power supply and 7805 reg confusion ??

hi, just looking up some specs on the 7805 regulator , one site says a max of 35vdc input and this says 25vdc max input

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,,,,,, i`m pretty sure the max is 35vdc ?

what is correct ?

also i have nearly finished building a fixed and adj power supply all in one , and after the filter cap i have a reading of 35.9vdc , now i`m relying on the max being 35 vdc for the reg , so how can i EASILY knock the voltage down to about 25 volts or so ?

thanks,'

mark k

Reply to
mark krawczuk
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Whatever the datasheet says for your brand of 7805 device.

Have you done your thermal calculations on your required heatsink? What is your max output current? A typical 1A capable output will dissipate 20W in the 7805 at max current with a 25V input. Whatever you use to drop the 10V will also dissipate 10W @ 1A.

Dave.

Reply to
David L. Jones

I wonder if a zener and a SCR might do the trick on the supply side.

Reply to
Wayne.

No transformer taps available?

If you're not bothered by the prospect of the AC input doing a +10% excursion, then just chuck an extra diode into the bridge output.

Reply to
spot the fake

max

in

=A0 knock

I always believed that it was 35v actually, but this may vary between manufacturers.

The simplest way is this item from Farnell, though note that its only

0.5A output:

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c/dp/1278561

(I have used this sort of device before to make a +5 V rail where only

28VDC was available, the only drawback these devices may have is they wont let you put a diode from the "Adj" pin to 0v to raise the output voltage. The ADJ pin must always go direct to the 0v rail.)

Alternatively: Unless you are only going to be supplying a small output current on the 7805, (say up to 100ma) then the only proper way is to reduce this

35v at the source, either by removing windings from the transformer, or installing an extra transformer to supply the lower voltage to your regulator setup. Something like 9-12V DC would be a pretty good choice for an input to a 7805, while allowing plenty of headroom to keep the +5v stable if the mains were to drop or fluctuate. A transformer like DSE.com.au part no M6672 would be a good choice. Add a rectifier and filter cap and its ready to go.

You can use a large resistor or pre-regulator (7815,7824) to drop it, but as said by David L Jones you will be dissipating lots of heat. For example if you plan on pulling 1a from the system, you will be dissipating 30 w (30v x 1A). (The 30v figure is the 35v in less the

5v out). Whether the 30w is dissipated in one or 2 regulator devices or in a HUGE resistor and a regulator, its still a lot of heat you will need to remove from the case, (and pay for on your electricity bill) probably will probably also need a fan to do this.
Reply to
kreed

**Depends on the application. You could use a suitably sized resistor. Better, would be to use a zener/series pass transistor ahead of the regulator. You could use a cheap, high power transistor to reduce the regulator input to 9 Volts or so, reducing the dissipation of the regulator. With a cap from Base to earth, you could gain some extra regulation too.

Read David's post. There are important questions/points there.

--
Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson

Whether a transistor, series diodes, resistor, or a pre-regulator is used, the excess power *will* be dissapted as heat directly related to the voltage to be 'lost' and current..

If the current is significant, then an additional (or different) transformer is the easiest approach.

geoff

Reply to
geoff

**Of course. However the history of the OP's ideas suggests that common sense is in short supply. He will likely take the most circuitous and inconvenient route to achieve his aim.
--
Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson

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