Can i connect +5v to the regulated side of a 7805 ??

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Hi Guyzz

I just bougt a cpl of Olimex AVR avr-p40b-8535 for my Mega32 project.

http://www.olimex.com/dev/avr-p4...
Schematic : http://www.olimex.com/dev/images...

I am going to use it in a chassis that allready have +5v regulated for a DVD drive , and wonder if i can just connect the regulated +5v to the "inside" if the 7805. Or would that be an absolute No..No

If not , i see two other options.

1: Dismount/Cut the trace to the 7805 output pin
2: Take the power from the DVD 12v (but i really dont wanna do that).

/Bingo

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Yes, that would be an absolute No..No You will fry the 7805

Randy

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Hmm

Damm ....

Well i'll prob desolder the 7805 then

/Bingo

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K5HJ wrote:
Yes, that would be an absolute No..No You will fry the 7805

Randy

I certainly wouldn't go quite THAT far, Randy.

"7805" is such a nebulous term, often used interchageably for "+5VDC regulator". They come in as many shapes and sizes as people.

There are many regulators that ARE very sensitive to being back-powered. But there are also many that are NOT. [The second category includes the ones we use. My normal dev. mode is to power the target through the ISP cable (STK500) for programming & bench testing. Some apps rarely get powered through the regulator except in production units.]

The OK regulators will have a bypass diode to the regulator input signal. Or you can add one to your design or test setup. Or sometimes the regulator has other circuitry so it won't mind being back-powered.

Summary to Bingo: If your regulator is one of the sensitive ones and you back-powered it, you very well could have fried it--if that is your question.

If your question is: "Can I use this alternate source of Vcc?" I say Yes. But first, check the datasheet for your regulator to see if there is a bypass diode. If not, install one yourself on your design.

Definitely not an "absolute no-no"--unless every 7805 in the world has no bypass diode.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Well it is a 5v Reg in question :lol:

@theusch

Well the design is OLIMEX not mine ...

The regulator is a 7805 , but i can't rekognice the manufactor logo

It lookc kinda like a "tilted" G or a C with a little dot at the end of the lower end on the C/G. Or a werry "New Half Moon" with the dot at the end of the lover part.

Anyone knows that logo

So i can search for a Datasheet

Hmm the bypass diode , shoult it run from in to out or ???

If i put a diode after the 7805-Out then i will loose 0.7v wont i ??

/Bingo

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Bingo600 wrote:

Hmm the bypass diode , shoult it run from in to out or ???

If i put a diode after the 7805-Out then i will loose 0.7v wont i ??

1) pointing from VOut to Vin--reverse bypass of the regulator. [Note that you are then powering anything else on the VIn side of the regulator with +4.n VDC.]

2) No, if you bypass as I mentioned above.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Ok

So i will in fact lead 4.3v from Out to IN

I think i have a couple of 1N4005 , they might be better than 1N4148 (Prob too low current)

Thnx

/Bingo

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You can connect seperate supplies together in certain cases. If you add some impedance (a resistor) of an appropriate value and size, then two linear regulators can be connected together (like the LM7805). Here is an example. Lets say that two 5V supplies (supply1 and supply2) need to be connected together. A typical tolerance is +/- 5% for each. This means that one supply may be as high as 5.25V and the other may be as low as 4.75V. There is a maximum of 0.5V difference between them that has to be dropped across our protection resistor. Lets also say that we can afford to give up 50mA of lost capacity to protect ourselves. Then a resistor of 10 ohms will do the trick (0.5/.050).

Here's the kicker. If both supplies are powered then there are no problems. However, if either supply is off, then one or both of the boards will be powered through the 10 ohm resistor. If current draws are low (say < 20mA) then there should be no problems dropping the extra voltage. However, if current draws are higher, then the resistor value has to be lowered. This will result in additional losses when the voltage mismatch is high,

Diode oring of the supplies is also an option. However, you will probably want to use schottky diodes for the lowest voltage loss.

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I dont want to feed power to the input side of the 7805 , as i would have to feed it with 12v from the DVD-Drive powerline , it will dissapate 7v on the 7805 , and there is no need to do that (even if its prob not much current).

I would just like to feed the regulated +5v from the DVD-Drive directly to the board VCC (The same PCB-Traces the 7805 Out)

/Bingo

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My method describes connecting two +5V supplies (regulated side) from two differnent sources.

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Quote:
My method describes connecting two +5V supplies (regulated side) from two differnent sources.

Your method is interesting, but it wasn't related to Bingo's question. :?

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Hi,

If you don't want to put the diode in, couldn't you just connect VIN and VOUT to each other and to 5V. This will be pretty much the same as connecting a diode in-line, the only reason to use the diode is so you can still power up the regulator if you want. However you don't sound like you care about that.

