To much heat from 7805

[kentken]

I have a small plastic box that houses a stepper control circuit and a Pic micro with small LED display.
It is to be powered by a 12v car battery. The stepper driver and Pic are 5v.

I have tryed a 7805 SMT mounted on the board, and it creates more heat in the box then I want.
It is to be hand held at times and I don't want it to get to hot.
I dont have my temp sensor for my meter handy, so I don't know the actual tempature. I will try to find out tomarrow.

Any Ideas on other voltage regulators, or additions to keep the heat down??

Thanks
Kent

 

[Someone Electro]

An quick fix would be mounting the reg on a hetsink that s sticking outisde the box.
These linear regs work by thurning the over voltage in to heat.So if you have 100 mA runing trough your reg you would make 0,7W of heat and whith no heatsink this can get realy hot.(Soldering irons have like 10 W)

An good fix would be to replace it whith an switchmode powersuply.These generate very litle heat.Becose the acses voltage is thurned in to amps rather than temperature.A lot of companys make switcmode controler ICs(Maxim,National semi,Microchip,Texas instuments...) Some have the intergrated fet so you just wire up a few resistors,capacitators and an inductor.For more power just use an external fet one and throw in an power MOSFET.

So you decide the ulgy simple quick fix or the slitly more complicated nicer good fix.

 

[motion]

Don't power the stepper from the 7805 regulator. If you run the stepper only briefly and power down the windings after moving, you can actually run it off the 12V supply. If not, you can use a chopper driver circuit. You can use the built-in PWM generator of the mid-range PICs.

 

[stevez]

The SMPS is probably a better solution but an alternative that might help is to add an external resistor at the battery/supply. The resistor value would depend on the maximum current.

You might decide to have the voltage to the regulator input be something like 8 volts to give it a little headroom. If the current were 750 ma then a 5.3 ohm resistor would do the trick (3 watts to dissapate). 5 ohms would probably a more standard value - the 750 ma is a guess by me so you'd need to calculate your own values and make adjustments for standard values. This will ease the burden on the regulator but not make the problem go away entirely.

Another voltage regulator at the battery doing the same thing as the resistor is also possible with proper heat sinking.

These aren't examples of good practice but might be a temporary measure.

 

[kentken]

Thank you for the replys.
What SMPS would you sugest? I am looking for simplest, and least amount of parts.
I need one that will work with a input of 9v to 14v with a output of 5v and takes up the least amount of board space, and can be homemade(not tiny smts)

Thanks
Kent

 

[Roff]

How much current does your load draw?

 

[kentken]

It draws about 500 to 700 mA.

800 to 1000 mA would probly be best.

Thanks

 

[Someone Electro]

This one would do but its in an smd packege that is dificult to solder.
**broken link removed**

 

[Roff]

Take a look at LM2575-5.

 

[kentken]

Thanks
I think that the 1575 or the 2598 would work for me.

Thanks for the ideas
Kent

 

[boncker]

I did the same thing "Someone Electro" told you, to put a power MOSFET and the 7805 drives the voltage. The MOSFET will be a current supply to your circuit.
I my circuit, a power supply, I put 2 BJTs running as current supply and a 7812 in the bases, as voltage reference.

Good luck!