Power Supply Voltage Question

[Bassybert]

Hello

I'm making a maestro fuzz 1A (the 'satisfaction' one) and have run in to a bit of power supply problem. The pedal itself only runs off 1.5vdc - I was going to use it with a cylindrical AA battery holder which would have been fine, but I would like to wire it for True Bypass with an LED indicator. Can I run an LED of this much power, or can I use a 9vdc adaptor with a zener diode to drop the voltage right down?

Regards

Bassybert

[Bassybert]

I was thinking along the lines of a 1N5343B Zener with a 5w rating and a voltage drop of 7.5v - Bingo 9 minus 7.5=1.5vdc!!!

[H.Manback]

I don't know how you are going to wire that zener, but the 5W part gets me a bit worrying :roll:

Realize that the zener will eat current for the 7.5 volt you are 'throwing away'... Not ideal when using batteries.

One solution is to use a buffer opamp, with a voltage divider stuck on to it.



Use that voltage follower and put a voltage divider on the +

To get 1.5 V from 9 V you have to solve the equation 1.5 = 9 * (R2/(R1+R2)) where R2 is the resistor to ground. Solving that gives 0.2R1 = R2, so take 500k and 100k for R1 and R2 respectively.

[Bassybert]

Thanks for the reply H.

Realize that the zener will eat current for the 7.5 volt you are 'throwing away'... Not ideal when using batteries.

I was going to power the 9vdc from an adapter not a battery so that I would have a constant 9v source

Why do you use a voltage follower if the output is the same as the input?

Regards

Bassybert

[H.Manback]

The advantage of using an opamp is a high input impedance and low output impedance. The zener solution can be acceptable if precision is required, but the point is that you are actually short circuiting the 7.5 V that you don't use, which will be turned in to heat. To combat that, you would have to put a resistor before the zener, but then you have to balance your power supply's output impedance to how much current you want to limit the zener to.

IMO zeners are not in their place here, since what you want is a small portion of the battery's voltage. A zener would be more in it's place if you were limiting to let's say 7 V.

With the opamp solution, you 'leak' very little current in the voltage divider, and your output impedance is low (in the order of 10 ohm IIRC, depending on opamp type). This means the voltage will stay the same for reasonably large current draws.

Edit: I misinterpreted what you wrote, sorry, I was under the impression you were using the zener to limit the voltage, by connecting the zener from battery to ground... Stupid, because that would of course require a 1.5 V zener instead of the 7.5 you said.

I am not sure what happens to the remaining 7.5 volt that is over the zener, but I think that it is ok to do it this way. So try that, and if it eats battery like hell or the zener gets too hot (unlikely), then you might consider my solution. Sorry for the confusion I created

[Bassybert]

No problem H, thanks for your time and your advice - i'm always open to others ideas.

I was going to use the zener in series to limit the voltage before it hits the board. I realise that a large percentage of the power will be lost as heat, but think the zener can take it - although it's not very efficient. I may just use a AA battery drawer instead. Is it possible to run an LED indicator off of 1.5vdc?

Regards

Bassybert

[H.Manback]

Well, check it with a AA battery, just don't stick the leads of the LED too long on there :wink:

My guess is it won't work, LED's usually have a higher turn on voltage AFAIK