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7805 help

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neoandrewson

New Member
Hi ppl,

I have made a simple circuit with 7805 with dc input voltage from a wall adaptor ( 12V 500mA), the circuit is from Simple 5V power supply. It states a current rating of 150 mA, but i've used 500 mA current rating. The output of the circuit is 11V. Have is screwed my chip? i've changed the chip and still it shows same voltage at output.
 

MikeMl

Well-Known Member
Most Helpful Member
Hi ppl,

I have made a simple circuit with 7805 with dc input voltage from a wall adaptor ( 12V 500mA), the circuit is from Simple 5V power supply. It states a current rating of 150 mA, but i've used 500 mA current rating. The output of the circuit is 11V. Have is screwed my chip? i've changed the chip and still it shows same voltage at output.
Several things. Do you mean that the Wall adaptor is rated at 500mA, or that you are drawing 500mA from the regulator on the load side?

Are you sure that the wall-wart has a filter capacitor in it?

Is it putting out half-wave rectified or full-wave rectified DC?

Which package is the 7805 are you using?

Methinks that the only way that a 7805 will put out 11V is if it is fried, or if you have it misconnected. Check the pin order. Look up the data sheet for the specific variant of the 7805 you are using.
 

neoandrewson

New Member
Wall adapter is rated 500mA

Yeah it has a filter capacitor and a full wave rectifier circuit

Package type is TO-220

pin order was fine. complete circuit is attached.

chip is 7805CT

I used two 7805 alternatively, both are fried?
 

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Mikebits

Well-Known Member
I used two 7805 alternatively, both are fried?
There's your sign. Your doing something wrong. Can you post model number of the wal wort?
 

neoandrewson

New Member
There's your sign. Your doing something wrong. Can you post model number of the wal wort?
Its a local wall adapter. It says Panasonic AE - 240, but i'm sure it aint panasonic, some cheap local made.

I used two 7805 alternatively, both are fried?
I meant, first i used a 7805 IC, once i got input = output, i replaced it into another 7805 IC. First one may be smoked, second one too? Or did i do some thing wrong with capacitors? Led's glowing fine.
 

MikeMl

Well-Known Member
Most Helpful Member
What is the load like? Are you seeing the 11V with no load connected?

Are you sure the middle pin is connected to the negative connection from the wall-wart?

I find the 100uF capacitor at the input of the 7805 strange. It's too small to act as the main filter capacitor which you said was inside the wall-wart anyway. It's too big to be used to prevent oscillation in the 7805. For stability, the input cap should be a Tantalum or other low inductance type; the 100uF is probably an aluminum electrolytic which is too inductive and resistive to be used for stabilizing the 7805. Check the data sheet. It specifies the value and type of capacitor to be used there.

It is possible that the reason that you are seeing 11V is because the 7805 is oscillating. It would take a scope to find out.
 

Mikebits

Well-Known Member
Well that part number did not help. I think your wall adapter is not what you think, or your not connecting 7805 properly. Hard to say?
 

Mikebits

Well-Known Member
What is the load like? Are you seeing the 11V with no load connected?

Are you sure the middle pin is connected to the negative connection from the wall-wart?

I find the 100uF capacitor at the input of the 7805 strange. It's too small to act as the main filter capacitor which you said was inside the wall-wart anyway. It's too big to be used to prevent oscillation in the 7805. For stability, the input cap should be a Tantalum or other low inductance type; the 100uF is probably an aluminum electrolytic which is too inductive and resistive to be used for stabilizing the 7805. Check the data sheet. It specifies the value and type of capacitor to be used there.

It is possible that the reason that you are seeing 11V is because the 7805 is oscillating. It would take a scope to find out.
Hmm, that sounds plausible...
 

neoandrewson

New Member
Originally Posted by MikeMl View Post
What is the load like? Are you seeing the 11V with no load connected?

Are you sure the middle pin is connected to the negative connection from the wall-wart?

I find the 100uF capacitor at the input of the 7805 strange. It's too small to act as the main filter capacitor which you said was inside the wall-wart anyway. It's too big to be used to prevent oscillation in the 7805. For stability, the input cap should be a Tantalum or other low inductance type; the 100uF is probably an aluminum electrolytic which is too inductive and resistive to be used for stabilizing the 7805. Check the data sheet. It specifies the value and type of capacitor to be used there.

