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'[EE:] Negative Voltage PS?'
2004\01\15@094543 by Hulatt, Jon

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Hi,

Given a source power supply nominally +12v (a car battery), can I use a 7805
and a 7905 together to get a +5/0/-5 regulated power supply? or am I
misunderstanding what a negative regulator actually does?

Thanks

Jon

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2004\01\15@095413 by Alan B. Pearce

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>Given a source power supply nominally +12v (a car battery), can I use a
7805
>and a 7905 together to get a +5/0/-5 regulated power supply? or am I
>misunderstanding what a negative regulator actually does?

You could, BUT be very careful that your 0V point does not come into contact
with the car body. If doing something like this then make sure that it is
not possible to short the internal "ground" to anything outside your box.

However on thinking about it a bit more, each regulator will have only 1V
drop across it, which is not enough to maintain a stable output voltage.
7800/7900 series regulators need something near 3V across the regulator to
maintain output voltage stability.

I would suggest that you use a 7805 to get +5V, and then a switching
regulator to get -5V from the +12V. This also means that your circuit ground
is at the same potential as the car body, so accidental connection of the
two is not a problem.

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2004\01\15@095826 by Mike Harrison

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On Thu, 15 Jan 2004 14:39:15 -0000, you wrote:

>Hi,
>
>Given a source power supply nominally +12v (a car battery), can I use a 7805
>and a 7905 together to get a +5/0/-5 regulated power supply? or am I
>misunderstanding what a negative regulator actually does?

You are misunderstanding - given a negative input voltage, the 7905 will  regulate it to -5v, but
will not make it negative to start with. You would need a dc-dc converter ofd some sort to get a true negative rail. The type of converter
required would depend on the current required.
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2004\01\15@100625 by Peter Onion

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On Thu, 2004-01-15 at 14:39, Hulatt, Jon wrote:
> Hi,
>
> Given a source power supply nominally +12v (a car battery), can I use a 7805
> and a 7905 together to get a +5/0/-5 regulated power supply? or am I
> misunderstanding what a negative regulator actually does?

You'll probably end up with +5 and +10 rather that +5 and -5.

You'll need a DC-DC converter to produce a real negative supply  (i.e.
negative with respect to the "-" terminal of the battery).

Peter

>
> Thanks
>
> Jon
>
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2004\01\15@102943 by Hulatt, Jon

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Thanks Guys, I get it now.

Now I'm looking at the 7660; need to do some sums to see if 20ma is enough.
it probably will be. What are my options if I need more current, say ~50ma?
(can't see that I'll need more than that).

> {Original Message removed}

2004\01\15@103321 by Mike Harrison

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On Thu, 15 Jan 2004 15:23:33 -0000, you wrote:

>Thanks Guys, I get it now.
>
>Now I'm looking at the 7660; need to do some sums to see if 20ma is enough.
>it probably will be. What are my options if I need more current, say ~50ma?
>(can't see that I'll need more than that).

If it's for a low-volume application, you can get ready-made DC-DC modules which will do 12v in to
+/-5 out, from Newport and others.
For >50mA you'd need to look at inverting switching regulators - there are many controller chips
available for this. Can't remember if any of NS's Simple Switcher range will do inverting
configurations.

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2004\01\15@163723 by Jinx

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> For >50mA you'd need to look at inverting switching regulators -
> there are many controller chips available for this

Best little chip I've been put on to lately (thanks Russell) is the MC34063

http://www.onsemi.com/site/products/summary/0,4450,MC34063A,00.htm

for d/s and application notes

Cheap, even from RS. Step-up, step-down and inversion with
a few simple external components

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