What is the purpose of this diode in Boss pedals?

[gutsofgold]

http://www.hobby-hour.com/electronics/s/schematics/boss-hm2-heavy-metal-schematic.png

D13 which sits at the cathode side of the LED? What is its purpose and what might be a proper sub for it?

[gutsofgold]

I should note that I put a 5mm LED in a boss pedal and it wasn't getting enough current to light up all the way so I kept dropping the LED resistor R46 until I was at 100ohm and it was still too much. I then shorted D13 and upped R46 to 1k and the LED was bright enough but the pedal sounded starved like it wasn't getting enough voltage.

[kurtlives]

D13 is a Zener, basically makes sure there is a constant voltage drop for the LED so it doesn't fry.

When the battery drops below the Zener voltage though it won't conduct and the LED won't turn on. So it also kinda acts like a low battery indicator. So you have FX but no LED, ok time to change the battery.

[DiscoVlad]

The service notes specify D13 as "RD5.1EB3", any 5.1V zener (eg. 1n4733) should work here.

I should note that I put a 5mm LED in a boss pedal and it wasn't getting enough current to light up all the way so I kept dropping the LED resistor R46 until I was at 100ohm and it was still too much. I then shorted D13 and upped R46 to 1k and the LED was bright enough but the pedal sounded starved like it wasn't getting enough voltage.

The zener stops the LED from loading down the switching circuit. RG explains how it all works here: http://geofex.com/Article_Folders/bosstech.pdf

[amptramp]

D13 is a Zener, basically makes sure there is a constant voltage drop for the LED so it doesn't fry.

The LED is a constant voltage device itself, so it could be replaced by a larger series resistor.  If you want to keep a LED from frying, you add resistance.  If your battery voltage went higher than 9 volts, the LED would be more likely to fry with a zener in series than with additional resistance.

When the battery drops below the Zener voltage though it won't conduct and the LED won't turn on. So it also kinda acts like a low battery indicator. So you have FX but no LED, ok time to change the battery.

This is correct - it is a method of getting a low battery indication without an additional indicator of any kind.

[gutsofgold]

So when I short the zener the LED is loading down the switching circuit? With that zener shorted the sound of the pedal changes considerably. I'm thinking I need to keep that zener in there and maybe try a high brightness 5mm?

[DiscoVlad]

there's 9V across the  string of components: LED - R46 (2k2) - D13 (zener) - Q9 (when switched on).

with each component having these approximate voltages:

Zener: this is easy, 5.1V
LED: assuming it's red, around 1.7V.
Q9 transistor: when switched on the voltage between collector and emitter "Vce(on)" will be around 0.5V (probably less) and for the purposes of lighting an led, can be safely ignored.

so, the voltage across the resistor is 9V - 5.1 - 1.7 = 2.2V.

Which from rearranging Ohm's law (I=V/R) means there's 1mA going through the LED. This is usually not enough current for a normal brightness 5mm LED.

If you want to use a 5mm LED here, high brightness would be the way to go as they draw less current for an equivalent amount of light than a standard LED. Leaving the Zener as is, means between the LED and the resistor, you've got 3.9V to share between these components. (a little more if you use a lower voltage zener).

In any case, it's probably a good idea to keep the current through the LED less than 10mA. The data sheet for the LED you're using should have minimum and maximum current  specs, and forward voltage which will help you select the right resistor value.

[Processaurus]

The service notes specify D13 as "RD5.1EB3", any 5.1V zener (eg. 1n4733) should work here.

I should note that I put a 5mm LED in a boss pedal and it wasn't getting enough current to light up all the way so I kept dropping the LED resistor R46 until I was at 100ohm and it was still too much. I then shorted D13 and upped R46 to 1k and the LED was bright enough but the pedal sounded starved like it wasn't getting enough voltage.

The zener stops the LED from loading down the switching circuit. RG explains how it all works here: http://geofex.com/Article_Folders/bosstech.pdf

I'd disagree.  They could have just used a bigger resistor in series with the LED to get it to load the circuit the same as the zener+LED+resistor combo.  The purpose of the zener diode here is likely to make the LED serve as a low battery indicator.  It makes it so the LED gets dim when battery voltage gets below where the audio parts in the pedal aren't working their best.  Without the zener, the LED would light bright and proud while the audio parts of the circuit have ceased to function.

It works because you need ~1.4v to turn on a red LED, so if you add a 5.1v zener diode in series, you need 7.5v to turn on the string of zener+LED and get the LED to make light.  Less than that, and it gets dim or turns off.

Zener diodes are great when you want to subtract a constant number of volts from a DC voltage.

EDIT: Didn't see Chris and Ron's posts. Beat me to it.

[Processaurus]

I should note that I put a 5mm LED in a boss pedal

Was it a blue or white LED?  If so, the specialty LEDs like blue, white, etc have a higher voltage drop, ~3v, rather than the ~1.4v you need with reds.  A 3.6v zener or so would make a white or blue behave like the original red LED.

[DiscoVlad]

I'd disagree.  They could have just used a bigger resistor in series with the LED to get it to load the circuit the same as the zener+LED+resistor combo.  The purpose of the zener diode here is likely to make the LED serve as a low battery indicator.  It makes it so the LED gets dim when battery voltage gets below where the audio parts in the pedal aren't working their best.  Without the zener, the LED would light bright and proud while the audio parts of the circuit have ceased to function.

It works because you need ~1.4v to turn on a red LED, so if you add a 5.1v zener diode in series, you need 7.5v to turn on the string of zener+LED and get the LED to make light.  Less than that, and it gets dim or turns off.

Zener diodes are great when you want to subtract a constant number of volts from a DC voltage.

EDIT: Didn't see Chris and Ron's posts. Beat me to it.

It also acts as you say like a low battery indicator.

As an aside, the bypass circuit used by DOD/Digitech worked similarly; the LED would stay on when the battery was low (and of course go out when the battery was dead).
e.g.. the classic fuzz schematic linked here: http://www.effectsdatabase.com/model/dod/fx/fx52