too hot. lm7824 burning up

Powering up a keyboard mixer based on the hamptone pre.

I've built a +24v psu - real basic, done this many times before, but this one is burning up lm7824s.
http://home.maine.rr.com/randylinscott/nov98.htm
I have a 25VCT transformer -CT left disconnected but once i pass through the bridge and ripple filter im up around 41 volts dc. (without regulator hooked up)
problem is the lm7824 has max input rating of 40v.

I tried putting a resistor inline on the + leg before the first filter cap but
that seems to raise the voltage:?

how do I get the voltage to drop.
It being saturday and me not wanting to wait till monday OR burn up my last regulator on hand.
Thanks

... A bigger resistor? Or better yet, a resistor and a shunt load (Better yet, a decent Zener) before the regulator, to make sure that the voltage doesn't rise too much off-load. (something that a series resistor alone in the incoming line cannot do...)

keith

Thanks SSLtech

Shunt Load? Is this a resistor between positive and 0v leg of the circuit?
I'm going to try that right now.

I'd like to drop down to around 30v from 40v...
I looked up some formulas for voltage drop, but I keep getting negative results on my calculator... had I known at the time I'd be needing this, I never would have slept through algebra class.

Kelly

That did the trick...a 4.7k dropped me down to 36v.
Much thanks again.
:grin:

about large series dropping R's... doesn't that worsen the transient response of the V.reg? when sitting btwn the reservoir cap and the reg. - when sitting btwn the rectifier bridge and the reservoir cap, in turn, it should worsen the filtering of the cap, resulting in 50/60 hz hum...?
but maybe you still can do that if you don't need too much current....?

about shunt zeners - i myself have this large 28v xformer (resulting in ca.39v dc after rectifying), and want the voltage to drop significantly before my lm317, or else it'll have to do all the regulating down to 20v dc by itself - which would make it pretty hot, even with a heatsink :wink:
but if i used a high-watt series R of higher than 5 or 10 ohms, i figure it would worsen transient response.
now if i used a shunt zener with those 700mA flowing, probably that would burn any zener, right...?!? :green: