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.
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.
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.
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.
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.
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?
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?
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.
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.
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..