Question on powering a 3VDC device from a 9VDC pedalboard power supply

smigeon

Hi all,

Question to the technical geeks if I may: 

I want to power a headphone amp (a Behringer p2 that takes 3 volts from 2 x AAA batteries) from a 9v tap on my Cioks pedalboard power supply. I remember from school that this is a job for a potential divider: if I put two resistors in series across a 9v output of the power bock, such that the ratio of resistor 1 to resistor 2 is 2/3, I will get the required 3v potential across the smaller resistor.

My question is: what are appropriate absolute values for the two resistors? Obviously, if they are both small, the quiescent current drain on the 9v source will be too high; but equally, if they are big, there may not be enough current to satisfy the headphone amp. 

Any suggestions as to appropriate values for the two resistors? (I don't know what the current consumption of the headphone amp is, but I'm guessing from the fact that it's powered by AAA batteries that it's low: 10mA maybe??)

robinbowes

I'd get one of these and stick it in a small enclosure:

https://www.ebay.co.uk/itm/162289623433

HTH,

R.

ecc83

You could just stick a 5v6 1W zener in series. Better would be a 7805 regulator. I very much doubt the amp would be bothered by 5V and that would give you more headroom.  Or, check the data sheets on the LM317 adjustable regs? I just feel a cheap dc-dc switching converter might be noisy for headphone use?

Dave.

robinbowes

ecc83 said:

You could just stick a 5v6 1W zener in series. Better would be a 7805 regulator. I very much doubt the amp would be bothered by 5V and that would give you more headroom.  Or, check the data sheets on the LM317 adjustable regs? I just feel a cheap dc-dc switching converter might be noisy for headphone use?

Dave.

OP wasn't sure about resistors, so a zener or 7805 regular circuit is probably not a good idea.

And for £1.99, a ready-built module is worth a try!

smigeon

Thanks both! I didn’t make it clear in the OP, but another consideration is that I’d like to cram the 9vdc->3dcv solution, whatever it may turn out to be, inside the Behringer P1’s battery compartment (2 x AAA battery size!). Two resistors making up a “potential divider” would fit easily and I’m guessing a zener diode would also...

robinbowes

smigeon said:

Thanks both! I didn’t make it clear in the OP, but another consideration is that I’d like to cram the 9vdc->3dcv solution, whatever it may turn out to be, inside the Behringer P1’s battery compartment (2 x AAA battery size!). Two resistors making up a “potential divider” would fit easily and I’m guessing a zener diode would also...

That module is 48 * 22 * 12 mm - might just be a bit big. Also, heat/ventilation.

Danny1969

I wouldn't use any switching regulator, rather use an LM317 ... if you connect the common between a 180r and 280r resistor your get about 3.1V 
So just a regulator and 2 resistors with maybe a cap on the output, that should fit in there

ecc83

robinbowes said:

ecc83 said:

You could just stick a 5v6 1W zener in series. Better would be a 7805 regulator. I very much doubt the amp would be bothered by 5V and that would give you more headroom.  Or, check the data sheets on the LM317 adjustable regs? I just feel a cheap dc-dc switching converter might be noisy for headphone use?

Dave.

OP wasn't sure about resistors, so a zener or 7805 regular circuit is probably not a good idea.

And for £1.99, a ready-built module is worth a try!

Sorry, don't understand the objection?

Dave.

robinbowes

ecc83 said:

robinbowes said:

ecc83 said:

You could just stick a 5v6 1W zener in series. Better would be a 7805 regulator. I very much doubt the amp would be bothered by 5V and that would give you more headroom.  Or, check the data sheets on the LM317 adjustable regs? I just feel a cheap dc-dc switching converter might be noisy for headphone use?

Dave.

OP wasn't sure about resistors, so a zener or 7805 regular circuit is probably not a good idea.

And for £1.99, a ready-built module is worth a try!

Sorry, don't understand the objection?

Dave.

No real objection, just that the OP wasn't confident enough to figure out what resistors to use so probably wouldn't be able to figure out how to use a zener or design a 7805 circuit. That's all.

R.

ecc83

robinbowes said:

ecc83 said:

robinbowes said:

ecc83 said:

You could just stick a 5v6 1W zener in series. Better would be a 7805 regulator. I very much doubt the amp would be bothered by 5V and that would give you more headroom.  Or, check the data sheets on the LM317 adjustable regs? I just feel a cheap dc-dc switching converter might be noisy for headphone use?

Dave.

OP wasn't sure about resistors, so a zener or 7805 regular circuit is probably not a good idea.

And for £1.99, a ready-built module is worth a try!

Sorry, don't understand the objection?

Dave.

No real objection, just that the OP wasn't confident enough to figure out what resistors to use so probably wouldn't be able to figure out how to use a zener or design a 7805 circuit. That's all.

R.

Ah! Well I have a process in such matters for forums! K.I.S.Sir. Tell OPs the options and then let them come back and ask further questions like.."Ok, so HTF do I do that?"

I have wasted my time before, going into intricate detail only never to see the guy or gal again!

Dave.

smigeon

Danny1969 said:

I wouldn't use any switching regulator, rather use an LM317 ... if you connect the common between a 180r and 280r resistor your get about 3.1V 
So just a regulator and 2 resistors with maybe a cap on the output, that should fit in there

Thanks Danny. I can’t quite visualise the circuit you have in mind from the description  - and when you say “280r resistor”, does that just mean a 280 ohm resistor (sorry for the naive questions!). And what value of capacitor are you thinking, and would this just be indicated if the circuit seems noisy without it?

smigeon

ecc83 said:

robinbowes said

No real objection, just that the OP wasn't confident enough to figure out what resistors to use so probably wouldn't be able to figure out how to use a zener or design a 7805 circuit. That's all.

R.

Ah! Well I have a process in such matters for forums! K.I.S.Sir. Tell OPs the options and then let them come back and ask further questions like.."Ok, so HTF do I do that?"

I have wasted my time before, going into intricate detail only never to see the guy or gal again!

Dave.

Ha ha - I appreciate all the suggestions and am learning a bit about “modern” electronics by looking up these various devices. When I was a kid I played with simple electronic circuits (and I’m wizard at soldering!) but devices like mini voltage regulators and even zener diodes are all pretty new to me.

robinbowes

smigeon said:

ecc83 said:

robinbowes said

No real objection, just that the OP wasn't confident enough to figure out what resistors to use so probably wouldn't be able to figure out how to use a zener or design a 7805 circuit. That's all.

R.

Ah! Well I have a process in such matters for forums! K.I.S.Sir. Tell OPs the options and then let them come back and ask further questions like.."Ok, so HTF do I do that?"

I have wasted my time before, going into intricate detail only never to see the guy or gal again!

Dave.

Ha ha - I appreciate all the suggestions and am learning a bit about “modern” electronics by looking up these various devices. When I was a kid I played with simple electronic circuits (and I’m wizard at soldering!) but devices like mini voltage regulators and even zener diodes are all pretty new to me.

I discovered many years ago that you need to pick your battles.

If you want to learn about electronics then look into how to calculate resistor values, and why that's not a great way to drop-down voltage, and investigate zeners and voltage regulators, etc. It's fun.

If you want to just get a job done, buy something to do it and move on.

Focus on what is most important to you.

R.