primitive circuit - where is my mistake?

I was putting together some bits for a power supply. Ive been away from electronics for too long and am trying to get back in the swing.

At this point here is what I have:

+18V---770 ohm resistor --- led -- ground | ----------4700 uF electrolytic ----- ground | ---------- (Vin)7809 voltage regulator (Vout)----- (nothing) | -------ground

when powered up the led lights but the vr gets v hot v quickly. There is no load on it. Where is my mistake?

is

result

The 3 terminal regulators are a hardy bunch but they are NOT indestructible! Spend some time studying the package and its pin layout. Take note of John's instructions regarding the caps and note that you are loosing 9 volts across the regulator IF it were to work correctly. That will generate lots of heat and will require a heat sink of sufficient size to cool the part. It is possible that your part went into thermal shutdown mode. Try applying not less than 11 volts to the input of the regulator.

Whether or not it's the cause of the overheating I don't know (I'd have suspected incorrect pin connections), but you are supplying the

7809 with only about 1.8V !

Swap the LED and R.

BTW, that's an odd value for R? How did you arrive at it?

--
Terry Pinnell
Hobbyist, West Sussex, UK

--- Dunno, but to consolidate some other replies, try this:

_____ | O | |_____| | | FRONT VIEW OF 7809 -->| | |_____| | | | IN-->| | |-----+-------+-------+----+ | +--+------+--->+9OUT | | | | | | [820R] |+ | | | | | [1000µF] [0.1µF] | [0.1µF] [1K] [LED] | | | | | |K | | | | | GND>------+-------+-------+------+----+------+--->GND

Solder the capacitors into the circuit as close to the 7809 as possible.

-- John Fields

Aye Terry,

You must have been very sleepy (or I must be now). Or, as I see now, you supposed to be the "schematic" in fixed font. As far as I can see it is not. So no need to swap R and LED. They are simply in series. As for the resistor, suppose they LED to need 2.4V. So you have (18-2.4)/770=20mA current through the LED. (Or, the other way around 15,6/20=780Ohm.) You and I should decide to take an 820Ohm resistor.

As for a 7809 becoming hot without load, it cannot be but defective. Maybe it has been flipped. One way to destroy an otherwise rugged 78xx regulator.

petrus bitbyter.

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Ive tried 2 different 7809s now with the same result. Why am I only seeing

2V from gnd to Vout?

Ill add the extra caps and put a small load on it and see what happens.

result

In that case I sugest it was the wrong way round to start with and was fried pretty quickly. if it wasnt it certainly is now ! instability might be a problem but i would be surprised if it cuases a hi curent through the gnd pin wich is the only place to go in order for it to heat up.

Best bet is to find a better drawing to look at, and get a new regulator.

Colin =^.^=

The OP said he couldn't tell which side was front.

The front is the side the lettering is on, and the metal tab is flush with the back of the plastic part of the package.

--
Peter Bennett VE7CEI 
email: peterbb4 (at) interchange.ubc.ca        
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html 
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Perhaps the problem is too much input V? How about if I put an 11V (or similar) zener in front of the voltage regulators? (the final circuit will have 4 vrs).

Thanks petrus, you're right - I mistakenly read it like this:

--
Terry Pinnell
Hobbyist, West Sussex, UK

"anonymous" schreef in bericht news:ESb9d.83937$He1.22203@attbi_s01...

Get a datasheet of that regulator, so you can read for yourself what max. Vin is allowed. If you have an 18V input *and* a 220nF decoupling capacitor on the input *and* another 100nF on the output *and* a load of let's say

1k/1W on the output *and* have connected your 7809 correctly *and* measuring only 2V on the output *then* either your voltmeter or your 7809 is gone. An 78xx regulator that becomes hot without significant load it is defective anyway.

petrus bitbyter

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--
If you look at the rear of the TO-220 package you won't see the
dividing line between the plastic part of the package and the tab.

However, if you look at the front of the package you will, and I've
shown the dividing line, above.  With that as a reference, the OP
should be able to hook it up as shown.

Ok. Thanks for all the input here. I got it working.

Are you certain that you have a 7809 regulator ? Could it be that you have a component with a date code from February1978? (9th week of 1978)

"Peter Bennett" schreef in bericht news: snipped-for-privacy@4ax.com...

That's one of the reasons he needed a datasheet.

petrus bitbyter

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