Output of 78L05 gives about 8 V - mystery solved

[todd.dukes]

I was having problems with my MAX9722 circuit and I noticed PVdd was 8 V.

I simplified the 9V -> 5V circuit to the above and am still seeing 8V. They are marked WS 78L05 301SB

I've tried two of them with similar results. Has anyone ever seen this behavior?

Thanks!

[davent]

Pin 3 is input, 1 is output.


dave

[ElectricDruid]

+1 agree. Regulator back to front is my guess. I've seen more volts out than I expected, and that was the problem. It turns out they don't regulate that well if not inserted correctly. Who have thunk it?!

It's my classic error. Somehow however many times I check I still screw this up. Luckily it doesn't seem to kill the parts. De-solder, flip them round, re-solder and it works. Hopefully you'll have similar good fortune.

Tom

[todd.dukes]

Thanks for the great suggestion. Unfortunately I already oriented it the way you suggested (wrong according to schematic).
I did try several of them both ways and got the same result. 9V in 8V out.

I don't know if I have used any of these from this package before. They are postmarked 2/2018 from china. I paid $0.98 for 20 pieces. I need a 7805 anyway instead of the 78L05. I am going to assume these are all bad and get some 7805s to use.

[MrStab]

Hi Todd,

after you reverse or replace the regulator, i highly recommend you add these components (in case you haven't already).



the capacitors will stabilise the voltage, and the diode will protect against failure if the output voltage ever becomes higher than the input voltage.
it's more typical for C1 to be 100uF and C2 to be 10uF, IIRC. ignore the red text (i think it's red? lol).

[Gus]

Are you testing the voltage without any load?
If so change the 1K to 470 ohms sometimes you need more of a load(min current) to get the regulator to regulate.

[MrStab]

IME the reading from a regulated supply should be the advertised output, give or take a few mV. i'm one for false memories, though.

[anotherjim]

It isn't wrong on the schematic for the 7805.
The little TO92 ones are actually the same way around and the flat side goes down against a heatsink in either case. However, since its easier to print the part number on a flat surface, the TO92 is marked on the back as you read it. To put it another way, the curved surface with the ejection pip in the middle is the front face of part.

Those regulators don't need loading in my experience, although you can always expect a tiny few mV's drop from that when loaded.

[todd.dukes]

I had the common pin tied to +9v. This is currently just bread boarded and the wire was bent to look like it was in ground, but when I was taking apart in frustration I noticed the pin came out of the +9V bus.

Hooking it up with common tied to the ground bus caused the behavior I expected, +5v at the output. Even with no load. Really noisy without a cap on the output to ground, but still obviously not 8V.

To be sure,  I hooked it back up with both input and common tied to +9V and I got +8V at the output.