A question about converting 12v to 9 v...

Dog1 · Post by **Dog1** » Fri Nov 09, 2007 12:46 pm

I'm in need of some help...I have a project in wich some of the lights are 9 volt, including a blinking stobe board and some that are 12v.

Is there an easy way to, for the 9v stuff, convert my incoming 12v to 9v?

I would like to do it using a 12v power source and a single on/off switch.

Thanks!!

Ziz · Post by **Ziz** » Fri Nov 09, 2007 3:27 pm

All you need is to branch off the 12v main connection inside the model into two, then take one of those and put a resistor on it to bring it down from 12v to 9v. Not sure of the math of what size resistor though.

Depending how many of each voltage light you have, you might also be able to just put a different size resistor on each individual one to bring 12v down to 9v at the last second.

tetsujin · Post by **tetsujin** » Fri Nov 09, 2007 3:42 pm

Ziz wrote:All you need is to branch off the 12v main connection inside the model into two, then take one of those and put a resistor on it to bring it down from 12v to 9v. Not sure of the math of what size resistor though.

It depends on how much current the 9v circuit is drawing... This presents a bit of a problem if the current changes (as you expect it would, for something like a "strobe circuit"...) - a series resistor, or even a voltage divider resistor network, can't give you a stable 9V under those conditions...

Basically you could use a voltage regulator to get the 9V from the 12V, or you could get a Zener diode with a 3V drop and put that in series...

Jonas Calhoun · Post by **Jonas Calhoun** » Fri Nov 09, 2007 4:25 pm

Better would be to use a voltage regulator--as the load changes on the 9v side, the requirements on the resistor can change...too flakey.

Dan

jwrjr · Post by **jwrjr** » Fri Nov 09, 2007 11:40 pm

If you use a resistor, the voltage will change as the load current changes (not to mention that it could get very hot).
I don't know if there is a '7809' regulator. If there is, that would be the easiest way (don't forget the heat sink).
One way which is definitely possible uses an LM317 regulator (Radio Shack may carry these, they have in the past) with 330 ohm and 2k resistors. That gives you 8.8 volts. You can get closer, but the resistors may be difficult to find. Again, if you are using much current, don't forget the heat sink.

Balok · Post by **Balok** » Sat Dec 01, 2007 1:55 pm

There are 7809s. I bought a few from Electronic Goldmine a few weeks ago. They will work with your 12V supply. You may need a heatsink.

Mr. Engineer · Post by **Mr. Engineer** » Tue Dec 11, 2007 4:55 am

IMHO,

There are two ways to go about it. (I could not understand why I chose the difficult way)

1. Convert all the lights of the 9v to 12v if you can so that everything is 12volts. Then you would not have a headache of worrying which voltage regulators dies first.

2. If you plan on using voltage regulators, I would usually prefer using the LM317 as it can give me current up to 1A. But I have to make sure that I have some heatsink standing by.

And I hope that you do not have any 5v circuits and need to convert down from 12v as in one of my experiments, the voltage regulator was quite hot.

DLMatthys · Post by **DLMatthys** » Tue Dec 11, 2007 12:57 pm

Mr. Engineer wrote:
And I hope that you do not have any 5v circuits and need to convert down from 12v as in one of my experiments, the voltage regulator was quite hot.

Was it hot enough to melt kit plastic? Would a heat sink take care of that issue?

Mr. Engineer · Post by **Mr. Engineer** » Tue Dec 11, 2007 9:11 pm

I have not tried a proper heatsink yet, just a piece of copper metal. But there are two reason, I suppose why it got so hot:

1) The voltage potential dropping from 12v to 5v

2) The current output of the voltage regulator.

Here, in one of my experiments, it got quite warm to the touch and personally, even if the heatsink sort of solved the problem, you'd know that its still warm inside the plastic. And so, in the long run, it would be sort of doing a slow melt there. So, when I comes to that, I prefer to have the voltage regulators, inverters, etc. all put outside the model and only the LEDs inside. This is because the LEDs are "cool" compared to bulbs. So, I do not have to worry about the model melting throughout the years. Ha ha

OK, this is my experiment. To build a Phaser Rifle Electronics. It got hot when the group of RED/White LEDs came on. It was already warm with the group of Orange LEDs and the circuit powered up. Later on, I think I switched from 7805 to LM317, thing sort of got better. But in the coming months, I will use a separate power supply for the RED/White LEDs. There is a Part II somewhere else in YouTube I forgot about.

The Phaser Rifle Circuit
http://www.youtube.com/watch?v=50oC5KuVz08