# Using 12 VDC Power Supply Question

Joined Jan 25, 2004
3
I am a "beginner" when it comes to electronics, I know just enough to be dangerous. I have purchased a 12 V (DC) power supply to be used with a prototyping "bread" board and I also got a bunch of different ICs (NANDs, NORs, Hex Inverters, ANDs, ORs, and some FlipFlops), LEDs, and resistors.

I believe that in order to safely use the ICs I purchased, I need a 5 V power supply, but mine is a 12 V power supply.

How can I use my power supply safely with the ICs? Or is this possible? Are transformers only to be used with AC curcuits?

Any suggestions will be greatly appreciated! Thank you.

Jared Schrag

#### Battousai

Joined Nov 14, 2003
141
If you're only doing DC stuff, just use a simple voltage divider (like a 7k and 5k resistor in series and take 5V off the 5k). You'll be wasting some power but besides that it might work.

Or you can just get a DC-DC regulator.

If you're doing AC stuff then you will need a transformer.

#### eldon

Joined Jan 24, 2004
14
You are correct in that 12 volts will quickly destroy most logic chips. 5 volts can be obtained from a 12 volt supply with a 7805 IC ($0.60 to$1.99). These can be rated as high as 1.5 amp if you use a small heat sink. The only external circuits needed will be a couple of capacitors, typicaly one on the input and one on the output. 10uF works well. It is also good practice to place a 0.1 uF capacitor near the power leads of each logic chip to suppress noise on the 5V line. For breadboarding you can usually get away with 1 or 2 of the 0.1uF caps. For higher current needs, then a new supply will probably be your best bet.

Transformers will only work with AC current. If you feed a transformer with a DC supply, one or the other will probably smoke rather quickly as the transformer will look like a short circuit to the DC supply and severly overload it.

Joined Jan 25, 2004
3
Okay! Thank you for all of the current replies. I have a CORRECTION to make. My power supply is actually 13.8 VDC. Most of the power supplies I saw were 12.8 VDC when I purchased this one. So, anyhow, I think that maybe I should have not purchase this particular power supply for use with ICs :-( (I'll pay more attention next time).

This being the case, could I still use a 7805 IC (power/voltage regulator) with this 13.8 VDC power supply?

Again, thank you for all of your help!!

Jared

Joined Jan 25, 2004
3
Well, I looked up a few data sheets on Positive 3-Terminal Volate regulators associated with the 7805 part number and found that most will accept a maximum input voltage of 30 - 35 V. So, I believe my questions have been answered.

Thank you to all who replied to my questions.

Jared

#### mozikluv

Joined Jan 22, 2004
1,437
:huh: hi,

as an added suggestion add a short protection by shunting a diode like 4001 or 4002 across the output and input of your regulator. you also mentioned that what you have is a13.8v secondary after being rectified you would have about 19v. since you will be using a 5v regulator that would be a lot of voltage drop of 14v. your regulator would be operating really hot. so be sure to put an adequate heatsink, and besides dont rely too much on its built in protection.

my suggestion for the shunt diode is for the input short circuit side. as for the output side also shunt a diode from output to grnd.