[sdiy] Power distribution board

[Scott Nordlund]

Linear regulators work by dissipating "extra" power as heat.  If you're regulating 15 volts down to 5 volts and supplying 100 mA, you've got 1.5 W coming in (15 V * 100 mA) and 0.5 W going out (5 V * 100 mA), and the remaining 1 W is heat.  That's a pretty considerable amount of heat, even without delivering much power.  If you've got a lower voltage coming in (keeping in mind that the 78xx regulators have a dropout voltage of about 2 V, so the incoming voltage should be at least 7 V), you can reduce the dissipated power by a lot.

A voltage divider won't work either as you're only supplying the correct voltage when there's no load.  You could use high power/low resistance resistors, but then you're shunting a lot of power straight to ground.  Trying this will only make more heat than the 7805 without providing any regulation or dependable output voltage.  You could use a low-current voltage divider (or zener diode) to drive a power transistor, but this is basically (as I understand it) a less precise version of what the 7805 does anyway- it won't solve the heat issue.

The efficient way to do it would be a switching regulator.  These waste very little power, but then you may have to be very careful with the design to avoid RF interference.

If you want to reduce power consumption, use CMOS chips.  The plain 74xx chips consume a lot of power.  74LS or 74ALS are better, but I believe the the HC/HCT/AC/ACT series (and also 40xx series) are most efficient.

----------------------------------------
> Date: Sat, 4 Apr 2009 10:30:00 -0700
> From: dixon at interchange.ubc.ca
> Subject: RE: [sdiy] Power distribution board
> To: rtellason at verizon.net; synth-diy at dropmix.xs4all.nl
> CC:
>
> This leads to my next question: Presuming that most have only +/-15V
> supplies in their modulars, what is the best way to handle digital power?
> I've only breadboarded circuits with TTL chips powered by a 7805 reducing
> +15V to +5, and was startled (and a little frightened) at how hot the
> regulator ran. I'm not sure I'd want anything that hot on any of my
> lovingly hand-crafted boards!
>
> I was thinking I would probably opt for just putting the +15V through a
> simple voltage divider to get +5V and be done with it, but I'm wondering: is
> this anathema? Does one find oneself excommunicated from the fold for such
> heresy?
>
>> -----Original Message-----
>> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
>> bounces at dropmix.xs4all.nl] On Behalf Of Roy J. Tellason
>> Sent: Saturday, April 04, 2009 10:14 AM
>> To: synth-diy at dropmix.xs4all.nl
>> Subject: Re: [sdiy] Power distribution board
>>
>> On Saturday 04 April 2009 05:50:29 am Julian wrote:
>>> Somthing ive wondered -
>>>
>>> Regulators are cheap - what about a power bus running over-voltage and
>> then
>>> a pair of dedicated regulators for each and every socket on that bus?
>>>
>>> Would that effectivly isolate every module?
>>>
>>> (im thinking back to my old doepfer lfos thatlld modulate the doepfer
>> vcos
>>> on the same bus...)
>>
>> That was the approach taken for the real early S-100 bus computer systems,
>> with the power buses being 8V, and +/-16V, which were typically regulated
>> down to 5V and 12V on each board. Some boards had to have multiple
>> regulators for the 5V, and there were heat dissipation issues. Later
>> systems put jumpers in place of the regulators and used switching power
>> supplies instead, though I've not much experience with them.
>>
>> But yeah, I'd say this would provide a lot of isolation.
>
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