Help - Transformer Voltage drop

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Godi

Godi

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Joined 2016
#1
Hello
I'm building an integrated amp using a Chinese TK2050 amp board class T, I will add a protection relay board (I don't like the ''POP'' when I switch ON or OFF), a tone stack, maybe a phono board and a relay switcher to select the input.

I have a 28VAC transformer and the project need 24VAC, how can I simply drop 4 VAC at the secondary of the transformer before entering the AC-DC power supply board????? I read about a voltage divider using two resistors but I don't even know if a voltage divider can work with AC voltage.

Thank you
 
#2
Godi said:
I have a 28VAC transformer and the project need 24VAC, how can I simply drop 4 VAC
at the secondary of the transformer before entering the AC-DC power supply board?
Click to expand...

You need something that doesn't waste much power and reduces the voltage by 15%.
If your line voltage is 120VAC, you could get a 120V:18V transformer with a secondary
rating equal to that of your present transformer's primary (about 3A?), or more.
Then use the secondary of the 120/18 transformer to buck the primary voltage
of your present transformer (connect it in series), lowering the 28V output by 15% to 24V.
 
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#4
Godi said:
I'm looking for a solution that will cost ''one dollar'', otherwise I could buy a 24VAC
without any modifications !
Click to expand...

Bear in mind that the 120:18 buck transformer would only have to be rated at around 1/4
of the wattage of the main transformer, so it would be smaller and cheaper.

There is no solution for $1, but maybe if you turn on your air conditioner, toaster,
and all the lights, your line voltage will drop by 15%.
 
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#5
A resistive voltage divider would waste 1/7 of the transformer output at all powers. If the stereo amplifier alone is assumed to have a real average efficiency over the full power range of say 88%, its current demand at full rated output will probably be around 3.5A. You would need at least 10W power resistors to reduce the supply to 24V for that use and these will become very hot as the amplifiers approach their full power rating with steady sinewave power output. On music at socially acceptable volume though, I don't think the heat will be a huge problem - just an inconveniently hot use of power that will take up a bit of space for cooling and airflow, but overall, no worse than class AB.

In the class D spirit and using due care and safety with these types of products, why not just fit a standard 24V DC SMPS module? You may even find that the start-up pop is reduced too. This is just an example:
AC110-240V To DC Voltage Transformer 24V 5A Switch Power Supply 120W Power Suppy | eBay
 
#11
You realise that that "amp" is charitably describable as Junk!
Tripath chip ..really? Tripath has been extinct for over a decade.
Pop, on turn / turn off is a clear.. do not use... warning.
Utter waste of your 20$ IMO. Really want to throw more $ at the mistake
But hey! one cannot cheat an honest man 😀
 
#12
That not the question, I want to know how to drop 4 VAC from a transformer.
Someone nice e-mail me and said :

''voltage divider works in AC as well. Choose R1:R2 as 1:6 so that R1 drops 4 V and R2 drops 24 V.''



Thanks and Regards,
Pradeep
 
#14
Godi said:
the project need 24VAC
Click to expand...

That board requires 12-30V DC. Look at the board. You will require a rectifier and smoothing capacitor to obtain a DC supply. When you rectify 24 volts AC, you get around 33 volts DC. You know all this, correct?

Your resistor network won't work the way you think it will. The amplifier is a variable load and the voltage will fluctuate greatly. Have you thought about what value resistors you will use? Tell us how you're going to figure that out and provide sample values.

The logical answer is to get a laptop SMPS supply. This piece of junk deserves no better. You can find one in a dumpster or at your local second hand store. Try to find one 24-28 volts with at least 2 amps. Don't expect much.
 
#15
The highest AC voltage you can safely put in is around 20Vac.
That will give a nominal 28Vdc but this will rise with mains voltage and will rise even more when the loading is low.
They specify a limit of 30Vdc. You have to assume this is the worst case highest that is acceptable and stay within whatever warranty they give you.
 
#16
Thank you Andrew T., my Power Supply : LT1083 Adjustable Regulated Power Supply Module Parts and Components DIY Kit
have an adjustable pot so I can adjust Output Voltage from 2.5VDC to 35VDC, however the input VAC needed to be under 27VAC so with 24VAC I'm safe. I will try the Voltage divider using two resistors to drop the Voltage a little from my 28VAC transformer.

I know guy that the TK2050 class T amp will not beat my Sansui AU-519 but I have the amp and the transfo on hands so I just want to have the fun of putting together this amp.
 
#17
Godi said:
Hi, I agree with you, but I'm looking for a solution that will cost ''one dollar'', otherwise I could buy a 24VAC without any modifications !
Thank you.
Click to expand...

What kind of transformer? Has it been dipped in varnish? If it is a single voltage transformer, unwind a few loops and measure it until you get the desired voltage.

Alternatively if there is space then wind a few winding on in the opposite direction, which would "subtract" from the voltage output.

That would cost you about a dollar's worth of sweat.
 
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#20
Normally you add turns to get more voltage.

To decrease voltage you preferably remove turns.

When you can't do that (buried winding or varnish-soaked), you "can" put on more turns over the total winding but connect them "bucking", in reverse.

Note that most smallish iron needs about 10 turns per Volt. So more than a few Volts of adjustment becomes impractical.

Transformer performance (efficiency and regulation) is decreased.
 
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