Subject: where does all the extra current go?
From: "anonymous"
[email protected]
Date: 10/11/2004 10:40 PM Central Daylight Time
Message-id: <pmIad.350483$mD.296451@attbi_s02>
WARNING - dumb question follows - WARNING
consider yourself warned.
So I have a nice transformer that puts out 25V and 1.5A. I want to step it
down to 9V with some voltage regulators. The regulators say they are rated
for 1A and 37V max (given proper heat sinking).
If the output of the regulator is drawing < 100mA - do I have to worry about
the remaining output from the transformer?
Not a dumb question -- just a newbie one.
First, I hope your transformer has a center tap on the secondary, because if it
doesn't and you're using a 9V linear regulator and are drawing 100mA, you're
talking about
(37V - 9V) * 0.1A = 2.8 Watts
Need a heat sink here. It might be somewhat easier to do it this way if your
25VAC secondary transformer has a center tap (view in fixed font or M$
Notepad):
25.2 VCT Sec ____
| | +
o-----. ,------>|--o---o---|7809|--o-----o
)|( | C1|+ |____| +|C2
)|( | --- | ---
) ,---. | --- | ---
)|( | | | | |
)|( | | | | |
o-----' '------>|--o---o-----o-----o-----o
| | -
| ===
=== GND
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
Use a 1000uF electrolytic for C1 and a 10uF electrolytic for C2.
This will give you about 18VDC peak on C1, which should make your 7809 a lot
more comfortable. With total power dissipation less than a watt, you should be
able to get away without a heat sink as long as the 7809 is in room temperature
ambient.
Oh, yes -- transformers are rated for maximum current with resistive load. By
definition, a transformer is usually rated so that the output voltage will be
within 20% of the rated voltage for the rated current. Usually, if you're
loading it lightly, the voltage will be higher than rated, and if you're
loading it toward its limit, the voltage will be lower. The transformer is an
on-demand machine -- it will produce the voltage as long as the current
requirement is within limits., There isn't any extra to take care of.
Good luck
Chris