Using single 12V supply to deliver 12V and 5V

[megadriver]

Voltage drop ...

Hi,

The last time I messed with ohm law was at highschool, a long, long time ago. I was searching the web and found this forum, so I thought I was a good idea to post here. I'm sorry in advance if I'm at the wrong place.

I have a little video decoder board which I'm trying to create an 'interface' for it. It has two power inputs, 12V and a 5V. So, I bought a power adapter of 12V and 500mA. Is it possible, using resistors, to drop its voltage from 12V to 5V? What resistor(s) should I use?

Thanks!
[]'ss
Nino

 

[nashwan]

Re: Voltage drop ...

hi
use LM7805 instead off resistor

https://www.fairchildsemi.com/ds/LM/LM7805.pdf

 

[IanP]

Re: Voltage drop ...

Is it possible, using resistors, to drop its voltage from 12V to 5V? What resistor(s) should I use?

If the current consumption of the 5V input is constant you can use a resistor, but I doubt it very much ..
By the way, do you know how much current will be drawn by that input??? and combined current by 12V pin and 5V pin, so your 500mA-12V power supply can deliver it ..

Otherwise it is safer to connect a simple voltage regulator that regardless of the load (to it’s rating level, for example 78L05 -> 100mA, 7805 –> 1A) will provide nice and clean +5V ..

Rgds,
IanP

 

[biff44]

Re: Voltage drop ...

Yep, while it is tempting to just use a dropping resistor, if the load current changes, or if the input voltage changes, the resulting voltage to your sensitive video chips will vary. If you go much above 5.5 volts, they can fry.

A fixed 5 volt output regulator is pretty simple to make. Take a LM7805, put a 0.33 uF cap between the input pin and ground, put a 0.1 uF cap between the output pin and ground, connect the ground pin to ground< and you are done.

Anywhere from 7 to 35 volts in, and you get a steady 5 volts out at one amp. If you have a big voltage drop across the chip, you will need to heat sink it with a piece of aluminum.

 

[threephase]

Re: Voltage drop ...

You didn't say, but if you are trying to interface to a PC, there's a regulated 5V and 12V in there, available on any socket that is provided for hard drives, CD drives, etc. Just a thought.

Failing that I'd concur with previous posters, i.e. 7805. Be aware that if current drain is more than a few milliamps the 7805 may get warm/hot. If necessary bolt it to a heatsink. Note that the device tab is at 0V so insulate if necessary.

If you are adding up current drain for both voltages, the current into the 7805 will be approximately the same as its output current (there's a bit of current down to ground through the middle pin but it's insignificant).

3phase

 

[abdullahsarwar]

there is a Ic of LM2576. so in that +5 volt voltage is not coming... so can any one help me in this sitution.. please reply soon.

 

[syeda amna]

the easiest solution is to use LM7805. it gives output of +5V.

 

[masifshahzad]

It you want switch mode supply then use LM2575-5.0 or LM2576-5.0 which is a 5V fixed regulator. And if u want linear type then LM7805 would be best. But switching regulators have far more advantages then linear one.

Plz see the schematic.

 

[abdullahsarwar]

the image which showed is right but. when we on it. its not on.......so if u please have the full diagram so just post it.....And if u know what can be the problem in this.....So please say me...

 

[masifshahzad]

Abdullah the image posted is complete. If your circuit is not working then might be there is some problem with your circuit. The schematic which i have posted is taken from datasheet of LM2576 and its 100% working.

 

[cool.man]

@Abdullah are you getting some voltages or there is no voltage (0v),double check your connections if your connections are right then might be the IC is damaged,try to conncect other IC and check the response,secondly make tracks as small as possible for Diode and Inductor.

Best wishes.