7805 heating

Hello..

i developed an application (12f629) to turn on or off my car using the alarm control. To stabilize the voltage i've used the 7805 regulator. I've used 4 relays in this project. It is draining at most 250mA when the 4 relays are on. This situation of 250mA draining can occur for 40seconds and no more than that. Is it normal for the 7805 to get hot? Should I use a heat sink?

ps.. i can let my finger on it when it's full loaded.

thanks...

Sylvio

Quote:

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Originally Posted by sirvo

Hello..

This situation of 250mA draining can occur for 40seconds and no more than that. Is it normal for the 7805 to get hot? Should I use a heat sink?

ps.. i can let my finger on it when it's full loaded.

thanks...

Sylvio

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Yes...and yes. You are dissapating ~8 volts at .250A which is two watts. I believe the regulator will eventually fail without an adequate heat sink. You can also put a 6 volt .25A bulb in series with the battery supply line to drop the voltage. The advantage to this is you will know when the relays are activated.
Ron

It should be normal if there is no heatsink.

Why don't you use a heatsink or a kind of metal plate to attach it?

Find a metal part around your circuit and use it as a heatsink.

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ok. Thank you both. i'm gonna find some heat sink.

thanks a lot..

sylvio

again.. do you think that a heat sink of 1 cm² ( 0,155 sq. inch) is enough?

reminding you that the ~2W are wasted in at most 40 seconds and rarely this will be wasted 2 times consecutively...

thanks again,

Sylvio

Quote:

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Originally Posted by sirvo

Hello..

i developed an application (12f629) to turn on or off my car using the alarm control. To stabilize the voltage i've used the 7805 regulator. I've used 4 relays in this project. It is draining at most 250mA when the 4 relays are on. This situation of 250mA draining can occur for 40seconds and no more than that. Is it normal for the 7805 to get hot? Should I use a heat sink?

ps.. i can let my finger on it when it's full loaded.

thanks...

Sylvio

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Hi, what if you kkep relay coils on unregulated voltage and release the burden from 7805?
Sarma

Quote:

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Originally Posted by mvs_sarma

Hi, what if you kkep relay coils on unregulated voltage and release the burden from 7805?
Sarma

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That is doable, I would switch the relays with a transistor to protect the pic.

it is working very well since i plugged the heat sink on 7805 regulator..

no more heating at all.. thanks, :)

mvs sarma.. i would like to do that but i've already bought some 5V relays.. and my supply is 12V ..

Joe S. - yes, that is what i have done.. i've used four bc337 transistors to switch the relays...

Sylvio

Quote:

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Originally Posted by sirvo

it is working very well since i plugged the heat sink on 7805 regulator..

no more heating at all.. thanks, :)

mvs sarma.. i would like to do that but i've already bought some 5V relays.. and my supply is 12V ..

Joe S. - yes, that is what i have done.. i've used four bc337 transistors to switch the relays...

Sylvio

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Hi, what if? you canadd a resistor series to the coil to limit the current, or you can use the transistor in emitter foller mode where the volage at emitter is less than the voltage at base.

Hi, Sarma

V. Good idea ...

If you use a series resistor AND a capacitor between resistor and ground ( say 100 to 470 µF ) you will have enough energy to power up the relay ... and the resistor will limit the quiescent current ...

Alain

sarma.. yes, it was a possibility.. but now it's done.. thank you for giving solutions... next time i'll use this information..


:)

Sylvio