# Have 5v regulator need 9 volts

Discussion in 'Electronic Basics' started by amdx, Jan 24, 2013.

1. ### amdxGuest

Hi all,
Tomorrow I'm working on a power supply to replace a 9v battery.
All I have is a *5 volt regulator. I think can use a voltage divider on
the output, then tie the normally grounded leg to the center of my
divider to bring it up to 4 volts. So I have 4 + 5 = 9 volts.
If I used 5.1K and 4.3k in series to ground. that would give me 9.1
volts. However there is some current from the reference pin.
So, I don't know how to calculate the proper ratio and how much
current do I need to flow through my divider.
Should I put a capacitor across the lower leg of my divider?
How big?
Thanks, Mikek

*My local Radio Shack only carries 5V and 12 V.

PS. In case this isn't feasible,
Here's the scene, I have three devices, an infrared transmitter (9v)
an infrared receiver (4.5v) and a walkie talkie (4.5v). I have tested
both of the 4.5v units on 5 volts and I'm comfortable they will be OK.
I'll be running this all from a 9v wall wart, we'll say it puts out
14v at no load. The big load is the walkie talkie during transmit
~350ma. At idle it is less then 10 ma for all devices combined.

2. ### Phil AllisonGuest

"amdx"

** All you need is a 3.9V zener in series with the ground lead.

.... Phil

3. ### Jasen BettsGuest

http://pdf1.alldatasheet.com/datasheet-pdf/view/89091/NSC/LM140.html

There it is right on the front page.

adding a capacitor between pin2 and ground will slow the voltage rise
at power-up but and may improve regulation under variable load
conditions,

4. ### amdxGuest

Yes, but I don't have one locally and I want to get done today.

Thanks, Mikek

5. ### amdxGuest

would be to high for my 9v device. Guess I could stick a diode in series
with the output giving me 9.4 Volts, pretty close to a fully charged 9
volt battery.
I think that might be an easy solution, need to reread thread, someone
mentioned quiescent current is a problem.

Thanks, Mikek

6. ### amdxGuest

For now, I'll try the stacking with a series diode on the output to
drop .6 volts.
The wall wart is not a problem, I have a box full of various voltages
and currents.
The wallwart is where my problem started. I used a 6v 700 ma, with a
no load output of 9.6 volts. All worked good until the walkie talkie
went into transmit. Then the wallwart voltage dropped to about 7 volts.
It still worked but I could hear hum on the receive walkie talkie.
If I went to a higher voltage wallwart then the voltage was to high for
the 9 v device. So, the need for a second regulator.
Thanks, Mikek

with s

7. ### amdxGuest

All that looks great but the objective of this exercise is to make it
work with the parts I have on hand today. The parts I have are two-5v
regulators. I have many wallwarts, so I have one that won't fall to low.
Hmm... I just recalled, there is an audio repair shop in town and the
guy sells parts. it's worth waiting till he opens and checking if he has
a 9v regulator in stock.
Thanks, Mikek
Thanks, Mikek

8. ### George HeroldGuest

I think that's pretty standard... did you look at any 5 volt regualtor
spec sheets? I bet they have a circuit.
I've got a LM79L05 spec sheet tacked to my wall. (I can never
remember pinoputs.)
It's got what you want on the back.
old ground)

Vout = -5V -(5V/R1 +IsubQ)*R2
5V/R1 > 3*IsubQ
0.1uF cap across R2.

All you need is the quesient current for your regualtor.

George H.

9. ### amdxGuest

Probably meant troughs rather than peaks.
Ok, project complete, boxed and working. Picked up a 9v regulator at
the audio repair shop. \$3.02 :-(
I used a 1000uf cap to filter the wallwart and .1uf at the output of
each regulator. There are input filter caps in the devices.
At idle, with every device powered the input is 13 volts with 100mv
of ripple. When the walkie talkie transmits the input voltage trough is
11.1 volts and 800mv of ripple. This is fine, there is no sign of any
ripple or droop in the output of either 9v or 5v regulator.
I used a 9v 1 amp wallwart.

Thanks for everyone's input.
Mikek

11. ### George HeroldGuest

Well the pinout on my wall is for the 79XX so that's what I quoted.
(I can remember the 78xx pinout. :^) Others have pin ups on their
wall, I've got pin outs, wire gauges, screw sizes and Drill tables
(And some drawings done by my kids when they were younger.)

George H.

12. ### amdxGuest

Hmm... I guess you just get hum if the troughs go below 10.7v.
Mikek

"amdx"

14. ### amdxGuest

Is that you Phil? It's seems a little weak.
Imposter!

Mikek

15. ### Guest

No, Phyllis is normally weak. ...or were you commenting on the
"little"?

16. ### amdxGuest

When testing, my line voltage was 123.3vac. When I got to my boat and
checked the line voltage it was 112.9vac with my heater running.
I decided to use a 12v wallwart instead of the 9v.