12 volts to 7.5 volts

[sammy004]

Hi Friends I have an old transformer plugin type I think it was from my speakers or something and I just wanted to know if there is any way I can bring the voltage down on that without too much work. Any help will be greatly appreciated.

Thank You
Sam

 

[Preher TV]

I am asssuming we are speaking D.C. here, but you could build a simple zener diode voltage regulator, you will need to also take into account the max current draw of the load that will be connected to the power supply in order to calculate the proper size of your zener diode and resistor.

This link will explain all you need to know.

 

[sammy004]

Hi thanks for the reply, yes this is DC. What I wanna do is I have a 12volt DC transformer laying around and it plugs right into a little piano my daughter has but the piano needs 7.5 volts and I don't want to hook that up because I will blow that little piano. I don't knkow how much that piano draws.

 

[Preher TV]

it should say somewhere on the unit the current that it draws or atleast it's power consumption. If not a model number so we can find out.

 

[sammy004]

OK here we go I found it. It says:

Casio SA-45
use AC adapter only Casio AD-1
Rated DC 7.5 volts 400mA or 5 (SUM-3, R6P, "AA" Size or Equiv. Batteries

I hope that info will work.

 

[colin55]

Measure the 12v supply.

It will possibly be 16v.

Get a 9v1 1watt zener diode and put it in the positive line so that the band on the zener diode connects to the positive wire from the power supply.
Put a resistor (100R) between the other end of the zener diode and the negative lead and measure the voltage across the resistor. It should be about 7v.

 

[audioguru]

The resistor and zener diode will supply only 69mA or less at 9.1V. But the electronic piano needs up to 400mA. Use an LM317 voltage regulator IC instead.

 

[Preher TV]

for a regulator like in the link I posted could use a 7.5V 5W(like an 1N5343) zener diode and an 11 ohm 2W resistor.


Or as audio Guru stated you could use a LM317 adjustable voltage regulator, which would offer better regulation.

 

[colin55]

I have the piano. It takes between 40 - 100mA. It just has five AA cells. You just need a 1 watt zener to drop the voltage from the plug pack.

 

[sammy004]

So I just need 1 watt zener diode and a 100 ohm resistor and set it up like this
**broken link removed**
and it will work fine?
And is the resistor closest to the wall plug or the piano plug?

 

[colin55]

That arrangment will give you between 40 - 125mA. Use 100R 1 watt. Put 470u across the output.

 

[sammy004]

That arrangment will give you between 40 - 125mA. Use 100R 1 watt. Put 470u across the output.

I'm still a little hazy on understanding a bit, 100R color code is brown, black, brown right? but what is the 470 ohms for is that the size of the zener diode?

 

[colin55]

The 470u is a 470 microfarad electrolytic. This will reduce some of the hum that might come from the power suplly.

It has 470/16 on the side of the body or 470/25v and has a black stripe down one side to indicate the negative lead.

 

[Mr RB]

It will be much more efficient (and simpler) to just use a zener diode in SERIES between the 12v power and the 7.5v output.

 

[colin55]

It will be much more efficient (and simpler) to just use a zener diode in SERIES between the 12v power and the 7.5v output.

That's what I suggested in the first place but since these small plug packs have such poor regulation, the voltage can rise as much as 3-4v between no-load and full-load and this may produce excessive voltage for the piano.

 

[sammy004]

The 470u is a 470 microfarad electrolytic. This will reduce some of the hum that might come from the power suplly.

It has 470/16 on the side of the body or 470/25v and has a black stripe down one side to indicate the negative lead.

Oh I see it's a capacitor!

That's what I suggested in the first place but since these small plug packs have such poor regulation, the voltage can rise as much as 3-4v between no-load and full-load and this may produce excessive voltage for the piano.

I don't want that?

Is it possible you can kidda draw it out for me because I'm kidda stupid when it comes to explaining stuff.

 

[Mr RB]

If it was my toy piano I would open it up and see how the 7.5v is used, ie if it goes to a regulator or just to the amp chip (LM386?) or whatever.

That might lead you to the best solution... At least you will know what you are dealing with which has to make the job easier.

 

[colin55]

It just goes to an amplifier chip in my sample. I cannot read the number as it is of Chinese origin but I think it would take up to 15v.

 

[Mr RB]

Those cheap music toy devices for 7v or 9v power almost always use the LM386, they can buy them for a few cents and they are so easy to source in China so there's little point using any other amp chip. They're fine on 9v or 12v.

He might have to watch if there's a micro they often use a resistor and zener to make the 5v for the micro, it might need changing the resistor value so the zener won't cook..

 

[sammy004]

Oh man you guys are confusing me, haha