Negative Voltage Regulator

[jsmoker]

I'm currently working on a graduate research project.  Our university electronics store has once again failed to update their inventory and are STILL out of +5V voltage regulators.  They do, however, have a -5V.  Is there any harm with just connecting a microprocessor so that -5V is on the Vss pin and the 0V is on the Vcc?  It's just a 5V potential right?

Thanks

PS  I know I can go to RadioShack, but it's not across the street like the university store is, and I would have to submit a reimbursement request instead of using a university account.

[MrWizard]

It is not the same....better cycle to Radio Shack....

[householdutensils]

You could use a -5v Regulator with a cmos hex inverter to get a +5v. That's how I designed my H-Brige, used inverters to get me a +5v and -5v pulse train for each pair of transistors (And the corresponding amps).

Edit: Actually...I'm not sure if you can use a -5v input on the cmos chip, can anyone confirm this?

Edit 2: Better yet, use Soeren's idea xD

[Soeren]

Hi,

Our university electronics store has once again failed to update their inventory and are STILL out of +5V voltage regulators. 

Shit happens, squeeze it and make lemonade... Err... What I meant to say was... Don't they have LM317 variable regulators or similar?

They do, however, have a -5V.  Is there any harm with just connecting a microprocessor so that -5V is on the Vss pin and the 0V is on the Vcc?  It's just a 5V potential right?

You are right... potentially.

As long as you connects the -5V to "circuit ground" and the 0V to "circuit +5V" you're good to go, IF you remember throughout the entire supply line that supply ground is positive.
An entirely possible task that you probably will blow anyway, in a stressful moment a short time before your submission is due.

PS  I know I can go to RadioShack, but it's not across the street like the university store is, and I would have to submit a reimbursement request instead of using a university account.

Reimbursement request??  It's your education, the cornerstone of your career, but don't let that inspire you to just spend a fortune (about 10% of a burger with fries) on a crucial part out of your own pocket.
Perhaps it's cheaper to get a hairdresser education or a taxi license? 

[Soeren]

Hi,

You could use a -5v Regulator with a cmos hex inverter to get a +5v. [...]

No you can't.

[householdutensils]

Yea, I realized that after I had posted it, because you'd need a +5v Vcc anyway xD

[Soeren]

Hi,

Actually, you could do it, but it would take a total lack of sanity to even begin such a project, as the voltage regulator in itself is enough.

You'd need to make an oscillator with both true and inverted outputs (better yet, a four quadrant oscillator, to give a bit of deadband, to increase the lousy efficiency), a couple of transistors to up the current and some "glue" components of course. Plus the negative regulator.

[householdutensils]

haha might as well just drop the $1 or whatever the regulator is gonna cost ya a radioshack xD

[jsmoker]

I think some (not all) of you missed my point.  I was thinking of putting the ground of the regulator on the + of the power supply on Vcc which is the positive terminal on the CMOS and the negative of the regulator input on the - of the supply and the -5V output on the Vss which is the negative terminal of the CMOS.  The net difference is +5V across Vcc to Vss as the reference...which is the specified required voltage of the dsPIC30. 

Sorry, I just wanted to clarify that I do have some logic to my suggestion.  I wasn't intending to put a -5V potential to a +5V requirement.

I figured that if I connected it so the + and - were on the corresponding traces on a PC board I wouldn't have to think about it again after the initial connection.

And for those who did follow my logic and said it wouldn't work, why is that?

as for

Reimbursement request??  It's your education, the cornerstone of your career, but don't let that inspire you to just spend a fortune (about 10% of a burger with fries) on a crucial part out of your own pocket.
Perhaps it's cheaper to get a hairdresser education or a taxi license?  Wink

I've made multiple reimbursement requests for over $300 for different projects.  This is part of my work as a research assistant and there are designated accounts for those projects.  Yes, $2 isn't much, but it sure can add up.

[Soeren]

Hi,

And for those who did follow my logic and said it wouldn't work, why is that?

It will work!
It wil not be a good idea though, as it sort of invites to a connection error that could kill lots of components.

I've made multiple reimbursement requests for over $300 for different projects.  This is part of my work as a research assistant and there are designated accounts for those projects.  Yes, $2 isn't much, but it sure can add up.

Sorry, I thought you were making your final project and in that context, I wouldn't even think twice to throw $300 at it - in fact, I spend several times that amount, when I took my engineering degree (and the ridiculous amount granted from the academy, I didn't max out).

My personal (hobby if you will) stock cotains around 400..500 µconrollers, a similar amount of voltage regulators etc. In short, I don't compromise on my hobbies/intrrests.

I would thing you could find a regulator for $1 btw.

[SmAsH]

wow, thats a bit expensive. i get my 7805's for about 20-25c each. and do you know how long until the university get more in stock?

[Admin]

I know I can go to RadioShack, but it's not across the street like the university store is, and I would have to submit a reimbursement request instead of using a university account.

Dood, you go to University of MD, and RadioShack is like a 5 min drive from your lab

(I know because I bought my $50 Robot parts from there)