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7.6 volts from a voltage regulator

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Electric Rain

New Member
Can someone help me figure out how to get 7.6 volts from a FIXED voltage regulator? I don't want to use an adjustable one. But can I like... take the voltage down from an 8 volt reg. with diodes or something? What should I do?
 

nettron1000

New Member
Why do you need to get the voltage down to 7.6 ?
 

stevez

Active Member
I've seen situations where the adjustment terminal had a diode inserted with the author indicating that it changed the output by 0.7 volts. Don't know if it works or how regulation is impacted. Author suggested multiple diodes to get other output voltages.
 

Mosfet

New Member
The short answer, use a 7808.
They are a little harder to find but the extra 0.4 volts shouldn't matter much.
If you need exactly 7.6 volts use a LM317 and dial it in.
Another way is to raise the ground reference pin of a 7805.
You could use a 2.6 volt zener diode.
But unless your current needs are very small the zener would need to be a 2 watt package.
These are expensive and difficult to find.
A high wattage zener can be made by adding a transistor to it.
See my post on pre-regulators.
http://www.electro-tech-online.com/threads/gps-roll-steering-coupler.2711/#post-13226
Replacing the Darlington pair, Q1 & Q2, with a standard voltage regulator lets you make a regulator at any voltage [within reason].
 
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laroche73

New Member
7.6v ?

I think Nettron 1K asked the right question. Not hard to do with a fixed regulator, but why do you think it's necessary? The problem you're trying to solve may be satistied without such specific voltage requirements.
It's sometimes hard to tell from short posts whether the requests come from a novice who doesn't know any better, or an expert who is looking for a very specific solution. There are clues, though.

I'm just pointing out that posts which provide some background as to the reason "why" often allow the responder to come up with a different (and maybe better) solution.

- CAL
 

Electric Rain

New Member
Sebi said:
You can do this with 7805 or 7806.
Hey, thanks Sebi! You always seem to answer people’s questions well. :D But I'm afraid that I'm new...ish to electronics, so I don't know what values the caps and resistors should be... :oops: Do you think you can help me with those please? :lol: :oops:
 

Sebi

Active Member
All condensers 100nF. With unmarked resistor You can adjust the output voltage. I'm too lazy to calculate it, always made with experiment.(start with 200ohm)
For calculations keep in mind: the 78xx regulators GND pin suck about 2mA (small depend from manufacturer).
 

Electric Rain

New Member
nettron1000 said:
Why do you need to get the voltage down to 7.6 ?
Why? Well, if you must know, I'm building an interface that lets me use my PSX Pad as a PC Pad. And allot of people say that you can use 9V; however, the rating on the PSX pad is 7.6V. And I might start selling these from my future website. So I don't want to take any chances. (I.G. Someone's pad(s) blowing up :lol: Disclaimer or no.) So anyway, that's why. :)

And Sebi, thanks! :wink: :D
 

laroche73

New Member
voltage regulators

What you do with that 7.6V is your own business, E-rain... I was curious, though. 2cd the good vibes on Sebi, always comes back with a thoughtful answer; Mosfet presented some good ideas, too.

I saw the method Sebi mentioned many years ago, and always liked it for simplicity. It uses the quiescient (kwhy or Kwee-es-scent, what the part consumes on it's own) current flowing through the regulators' ground pin to raise the output voltage by a fixed amount. Check the following links, you'll find this circuit with equations near the end.

http://www.electro-tech-online.com/custompdfs/2003/08/LM7805-1.pdf

http://www.electro-tech-online.com/custompdfs/2003/08/LM78L05.pdf

Keep in mind that Iq is loosely spec'd, ~ 2-8 ma. No problem if you only make a few units and hand-select the resistors, but it's not something you would depend on for higher volume production. The regulators you have on hand may have closely matched Iq's (no pun intended), but then they probably come from the same batch. Always use the worse-case specs from the data sheet when making a production design, and then de-rate by another 10%, if the cost is small.

Here's why I asked for the end application. The first thing I'd do is take apart the PSX pad and see if it has it's own 5V regulator. The numbers mentioned (7.6V - 9V) suggest there's one inside. 7.6V is a typical minimum voltage a designer would spec to account for the insertion loss of a 7805 type regulator (~ 2V). It's kind of redundant to regulate the regulator if that's the case (there are situations where you may want to do this, such as spreading heat dissipation around the various physical components. It's inefficient to do so, and you're better off reducing power dissipation in the first place. Ex: using switching regulators for digital logic).

A quick web search didn't turn up the internals for the PSX pad. Sometimes it's a quicker to figure things out on your own. Don't be afraid to take things apart, even if you can't put them back together. Learning's worth more than stuff. Good luck!

- CAL
 

Roff

Well-Known Member
Sebi's circuit doesn't intentionally use quiescent current (Iq) to raise the output voltage. In fact, he specified a large feedback current (200 ohms suggested, which yields 25ma) to swamp out Iq.
The output voltage will be

Vout=5+100*(5/200+Iq)

If Iq=2ma, then Vout=7.7v

You will need a least 10 or 11 volts on the input to make this work.
 

laroche73

New Member
you're right, Ron. perhaps I should stick to digital... :oops:
The feedback current should be >= 5x Iq, according to the datasheet, so the effect of a varying Iq is about 0.1V/mA in this circuit (Vo = 8V @5mA Iq). Iq varies a bit with temp and input voltage.

And you're correct, the input voltage should be ~ 2-3V above the regulator's output voltage for the 78xx series.

My money's still on a regulator already being present in the PSX pad. Anyone care to verify/disprove? And Ron, you can be my proofreader anytime :wink:
 
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