Blue LED Indicator

[Liam]

Got a Boss BD-2, and would like a blue indicator LED. However the higher voltage and current needed has left me with some questions.

The standard circuit goes something like:

Battery
Red LED
1.2k dropping resistor
5 Volt Zener
Grounded through FET switch

The red LED is dropping about 1.85 volts, the 1.2 K resistor drops just over 1 volt (can that be right, about 1 mA through the LED?)

All seems to add up to 9 volts anyway....

The blue LED I have got will need to drop more like 4 volts. I'm willing to play about with resistors, but does the zener need to be there? I could do this with a 2 volt zener, but can I just bridge it out and drop 5 and a bit volts through a resistor?

I guess my question is: Why the Zener?

TIA

Liam

[Mark Hammer]

I guess my question is: Why the Zener?

Get ready to smack your forehead and mutter "Of course".

Why the zener? Because the voltage coming off the zener doesn't change, while the voltage coming off the battery does. The stability of the zener permits any settings based on specific resistor values to remain valid, even as the supply voltage starts to slip.

[Liam]

Not smacking my forehead yet. If the zener and the dropping resistor were replaced with a higher value dropping resistor, the proportion of voltage dropped over the resistor with falling battery voltage (and therefore diode current) would be far less than with the zener arrangement. Wouldn't it?

[Liam]

If it's any consolation, I just smacked my forehead quite hard. With the zener in circuit the brightness of the LED varies more drastically with battery voltage. So it works as a battery voltage indicator as well as telling me when the bypass circuit is inactive.

Thanks Mark!

Liam

[Sweetfinger]

Back before I knew that I would have to do math and figure out what values to change diodes and resistors to in order to accommodate the different current and voltage requirements of blue LEDs, I just stuck 'em in anywhere there was a red one, and nothing bad ever happened, and they always worked. Weird, huh.

[Liam]

I'm sure ignorance is bliss, but unfortunately some unkind people taught me about non ohmic IV relationships and derating curves, and anything like this turns into a major job!

(In all honesty I quite enjoy the math side of electronics)

[Mark Hammer]

Use of zeners and other forms of regulation (e.g., 3-pin LM7805) for the purpose of having a dependable baseline is actually fairly widespread. One of the reasons why the 2nd generation of bucket-brigade chips (MN32xx) was developed and widely used was because they could be powered by 5vdc. That meant that if you powered them from 5v, and set the bias from a stable 5v, the bias voltage that they require at their input in order to function could be set and would remain valid until the battery itself dropped down to 7v (at which point regulator would not function properly but the chips in audio path were probably wanting anyways at that point).

Same thing with the biasing of FETs in FET-based phasers. Those suckers need a bias voltage, and since bias voltages are essentially passively divided-down voltages, you need to know what the heck it is you are dividing down in the first place. Zeners and other absolute voltage sources provide that assurance, and make it possible to set the bias and know that it will remain properly set.

[Liam]

This one was put there for another reason. It only controls voltage to the LED indicator circuit, and roughly halves the voltage across the LED and dropping resistor. That way when the battery voltage has dropped by a volt, the voltage to the LED circuit has effectively dropped by about 20% rather than about 10%. At least as far as I can see. I notice quite a few effects pedals seem to use this feature now I look over some schems.

But yes, you're quite right, what the zener achieves is a steady voltage drop whatever the current. And it does it very well!

Swapped the zener for a 3k9 resistor and I'm getting about 1 mA through a blue LED circuit. The brightness seems about right. Just for the sake of it, I tried popping the blue LED in with the dropping circuit left stock. Much too dim, only drew microamps.

Liam

[raycroft]

MAN!!! You guys are some nerds! I can't wait to be a nerd I think approx. 90% of that conversation went right over my head. Good read, though, thanks!

[Enzo]

Then you should ask about the parts you missed.

[raycroft]

I wouldn't begin to know what to ask about. I just built my first pedal, The Rocket, by Aaron Nelson ; and I'm getting ready to post for some help. Thanks anyway, I know you are all more than willing to help . Us newbs appreciate it!!!!

[darthpoodle]

What is the worst that could happen if you didn't change anything and stuck the blue LED in any way? With the new micro stuff you cant really change anything but the LED.

[Liam]

They end up too dim to be visible. Are you talking about surface mount components being hard to change? Not easy, but it can be done.

Liam

[rockinrendell]

yes, surface mounting can be done. you need an hot air desolderer/solderer. i used to run a surface mounting line and some do have to get reworked. now, if the board has been conformal coated this can be tricky and ruins the coating in that area. the hard part about surface mount components is identifying them. very,very tiny writing on them.