Voltage regulators (e.g. LM7805): fed reverse polarity, need info

Hello, I will briefly explain my "project": I'm trying to create a variable voltage regulated power source for my breadboard/test circuits. What I thought to do is to use a set of buttons/mechanical switches to determine which "+" comes through. I will have a 9V battery and then at least 3 voltage regulators (3.3V, 5V and 6V). The output of each one of these will be connected to the output of my "tool" and the 9V will be fed only to the desired regulator. I thought, though, that this way if I have the 5V selected, the 5Volts are fed also to the output pin of the 3.3V regulator (the output is common, but the 3.3V regulator won't be connected to the 9V plus, only to the 9V minus) and this may damage my components, or cause them to dissipate energy/current when not needed. I hope I have been clear; do you think there is a problem with my configuration? Should I have some kind of switching on the output too (i.e. something that leaves unconnected the unused regulator outputs)? Thanks!

You need to draw a schematic so we can check, words are not normally enough.

The output of each one of these will be connected to the output of my "tool" and the 9V will be fed only to the desired regulator. I thought, though, that this way if I have the 5V selected, the 5Volts are fed also to the output pin of the 3.3V regulator (the output is common, but the 3.3V regulator won't be connected to the 9V plus, only to the 9V minus) and this may damage my components, or cause them to dissipate energy/current when not needed. I hope I have been clear; do you think there is a problem with my configuration? Should I have some kind of switching on the output too (i.e. something that leaves unconnected the unused regulator outputs)?

Your description sounds all wrong. Post a schematic.

I thought, though, that this way if I have the 5V selected, the 5Volts are fed also to the output pin of the 3.3V regulator (the output is common, but the 3.3V regulator won't be connected to the 9V plus, only to the 9V minus) and this may damage my components, or cause them to dissipate energy/current when not needed. I hope I have been clear; do you think there is a problem with my configuration?

Yes, everything is wrong about it. I can't tell if the problem is in the way you described it or in the way you actually did it but if you did it the way you described it then you have some serious issues.

you are using 3 separate voltage regulators when you could do the same with one adjustable LM317 with a variable resistor or 3 resistors actually 4 this is only for positive regulation. If you need a +gnd- supply for op amps you would need 7805 and 7905 or LM 317 and LM 337 there are many calculators on the net for using LM 317 as a variable voltage regulator

Draw a schematic on a blank sheet of printer paper and post a photo of it.

travis_farmer:
I have attached two schematics showing the general idea of what I think he was describing, and my revision. these are ROUGH schematics, as I have not added ANY capacitors. they are only to convey the general idea.

If those diodes can handle the current… then ok. Power diodes.

travis_farmer: based on his design, I would suggest some rectifier diodes on each of the regulator outputs to isolate the outputs.

~Travis

That will work,but after you will end with 2.6 - 4.3 - 5.3 volt on outputs. A trick to mantein the proper voltage is to add a second diode for each regulator anode to reg. gnd pin and catode to ground.

Ciao, Ale.

Yeay - it should work - but rogertee's idea of using an LM317 is neater, cheaper , and more accurate.

regards

Allan

Does anybody care that the 9V battery is wired wrong ? (it's reversed), so NO , it would NOT work the way shown in the schematic.

@Travis Farmer, research battery schematic symbol. The short bar is the negative, the long bar is the positive. You have the long (wide) bar connected to GND instead of the other way around. TSK TSK...

raschemmel: Does anybody care that the 9V battery is wired wrong ? (it's reversed), so NO , it would NOT work the way shown in the schematic.

Maybe the layout is ok.... just stick a minus sign on the left of the '9V' symbol and it's all good. They did mention rough schematic. It was to convey general idea only.

Hello ! You can't have a negative input for positive regulators. A "rough" schematic is one that is unfinished, not one that is incorrect.

Get your terms straight call it a bad schematic.

They managed to convey the general idea. That's a win in my books.

Any schematic that leads to damage is loss in my book.

Here on the forum we like things to be correct. The whole world is watching. How would it look if we treated our posters as if they were 5 years old and let them get away with posting schematics with reversed batteries . What would the rest of the world think ? I'm sorry but regardless of age or experience, schematics posted here on the forum need to be correct, and we will see to it that they are. That's how we roll...

In a forum built on EDUCATING people, why would you start a flame on a simple mistake?

You learn when someone corrects you . Do you want to spend the rest of your life drawing it wrong ? Obviously you didn't know so I corrected you. It's not a "flame". It just HAPPENS so be our JOB here to CORRECT you. If you don't want to be corrected, then don't post incorrect stuff on the forum. That's how it works. Consider your paper just corrected: Grade "C+"

While we're on the subject , your battery symbol should be rotated 90 degrees clockwise to be correct. It's never shown sideways like that . The GND should be at the bottom and the + at the top.

And your welcome.

I agree - not a flame, just a (needed) correction.

PS: it's "you're welcome", not "your welcome" :)

But as I should have apparently been treated as a five-year-old, it is apparent that clearly I am nothing to your years of experience. I shall henceforth refrain from spreading around my caring and generous nature to help people, and will thusly insult those not up to my level of skill.
Because that is how you roll.

You need to reread my post.

The whole world is watching. How would it look if we treated our posters as if they were 5 years old and let them get away with posting schematics with reversed batteries . What would the rest of the world think ?

Did I let you get away with it ? No, because you’re not 5 years old.

I said we would NOT correct a 5 year old. (for obvious reasons) The wording was correct. The meaning was clear. IF you were a 5 year old, you would NOT be corrected.
Your interpretation (incorrect): you are being corrected BECAUSE your are a 5 year old.

There was nothing rude . It was a bad schematic and were corrected because you are not a 5 year old.

There is something wrong with the attitude that it is wrong to correct you. I won’t say what it is, but suffice it to say that it’s not the right attitude to have if you want to learn.

You need to accept constructive criticism.

PS: it’s “you’re welcome”, not “your welcome”

THANKS ! How did I miss that !?

Keep it civil, children, or find yourself on the wrong of forum time out. Again.