piclist 2001\11\12\113219a >
Thread: PIC overheating a 7805 !
www.piclist.com/techref/power.htm?key=7805
BY : Mike Blakey email (remove spam text)

Hi Kat,

I Think I misunderstood the original point of the thread and only interperated what
I thought.

According to National Semiconductors data sheets the TOTAL Power is

Pd = ((Vin -Vout) * Load Current) + (Vin * Quesient Current)

MAximum allowable temperature is Tr(max) = Tj(max) - Ta(max)

Tj(max) is 125deg C    and Ta(max) is the maximum ambiant for your application.

Thermal Resistance(ja) = Tr(max) / Pd

Use this to calculate the TR of the heatsink.

I hope that helps?

katinkaMAGESTOWER.COM on 12/11/2001 22:28:47

To:     PICLISTMITVMA.MIT.EDUINTERNETwtgw
cc:
Paper Mail:
Subject:        Re: [EE]: PIC overheating a 7805 !

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Hi Mike,

I have searched data sheets from natsemi and TI, and cannot see how to
calculate power disapation.

Could you please provide a reference.

I disagree with your comment, to me a linear device working in a linear
region has to dissipate the power not consumed by the load ie

If I have 24 volts in, 5volt out to the load, load consumes say 1 amp.

Power into regulator is 24W, power out is 5W, so the regulator must consume
the missing 19W.

With your idea, the max power consumption would be 152mW ( vin - vout * Qc)
= (24 -5) *0.008A (from the TI data sheet maximum value)

Regards,

Kat.

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