General > General Technical Chat
78L05 placed in reverse?
Medved:
--- Quote from: magic on November 22, 2019, 04:04:12 pm ---10V zener across the transformer and then a series resistor and capacitor? Clearly some garage entrepreneur designed that thing ;D
--- End quote ---
That Zener is maybe the only thing here that is actually not wrong. It is not supplied from any transformer, but from (nearly ideal) current source formed by the input series capacitor.
Removing the Zener would mean the output voltage would rise up to virtually 320V, but in reality to a level where something blows up.
What is wrong with it?
Now assume the thing worked when it was new, regardless how crazy it is connected. With these things supplied via such capacitive dropper circuit there use to be two common problems:
1) The input ballasting capacitor (the few 100's nF/275VAC in series with the line input) loses capacitance. For safety reasons it must be of a "self recovery" type, but the "selfrecovery" actually means it by itself just clears the short circuit once its dielectric breaks down, it uses to do it by either evaporating part of the conductive layer around the damaged spot (mainly older "metallized paper" designs), or by splitting the conductive layer into many sections and each connected via a thin trace acting as a fuse, which then disconnects the damaged section. Either way, as the capacitors connected like that do suffer dielectric breakdown events (because of teh various mains spikes and generally long time exposure to rather high voltage for the dielectric material thickness), the consequence is the capacitor as whole gradually looses its capacitance. And that means the current feeding the circuit gradually decreases, till it goes below a level where the circuit is still able to work. Typical consequence is erratic operation (after it "turns ON" the first time, it does not want to turn OFF anymore) and very low LED brightness or when the thing uses relay to switch the higher power load, the relay gets insufficiently driven, starts to rattle and so usually fails very soon after.
2) Second problem uses to be dried out filter capacitor behind the rectifier. Part of the reason is the usual proximity of that Zener which dissipates quite some power so generates heat, part because of the permanent ripple current load. Consequence when the capacitor is not really good quality, it tends to fail (the rubber seal rots out, allowing the electrolyte to loose water). As a consequence, it becomes unable to maintain the supply ripple within acceptable limits and so the circuit fails. Usually into a "motion permanently detected"
Medved:
I think the design originally asked for the 78L05 to be the normal way, just for some error it got reversed. The error could come either from the thing to be originally designed with some other similar regulator with different pinout, which then was supposed to be replaced by the 78L05, but someone did not checked the correct orientation. Or the information about the need to assemble it "against the silkscreen" got lost on the manufacturing floor over time, as they were cheap to update the silkscreen on the PCB.
Brumby:
--- Quote from: SL4P on November 22, 2019, 10:12:07 pm ---Doesn’t everyone KNOW that a regulator installed backwards, becomes an UNREGULATOR. ;)
--- End quote ---
It was my impression that they became a DEregulator.
Apparently they have been utilised in some government applications - with very mixed results.
S. Petrukhin:
Most likely it was a 7805 made upside down. This was known at the production.
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