# 25v to lower voltage

Discussion in 'Electronic Basics' started by Guy Paddock, Jan 28, 2004.

Is there a quick and easy way to drop the voltage in a circuit down from 25
volts, to, say, 9v? I'm assuming a transformer, but I was wondering if there
was a resistor that might handle it, or something of the sort...

Also, if a transformer is required, where would I get one to match those
specs?

--Guy

2. ### JD SteffenGuest

Hi,

Might check into the LM7809 voltage regulator. I don't remeber what
the voltage limit is for it, so you'll have to check out the data sheet. Try
Motorola or Linear. This way you don't need a transformer. The
regulator will output a clean 9 volts.

JD

3. ### Robert C MonsenGuest

Are you interested in 9V A/C? Transformers don't work with DC. For A/C,
it'll be hard to find a transformer that will take 25VAC -> 9VAC.

If its DC, using a 7809, like M. Steffen suggested, would work:

25V .-----. 9V
---------|7809 |------+--------
| | |
| | ---
| | --- 0.1uF
'-----' |
| |
| |
| |
-------------+---------+-------- GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

However, the 7809 will consume 64% of the power through the circuit. Its
also limited to some small current, say 500mA.

There are more exotic chips used to drop DC voltage called 'switch mode
power supplies' that are more efficient, and thus will generate less heat
for larger power requirements. They are also more expensive.

Regards,
Bob Monsen

4. ### OlafGuest

is the 25V AC or DC, do you need 9V AC or DC, what current do you need at
9V? Without this information it will be quite impossible to give some

bye, Olaf

I need 9v to power an electromagnetic coil. The current 25v makes the coil
very powerful, but makes it heat up too much. I need it to run with less
heat.

--Guy