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Join Date: Oct 2007
Posts: 48

Voltage Regulator Chips

10/30/2007 4:43 AM

A warm hi to all!

I have query regarding regulator ICs available in the market.Say for e.g 7805 (1A).

If 7805 regulator is supplied with +12 at its input terminal in one case and in another case If it suppied with +24 v at its i/p terminal , then does this i/p voltage makes any diffrence in heating of the IC? (In both cases, assume the same load.)

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Guru
Engineering Fields - Control Engineering - New Member China - Member - New Member

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#1

Re: Voltage regulator chips

10/30/2007 6:07 AM

is i/p meaning current and power?

if you connet 24v at input, the power certainly changs. no problem.

the current depends on lyou r load. if load constant, the current dont change.

i= v/r

as your voltage increase, the power dissipate increase. your chip iwll be hotter than front.

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#2
In reply to #1

Re: Voltage regulator chips

10/30/2007 6:20 AM

I mean to say:

If 7805 is used to provide regulated +5v to one ciruit, with +12v at its input terminal in one case and with +24v at its input terminal in another case with the same ciruit(i.e. load does not get changed but only input voltage to the 7805 get doubled.)

As fas as i think as per your comment 7805 will dissipate more heat with +24 v at its input terinal than +12 v input.

Is it right?

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Associate

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#3

Re: Voltage regulator chips

10/30/2007 7:59 AM

As other said the power dissipation goes up with the input voltage. The power dissipation of t he 7805 is (Vin - Vout) * Iout. The efficiency of the power conversion is Pout / Pin or Vin / Vout is a very close approximation for linear regulation. If you can afford the power dissipation and want to keep the 7805 cool you can pre-regulate with an LM137, a zener in series or just a properley sized resistor in series. Either way you will probably need a heat sink. If you want to save some power and their is a few extra bucks in the budget try a switching regulator.

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Anonymous Poster
#4

Re: Voltage Regulator Chips

10/30/2007 11:22 AM

Well , your warm Hi to all is quite apropriate.

Using your example , the 7805 is a 3-pin +5V 1A voltage reulator.

To calculate the power dissipated in the regulator , you subtract the output voltage ( 5V in tis case ) from the supply voltage ( say 12 V) and multiply by the load current.

The load current wil vary somewhat, but lets assume full load of 1A.

then the power dissipated is (12-5)X 1 = 7 Watts.

If the supply vlotage was 24v then the power dissipated would have been 19 Watts.

Your heatsink wil have to be sized to dissipate the heat from the regulator.

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Anonymous Poster
#5

Re: Voltage Regulator Chips

10/31/2007 12:41 AM

Hai everybody....

Actually the 7805 Regulator is a +5V regulator. When it is fed with +12V it will regulate the o/p voltage to +5V consuming some current internally... When the 7805 is connected to +24V input the o/p anyway will be regulated to +5V but the current taken internally to regulate is more than compared with the +12V input..

The load current depends upon the load. The total current is the summation of the Load current & the device internal consumption current.

Thanks & Plz. reply I am wrong.....

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#6

Re: Voltage Regulator Chips

10/31/2007 1:30 AM

Thank you very much Everyone here to contribute your valued experience.

My concept gets cleared regarding regulator chip.

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Guru
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#7

Re: Voltage Regulator Chips

11/04/2007 9:33 PM

Does anyone know what the maximum input voltage is for the 7805?? I don't have a book handy, but I would check that parameter before running it from 24 volts.

Bill

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#8
In reply to #7

Re: Voltage Regulator Chips

11/05/2007 2:46 AM

Yup!

definately i knew it!

Max. i/p to 7805 is 35v

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