7805 getting heat

cakrawibawa · Apr 30, 2009

Dear all,

I'm creating a 7805 based circuit for charging my cellphone when driving my motorcycle.

This circuit powered from the accu. It work normal when the motorcycle is off. but when I'm driving, it getting heated.

Is it caused by the high current from the alternator/coil?
What should I modify to make it work normal when I'm driving my motorcycle?

any help is highly appreciated.
Thanks, Cakra

picbits · Apr 30, 2009

Do you have a heatsink on it ?
What is your measured voltage across the input pins when your motorcycle is running ?
What is the output current ?
Have you got filter capacitors on the input and output ?
What is the ambient temperature ?
If you are using a heatsink, what size is it ?
Is there any airflow around the heatsink ?

mvs sarma · Apr 30, 2009

cakrawibawa said:
Dear all,

I'm creating a 7805 based circuit for charging my cellphone when driving my motorcycle.

This circuit powered from the accu. It work normal when the motorcycle is off. but when I'm driving, it getting heated.

Is it caused by the high current from the alternator/coil?
What should I modify to make it work normal when I'm driving my motorcycle?

any help is highly appreciated.
Thanks, Cakra

while traveling if the headlight is OFF during day time , the voltage at the battery terminals would be almost 14V. Think that you are trying to draw 200mA approx. The difference is 7V and current is 0.2 A, hence 3.5watts is wasted across 7805 which causes the heat. Better follow the suggestion given by Picbits by providing a suitable heatsink.

ayushsk · May 12, 2009

while traveling if the headlight is OFF during day time , it means ic7805 drive on more current .One thing you can do - one low value resistance approximatly 10 ohm 1- 2 watt, added in series with input terminal of LM7805 ic as well as providing heat sink to the IC LM7805. and insure cooling of charger should be there.

regards,

s.k.singh

Hero999 · May 12, 2009

Try the black regulator, 5V 200mA output, 88% efficient so no heatsink required and probably costs the same or less than the LM7805 plus capacitors and heatsink.
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7805 getting heat

cakrawibawa

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7805 getting heat

cakrawibawa

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