Driving an audio transformer

Thread Starter

atferrari

Joined Jan 6, 2004
4,120
I run across the PSD (phase sensitive detector) concept few days ago.

To test the basics (with no particular goal in mind) I plan to drive 1:1 audio transformer with a 3 KHz - 6 V p to p signal.

I know almost nothing about how to drive it: transistor, opamp, power opamp, small audio amp. Balanced or not?.

What are the basics to calculate in / out impedances and from there how to design something to drive it safely.

Honestly, as in many cases I would like to spend little time in getting something more or less functional. I am certainly more interested in what comes next: the PSD concept.

In this case, I found that reading A LOT, didn't help. I would say it confused me more than it helped.

I have been dreaming of a LM386 working as a power oscillator, or a balanced driver with common opamps to see what I get even if a tiny signal at the output.

I need help to avoid the most obvious false starts here.

Gracias.
 

JDT

Joined Feb 12, 2009
657
The easiest way to do this if you have no other equipment is to use your computer's sound card to generate the sine wave.

Connect the output of your computer to a small audio amplifier to drive your transformer. A DVM on the AC voltage range can be used to measure the amplitude of the transformer drive signal.

Don't forget the multimeter measures the RMS voltage. The peak voltage is 1.414 times this value.

Try this software to generate your sine wave:-
http://www.electronics-lab.com/downloads/pc/005/index.html

Or you can Google for other applications.
 

Thread Starter

atferrari

Joined Jan 6, 2004
4,120
Thanks for replying.

I am building a trafo much in the way they are used in LVDTs. Hence my interest about calculating impedances and how to drive it.

DSP will come later.
 
Last edited:

t06afre

Joined May 11, 2009
5,934
The heart in every lock-in amplifier is a phase sensitive detector. You can build phase sensitive detector using both an analog or digital approach. A very common analog building block is the AD630. It is somewhat expensive but at low frequencies it do the job
 

t06afre

Joined May 11, 2009
5,934
The basic principle behind phase sensitive detection is to compare a reference signal with a measurement signal. Both the amplitude and phase of the measured signal will influence on the measurement result. So any added phase difference caused by say a filter in the signal chain may cause large inaccuracy in the measurement result.
 

t06afre

Joined May 11, 2009
5,934
Have you given the core phase sensitive detector(psd) unit any thought? I think this is more important than the input stage ;)
 
Hi Agustin,

I've been wrestling with similar questions lately, and have gotten a bit of an education.

Here are a few tips I'd give you:

1. Because the transformer is 1:1 --- isn't it? --- the impedance presented across the primary to your driving circuit will be the same as the impedance connected to the secondary. Are you connecting a speaker to the secondary? If so, a 1:1 transformer will then reflect that very low impedance...about 5 ohms...to the driver. It's not easy to make driver that will drive 5 ohms directly...so, for starters, I need to know what the transformer load will be to give you advice on how to drive the tx.

2. You also need to know if the transformer is going to be push-pull, single-ended, or if it will have an air-gap...otherwise you'll saturate the core and possibly magnetize the core, rendering the tx useless. Let's assume this transformer has a simple primary and a simple secondary...that is, two wires in, two wire out...and it is non-gapped (you'd know if it was gapped, that's a special type). Driving this sort of transformer is not a problem, you just have to know that you can't have a DC component, a DC offset, in the signal driving the primary. If there is DC in the signal, there will be current going through the primary in one direction only, all the time...and the result will be saturation of the core (so the tx doesn't work at all) and eventual magnetization of the core (so the tx will be permanently damaged - pun intended :)

Does that help at all? If yes, and you want to give more detail, I can help with more driver circuit detail.

best, charlie
 

Thread Starter

atferrari

Joined Jan 6, 2004
4,120
The actual transformer is the type used in LVDTs (one primary and two secondaries) axially in line (with a movable core).

The idea is to make sure I build something able to drive properly to later dedicate my efforts in the phase difference detector. But as I said, many times already, my focus is on the trafo now.

Honestly I do not want to start by shooting in the dark.

Charlie, thanks for any further help.
 
Top