# CT Energy Harvest circuit

#### t.bishop

Joined Mar 21, 2023
2
I'm trying to add energy harvesting to my current transformer measuring circuit.

I'm measuring a current with a CT like this right now, measuring the voltage across R3. R1 and R2 bias the CT to Vcc/2.

And I am pretty sure I could harvest energy from a CT, in a completely separate circuit, like this:
(As long as the CT secondary current exceeds the current draw of the circuit)

But how can I combine these circuits? I think I need an additional little toroidal transformer T4, like this:

Does this have a chance of working? How can I compensate for phase shift introduced by T4, so that the voltage across R6 is not phase shifted?
And how do i specify T4? How do I calculate the instantaneous or rms voltage drop across T4 at 60Hz? What else do I need in this circuit - if this is even in the right direction?

Thanks for any ideas or criticisms.

-Tim

#### WBahn

Joined Mar 31, 2012
28,465
Energy harvesting for what purpose?

If you are trying to make a measurement, any attempt to extract energy from the circuit is almost certainly going to have a negative impact on the measurement.

If you want to extract power from whatever is driving the input of your current transformer, then use a separate transformer to do so.

#### Ian0

Joined Aug 7, 2020
7,441
I'm trying to add energy harvesting to my current transformer measuring circuit.

I'm measuring a current with a CT like this right now, measuring the voltage across R3. R1 and R2 bias the CT to Vcc/2.

View attachment 290350

And I am pretty sure I could harvest energy from a CT, in a completely separate circuit, like this:
(As long as the CT secondary current exceeds the current draw of the circuit)
View attachment 290347

But how can I combine these circuits? I think I need an additional little toroidal transformer T4, like this:

View attachment 290349

Does this have a chance of working? How can I compensate for phase shift introduced by T4, so that the voltage across R6 is not phase shifted?
And how do i specify T4? How do I calculate the instantaneous or rms voltage drop across T4 at 60Hz? What else do I need in this circuit - if this is even in the right direction?

Thanks for any ideas or criticisms.

-Tim
Yes. It will work, but don't think you are getting something for nothing! Your 3.3V power supply will simply add to the burden of the current transformer, and cause a proportionately higher voltage drop across the primary of the current transformer.
If you are measuring the mains current, then simply using a mains transformer connected to the mains voltage will probably be more efficient, and won't detract from the accuracy of your current measurement.

#### t.bishop

Joined Mar 21, 2023
2
Energy harvesting for what purpose?

If you are trying to make a measurement, any attempt to extract energy from the circuit is almost certainly going to have a negative impact on the measurement.

If you want to extract power from whatever is driving the input of your current transformer, then use a separate transformer to do so.
Thanks a lot WBahn,

The energy harvest is to power the measurement circuit itself. Perhaps with a large capacitor the circuit could switch between measuring for 16ms and then harvesting power for 984ms, repeatedly every second.

I'd really like to only use one (current) transformer to measure and power the circuit. If the measurement is not perfect, that is ok, as long as it is close.

Here is an example Current Transformer that I am thinking of - it is meant to be field installed.

https://www.amazon.com/SCT-013-000-Non-invasive-Current-Transformer-Sensor/dp/B0789JYRH8?th=1

Thank you

#### BobTPH

Joined Jun 5, 2013
6,508
Rectify the output, put the rectified DC out through a resistor. Now the voltage developed across the resistor is proportional to the current in the primary. You can run a sensitive voltmeter from the voltage across the resistor.

What range of currents do you need to measure? You might be able to get it to work down to 1 or 2A. If it goes down to zero, it ain't gonna work.

#### ronsimpson

Joined Oct 7, 2019
2,708
That CT makes about 1V rms. It is hard to get much energy out of it. 1V @ 50mA assume you have a 100% officiant circuit 5V @ 10mA.
The CT can withstand 1V for 1/50 of a second. (there is a VT product) If you try to get 5V the transformer will saturate and the voltage will drop to zero. and there are 1.7V of loss in the diodes.

#### MisterBill2

Joined Jan 23, 2018
14,666
Certainly "energy harvesting" from a current transformer will work, HOWEVER, it will be at the expense of accuracy, which will no longer be present. A current transformer is made to provide power to a meter circuit and unless the harvesting circuit can exactly duplicate that, no accuracy will be available.

#### Ian0

Joined Aug 7, 2020
7,441
Changing the burden resistor changes the voltage drop across the primary, not the current in the secondary (within a reasonably large window) So adding a series resistance in the secondary will not change the meter reading.
Things may be less predictable with a non-linear load such as a bridge rectifier and capacitor in series with the meter.
@BobTPH has the most important point: No current in the primary = no "harvested" power to run the electronics.

#### MrAl

Joined Jun 17, 2014
10,057
I'm trying to add energy harvesting to my current transformer measuring circuit.

I'm measuring a current with a CT like this right now, measuring the voltage across R3. R1 and R2 bias the CT to Vcc/2.

View attachment 290350

And I am pretty sure I could harvest energy from a CT, in a completely separate circuit, like this:
(As long as the CT secondary current exceeds the current draw of the circuit)
View attachment 290347

But how can I combine these circuits? I think I need an additional little toroidal transformer T4, like this:

View attachment 290349

Does this have a chance of working? How can I compensate for phase shift introduced by T4, so that the voltage across R6 is not phase shifted?
And how do i specify T4? How do I calculate the instantaneous or rms voltage drop across T4 at 60Hz? What else do I need in this circuit - if this is even in the right direction?

Thanks for any ideas or criticisms.

-Tim

Hi Tim,

Not exactly sure what the heck you are trying to do there but it already looks way too over complicated.

