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78L05 placed in reverse?

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RoGeorge:
I've thrown away some dead batteries yesterday, and found in the electric waste dumpster bin a defective ambient light lamp with movement sensor, the ones you plug into a wall power outlet, and the lamp automatically turns on a few LEDs when somebody walk through the room, and only when it's dark in the room.  Took it home for parts, in the hope that the PIR sensor will still be good to play with in some other project.

The surprise was when I opened the lamp.  The design is faulty, and apparently the lamp shouldn't work at all, in the first place.  I don't get it.

Inside there is a 78L05 in reverse, fed with voltage at its output pin.  It is not an assembly error, the footprint layer drawn on the PCB is also wrong, and the 78L05 was soldered according to the footprint draw.

I desoldered the 78L05 and placed it correctly.  It was not dead, just that in reverse it outputs some 3.4V instead of 5V (tested this outside the board, with a power source limited at 10 mA).  Now the +5V is OK, but the lamp still didn't work.

While checking the rest of the parts, the schematic (raised from the PCB) looks like nonsense.  I'll double check this afternoon, but still, the 78L05 was in reverse for sure, no doubt about the 220Vac to 5Vdc part of the schematic.

My question is, how did this schematic worked, in the first place?

Is this some 78L05 trick I didn't know, or is it just a wrong design?  Maybe a fault clone of some other working lamp?  Didn't they noticed in production that none of the lamps were working?  Even if they didn't test it, how did they manage to sell something that never worked?

 :-//

T3sl4co1l:
Drops a few volts, AFAIK.

It's zener regulated in the first place and the opamp is probably good for 3-30V (LM358??), it doesn't really care.

More surprising actually that they bothered with a 78xx at all.

Tim

MagicSmoker:
I can't imagine anything useful from connecting a 78xx regulator backwards. In fact, the datasheets recommend an inverse bypass diode from output to input if there is significant capacitance hanging off the output terminal to protect against reverse biasing the emitter-collector junction of the internal pass transistor.

But for more fun 78xx regulator stuff check out this classic web page: http://sm0vpo.altervista.org/info/regulator.htm

magic:
10V zener across the transformer and then a series resistor and capacitor? Clearly some garage entrepreneur designed that thing ;D

If you look at the schematic on that site above you will see at least two obvious paths from the output to the input:
1. through forward biased BC junctions of the current limiter transistor and the output pre-driver, plus that series resistor before the former.
2. through avalanched EB junction of the output driver.

For very low currents 1 is used an nothing bad happens at all, at higher currents 2 happens and damage is possible if the transistor overheats. Perhaps some loss of performance too if it doesn't melt outright, I don't remember what the effects of EB breakdown were supposed to be.

duak:
I remember being caught up in the 70's with the same basic regulator (78xx or 79xx) pinouts notated differently between the manufacturers (Fairchild, National and Motorola).  I can't remember if it was an error or something to do with corporate naming conventions or if it was related to the packaging.  I don't think I had got to the PCB stage yet so I was able to sort it out in the prototype.

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