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Need Help on the Function of a Diode in a Pedal

ked

Member
Messages
911
Hello guys. Need some help, does the D5 diode act as a protection against reverse voltage in this layout? The link to the layout is below. I have a pedal that has quit working and I'm trying to figure out if this is the problem. If it blew, would the pedal stop working? Testing the diode in circuit, I get the same reading both ways. I know a diode should pass voltage only one way but the in circuit test may be causing my reading to be useless.

http://www.generalguitargadgets.com/pdf/ggg_its8_lo.pdf

Thanks,
 

Blue Strat

Member
Messages
30,192
Now it's easy! It protects the circuit from someone momentarily connecting the battery backwards.
 

ked

Member
Messages
911
Thanks Mike. If the diode blows will the pedal stop working? Is voltage being passed to ground when the diode blows?
 

jay42

Member
Messages
7,054
sounds like you should remove the diode and check the op amp...hopefully it's socketed. Blowing that diode should be difficult with a battery, but a power supply might kill it. The pcb probably won't take a 1N400x, which is ime, a bit more typical. Be careful. I've only built a couple GGG boards and I can't remember how robust the traces are.

Does your meter have a diode setting? Also, reading components in a circuit is often impossible because of the way the meter works...tends to charge capacitors.
 
Last edited:

ked

Member
Messages
911
Thanks jay42. My DMM was giving me readings with the probes placed at either position (negative to cathode and positive to power in or vice versa) so I went ahead and lifted one leg of the diode and the pedal began to work. It's going to be fun getting my big mitts in that tiny tight space to solder in a new diode. One other thing, some how it managed to burn out the On-Off LED indicator in the process.

I built the pedal and gave it to my future son-in-law. All I can figure is he tried to use the wrong power supply and blew out the protection diode.
 
Last edited:

J M Fahey

Member
Messages
2,665
In these cases, very low consumption pedals with just 1 or 2 Op amps or a double or a couple transistors (many Classic ones fit that description) I add a 10 ohms 1/8W resistor in series with +9v (between the battery or connector and the +9v marked pad in the actual effect board).
Almost "invisible" in normal operation (drops a few mV) but both limits current and blows in seconds .

Not my idea, I always saw them in every +and - Op Amp power pin in large mixers, which mightb have 40 to 200 op amps.

The reason was triple:
* protects the supply by limiting current even in case of gross short (15V/10r=1.5A max.)
* blows like a fuse while being so cheap that you could fit them everywhere.
* disconnects the bad chip from main supply, in principle others keep operating
* is very easy to find, then toasted resistor sticks out like a sore thumb.

That said, another popular value was 100r per device, but in this case with a 10uF decoupling cap.
 

ked

Member
Messages
911
In these cases, very low consumption pedals with just 1 or 2 Op amps or a double or a couple transistors (many Classic ones fit that description) I add a 10 ohms 1/8W resistor in series with +9v (between the battery or connector and the +9v marked pad in the actual effect board).
Almost "invisible" in normal operation (drops a few mV) but both limits current and blows in seconds .

Not my idea, I always saw them in every +and - Op Amp power pin in large mixers, which mightb have 40 to 200 op amps.

The reason was triple:
* protects the supply by limiting current even in case of gross short (15V/10r=1.5A max.)
* blows like a fuse while being so cheap that you could fit them everywhere.
* disconnects the bad chip from main supply, in principle others keep operating
* is very easy to find, then toasted resistor sticks out like a sore thumb.

That said, another popular value was 100r per device, but in this case with a 10uF decoupling cap.
Thanks JM, that's a good idea. I'll do that next time I build a pedal. Luckily, after I replaced the protection diode and the LED, everything is working like it should.
 

oldhousescott

Gold Supporting Member
Messages
2,811
I like designs that use a 1N5818 in series with the 9v. It only drops a half volt from the supply, and nothing blows up if the power is hooked up reversed.
 




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