If you need a DC voltage from the same transformer then you may be able to move to a half wave rectifier, but do you really need both output terminals of T3 to be floating (more or less)? If you can ground pin 3 you can use a half wave rectifier to power the DC circuit.

I realize that for some apps like this you can not use a mains transformer you only have that current transformer, but you should be able to tap power from it without resorting to another transformer. If you can not ground pin 3 of T3 then perhaps you can capacitively couple the output of T3 to your DC circuit and rectify that.
Alternately, since you said you want to 'harvest' the energy for a time and then at another time use that energy to power the different part of the circuit, then a relay is another possibility if you can put up with that. Tap the power with the relay closed one way, then run the circuit with the relay closed the other way, contacts either SPDT or DPDT, whatever works best for you.

#### ronsimpson

Joined Oct 7, 2019
2,708
I have tried this energy harvesting game.

I am using CTs that saturate at about 1V. I thought I could connect LEDs across the CT, without resistors, and see the light. My problem is that the LEDs need 1.5V to light up. Even with no load I cannot get enough voltage to fire a LED.

People tell me that the output of a CT is "constant current" and without a load the voltage will go to "mega volts". I find, below saturation the constant current idea is right but at and above saturation there is no voltage. The core is over driven the transformer stops working.
--------------------------------
Next thought. At low voltage the transformer is acting like a constant current source. Now lets try to get power out of it. Start out at 1V and 20mA = 20mW. That might run some electronics. If you reduce the resistance to 1/2 you will get 0.5V @ 20mA = 10mW. Again 1/2 the resistance you get 0.25V @ 20mA = 5mW. If the resistor goes to 0 Ohms you get 0V @ 20mA = 0mW.

My first try, I made a circuit that opens up the burden resistor, and tries to suck off energy at as high voltage as I can get. (higher voltage higher energy) If the load pulls too little current the voltage goes up and the transformer saturates which results in no voltage and no energy. I had to add a 1.25V limit on the voltage. If the load pulls too much current, say 21mA, the voltage also drops to zero.

I built a strange circuit that responds fast to changes in load and current. If the CT voltage is above 1V we pull more current. If the voltage is below 1V we pull less current. (current is stored on a large cap) If the cap is over charged we dump the power into a Zener.

There are some nice Energy Harvesting ICs that work at less than 1V and make 3.3 or 5V.

I don't remember which IC I used for harvesting. Might be in this family. It takes voltages above 20mV and tries to charge a battery or super cap. Read Analog devices information on energy harvesting.

#### MisterBill2

Joined Jan 23, 2018
14,666
An interesting thing, I had never considered what a current transformer would do with an overload. I have some shunts and a couple of current transformers, so at some point I may do a bit of experimenting.
Of course, if the TS wants to harvest some power it should be simple to wind a bit of wire over the existing core and draw power from the second winding. That might not affect the accuracy at all. Or it might. Testing to see should be easy.

#### MrAl

Joined Jun 17, 2014
10,057
I have tried this energy harvesting game.

I am using CTs that saturate at about 1V. I thought I could connect LEDs across the CT, without resistors, and see the light. My problem is that the LEDs need 1.5V to light up. Even with no load I cannot get enough voltage to fire a LED.

People tell me that the output of a CT is "constant current" and without a load the voltage will go to "mega volts". I find, below saturation the constant current idea is right but at and above saturation there is no voltage. The core is over driven the transformer stops working.
--------------------------------
Next thought. At low voltage the transformer is acting like a constant current source. Now lets try to get power out of it. Start out at 1V and 20mA = 20mW. That might run some electronics. If you reduce the resistance to 1/2 you will get 0.5V @ 20mA = 10mW. Again 1/2 the resistance you get 0.25V @ 20mA = 5mW. If the resistor goes to 0 Ohms you get 0V @ 20mA = 0mW.

My first try, I made a circuit that opens up the burden resistor, and tries to suck off energy at as high voltage as I can get. (higher voltage higher energy) If the load pulls too little current the voltage goes up and the transformer saturates which results in no voltage and no energy. I had to add a 1.25V limit on the voltage. If the load pulls too much current, say 21mA, the voltage also drops to zero.

I built a strange circuit that responds fast to changes in load and current. If the CT voltage is above 1V we pull more current. If the voltage is below 1V we pull less current. (current is stored on a large cap) If the cap is over charged we dump the power into a Zener.

There are some nice Energy Harvesting ICs that work at less than 1V and make 3.3 or 5V.

I don't remember which IC I used for harvesting. Might be in this family. It takes voltages above 20mV and tries to charge a battery or super cap. Read Analog devices information on energy harvesting.

View attachment 290543
Hello there,

I've used a lot of current transformers and if they are designed to output more than 1 volt they certainly do. I have one or two now that can easily light an LED. In the past i have used parts that can easily output 12 volts ac. It just depends on the design as to what it was intended to be used for.

Yes, the constant current idea is to allow us to think about these things in terms of output current, but of course there are always going to be limitations.

#### ronsimpson

Joined Oct 7, 2019
2,708
It just depends on the design as to what it was intended to be used for.
The CT he has is rated for 1V. It probably will not make more than 1.5V. My point is that, through a full wave bridge, it will not make 5V.

#### MrAl

Joined Jun 17, 2014
10,057
The CT he has is rated for 1V. It probably will not make more than 1.5V. My point is that, through a full wave bridge, it will not make 5V.
Oh ok, well that's no good then he needs a better component or some other ideas.

#### MisterBill2

Joined Jan 23, 2018
14,666
There are really cheap current transformers made for monitoring heater current, that are used with injection molding dies. They typically will light an LED and trigger a PLC input.