Hello, i have a wireless camera that wants 8 (or 9) volt input. That's a little weird i think.. Anyway, i have a 12 volt battery, is there a way i can get the 8 volt?
I have a spare 3V regulator and a 5V regulator (the classic L7805 something). Is there a way i can use them together to get the 8 volt?
Schematics appreciated :)
Thanks!
7805 with some pot’
Hi,
From what I remember, You can control the output of a 7805 by potting its GND
I found this to confirm:
http://www.elecfree.com/electronic/5v-to-24v-dc-variable-regulator-by-7805/
You can see here that a simple voltage divider controls the 7805’s outpout
Quiescent current (new term I’ve just discovered)
Yes!
all the explanations here
(from http://www.cpemma.co.uk/diodes.html )
If the voltage at the “common” pin is raised above ground, this is added to the 5V regulator output, up to a limit about 2V below the input voltage – so a 12V supply can give 5-10V output to fans.
Text-book method shown is with a two-resistor potential divider on the output. Voltage across R1 is 5V, which produces a current of 5/R1 amps. This, plus the regulator’s current from the “common” terminal (quiescent current Iq), flows through R2 producing a voltage across R2 of R2(5/R1 + Iq) volts. This voltage across R2 is added to the 5V across R1 to give the raised output voltage.
Formula for the output voltage Vo is
which rearranges to
Iq is typically about 4mA (.004A), so if R1 is 240 ohms and R2 is 150 ohms,
To minimise the effect of Iq variations between regulators, the divider current needs to be significantly higher than Iq, so R1 should preferably be under 330 ohms.
Check the output with a multimeter, as small changes in Iq from one regulator to another have a significant effect. Or use a 220-ohm preset potentiometer for R2 and adjust as required.
