Help needed identifying audio transformer - AC injection to DC power

Thread Starter

Gipper

Joined May 20, 2021
18
I need to test a system by injecting a +/-4 Vpp signal onto a 28 VDC power line. The current draw for the system is 300 to 350 mA DC. The frequency range for testing is 20 Hz to 20 kHz (audio). The recommended test configuration is attached.

I have a function generator that can produce the AC frequency content up to 2.5 Vpp. This signal can be amplified further.

I am struggling to identify an appropriate transformer. Most audio transformers do not list their saturation currents as they are for specific audio applications. What should I be looking for when selecting a transformer? Specific example would be much appreciated.

The secondary challenge here is the power supply regulation, but I think I can identify a choke to help with the various frequencies. The 28 VDC voltage level doex not need to be exact, so I'm thinking a battery setup may work here, too.
 

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Ian0

Joined Aug 7, 2020
10,041
The only audio transformers you are likely to find that will withstand a standing DC current would be telephone transformers. They are unlikely to manage a 20Hz to 20kHz bandwidth.
 

Thread Starter

Gipper

Joined May 20, 2021
18
How about this audio transformer? Hammond 1650TA.

https://www.digikey.com/en/products/detail/hammond-manufacturing/1650TA/2358093

  • 403 mA max DC current per side
  • 1900 ohm primary impedance with center tap
  • 4, 8, and 16 ohm impedance on secondary
  • 30 Hz to 30 kHz range (maybe I can push it a little lower to 20 Hz?)

If I drive the primary using the center tap (Zp = 850 Ohm) and the secondary does not use any center taps (Zs = 16 Ohm), that results in turn ratio of 7.29. I'd have to drive the primary up to around 30 Vpp, which seems high, but it is doable.

Can someone confirm if this is appropriate? Am I missing something?
 

MisterBill2

Joined Jan 23, 2018
18,996
The application is similar to modulation of an AM transmitter. So the power that you need to feed into the transformer will be, under perfect conditions, about half of your load power, 28 V x 0.35 amps. So a ten watt amplifier will be able to provide enough power to develop four volts variation in the 28 volt line.
The resistance of that winding, which determine the wire size, should not drop the voltage more than one volt. So 3 ohms at about 0.3 amps will do.
So the cheap trick would be to get a mins- power dual winding filter rated for about ten amps, and use it as a transformer instead of as a series choke.. Then connect the other winding to the four ohm output of your ten watt amplifier.
 

Ian0

Joined Aug 7, 2020
10,041
The application is similar to modulation of an AM transmitter. So the power that you need to feed into the transformer will be, under perfect conditions, about half of your load power, 28 V x 0.35 amps. So a ten watt amplifier will be able to provide enough power to develop four volts variation in the 28 volt line.
The resistance of that winding, which determine the wire size, should not drop the voltage more than one volt. So 3 ohms at about 0.3 amps will do.
So the cheap trick would be to get a mins- power dual winding filter rated for about ten amps, and use it as a transformer instead of as a series choke.. Then connect the other winding to the four ohm output of your ten watt amplifier.
Are you referring to common-mode chokes? They have an ungapped ferrite core, so rely on a balancing low-frequency current in the other winding to prevent saturation.
 

MisterBill2

Joined Jan 23, 2018
18,996
Are you referring to common-mode chokes? They have an ungapped ferrite core, so rely on a balancing low-frequency current in the other winding to prevent saturation.
YES, that is the sort that I was describing. SOME of them make reasonable transformers. But they need to be the ones with quite a few turns. And they do need to be connected like transformers rather than chokes, for the alternate application.
It is absolutely an unintended application but it often works well. But just the suggestion does upset some folks terribly.
 

Danko

Joined Nov 22, 2017
1,843
I need to test a system by injecting a +/-4 Vpp signal onto a 28 VDC power line. The current draw for the system is 300 to 350 mA DC. The frequency range for testing is 20 Hz to 20 kHz (audio). The recommended test configuration is attached.
I have a function generator that can produce the AC frequency content up to 2.5 Vpp. This signal can be amplified further.
I am struggling to identify an appropriate transformer. Most audio transformers do not list their saturation currents as they are for specific audio applications. What should I be looking for when selecting a transformer? Specific example would be much appreciated.
You do not need transformer at all.
You can put 100 kHz and higher
and 10 Hz or 1 Hz if you want.
So here V IN = 2 V p-p,
V OUT = 4 V p-p:
1674244515024.png
АDDED: Power, dissipated by transistor Q1 is 950.4mW.
 

