MC7808BTG drop out voltage?

[The Dude]

Yes, you can add as many diodes as you like to elevate the ground pin of the regulator. The regulator output will be its rated output plus the voltage drop of the diodes with respect to ground. You could also use a zener diode to get the regulator output to a more specific voltage, as regular diode voltage drops are not nearly as consistent. Of course, cathode to regulator if using a zener.

[daz]

Yes, you can add as many diodes as you like to elevate the ground pin of the regulator. The regulator output will be its rated output plus the voltage drop of the diodes with respect to ground. You could also use a zener diode to get the regulator output to a more specific voltage as regular diode voltage drops are not nearly as consistent.

Very good, thanks Dude. So i guess these achieve their rated voltage by diodes on the chip engraving (or whatever you call it) and i'm simply adding external ones to further change the voltage?
I dunno what i would do w/o this place ! You guys are freakin electronics gurus. Thanks to everyone for helping me with this.

[The Dude]

There's more inside a regulator than just a zener, but conceptually you have the idea. A regulator's output is rated with respect to its ground pin. If we elevate the "ground" pin to some higher voltage, the regulator output is raised accordingly with respect to circuit ground.

I'll add: If you want 9V instead of 8.5, it would be easier to just use a 9V regulator and forget about the diodes.

[daz]

I know, and i thought about that. But i decided i'd like the option of either depending on what voltage i eventually come to decide i want. Can't drop a 9v to 8.5 right? I will likely go with 8.5 because that seems to be where alkalines spend most of their life and the sound that seems slightly better to me. It's subtle but for peace of mind i'd like to have either option. Could even put one diode on a switch to bypass it so i can compare 8.5 to 9. But in reality the main reason i am doing this is these particular rechargables charge way higher then others and are 11.6v when i pull them out of the charger. And they take a quite a few hours of use before they even get down to 9. So i just want the tone to stay consistent for their charge cycle.

[Enzo]

daz, when you said in series initially, I thought somehow you were envisioning a diode in series with the voltage. If I have a 9v source and stick a diode in series with it, the result will be the 9v minus the diode drop. You see what I intended. If I misunderstood your description back there, I apologize.

[daz]

daz, when you said in series initially, I thought somehow you were envisioning a diode in series with the voltage. If I have a 9v source and stick a diode in series with it, the result will be the 9v minus the diode drop. You see what I intended. If I misunderstood your description back there, I apologize.

R U kidding.....no need. You've helped me so many times you'd need to kill someone in my family to require an apology !

EDIT: so you CAN reduce the voltage too by putting a diode on the + leg? Not that it matters at this point but good to know you can go both ways if thats the case.

[Enzo]

Sure, sometimes folks use that to reduce heater voltage

[Jazz P Bass]

I don't want to scramble the noodles but they do make 'adjustable' voltage regulators.

[daz]

I don't want to scramble the noodles but they do make 'adjustable' voltage regulators.

I know, thats been mentioned already but to find that AND very low drop out voltage in a single regulator was hard. It was hard enough just finding one with a low enough DOV., No matter, it IS adjustable anyways using a diode or 2 as Enzo described and thats why i got 8v instead of 9, so i could get voltages from 8 to 9 and inbetween.

[Mick Bailey]

Some fixed 3-terminal regulators (78xx for example) can also use a voltage divider instead of a diode to set the ground reference and even a trimmer to give some adjustment (though not through the range of a proper adjustable regulator). The bias current through R2 can be higher and has to be factored into resistor values - you can't go too high, and regulation isn't quite as good. But the method can work out well.

[daz]

Some fixed 3-terminal regulators (78xx for example) can also use a voltage divider instead of a diode to set the ground reference and even a trimmer to give some adjustment (though not through the range of a proper adjustable regulator). The bias current through R2 can be higher and has to be factored into resistor values - you can't go too high, and regulation isn't quite as good. But the method can work out well.

Is there a reason to do it that way instead of diodes?

[Mick Bailey]

Not perhaps for your application, but a diode's forward voltage varies with temperature so the regulator output voltage varies according to how many diodes are in the string and how much the temperature changes. Metal film or metal oxide resistors give a more stable output under widely ranging temperature. The other advantage is you can use a trimmer plus series resistor instead of R2 to give an adjustable output, though not below the regulator's output voltage. You can get fine control of the output - with diodes you have steps according to forward voltage drop. So about 0.6v for a regular silicon diodes or 0.2v (or less) for Schottky types.

The resistor 'trick' may or may not work for LDO regulators and I haven't tried it with these, but it works for 78xx types. I don't have any suitable 3-pin LDO regulators to experiment with, otherwise I would have breadboarded this and let you know the outcome.

[daz]

One more question if you guys don't mind. I wired up the 8v regulator to test it with a diode in series with the ground leg and a 22uf across the output. Works great at least with a meter, hopefully once installed on the board won't experience any noise etc. But i'm wondering.... You have the battery now with a diode and cap between ground and hot, so will this continuously drain the battery even when i switch it off? Heres the way i have it wired now w/o the regulator in yet......battery ground goes to a switch where it lifts the battery from the signal ground thereby lifting all pedal grounds from the battery. Will i have to remove the switch from that location and put it at the battery before the regulator in order to keep the battery from draining when i switch it off? I ask because theres a bit more to swapping places due to having a tuner also on that switch that uses it's own battery and there would be some juggling to do.
So the question in a less confusing way is, If i'm cutting the ground with a switch AFTER the regulator output. Will the regulator circuit continue to drain the battery that way?

[Mick Bailey]

The regulator passes a small reference current to ground via the GND leg, even if no load is connected. The ground needs cutting before the regulator, otherwise it will slowly drain the battery, but how long this would take depends on the current draw. You can check with your DMM set to mA in series with the battery with the load disconnected.

[daz]

Thanks Mick