Almost ready to power up

[Grant Stockly]

I wonder if its possible the 7805 was damaged when the power was applied with the bypass resistor and no load?

Can you desolder the large resistor and check the 7805? You should be able to put a 1A load on it without too much of a voltage change.

[Grant Stockly]

I see you are experimenting with VTL!

THAT is amazing code. 768 bytes!

[Geoff Harrison]

Originally Posted by Grant Stockly

I wonder if its possible the 7805 was damaged when the power was applied with the bypass resistor and no load?

That's an interesting thought, I've read 7805s don't like to see high voltage on their output side.

I unsoldered the resistor and the regulator is putting out a solid 5v now, but I don't know whether its load handling capacity has been damaged in any way. I may swap it out just to be safe.

MITS used that bypass resistor across the regulator on their 4k board too. I don't know much about circuit design, but doesn't that kind of negate the purpose of a regulator? I assume that when the circuit is loaded normally all the current flows through the regulator, and the resistor comes into play only if a heavy load is added (say, a couple of 16k memory boards), but then what ensures that the voltage stays constant?

I guess when you're testing an unloaded board with this kind of power regulation, it's probably a good idea to put some kind of load on it.

Originally Posted by Grant Stockly

I see you are experimenting with VTL! THAT is amazing code. 768 bytes!

And it runs happily with just 1k of RAM!

I'll put my 16k board in later and try to download basic. It should work, it runs fine on the vintage machine.

Geoff.

[rfeecs]

Since the 7805 can only sink a few mA, it will not be able to regulate without a load because of the 15 ohm resistor. Probably the resistor is added in an attempt to boost the maximum current capability. With almost 12 volts input and 5V output, the load has to sink roughly 450mA before the regulator can function.

As you found, if the load current drops below 450mA or so, the output voltage goes up. Pretty scary regulator design, no doubt to save a dollar or two.

Good thing the 680 doesn't have a low current sleep mode.