[sdiy] When to use voltage regulators and when to use resistors?

[KA4HJH]

>Probably a pretty silly question but I'm wondering when to consider voltage
>regulators versus knocking down voltage with resistors.  In my particular
>case, I'm adding some modules to my rack mounted SEMs.  The SEMs run off of
>18.5v but the additional circuitry I want to add expects 15v.  Should I put
>a 15v regulator on or can I just pad down the 18.5 volts coming from the
>power supply with appropriate resistor values?  I have plenty of spare
>amperage in the supply to handle the additional modules, I'm just not so
>sure about getting the voltage down to the 15v. Thanks!

The SEM's have on-board regulation which drops the +/-18.5 to +/-15V
(there's one spot that actually uses +18.5V, IIRC). The easiest thing to do
is duplicate this scheme. Resistors are a bad idea and TTR's are cheap.

The one snag is that you have to use a regulator that can operate with an
input only 2.5V above the output. A 7815 won't work. Tom used a 723 and a
discrete transistor along with all the extra circuitry. Are there low drop
15V TTR's these days? 5V ones seem to be common enough.

How much current do you need? You always swipe a little from the SEM. I get
the impression  you're doing something which will require more current.

You could always go back to your 18.5V supply and add a 15V section...

Any mistakes in this posting are someone else's fault.

-- 

Terry Bowman, KA4HJH
"The Mac Doctor"