Voltage Divider

Hey guys Im designing a 500 series pre-amp based on an old schematic that requires a +/-15Vdc power supply. 500 series racks provide +/-16Vdc can I simply us a voltage divider like
Input +/-16Vdc =>R1 250ohms, R2 3.75kohms= +/-15Vdc
or is this a bad idea?

Crude but you can put two diodes in series with each rail to drop the V about 1V.

JR

Monksound said:

Hey guys Im designing a 500 series pre-amp based on an old schematic that requires a +/-15Vdc power supply. 500 series racks provide +/-16Vdc can I simply us a voltage divider like
Input +/-16Vdc =>R1 250ohms, R2 3.75kohms= +/-15Vdc

Click to expand...

No. Use ohms law:

V = I * R

So the voltage varies with current. Meaning you won't get +-15V.

But it doesn't matter anyway. You can almost certainly use +-16V no problem. Just plug it in and see. If it doesn't burst into flames, you're good!

Need more details.  The amount of current drawn by this "mystery circuit" will impact the voltage dropped through the divider, and that current draw may vary depending upon many factors, such as audio levels varying from nothing to full-tilt.

Many circuits won't care if the voltage is a bit on the high side.  Adding a diode (like a 1N400x) in series with each rail will drop the voltage by approx. 0.6 VDC on each rail.

Bri

edit...lol....looks like two similar suggestions from John and Squarewave as I was typing!

The rack guts are normally +/-15V. The rack bus supplies +/-16V SO THAT you can add an R-C filter on each card's power line so your cards don't cross-talk to each other.

You need "some" idea of your current demands to pick the resistors.

Almost certainly all the stuff on your cards is "36V process" so can stand +/-18V for a decades. +/-15V is safer long-term. +/-15.7V is also fine.

If you can't reason-out why a voltage divider is a wasteful solution you should review basic electricity.

I don't want to scare new posters with too much information about possible risks.... but this is the WWW and posters (including me) routinely share everything they can think of vaguely on topic, or not.  :

As others have shared most modern ICs are fabbed on 36V process technology, so any supply rails less than +/- 18V are unlikely to release their magic smoke.

That said I can think of one possible case where rail voltage matters. Some DOAs (discrete op amps) can experience excessive internal bias current and heat dissipation from higher than designed for rail voltages. 16V is not that much more than 15V to obviously be risky but just be aware it "could" matter.

If using conventional ICs you are probably cool (literally), if using discrete circuitry maybe check for unexpected heating.

JR 

Thanks for the replies everybody. You are all helping me and i'm learning a lot. Group DIY is a vast resource of knowledge!