-Colin

Edit: by diode in-line I mean between VOUT and VIN pin

Last Edited: Wed. Sep 15, 2004 - 10:46 PM
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Quote:
I am going to use it in a chassis that allready have +5v regulated for a DVD drive , and wonder if i can just connect the regulated +5v to the "inside" if the 7805. Or would that be an absolute No..No

I assume that the DVD is part of the project. If this is the case, and since both the Olimex and the DVD are 5V devices, then just connect the common from the Olimex to the common of the DVD player. Power the Olimex from a "wall wart" power supply that is run into the DVD chassis. There is no need to connect the +5V supplies together for debugging and development.

For a more permenent solution, you could remove the 7805 on the Olimex (or cut the trace to the output) and power the Olimex from the DVD.

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Hi,
I think there is nothing happened, look at the schematic in datasheet (attached). Even both regulators are connected together by output pins and powered from different sources. If the outputs are different the lowest one just closes output transistor by differential control circuit.
Regards, Alex

Attachment(s): 

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Quote:
If the outputs are different the lowest one just closes output transistor by differential control circuit.

Another very interesting answer, but it's not an answer to Bingo's question. Perhaps you didn't understand the question, he intends to have 0V at the input of his regulator while applying 5V at its output.

There is no power applied to the differential control circuit except through paths perhaps not forseen by the designer. The 7805 schematic does not clearly show what would happen in this case.

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Sorry, it is not the answer, it is writen in the schematic. And just my comments.

Well, if no power on input site so everything is just closed, the only leakage current through the R17-R18 is about 1-2 mA, probably much less (I don't know the R17 value, you are so interested you could measure the resistance between OUT and GND).

Regards,
Alex

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I'm interested in this thread because I have an application where an external regulated 5V input could be used instead of the onboard regulator.

mneary wrote:
he intends to have 0V at the input of his regulator while applying 5V at its output

How is 0V being applied to the input side of his regulator? Wouldn't it just float? Is it bad to connect 5V to the regulator's OUT while leaving the input floating?

/* John Butera */

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I was hoping that i could leave the input floating

And just connect the regulated +5v from the dvd players PSU.

I'm remodelling a KISS-DP330 (Wife likes that chassis) , and its "just a DVD player".

I have gotten the M32 (in my STK500) to control the PT6311 Frontpanel , now i need to connect the Olimex board instead.

Then i'll have a go of the IR receiver on the frontpanel (its RC5) , and danni has made some nice C routines.

Then i hope VIA has released their Nano-ITX boards , as i have 14x14 cm space in the chassis , to small for a Mini-ITX but ok for a Nano-Itx.

So i might end up with a PC (Oopzz a DVD) in the living room anyway :lol: :lol:

/Bingo

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I think if I wasn't ever planning to USE the regulator, that I'd just rip it off of there to be sure it wasn't drawing current and getting hot.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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Well

I have thought about desoldering the 7805 , and put a wire between In and Out then i can even use the "Plug" to feed the board.

But i better desolder the Diode also then , or i would end up with 4.3v.

Maybe i should just do the reverse diode trick anyway ....

/Bingo

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If I may be definitive here:

The original 78XX regulators would not survive a short circuit on the input, because they lacked a reverse protection diode. It's not a problem with having the output at or over the regulated volttage without input, it's a problem with current flowing backward, to the power supply input caps, or the short on the input.

If you add a diode from input to output, with the banded end of the diode on the input pin, then there will be no problem.

SOME newer designs have internal protection diodes, but unless you know for sure that yours has this, then assume it does not.

Dave's Engineering Page: www.dvanhorn.org
Barcode and other AVR software contracting.
765 287 1987 dvanhorn@dvanhorn.org

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Hello

there are v-regulators that can handle if voltage is applied to their outputs.
LT1121 has "reverse output current" specified.

Klaus

Klaus
********************************
Look at: www.megausb.de (German)
********************************

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I did the Reverse diode trick as proposed here.

No smoke yet :lol: :lol:

/Bingo

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The Input side is not connected to anything , so all the 4,3v sees/hiots is the 7805 input capacitor.

/Bingo

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Bingo600 wrote:
The regulator is a 7805 , but i can't rekognice the manufactor logo

It lookc kinda like a "tilted" G or a C with a little dot at the end of the lower end on the C/G. Or a werry "New Half Moon" with the dot at the end of the lover part.

Anyone knows that logo


Try to find the logo here at this great site and click at the logo, to find out who the vendor is:
http://www.elnec.com/iclogos_l.php

You could also upload a picture of the logo, the we might be able to tell which vendor it belongs to.

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Hello,

Is there a common answer to..
... Can i back-power switching regulators or they are sensitive...?

what do u advise me to check to realize if mine is sensitive or not?

thnk u all :lol:

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Wayward717 wrote:
Hello,

Is there a common answer to..
... Can i back-power switching regulators or they are sensitive...?

what do u advise me to check to realize if mine is sensitive or not?

thnk u all :lol:


What type of switching regulator?
Buck, Boost, Buck-Boost, Flyback etc.

The easiest would be to look at the datasheet for you switching regulator IC, if you use an IC.