Author Topic: Negative voltage regulators  (Read 1607 times)

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dsharp02

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Negative voltage regulators
« on: June 08, 2016, 08:14:08 pm »
This is probably a stupid question, but if all you have is a positive voltage supply, and you want to power a negative regulator, can you simply hook it up "backwards"?  For that matter could you simply use a positive regulator and hook your circuit up backwards?  If so, why do negative regulators exist?

I'm having a bit of trouble wrapping my mind around how negative voltages work.

Thanks,
Dave

jitter

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Re: Negative voltage regulators
« Reply #1 on: June 08, 2016, 08:38:58 pm »
This is probably a stupid question, but if all you have is a positive voltage supply, and you want to power a negative regulator, can you simply hook it up "backwards"?
For that matter could you simply use a positive regulator and hook your circuit up backwards?

If you designate the higher voltage as common/ground (the reference to which you measure) then the lower voltage will be negative and vice versa.
By making (designating!) the + to be "0" then the other terminal will be more negative, i.e. a negative supply.

So, yes you can hook up your circuit backwards. But beware, you cannot hookup additional circuits unless they too have to be connected "backwards".

Quote
If so, why do negative regulators exist?

For symmetrical power supplies with centre tap as ground.

Quote
I'm having a bit of trouble wrapping my mind around how negative voltages work.

We tend to designate ground/common as 0 V and take measurements against it. Higher voltages will be positive and lower voltages negative with respect to that point.
But two positive supplies wired in series could become a negative and a positive supply if you designate the point at which they connect as common/ground. That's all there is to it.

Take two completely separate (e.g.) 15 V supplies and hook up the + of one to the - of the other. This connection is then your common with respect to which you measure the voltages and to which your circuit common (aka ground) connects.

Put the black lead of your DMM on this common it and touch the unconnected + of one supply with the red lead. The DMM reads 15 V. Then move the red lead over to the unconnected - of the other supply, keeping the black lead where it was, it will read -15 V.

Now swap the black lead to the unconnected - and move the red lead to the +- connection between the two supplies, it measures 15 V. Move the red lead over to the unconnected + and it reads 30 V.
And if you repeat the above with the black and red leads reversed, they would be -15 V and -30 V.

Voltages are relative.

To make this even more clear is the following:
I had a colleague who shocked himself by touching a "ground"  in a circuit. He was measuring between this and some other point in the circuit, and it measured lowish voltages, say 5 V. But what he didn't realize was that that "ground" point was some 300 V above earth potential. Ouch. So, if he had measured between earth and that "ground", he would have measured 300 V, had he then gone on to measure the "5 V", it would have measured 305 V. Again, voltages are relative.

For this reason I prefer "circuit common" or "circuit reference" over "ground" or "0 V", as they may not always really be 0 V.
« Last Edit: June 08, 2016, 09:16:13 pm by jitter »

uncle_bob

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Re: Negative voltage regulators
« Reply #2 on: June 09, 2016, 01:47:51 am »
This is probably a stupid question, but if all you have is a positive voltage supply, and you want to power a negative regulator, can you simply hook it up "backwards"?  For that matter could you simply use a positive regulator and hook your circuit up backwards?  If so, why do negative regulators exist?

I'm having a bit of trouble wrapping my mind around how negative voltages work.

Thanks,
Dave

Hi

A lot depends on exactly what your constraints really are:

If you have a bench supply with a + and a - out, you can ground either side. Ground the + out and you have a negative supply.

If you have a wall wart that has one side hooked to ground at the wall, hook up your negative voltage load "backwards" on the supply. As long as it connects to nothing else all is fine. Most circuits *do* connect to something else so ... all is not fine.

If you have a positive supply and are using it for part of your circuit, *and* you need a negative voltage as well for the same circuit: Now you need a second supply. One way or another you have to come up with an independent voltage source. There are switching regulators that will do this. There are a number of other options.

Bob

Smf