How to drive a transformer

Thread Starter

bug13

Joined Feb 13, 2012
1,996
Hi team

I need to driver a transformer with 625Hz sine-wave, I will need to output power of approx 5W. The purpose of the transformer is for isolation only. How do I go about it? Whats are the options. Driving voltage is flexible, but would like to keep it under 12VDC.

Thanks team!

Bullets points:
  1. Drive a transform with 625Hz sine-wave.
  2. Output of the transformer need to be approx 5W.
  3. Transformer is for isolation only.
  4. Driving voltage ideally need to be under 12VDC.
 

nsaspook

Joined Aug 27, 2009
8,910
Is this 625Hz sine-wave a signal you need to generate/synthesize for the transformer output or is it a existing signal that needs a driver for a transformer input coil to produce the desired output.
 

Thread Starter

bug13

Joined Feb 13, 2012
1,996
Is this 625Hz sine-wave a signal you need to generate/synthesize for the transformer output or is it a existing signal that needs a driver for a transformer input coil to produce the desired output.
Good question, didnt think of that.

I was under the assumption that if I feed a 625Hz sine wave to a transformer, the out put will be 625Hz sine wave as well. The sine wave will be a signal generated by a circuit before transformer driver.

The end goal is a 625Hz isolated transformer output, can delivery approx 5W.
 

nsaspook

Joined Aug 27, 2009
8,910
Good question, didnt think of that.

I was under the assumption that if I feed a 625Hz sine wave to a transformer, the out put will be 625Hz sine wave as well. The sine wave will be a signal generated by a circuit before transformer driver.

The end goal is a 625Hz isolated transformer output, can delivery approx 5W.
Yes the output will be a 625Hz but you need to look at transformer ratios, input/output impedance loads to select/design a proper driver for that 5W. 5W at that load impedance? A simple audio amplifier could handle the task driving the voice coil input of a line matching transformer like this if the system volts matches.
https://www.hammfg.com/electronics/transformers/audio/117
 

Thread Starter

bug13

Joined Feb 13, 2012
1,996
Yes the output will be a 625Hz but you need to look at transformer ratios, input/output impedance loads to select/design a proper driver for that 5W. 5W at that load impedance? A simple audio amplifier could handle the task driving the voice coil input of a line matching transformer like this if the system volts matches.
https://www.hammfg.com/electronics/transformers/audio/117
OK, just look at one thing at a time as I haven't work with transformer much.

Can I look at the input impedance of a transformer @ 625Hz, then find a driver that can delivery 5W + (?? loss power at output) at that input impedance? (forget about efficiency and impedance matching for now).

Then look at the output impedance, and my output impedance need to match my load to delivery 5W?
 

nsaspook

Joined Aug 27, 2009
8,910
OK, just look at one thing at a time as I haven't work with transformer much.

Can I look at the input impedance of a transformer @ 625Hz, then find a driver that can delivery 5W + (?? loss power at output) at that input impedance? (forget about efficiency and impedance matching for now).

Then look at the output impedance, and my output impedance need to match my load to delivery 5W?
You usually sort of work backwards with transformer output loads (power@X voltage @Y impedance). They are defined first, then design/look for a transformer that will match the driver to the transformer input.
 

Thread Starter

bug13

Joined Feb 13, 2012
1,996
You usually sort of work backwards with transformer output loads (power@X voltage @Y impedance). They are defined first, then design/look for a transformer that will match the driver to the transformer input.
Yes that makes sense.

Do I need to look at the input impedance together with the output impedance? Or I can look at input/output impedance independently? Or is this something from a transformer datasheet?
 
Last edited:

nsaspook

Joined Aug 27, 2009
8,910
Yes that makes sense.

Do I need to look at the input impedance together with the output impedance? Or I can look at input/output impedance independently?
Transformers by their very name transform impedance so it's a usually a feedback loop of design for a working transformer (off the shelf) turns ratio. What is the load impedance for that 5W output?
 

Thread Starter

bug13

Joined Feb 13, 2012
1,996
Transformers by their very name transform impedance so it's a usually a feedback loop of design for a working transformer (off the shelf) turns ratio. What is the load impedance for that 5W output?
I haven't got to the output load yet. As I am still trying to understand how to drive a transformer to delivery the output power I need. But it mainly is a resisted load, it's about 2 ohms.
 

crutschow

Joined Mar 14, 2008
28,224
Do I need to look at the input impedance together with the output impedance?
Transformers do not have a intrinsic impedance as such.
They can be designed to work with a specific operating impedance (such as 600Ω audio transformers) but that's not really needed here.

So you design the transformer to work with the output impedance you have (about 2Ω) and select a turns ratio that will give that 5W of power with a 12V driver signal (12Vpp).
Thus 5W into 2Ω requires 3.16Vrms or 9.0Vpp for a turns ratio of about 1.33:1 primary to secondary with a 12Vpp drive signal.

