# Current transformer with phase cut rectifier

Discussion in 'The Projects Forum' started by eem2am, Jan 8, 2013.

1. ### eem2am Thread Starter New Member

Jul 22, 2012
9
0
Hello,

How can I implement a variable phase-cutting rectifier on a 50KHz current transformer secondary?

I have a current transformer with a split secondary which is charging a battery from its secondary. The primary current is a fixed sine wave.

Schematic: Full wave rectifier on current transformer output
http://i46.tinypic.com/28jxsgg.jpg

As you see, there is a full wave rectifier which puts DC into the battery from the secondary as follows..

Full Wave Current waveform into battery:.
http://i46.tinypic.com/ipb989.jpg

The primary current is fixed and is a sinusoidal 50KHz.

I now wish to reduce this current by phase cutting the rectified sinusoidal output. I can do this by periodically shorting out the rectifier diodes.

Here is the waveform that I now require into the battery(0.3 duty cycle)
http://i48.tinypic.com/qq4ksi.jpg

as you can see its phase cut version of the initial waveform and has a 0.3 duty cycle.

I also want to be able to do any duty cycle from 0.1 to 0.9.so for example, I may sometimes want the following waveform to charge the battery

Current waveform into battery (0.7=duty cycle)
http://i49.tinypic.com/2wo9e7d.jpg

Here is a rough version of the phase cutting circuit..
http://i48.tinypic.com/2vhyq04.jpg

as you can see, the controlled voltage source switchs the FETs on and off periodically via the FET drivers, shorting out the rectifier diodes as it does so.

Though my question is, how can I most simply provide this phase cut waveform?
What circuitry can most simply do this job?

on any particular product I only need one duty cycleand it might be anywhere between 0.1 to 0.9..(so that makes things easier)

2. ### crutschow Expert

Mar 14, 2008
13,475
3,361
I don't understand how "shorting" the diodes will reduce the current. You want to open the diode circuit, not short across it.

3. ### eem2am Thread Starter New Member

Jul 22, 2012
9
0
I understand what youre meaning...but shorting the diodes makes the current flow in a loop through the secondary , and through the diodes, and thus it does not go into the battery.

....so, shorting the diodes "diverts" the current away from the battery, which reduces the current into the battery.

This is part of a system which is a waterproof lighting system with an emergency battery..............instead of a voltage source, it uses a current source (50KHz) which is coupled into the luminaires via current transformers, which are waterproof as they are not like normal connectors.
..the battery charging is also done by a coupler, as you see represented in my pics

4. ### crutschow Expert

Mar 14, 2008
13,475
3,361
OK, that makes sense. I forgot that is was a current transformer.

Since it's a battery load do you really need to "phase cut" the charge current. Perhaps you could just do a ON/OFF duty-cycle type of control with the ON and OFF periods being several cycles. All you need for that is a low frequency PWM type of circuit. Then you wouldn't have to synchronize the switching with the incoming waveform. I don't think the battery would care. It's the average current the battery sees over a long period that is important.

5. ### MKCheruvu Member

Nov 20, 2012
30
6
can you provide the IC Part Number(I assume it is from Linear technology)

6. ### crutschow Expert

Mar 14, 2008
13,475
3,361
As a suggestion, do not post schematics as JPEGs since it makes them blurry and hard to read. Better to use a GIF, PNG, or other format designed for graphics, not pictures.

7. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
Put a single SCR ink your circuit in place of D3. Phase control the gate at 100cps rate.

8. ### crutschow Expert

Mar 14, 2008
13,475
3,361
The AC frequency is 50kHz. How will phase control at 100Hz work?

9. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
NO. It will need 100KHz, I missed the K on first read. I ASSUMED (yes, I have experience in that field) that we were talking about 50HZ line frequency. Strange to charge a battery with rectified 50KHZ

10. ### eem2am Thread Starter New Member

Jul 22, 2012
9
0
This method is used for the battery charging, but its mostly used with the LED luminaires.........ie the inductive coupler drives a chain of LEDs.

Its 50KHz because the high frequency makes the coupler smaller.

Do you think the cap C1, in the first schematic above, corrects the power factor?