[Elecraft] Power Supply

[Jack Brindle]

> I finished my K2 a couple of weeks ago and it is working great, and I 
> love it!  Now, I just received the KPA100 yesterday and have started 
> construction on it.  My question is that I have stumbled upon an old 
> GE power supply rated at 35 amps, and it is really built hefty.  
> However, it is unregulated and puts out 15.5 VDC with no load.  I 
> would not expect the voltage to drop much when loaded with my K2/100, 
> but I know the manual says the operating voltage should be between 9 
> and 15 volts.  Will the 15.5 volts be too much, and would the 
> unregulated output present any problems?  I would love to use this 
> thing because it is filtered so well and looks to be built like a 
> tank.  Note:  It came out of a commercial two-way radio repeater 
> station.

I don't think I can give you an absolute answer here, but maybe an 
explanation might help. The K2 and its peripherals use small 78LXX 
regulators which come in TO-92 packages for voltage regulation. These 
are linear regulators which must dissipate, in heat, the difference 
between what comes in and what goes out. For example, suppose the 
regulator is a 5-volt unit and is supplying 50 mA to its circuit. the 
circuit itself is using 250 mW of power. if the input voltage to the 
regulator is 10 volts, the regulator itself is dissipating another 250 
mW. At 15 volts input, it goes to 500 mW (half a watt). So you see that 
the higher the voltage, the more power you waste. The TO-92 package is 
rated at about 600 mW of dissipation at 30 degrees C, so you are 
pushing the package against its limits when you go above the 15 volt 
level. If the current drain is above 50 mA (quite likely), then you are 
already outside the limits of the package, and are risking device 
failure. As you can see, it is the heat that is the limiting factor in 
this (and most other) semiconductor design.

In general, 78LXX regulators have a maximum input rating well above 15 
volts, although some of the devices we use have a limit of 16 volts. 
Your power supply might work quite well for you, but it could also 
cause some problems when voltage spikes hit your power lines, 
especially if they induce an output spike above the 16 volt Max rating 
of these regulators. If the supply is well filtered, you could probably 
get away with using it with the K2 with little, if any problems. But 
remember, it _is_ out of the recommended range (even if barely)...

-Jack Brindle, WA4FIB
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