Author Topic: 7805 getting very hot.  (Read 4511 times)

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Offline minesapintTopic starter

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7805 getting very hot.
« on: February 01, 2008, 04:06:31 PM »
Dear All,

I have a circuit that I have been using on and off for the past year, while I try and teach myself robotics/electronics.  Everything has been up til now working fine - Ive just been turing leds on/off, using continous/non continous servos, ultrasonic sensors and an lcd.

I have decided to move from using servos to using motors and I have a couple of 1.5-3V motors connectted to a l298n h-bridge.  When both motors are running the 7805 gets very hot very quickly and the pic seems to brown out.  I am currently running the circuit through a 7.5 V mains power pack, the power for the motors comes from the regulated 5V that I use to power the pic - I figured this would be ok as they are 1.5-3V so the voltage drop from the l298n would still supply sufficent power for the motors.

Does anyone have any ideas as to what is causing my problem.

EDIT: If I only connect one motor, the 7805 still gets warm to the touch but the motor continues to run, so the problem is with 2 motors.  Any help would be appreciated.
« Last Edit: February 01, 2008, 04:08:56 PM by minesapint »

Offline Del

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Re: 7805 getting very hot.
« Reply #1 on: February 01, 2008, 04:29:55 PM »

My guess would be that 2 motor's are drawing too much current.
How much does one motor draw, and how much current can the
7805 source?

It might be better to get the power for the motors directly from the battery,
and let the microcontroller get it's power from the 7805.

HTH

Offline ed1380

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Re: 7805 getting very hot.
« Reply #2 on: February 01, 2008, 04:45:31 PM »
option1- hbridge to get power directly
option2- swirching regulator. much more efficient
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Offline SmAsH

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Re: 7805 getting very hot.
« Reply #3 on: February 01, 2008, 05:18:56 PM »
by my understanding h-bridges are quite sensitive right???
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paulstreats

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Re: 7805 getting very hot.
« Reply #4 on: February 01, 2008, 05:54:01 PM »
The startup power of motors exceed their normal operating conditions because they have to get the weight moving (storing up inertial energy), In consequence, the mcu will brown out causing the motors power to stop moving. When the mcu has enough power to start again and the motors start you keep going in the loop.

It might help to add some large capacitors to your power rails to help your power source cope with it. You really should try and use them in a way that doesnt go through the linear regulator though(and youll still probably need the capacitors).

Smash, what do you mean by h-bridges being sensitive?

Offline SmAsH

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Re: 7805 getting very hot.
« Reply #5 on: February 01, 2008, 06:57:35 PM »
by like to much voltage going through them...like spikes and stuff. ive killed one before with my antics.
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Offline minesapintTopic starter

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Re: 7805 getting very hot.
« Reply #6 on: February 02, 2008, 03:38:46 AM »
thanks for the prompt replies.  I'll try the suggestions of seperate power supply for the motors, see if that sorts out the problem.

Offline Rebelgium

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Re: 7805 getting very hot.
« Reply #7 on: February 02, 2008, 07:46:34 AM »
or the easy way out: just place two or more voltage regulator in parallel.
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Offline rargus

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Re: 7805 getting very hot.
« Reply #8 on: February 03, 2008, 11:25:00 AM »
I would not suggest putting regulators in parallel. Small differences mean that one may take almost all the load.

Look at figure 15 here http://www.fairchildsemi.com/ds/LM/LM7805.pdf

This is a better arrangement since the regulator's shutdown circuits still work

Offline gamefreak

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Re: 7805 getting very hot.
« Reply #9 on: February 03, 2008, 11:30:06 AM »
well if a regulator started doing all of the work then it would shutdown and the other would take over, I think, so then the two regualtors would be switching on and off, maybe leading into short brown outs...

I know that with MOSFETs you can put them in parallel and get extra current, but you cant do it with transistors, so does a regulator count as a transistor or a MOSFET?
« Last Edit: February 03, 2008, 01:14:35 PM by gamefreak »
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Offline Admin

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Re: 7805 getting very hot.
« Reply #10 on: February 04, 2008, 11:08:10 PM »
You shouldn't regulate the voltage to a motor, for this very reason.

Only have the regulator power the PIC.

 


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