decreasing voltage to regulators.

[mark krawczuk]

-- HI, i am in the process of building up a power supply using lm 7805
regs
etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k



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[Dr. Anton T. Squeegee]

-- HI, i am in the process of building up a power supply using lm 7805
regs
etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k

Indeed there is. Use a transformer with a lower output voltage.
All you really need is a 12V center-tapped secondary rated for two or so
amps.

73 de KC7GR

--
Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
(Known to some as Bruce Lane, ARS KC7GR,
kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
"If Salvador Dali had owned a computer, would it have been equipped
with surreal ports?"

 

[Byron A Jeff]

-
-
--- HI, i am in the process of building up a power supply using lm 7805
-regs
-etc.....
-but the voltage before its had a capacitor across it , staright of the
-rectifier is about 37 volts dc, which is too much for the reg, plus with the
-cap across it would increase it abit.
-
-is there a simple way to reduce the voltage to the regualtor ?

Almost all have some consquence related to heat dissapation
associated with them. That 22 volts at whatever current draw
you're pulling has to go somewhere.

This is one of those times where you really need a switching
regulator. Since almost all of the power goes to the load the
heat generated is greatly reduced.

A LM2574 (or 2594 or similar) operates a extremely high
efficiency up to 37V of input.

But a couple of high wattage resistors can make for an
effective voltage divider if necessary.

BAJ

 

[John Popelish]

-- HI, i am in the process of building up a power supply using lm 7805
regs
etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k

Sure there are ways, but why start with a difficulty? What is the
source of the 37 volts DC? It might be possible to rewire it to lower
its output.

 

[cpemma]

-- HI, i am in the process of building up a power supply using lm
7805 regs
etc.....
but the voltage before its had a capacitor across it , staright of
the rectifier is about 37 volts dc, which is too much for the reg,
plus with the cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k

Admittedly the easiest & cheapest solutions waste power, but if your maximum
current requirements are fairly modest, knock the voltage down with a
resistor + zener diode pre-regulator first. You can series 2 or 3 zeners to
increase the voltage, spread the heat. Leave at least 3V headroom.