Basic 12v to 8v drop

[spaceman]

I have a 12vdc backup battery that I'm trying to use to backup an 8vdc phone system (300mAh). So no vehicle startup involved.

This almost seems too simple to ask, but I'd like to get it right the first time and I only dabble in this stuff with the help of those that know more than I.
Thanks.

[mariushm]

Re: Basic 12v to 8v drop

Just buy one of these :

http://www.ebay.com/itm/1-Pcs-DC-DC-...item2ec2650df2
http://www.ebay.com/itm/1-Pcs-DC-DC-...item2323fc044a

The parts alone are more expensive than the price of this converter, if you were to buy them yourself. You just adjust the potentiometer to get the voltage you want.

If you want to actually make one yourself, get a 34063 :

http://www.digikey.com/scripts/dksea...177&PV1291=862

a suitable inductor (1a peak, 20+ uH), a few resistors and capacitors and a 1n5819 diode and you're done.

This page does all the formulas for you: http://dics.voicecontrol.ro/tutorials/mc34063/ (C0 and L are minimum values, you can pick the first size you find that's bigger, the rest have to be as close as possible to those values)

or if you don't care about efficiency, just get a linear regulator, a 7808:

http://www.digikey.com/product-searc...2556290?k=7808

one capacitor on input, one on output and you have 8v dc

[spaceman]

Re: Basic 12v to 8v drop

I knew that I could count on you all. Thank you for the very quick reply. And although I wouldn't mind trying my hand (probably a few times), the pre-made one looks like a winner to get me onto the next project.

Though this one looks to be US based with quick shipping. Fewer reviews, but the only difference that I can spot is a little smaller text on the caps.
http://www.ebay.com/itm/DC-DC-Buck-C...item337ebc9fdc

Thank you again, this place is a diamond in the internet rough.

[spaceman]

Re: Basic 12v to 8v drop

I've been using your suggestion for about a month now and although it does work well for powering it. Thank you.

Though there is now a fair bit of crackle and static on the phone line. This is probably from the 12v UPS (doubt it has any conditioning), but I'm not sure how to get rid of it.

In theory, I could use a relay to provide mains power until there's a power outage and switch over, but I'd rather just remove the noise all together if possible.
Any ideas?

[tom66]

Re: Basic 12v to 8v drop

It's probably because the converter hasn't got a clean output. A pi filter may be necessary.
Here's one I made earlier.
http://static.rcgroups.net/forums/at...i%20filter.jpg

It's basically two capacitors with an inductor between them.