questions on safety diodes for a Symetrix 522

[changeng]

HELP! i need some input so I don't hurt innocent humans...

with some further research, and PLEASE correct me if i'm wrong! the circuit board on the 522 is not easy to eyeball, and i don't have a meter to measure voltage - so looking at the schematic...
Where can I find Schematics for the 522 Compressor / Limiter / Expander ?

it appears that the "1" pin (or 'in" pin to the left on the regulator) goes to the 1000uF cap and the "3" pin goes to the 4.7uF. Although the schematic isn't too clear to me either, I'm thinking it's the same layout for all four regulators? (if i'm wrong, please tell me before I burn down Los Angeles?)

Since the UNregulated cap is the 1000uF cap and the REGulated cap is the 4.7uF cap,

For the POSITIVE regulators (the 78M's)I need to put the banded side of the diodes on pin "1"

For the NEGATIVE regulators, (the 79M's) I need to put the banded side of the diodes on pin "3"

and I am assuming the non-banded side of each diode is soldered to the non-earth pin on each regulator - so each regulator has it's own "closed-circuit" diode?

my head i turning to noodles.

Please help and many thanks!!!!

Stuart

[changeng]

did that make sense? Is it correct? i'm going to recap the 522 and was advised to add the safety diodes.

[LeeYoo]

Hi Stuart,
Never heard of a safety diode. There are only normal and zehner diodes used in regulator circuits. A 78XX regulator is to regulate a positive voltage into the voltage were the xx are, e.g. a 7812 is to make from any voltage above 15volt a stable 12 volts. These can handle 1Amps, while the letter M in the middle is the 0.5Amp version and an L in the middle is the 0.1Amp version. The small cap (4u7) on the output pin3 is to reduce noise and oscillation. The big cap on input pin1 is to smoothen humm from the AC transformer. If there is a chance of reverse polarity like supply with a plugpack, a diode is sometimes used here. The diode only conducts if you connect a supply with the wrong polarity. The band goes to pin1 and the other side to pin2 (ground). Sometimes a zehner is used here if there is a chance of too high voltage or spikes e.g. a 24volt zehner. Connect the same as the normal diode. A 79xx is just the same, but for negative voltages. The diode bands go the other way and so do the caps. Take care here. The 79xx has another pinout. 1=ground, 2=in, 3=out. Hope this helps..

[JohnRoberts]

These are protection (to protect chips, not human "safety") diodes often used with older 3 terminal regulators to prevent latch up. Some older regulators could latch if say the - supply comes up before the +. Putting the diode in series with the ground lead, prevents this reverse latch up, but now the regulation voltage is .6v higher, and regulation is slightly deteriorated as diode voltage varies with temp and current.

A second diode is often used, backwards across the regulator to prevent reverse current flow in the scenario where the unregulated supply drops faster than the regulated, and post regulator reservoir caps discharge back through the regulator. This is not that common but can occur if multiple regulators are driven from a common unregulated, with large post regulator caps.

Note: the polarities of protection diodes will be reverse for the positive and negative regulators. Since valid current flow in a diode is from anode to cathode, in the ground leg of positive regulator the diode cathode (banded end) goes to ground and anode end to regulator ground pin. Reverse flow clamp has anode connected to + regulator output and cathode to + unregulated, so no current flows under normal voltages.

Minus regulator diode polarities are reversed to accommodate reversed current flow.

JR

[changeng]

thanks, guys, but yikes. now I have three versions of where the ends of the diodes go. The circuit board itself makes it hard to trace which pins on the regulators are going to the 1000uF and which are going to the 4.7uF caps, so I'm surmising their placements from the schematic.

If I get this wrong, will I destroy the regulators or worse?

[JohnRoberts]

yup

[LeeYoo]

Very confusing for a non tech person.
Will try to make it easier.
For the 78xx, the diode goes between pin1 and 2, with the band on pin 1 (pin1=in, pin2=ground).
For the 79xx, the diode goes between pin1 and 2, with the band on pin1 as well, because pin1 of the 79xx is ground and pin 2 is in (different pinout).
A reversing diode from out to in is not needed because the regulators have one inbuild as far as I know. These regulators are very reliable. I had not working ones, but never a shorted one, so a crowbar type Zehner diode from output to earth is overkill.

[changeng]

Thanks, guys. I really appreciate the help!