Transformers don't like DC going through them so you could connect the 0-12V driver signal to the transformer through a large capacitor.

The transformer must, of course, have an intrinsic magnetizing inductance that will keep the magnetizing current below the core saturation value at 625Hz and the maximum signal voltage.
 

Thread Starter

bug13

Joined Feb 13, 2012
1,996
Transformers do not have a intrinsic impedance as such.
What do you mean by this? Do you mean I should not see a transformer as two inductors join together?

So for the input, I just need to make sure the core does not saturate @ 625Hz and 12V? And at the output, I need to delivery 3.16Vrms to have 5W push into a 2ohm load.

How about impedance? I don't need to worry about impedance matching??
 

Ian0

Joined Aug 7, 2020
3,783
Just drive it with a small audio amplifier, like what would happen if you were to put a 625Hz tone through a 100V line public address system.

The most important factor to consider is the Design of the transformer. A 50Hz transformer will work adequately at 625Hz, but it you design one for 625Hz it can be much smaller. You also need to know the input and output voltages. The transformer is the tricky bit, driving it is easy.
 

vu2nan

Joined Sep 11, 2014
189
The 625 Hz, sine wave output of a signal generator, could be amplified by a 10 W audio amplifier with its gain control set for 5 W output.

The amplifier output could be matched to the 2 Ω load through two 100V 10W audio line matching transformers, connected back-to-back as shown.

1.JPG
Nandu.
 

crutschow

Joined Mar 14, 2008
28,224
What do you mean by this? Do you mean I should not see a transformer as two inductors join together?
Yes, but that determines the magnetizing current.
It is unrelated to the source/load impedance.
How about impedance? I don't need to worry about impedance matching??
The turns ratio needed to transform the 12V input to the output load voltage is what you need to worry about.
You would not match it to the source impedance, for example.
 

Thread Starter

bug13

Joined Feb 13, 2012
1,996
The most important factor to consider is the Design of the transformer. A 50Hz transformer will work adequately at 625Hz, but it you design one for 625Hz it can be much smaller. You also need to know the input and output voltages. The transformer is the tricky bit, driving it is easy.
So if I go to a transformer manufacturer, and tell them I need to drive a 625Hz 12V sine wave to the input, and the output needs to delivery 5W at 2ohm load. Do you think it will give them enough information to make the transformer for me?
 

Thread Starter

bug13

Joined Feb 13, 2012
1,996
Yes, but that determines the magnetizing current.
It is unrelated to the source/load impedance.
The turns ratio needed to transform the 12V input to the output load voltage is what you need to worry about.
You would not match it to the source impedance, for example.
I may sounds like I am not listening, and I think I understand what you are saying.
Just to make sure I understand it correctly. So I don't need to worry about the resistive component of the transformer (input and output). Or are they not something I need to worry about in practice, because it's only 5W power and the loss will be insignificant??
 

DickCappels

Joined Aug 21, 2008
7,997
So if I go to a transformer manufacturer, and tell them I need to drive a 625Hz 12V sine wave to the input, and the output needs to delivery 5W at 2ohm load. Do you think it will give them enough information to make the transformer for me?
Probably. Back when I was using copper and iron transformers for 50/60 Hz, that's about all the transformer designer needed to know. He would then ask questions to pin down any details he needed, like the desired maximum magnetizing current, maximum input, regulation, etc. Get to know your local transformer designer (if they still exist) -it is in his job and in his interest to figure out how to give you what you need to succeed.
 

MisterBill2

Joined Jan 23, 2018
9,893
If the load impedance is two ohms resistive and the power is intended to be 5 watts, then the current would need to be about 2 1/2 amps from the transformer. That is a reasonable current.
If you are using an audio amplifier with a ten watt max output capability you can simply connect your load to the four ohm output and adjust the gain of the amplifier to provide the power required at the load. The fact is that with such a low load resistance you will also experience some voltage drop in your wires, and that will be a challenge to know in advance.
And depending on the actual amplifier circuit it might not be as isolated as you are seeking. So you need to check that in advance.

What sort of application is this, anyway??
 

Ian0

Joined Aug 7, 2020
3,783
I was trying to think where one might find a suitable transformer. It's well within the "telephone" bandwidth of 300Hz-3.4kHz, but I can't think of a telephone application that would need 5W. It's about the same spec as an output transformer for an early transistor power amplifier, but no-one has made transistor amplifiers with output transformers in donkeys' years.
What sort of application is this, anyway??
I'm curious to know as well!
 

MisterBill2

Joined Jan 23, 2018
9,893
Really, one type of transformer that could work very well is one with two similar windings intended for series or parallel connection, either primary or secondary side. that would include one specifies as 12/24, where it would provide 12 volts at 2 amps or 24 volts at one amp, depending on the connection being series or parallel. Those arrangements are available from a number of companies. The mains connection would be unused.
 
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