pulse transformer

Thread Starter

fls_design

Joined Apr 6, 2015
7
Hi everyone: I have been designing a circuit that will output a very low level pulse. The pulse train is two pulses, with each pulse at 10 mV for 50 uSec, and -10V for 50 uSec. This is repeated every milliSec. The outpus needs to be isolated, which (at least in my current attempts) has employed a transformer. I have all the drive and controls worked out and prototyped. I can't seem to get a good pulse across the transformer. The circuit runs off a single AAA alkaline cell with a boost controller to produce 3.3V. If everything works, I may reduce that to about 2V.
Can some one suggest a way to get to a solution?

Thanks in advance.
 

Thread Starter

fls_design

Joined Apr 6, 2015
7
Can you supply a part number and specs on the pulse transformer?

Have you considered using an opto-isolator?
Thanks MrChips:

The problem with an optoisolator is the need for some kind of power on the isolated side. I have none, and can't put one there in this project.

I have tried a number of transformers:
The 100o series from Murata,
The 786 series from Murata,
Miniature SMT transformer T6437-DL from coilcraft.

One of the controlling factors for a pulse transformer is the VuS product, how long it is designed to holdup a particular voltage in uSec (on the primary). Based on the frequency of the square wave I am passing, that would be 50 uSec multiplied by the voltage on the primary.
Also, I am using a center tapped primary on the transformer, with each leg driven to ground by a pulse on a 2N7000/2N7002 transistor.

On the output side, I can resistor divide down to get the voltage levels I need.

I did neglect to mention that the output load is a nominal 600 ohms. I had thought to use an audio transformer, but cannot find enough info in the specs to be able to make a decision.
 

Thread Starter

fls_design

Joined Apr 6, 2015
7
Sure. First step is to post your circuit here so we can get a look at it; then maybe we can figure out what's going wrong.
You are right:
I have posted the output circuit I have been working with. The NPN transistor is currently a 2N3904. BV1 is a AAA alkaline battery. VCC is 3.3V from a step up controller. The transformer CT can be attached to any of these at this time, and will stay attached to the source that actually works.
 

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OBW0549

Joined Mar 2, 2015
3,092
Hmmm... all I can think of, looking at the datasheet for the T6437-DL (as an example) is that perhaps you are exceeding the V*T rating of the transformer; if that's the case, the only remedy I can see (other than finding a transformer with higher V*T) would be to drive the transformer from as low a voltage as possible, and then not attenuate its output so much. You're looking for output pulses of only +/- 10 mV, correct?
 

Thread Starter

fls_design

Joined Apr 6, 2015
7
Hmmm... all I can think of, looking at the datasheet for the T6437-DL (as an example) is that perhaps you are exceeding the V*T rating of the transformer; if that's the case, the only remedy I can see (other than finding a transformer with higher V*T) would be to drive the transformer from as low a voltage as possible, and then not attenuate its output so much. You're looking for output pulses of only +/- 10 mV, correct?
I agree with your initial finding. That's why I tried using the op amp current source (it is compliant when there is no pulse, with about 100 mV on the output.) That is also why I tried the T6427. It has a much higher Vt rating. However, it is designed for 10 KHz operation, and it does not do so well at 1 KHz.
 

OBW0549

Joined Mar 2, 2015
3,092
Do you have any scope traces you could post? It might help if we could get a better idea of the waveforms you're actually getting from this circuit...
 

Thread Starter

fls_design

Joined Apr 6, 2015
7
Do you have any scope traces you could post? It might help if we could get a better idea of the waveforms you're actually getting from this circuit...
I will put them together and post. It may take me a couple of days though, I am on vacation and my lab is not available (also don't want to upset my wife too much by working).

I will do that next week.

So far you have all been extremely helpful and I appreciate that a great deal. Thanks.
 
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