It is possible that the reason that you are seeing 11V is because the 7805 is oscillating. It would take a scope to find out.
Hmm, that sounds plausible...
I'm seeing 11V at no load, using multimeter. In ac mode it shows 24V (i dont know if its of any help) both at input and output. So i reason that 7805 is acting as a short.
I have connected the negative terminal to pin 2 of 7805
I guess i'll get another 7805 and try with 100 nF cap values both at input and output.
Btw, the cap in adapter is 1000uF.
and 7805 heats in few minutes.
also is it possible that 500mA current is killing it?
 
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Torben

Well-Known Member
What size is the filter capacitor on the wall wart? [Edit: Never mind, I was composing this message when you posted the answer.]

How much current are you trying to draw through the regulator? Or are you just measuring its voltage and not attaching a load yet? [Edit: Ditto.]

The 78xx datasheet shows a 0.33uF cap from pin 1 to gnd, not a 100uF cap. Does changing that make a difference? And while the value of this doesn't have to be exact (I've heard that people often use 0.1uF with no trouble), 100uF is 303 times as large as 0.33uF. That seems excessive. [Edit: It appears that I totally missed the second paragraph in MikeMI's post. My bad. But at least now I know that I'm not completely insane.]


Torben
 
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Mikebits

Well-Known Member
I think he is using an AC wall wort. Putting out AC not DC. Hard to say though.
 
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Torben

Well-Known Member
I think he is using an AC wall wort. Putting out AC not DC. Hard to say though.
It could be. But I don't know why it would read 11VDC and 24VAC on the meter. Measuring the mains here now with my meter shows 118V on the AC setting and 0V on DC. I think that means I have a crap meter. :)

Also, he said the wall-wart had full wave rectification and filtering and was able to identify the size of the filter cap, so that makes me think he is using an AC to DC wall wart.

I haven't done the math but a 500mA load, with only a 1000uF filter cap, no heatsink, and a 100uF input cap on the 7805, all running from several volts over 5VDC, sounds like a recipe for a dead 7805. That's a lot of extra power to dissipate.


Torben
 

Mikebits

Well-Known Member
I am thinking 12VAC output of wall wart. I dunno, it does not make sense. A 12 VDC supply into a 7805 should not smoke it regardless of .33uf caps or 100uf caps. Also wall wort could be 10 amps, but unless 7805 is being overloaded it should not matter. I suspect we are not getting the full picture. Something does not make sense.
 

Techie7

Member
Pin conf of 7805 is 1,2,3 when looking on to the face where part no. is written. Check out for reversed connection if any.
 

Torben

Well-Known Member
I am thinking 12VAC output of wall wart. I dunno, it does not make sense. A 12 VDC supply into a 7805 should not smoke it regardless of .33uf caps or 100uf caps.
If the wall wart has a 12VAC output that could do it. For that to be the case though, either the OP is seriously misinformed as to what wall wart he's using, or he's lying.

Also wall wort could be 10 amps, but unless 7805 is being overloaded it should not matter. I suspect we are not getting the full picture. Something does not make sense.
He did say it takes a few minutes for the regulator to heat up, which if you combine the excess heat dissipation and the possibility of oscillation, seems reasonable.

But yeah, I also I don't think we're getting the whole picture. Something is definitely fishy with those meter readings.


Regards,

Torben
 

Mikebits

Well-Known Member
All I can say at this point is that a 12vdc 500ma Wall wort should not smoke a 7805. OP is doing something very wrong. I dunno.
 

Torben

Well-Known Member
All I can say at this point is that a 12vdc 500ma Wall wort should not smoke a 7805. OP is doing something very wrong. I dunno.
No, I agree. But if he tries to pull 500mA through it and it has poor filtering, and oscillates, and has no heatsink, it could be bad news. It might not fry instantly but I don't think it would last long. It might also just be shutting itself down. We don't know enough to say for sure. (At least, *I* don't ;) ).

I know the current rating of the wall wart doesn't come into it. :)


Torben
 

forumlicker007

New Member
Mike you are very right. 7805 is rated for handling around 1A and somethin upto 35v. He might be doin some strange things.
 

Torben

Well-Known Member
Mike you are very right. 7805 is rated for handling around 1A and somethin upto 35v. He might be doin some strange things.
With 35VDC input and a 1A load you need some serious heatsinking or the 7805 is going to die. You have to read the rest of the datasheet. And build a few power supplies with it.

He is definitely doing some strange things. That's why his circuit is not working.


Torben
 

flat5

Member
As stated above, polarity or connecting to the correct leads of the device is very important to not destroy it.
A clear picture of how it is connected is needed to help say what the problem is.
 
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