Folks thanks a lot for the ideas. I've been doing a little more digging
around on the PCB of the old amp and it has a LM337 and a LM 317 to provide
a lower voltage (-12..+12) for the input stages of the amp. These regulators
are bolted to a huge heatsink that forms part of the case. Since I was
planning on using the case of the amp for this one off project anyway it
looks like that with change of the ref. voltage resistor I can get +5 from
this. Failing that I think I'll just invest in a wall wart! It's amazing how
much of this old amp I think I can re-use. Both parts of the power supply
(including smoothing caps), the case/heatsinks and I think I can make use of
most of the input sockets to provide outputs to the steppers and inputs from
limit switches! Oh and I get a nice safe mains switch and power setup thrown
in as well!

I've also learned a lot about switching supplies and the other non linear
regulators... I'm sure that will come in useful in the future. Once again
this list provides just the right amount of help....

Thanks


Andy

----- Original Message -----
From: "Russell McMahon" <apptech@PARADISE.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, May 08, 2003 1:18 PM
Subject: Re: [EE]: 5V from 40V


> >you do not want a linear reg. - it will dissipate nearly 9 watts at 250mA
> >A switcher will cost less than the heatsink!
>
> Better still, try to use a more sensible input voltage. From what you say
it
> appears the supply isbeing used "because its there". Dissipating most of
the
> voltage or using a switcher are both introducing complications you don't
> need. Suitable transformers or plug packs (wall warts?) cost very little
> compared to the value of the system to you and other costs.
>
> If you MUST use a linear supply, place a series resistor from 40v to the
> regulator input. Design this to drop most of the voltage drop needed at
full
> power. Rmax = (Vin-Vregulator_in-Vsafety)/Imax.
> eg Vin = 40, Isafety = 3v, Vreg in = 7volt say.
>     R = (40-7-3)/0.25 = 120r.
> This will drop V=IR = 30v at max current and dissipate P = I^2R = 7.5
watts
> at full current.
> Use AT LEAST a 10 watt resistor.
>
> There areany comercial IC switching regul;ators that will meet your
> requirement, but introducing a switcher unnecessarily is seldom a good
idea.
>
>
>
>         Russell McMahon
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics