psu volts > regulator question.

[harps]

hello electro techs.

i purchased a pre made circuit board yesterday that needs between 24v - 30v to operate. ( as it says in the instructions 24v>30v for single rail power and +/-12v->+/-15v for dual rail.

i have an old hewlett Packard (HP)PSU from a printer that passed away a while ago that outputs 40.6 volts and I have just found a voltage regulator on the net that can deliver 24volts.

the voltage regulator has a max input of 40v to operate but as i say my HP PSU ouputs 40.6 volts, so its a little high perhaps for real comfort? Could I simply place a resistor in series between the psu and regulator to bring this down a tiny bit so i've nothing to worry about when its up and running?

Voltage regulator,MC7824CT 24V 1A
circuit board power consumption 12ma ( i hope to power two cards in the future from this one supply)

thanks for help on this : )

 

[k7elp60]

I would connect a couple of 1N4001's in series with the input, that would drop input voltage by about 1.5V. A resistor voltage drop would vary greatly with the current drawn by the load. The diodes will drop a nominal 0.7V each but it could go as high as about 1.1V depending on the load current.

 

[MikeMl]

Use an LM317. It has an Input-Output max rating of 40V. Since your output will be 24V, that means the differential will be 40.6-24 = 17V, unlike the 7824, where the full input voltage appears between its Input pin and its GND pin.

 

[crutschow]

Use an LM317. It has an Input-Output max rating of 40V. Since your output will be 24V, that means the differential will be 40.6-24 = 17V, ...

As long as you don't accidentally short the output.