage ratings of LEDS I've seen online) I think I need about .150 ma throught the LED load across the secondary, so the LED will work. OR, is there such a thing as a CT which would be 1:1 so I could use a C-7 120v LED Replacement Holiday Bulb?

Last edited: May 9, 2009
2. ### DonQ Active Member

May 6, 2009
320
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The spec sheet for the TRIAD CST206-1A, says: "These transformers ... operate over the frequency range of 20kHz-200kHz."

Pretty sure your line frequency isn't in this range.

Also, type of transformer you are using is called a current transformer. You should be thinking in terms of current, not voltage. A certain current in the primary makes a certain current in the secondary (within reason). This is great for what you want, because an LED is a current operated device. It is often run off of a voltage by putting a current limiting resistor in series to turn the voltage to a reasonable current.

In a current transformer, you already have current. It's just that the transformer you selected does not provide the current ratio you want (even if it was designed for 60 Hz.)

Back to the drawing board.

You might be able to get what you have to work, but it won't be by using the device as designed. One way might be to loop the wire through the hole more than one time. But at this low a frequency, compared to its design frequency, you may already have exceeded some spec (but it's easy to try and won't hurt anything).

If you get to the point where you can ask some more defined questions, you might be able to get some more specific answers.

3. ### RmACK Active Member

Nov 23, 2007
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Some good advice there from DonQ.

I might add that it would not be a bad idea to connect a diode across your LED so that current can flow either one way via LED or the other via diode, to prevent very high voltages developing in reverse across the LED.

Indeed think current, so turns ratio in reverse. 1.67A through the 1 turn primary suggests that at least 84:1 turns ratio will give you under 20mA which is the right sort of figure for the LED.

4. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
To reduce the secondary current on a current transformer, add more turns to the secondary.

So if one wants 1mA secondary current for a one turn 1A primary current, its 1000 turns of secondary.

5. ### DonQ Active Member

May 6, 2009
320
11
Except this poster is using a pre-built device... no access to the secondary.

6. ### BillB3857 Senior Member

Feb 28, 2009
2,400
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I salvaged the torroid from a GFI and it worked great to drive an LED directly. I split the AC line and ran the hot lead through the torroid and whenever there is current, the LED is on. My circuit is used to trigger a counter on my basement sump pump and has been in operation for several years.

7. ### stmijaar Thread Starter New Member

Mar 18, 2009
5
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You've got me on the right track I believe. Yes, I ordered the wrong CT! I think the Triad CST-1005 is the one i should have ordered.
I would the current wire up to 4 times through the hole in the donut and only got the voltage up to about .5vac I have no idea what the current is as yet. I will measure it once I get the new CT, but the Iin is about 1Amp. Also, I had wonderd about using a diode, but didn't realize that back voltage could build up.
And, PLEASE forgive my ignorance, but my electircal knowledge is severly stunted post 5U4's and 12AU7's, so I'm confused. Granted the LED is a current device, but doesn't it have a voltage limit? What happens regarding voltage? Some LEDs show a voltage rating, so don't I have to keep within a certain limit?

8. ### tkng211 Well-Known Member

Jan 4, 2008
65
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LEDs are sensitive to reverse voltage and break down easily when a reverse voltage larger then 6V is applied. When using a CT to energise a LED, better connect a diode (1N4001 or 1N4148) in parallel with the LED but in reverse polarities to protect the LED. Besides, the forward voltage drop of some red, amber, greenish yellow LED is approx. 1.9V and that of the blue, green, white LED is approx. 3.2V, when the current passing through it is around 20mA. A current limiting resistor is needed in series with the LED. You may experiment with the value through 10 to 1K Ohm.

Last edited: May 11, 2009
9. ### DonQ Active Member

May 6, 2009
320
11
A current limiting resistor is needed when driving an LED from a voltage source. It's different when a current source is being used. With a current source and a series resistor, the voltage will just rise to drive the same current through the increased resistance. The CT (current transformer) acts like a current source.

This is the problem mentioned with the reverse biased LED. For example: if the current source is going to provide, say, 10mA... how high will the voltage have to go to force this current through an open circuit? Answer: Really high!

Fortunately, this is not an ideal current source (or there would be really long sparks all over the place), but it can easily go high enough to exceed the reverse breakdown of the device. If a parallel diode forward conducts at about 0.7V, the given current will flow at that voltage and no one gets hurt.

In the forward direction of the LED, the current will start flowing at the forward voltage of the LED. Again, nobody gets hurt as long as the current source does not provide more current than the LED can handle. If it does, a series resistor will not help when using a current source. In this case, a parallel resistor can shunt some of the current around the LED. Do the calculation for the parallel resistor using the forward voltage of the LED. If you have 50mA, but only want 20mA to go through the LED, and the LED forward voltage is 2V (example), then:

2V/(50mA-20mA) = (2/0.04)Ω = 50Ω, in parallel.

10. ### stmijaar Thread Starter New Member

Mar 18, 2009
5
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Thank you very much "tkng211" and "DonQ" You have given me the answers I have needed, but couldn't find, having gone to a reference point which I understand, wheras in ALL my searches, LED suppliers assume (and rightly so) that prospective purchasers know how these things work.
You have generously and gently led me (no pun intended) through a lesson on these devices and I appreciate it.
I will let you know through this post how things work out.

11. ### thephotoguy New Member

May 30, 2016
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"My circuit is used to trigger a counter on my basement sump pump and has been in operation for several years."

Could you please give me some more specifics on your toriod current transformer with gage of wire and turn#s etc.? I'm trying to use something like this to trigger a pool pump (mains 120vac) to turn on only when my salt/chlorine generator is on. The salt/chlorine generator is self-timed so will draw a very small current to run internal electronics/timer but will kick up current when it begins to generate chlorine. I don't know the current differential but hope to trigger a transistor or arduino to turn on the water pump while salt/generator is on.

12. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,777
1,103
You realise this thread is 7 years old? Hopefully the relevant person is still around to answer your question.
Welcome to AAC!

thephotoguy likes this.

Jul 18, 2013
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thephotoguy likes this.
14. ### thephotoguy New Member

May 30, 2016
2
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Thanks and yes I knew it was. Even the automated system warned me but I persisted... I was trying to reply directly to BillB3857's comment about scavenging a toroid to trigger a counter for his sump pump. I looked and saw that his profile had been used recently and thought that even if the thread was old, he might see my comment. It appears that my comment was a general one and not specifically to his comment as I'd intended.

I've been enjoying looking at things here on AAC for a while but only just signed up so thanks for the welcome! Sorry if I was off in etiquette in my reply, I'll learn the ropes hopefully!