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MisterBill2

Joined Jan 23, 2018
18,996
Certainly a series circuit can be used. The benefit of using a transformer is that it allows the test hardware to be fully isolated from the power circuit, which can be a great convenience if there is a need for isolation. another benefit is greater efficiency.
 

Thread Starter

Gipper

Joined May 20, 2021
18
Thanks for the responses. The conversation is helpful.

In my haste to get this test completed, I ordered the 30Hz-30kHz transformer linked in my second post. I want to stay as close to the original high level testing diagram as possible as this test method and results will need to be verified by a third party, and that was the recommended configuration.

I will post my results in a week or two.
 

MisterBill2

Joined Jan 23, 2018
18,996
It will be interesting to see how that 30 to 30 kHZ transformer compares to a higher powered common mode choke. I see possibly some similarities, except the transformer will probably have different primary and secondary windings.
 

Thread Starter

Gipper

Joined May 20, 2021
18
I breadboarded the circuit and did the initial test today. It works great, for the most part. My implementation schematic and a couple scope shots are attached for reference.

The transformer works well down to 20 Hz and didn't require much more power to drive it lower, so that's good for my final test.

The op amp surprisingly takes quite a bit of power for all frequencies. It's 13-14W on the negative rail and 0.3-0.4W on the positive rail. Not sure why there is a significant difference. This adds up to a lot of excess heat. Had to add a heatsink and a small fan. Op amp temperature is about 60C according to my thermal camera.

I thought I may need a RC low pass filter out of the power supply to keep the supply from trying to control the ripple, but it didn't require any resistors. A 1000uF on the output is all that was needed.

I have one power supply providing +/-30V on the op amp/primary side, and I have another power supply providing the 28VDC on the secondary side.
 

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Ian0

Joined Aug 7, 2020
10,041
The op amp surprisingly takes quite a bit of power for all frequencies. It's 13-14W on the negative rail and 0.3-0.4W on the positive rail. Not sure why there is a significant difference. This adds up to a lot of excess heat. Had to add a heatsink and a small fan. Op amp temperature is about 60C according to my thermal camera.
Check the offset voltage. It can be as much as 10mV and you are amplifying it by 10. You could have 100mV DC offset on the output.
Try putting a capacitor in series with R4.
 

MisterBill2

Joined Jan 23, 2018
18,996
The reason for the unbalanced amplifier current is that the modulation source has to supply all of the extra power to the load as the load voltage is increased. Probably the load current increases quite a bit as the source voltage rises above the specified DC level.
That was the case with AM transmitters used by radio amateurs back when they used AM, and it is undoubtedly still the situation with AM broadcast transmitters still, although I doubt that they use modulation transformers today.
But back in the day of AM, an amateur transmitter putting out 500 watts of RF carrier would need a modulation amplifier able to deliver 250 watts, to have 100% modulation, at least on the positive peaks.
 

Thread Starter

Gipper

Joined May 20, 2021
18
Try putting a capacitor in series with R4.
I added a 0.1u in series with R4 to the low side. Power was reduced to 3W on the negative rail, but the gain was also reduced. It went from +/-4V to about +/-1V with the same potentiometer setting. I did not try increasing the gain.
 

MisterBill2

Joined Jan 23, 2018
18,996
Check the offset voltage. It can be as much as 10mV and you are amplifying it by 10. You could have 100mV DC offset on the output.
Try putting a capacitor in series with R4.
That must be quite an op-amp to deliver that much power. I was thinking about using an audio amplifier rated at least 20 watts.
 

MisterBill2

Joined Jan 23, 2018
18,996
I recall that TI also makes AUDIO power amplifiers able to deliver 5 or 100 watts. Given that the load is transformer isolated there should be some way to utilize one of them. But it is a bit late now. Does the setup satisfy the requirements for the test??That certainly is the bottom line here.
 

Thread Starter

Gipper

Joined May 20, 2021
18
I recall that TI also makes AUDIO power amplifiers able to deliver 5 or 100 watts. Given that the load is transformer isolated there should be some way to utilize one of them. But it is a bit late now. Does the setup satisfy the requirements for the test??That certainly is the bottom line here.
Yes, this setup will work for what I need. I'll have to keep an eye on the power and temps during the test, though.

Thanks for the input and conversation.
 
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