Altair 680 power supply...

Chuck Guzis cclist at sydex.com
Sun Jan 17 17:01:00 CST 2010


On 17 Jan 2010 at 17:32, Bill Sudbrink wrote:

> Yes, there is an 18 Ohm 2 Watt resistor between the +9 and +5 (the
> input and output pins of the 7805).  I was wondering what that is
> supposed to do. Is this resistor causing the +5 side to "look higher"
> than it really is? Is everything actually OK, even though my VOM and
> scope both agree the +5 rail is at +9.2 _WITH_ automobile light dummy
> loads?

Back to what I said.  Your load isn't heavy enough.  At an 11 V high-
side supply, an 18 ohm resistor will produce 9V at a load current of 
100 mA, even if the 7805 is completely kaput.

Put a load that pulls about 2A at 5V  (a 10W 3 ohm resistor should 
do) and see what your readings are.

Better yet, ditch the 7805 and resistor and install an LM323 with 
suitable heatsink.  Or use one of the switching buck regulators.  
MITS was about shaving pennies from their designs.

---Chuck




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