Author Topic: Regarding wiring a L7805 V.reg  (Read 1480 times)

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Offline SuperJeep

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Regarding wiring a L7805 V.reg
« on: February 17, 2013, 02:16:32 AM »
Hi, Im using an old satellite receiver powersupply to build a small PSU for my breadboard projects but sadly it only has rails for 3.3v, 7v, 12v, and 14v and no 5V

I have an old L7805 voltage regulator that I plan on using to make the 12v or 14v rail adjustable to get 5v or anything in between.

My question is one that might be stupidly simple but hopefully someone can clear my ignorance in the area of grounding:

Is it recommended that I connect the ground pin of the IC to the case (where the metal backing of the IC will be connected)?
To clarify, it is a TO-220 package, and the reason I ask is in some of the datasheet's recommended applications, they only show the metal back of the package as GND and the center GND pin on the chip itself is not connected.

Any feedback is greatly appreciated! Thank you.

Offline mariush

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Re: Regarding wiring a L7805 V.reg
« Reply #1 on: February 17, 2013, 02:49:51 AM »
L7805 is a fixed voltage regulator, so it wouldn't be a good idea to make it adjustable, if it's even possible.

The ground pin should always be connected to the tab, you can test that with your multimeter on continuity or resistance test and placing the probes on tab and pin.

I would use the pin always. The tab is for heatsink, and the voltage regulator will get hot if you use a lot of power for your projects (more than let's say a microcontroller and a small lcd screen)

Remember, the regulator will dissipate the voltage difference as heat, so if you want 5v at 0.1A and you feed the regulator with 12v, the regulator will dissipate  (12v - 5v ) * 0.1a = 0.7 watts.  Without heatsink, the regulator can probably handle up to 2 watts for short periods of time.

Offline SuperJeep

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Re: Regarding wiring a L7805 V.reg
« Reply #2 on: February 17, 2013, 02:53:32 AM »
Thanks for the prompt reply! I've decided on just feeding it with the 7v rail and using it as it was intended as a 5v regulator. I have the heatsink from the old device it came out of so ill be sure to use it. Thanks again

Offline funk1980

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Re: Regarding wiring a L7805 V.reg
« Reply #3 on: February 17, 2013, 06:00:08 AM »
Fot adjustable regulated output voltages, use a LM317.

Offline Smatek

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Re: Regarding wiring a L7805 V.reg
« Reply #4 on: February 17, 2013, 08:58:57 AM »
To use the 7V rail is a good idea. The efficiency is mucht better.

I would use a LM317. With a 5k pot you have the opportunity to choose the output voltage.
It is alswas a good idea to look at the data sheet. You will find a typical application for the chip. Look at http://www.ti.com/product/lm317

Offline MikeK

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Re: Regarding wiring a L7805 V.reg
« Reply #5 on: February 17, 2013, 09:23:00 AM »
What's wrong with using an LM7805 as adjustable?  An LM317 is fixed too, it's just fixed at 1.25V, right?  The LM7805 can be adjustable, you just won't get lower than 5V.

Offline c4757p

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Re: Regarding wiring a L7805 V.reg
« Reply #6 on: February 17, 2013, 09:30:12 AM »
Sure, but its usefulness as an adjustable regulator is going to depend on how small and predictable the current out of the ground pin is. If it's too large it will create a voltage offset. LM317 limits this current to 100 uA. I cannot find a specification for it on a 7805 and I would not depend on it being nearly the same from device to device.

Offline mariush

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Re: Regarding wiring a L7805 V.reg
« Reply #7 on: February 17, 2013, 09:45:31 AM »
The IC already has two resistors between output and ground, inside it.

See http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000444.pdf  , page 5 shows the diagram.

Could you adjust it? Yeah, you probably could, but it gets complicated, you may push too much current on the ground pin and other stuff.

The datasheet has an adjustable regulator design on page 29, but it's not quite as simple as a couple of resistors, they use an opamp.

Everybody recommends 7805 because it's very cheap and simple to use but nowadays there are much better, and still cheap, components.

funk1980, you have to be aware that this 7805 tipically needs as much as 2v above the output voltage to keep the output smooth, stable and so on.

If you pull 1A, to get stable 5v output, it needs 7v.   At less current, it needs less input voltage, but would probably still need at least 1.5v

Depending on how that power supply you have is built, that 7v may hover around 6.8-7.2v, so you have to be careful. 99%  you won't have a problem.

But if you can, go for a voltage regulator that's designed to be adjustable from the start and not demand so much voltage above its output setting, as 7805 and 317 do.

For example, I personally like MIC2941 and LT1084/LT1085/LT1085 but they're a bit expensive at $3-4 a piece

http://uk.farnell.com/micrel-semiconductor/mic2941awt/ic-reg-ldo-1-25a-adj/dp/1556737

http://uk.farnell.com/linear-technology/lt1086ct-pbf/v-reg-ldo-adj-1-2-29v-1086-to-220/dp/9559868

MIC2941 has a maximum dropout of 0.35v while LT1086 is guaranteed to be under 1v ... datasheets are there in the links above for both.

Offline smashedProton

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Re: Regarding wiring a L7805 V.reg
« Reply #8 on: February 17, 2013, 04:13:25 PM »
Dude, this is what breadboards are for   :-/O
http://www.garrettbaldwin.com/

Invention, my dear friends, is 93% perspiration, 6% electricity, 4% evaporation, and 2% butterscotch ripple.

Offline SuperJeep

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Re: Regarding wiring a L7805 V.reg
« Reply #9 on: February 27, 2013, 12:03:46 AM »
Thanks for all the insightful comments. I had a few LM317's but decided not to make this project variable (I was in a rush to get it done because I've been using a 5v and 12v dc wall adapter for all of my projects-never owned a powersupply). It's all done now! I would link the documented project on a new blog I made for my projects but im not sure if such external links go against the terms of service for this forum:)

Thanks! This community has always provided quick and helpful assistance.



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