LM7805 regulator heat problem

[wakoko79]

Hi again...
I'm now building a driver box for a robot arm. My problem is about the supply of its encoders..

The encoders, of course, needed a supply for its operation, in this case its 5v. There are a total of 5 sets of these encoders (A, B, Z and Limit switch). My main supply is a 24V DC source (ALL power is taken from it {motor supply, IC supply, optocouplers, etc..}building it is part of my project), but for now, I'm using a power supply off the shelf.

As you can guess, the 24V is the input to the LM7805 which outputs 5V. I know its not very good to do that for efficiency... I only did that because I thought a SINGLE LM7805 could handle the supply for all the encoders... I was astounded when the current reading was around 0.75A.... The LM7805 went really hot. Nearly-skinned-my-finger-damn-hot.
I mean P= (24-5)*0.75=14.25 watts is NOT negligible.

I searched the net for solutions about this without considerable success.. I know something about this could be done since I'm just "copying" the design from the old design of my adviser.

I wanted to find out the components of his circuit so I can copy it (the encoder conditioning part of my project is really just an extra, its just couple of 74LS245 buffers to buffer the encoders from the external microcontroller), but there is so much wiring in the driver that to see the markings of components, I have to disassemble it. I can't disassemble it now since someone is using it and if I break it I'll make someone really mad..

I already sent an e-mail to my adviser, but for the mean time, I just want to find out help from the forum..

Can anyone suggest an alternative to the LM7805? thanks!!!!!!!

 

[Boncuk]

Hi,

the regulator has to dissipate 8.93W which it converts to heat.

Use a heat sink to get it cooler.

Boncuk

 

[wakoko79]

I already considered that.. Isn't there any switching regulator IC out there? I've read that its really efficient.. But I don't want to build one just for the sake of the output... I mean, building the 24v supply is the other half of my thesis.. I think you can get my point..

 

[Gayan Soyza]

I'd simply use LM7805 with a high current pass transistor & give more heat on the transistor.

 

[Grossel]

I'd simply use LM7805 with a high current pass transistor & give more heat on the transistor.

Just go ahead and do this. It's the second best solution behind PWM.

Method is described in the LM7805 datasheet.

 

[crutschow]

I'd simply use LM7805 with a high current pass transistor & give more heat on the transistor.

How does that help? You still have to dissipate the same amount of heat with a heatsink.

 

[Dragon Tamer]

If he is running a robot that has a 24V power supply then it is safe to assume that the power supply is just two 12V batteris connected in series right? Just supply the 7805 with power from one battery.

 

[audioguru]

I have seen some switching regulators in the same 3-pins TO-220 package like a 7805. They do not get hot.

 

[Diver300]

https://uk.farnell.com/1703410

That is the sort of thing. That particular one is only 0.5 A, but there are many others.

 

[OlPhart]

One simple trick I use is to put power resistors in the feed of XX(24V your case) to the 7805 to drop it to ~8V at max(+ some margin) current. 78xx needs min 3V differential, and then it'll settle up higher at lower currents.

Allow enough room to spread the heat and stay well within total rating. I prefer being <1/2 total rating and multiple resistors to get there.

'Mem to put stabilizing caps right at the 7805 input, after the resistors.

Making a 78xx arbitrate 19V is just Mean, give the guy a chance... <<<)))

I've also used power (3+A) diodes in a string to the same result, but a more deterministic drop per unit.
I'd stay resistive: 19V is a Long string... season to taste.

 

[bountyhunter]

I already considered that.. Isn't there any switching regulator IC out there? I've read that its really efficient.. But I don't want to build one just for the sake of the output... I mean, building the 24v supply is the other half of my thesis.. I think you can get my point..

Tons of them. The 'simple switcher" line like the LM2575 would do it easily.

 

[Mr RB]

Fix the encoders. "5 sets of encoders" that draw 0.75A is just silly!

Encoders can work fine with 5mA of LED current, and even with 2 LEDs per encoder thats; 10 LEDs * 5mA = 50mA.

 

[wakoko79]

Fix the encoders. "5 sets of encoders" that draw 0.75A is just silly!

Encoders can work fine with 5mA of LED current, and even with 2 LEDs per encoder thats; 10 LEDs * 5mA = 50mA.

yes, that's what i thought... my adviser said that the rotary encoders really won't get damaged it i connected the wrong wires, instead, the buffer inside the chassis of the arm will be burnt.... I really don't get it but the buffered encoders inside the arm needs to be buffered again before passing the signals to an MCU...

 

[wakoko79]

One simple trick I use is to put power resistors in the feed of XX(24V your case) to the 7805 to drop it to ~8V at max(+ some margin) current. 78xx needs min 3V differential, and then it'll settle up higher at lower currents.

Allow enough room to spread the heat and stay well within total rating. I prefer being <1/2 total rating and multiple resistors to get there.

'Mem to put stabilizing caps right at the 7805 input, after the resistors.

Making a 78xx arbitrate 19V is just Mean, give the guy a chance... <<<)))

I've also used power (3+A) diodes in a string to the same result, but a more deterministic drop per unit.
I'd stay resistive: 19V is a Long string... season to taste.

thanks.. I have a couple of 1ohm 5w power resistors.. can I use them for the divider? I think it will exceed the power rating, but what the hell, let's hear it from the experts...