12v to 5v DC circuit help

[lofar]

Hello everyone, just looking for some help.

I'm moderately versed in electronics but am rather stumped at this simple circuit i'm trying to put together.

I'm trying to make a smallish device that will go in my car and convert 12v DC current to 5v DC current to power a couple of things. I have the base of the circuit built which is built around a 7805 regulator putting out my 5v current. My main problem is that the 7805 regulator works fine for small devices when I plug anything significant into it it immediately gets hot and shuts down. I would like to keep this device small so I'd rather not just slap a huge heat sink on this to solve the problem. I've tried some simple divider circuits and just plain old resistors to drop the input voltage but any of those solutions won't allow me to draw enough amps to power anything. The devices i need to power draw around 500ma and it appears that with a 12v input the max the 7805 will handle is around 100ma.

Any thoughts on what I can do to drop the 12v input to 5v and provide adequate amperage without going to a large heat sink?

 

[ericgibbs]

hi lofar,

You say you require about 500mA current.

In free air the 7805 will get hot if you try to draw over 100mA. Thats why it has the heatsink plate and fixing hole.

An important point to remember is that when the car engine is running the battery could up around 14 Volts.

So at 500mA and [14V-5V] you have 4.5Watts.

Whats the reason why you don't want to use a heatsink?.

EricG

 

[Gayan Soyza]

Any thoughts on what I can do to drop the 12v input to 5v and provide adequate amperage without going to a large heat sink?

Then you give the supply from a 4.8V battery pack.
Otherwise use a regulator or a transistor with a zenner both needs adequate heatsinks.

 

[dch222]

There is no way around it.
Unless you use a switching regulator, you have to get rid of that 4.5W somewhere.

You don't need a purpose built heatsink - just bolt the regulator to a bit of metal making sure you use the correct insulating kit to insulate the regulator from the metal if the the metal is part of the car, or could touch it at any time. You won't need the kit if you can keep the metal insulated from the car.

If the current is constant at 500mA, you could dissipate some of the power in a seies 8.2 or 10 ohm 5W resistor before the regulator. The regulator may then not need a heatsink, or just a little one.

 

[lofar]

Thanks. The main reason is to keep the overall size of the unit as small as possible because it has to fit in a pretty tight area.

 

[Hero999]

https://www.electro-tech-online.com/custompdfs/2007/03/Max649.pdf

 

[Wireman23]

I have a similar circuit I'm putting together. I'm looking at the LM2940 drop out regulator. 5V in and 1A capacity. It seems ideal for automotive use (12V) as it claims to withstand max. input voltage of 26VDC. It also has protection against 2nd-battery jumps w/reverse polarity.

Does anyone have any comments about this item? Any drawbacks?

thanks

 

[Hero999]

The LM2940 is just another linear regulator and if you're going to use it at 12V has no significant advantage over the LM7805.