[sdiy] Newbie warning! What is negative DC power?

[Andreas Gaunitz]

Thanks for the info!

However, if I use the +- supply during development then I will have 
problems when I put it on PCB - I will need to create a neg current by 
then! And I will (of course) connect an audio cable sooner or later, so 
I can't use a positive ground to get minus DC either.

It's just strange that this is a problem... I looked for DC wall warts 
and PCB mounted transformer components that can produce -10 : gnd : +10 
volts and I had a really hard time!

So how do you people make a neg DC supply on a PCB?

regards
-Andreas


---------------------------------------------------------
2005 jan 04  kl. 15.12 skrev Paul Perry:

> From: "Andreas Gaunitz" <andreas.gaunitz at konstfack.se>
>
>> I'm planning to buy a DC power supply, with limited current and 
>> digital
>> readout etc, for my upcoming projects.
>>
>> Now, I am choosing between two fairly cheap ones: One that has +-15
>> volts, and the other that has 0-30 v. Now, I always thought that a
>> negative DC power was a breeze to produce: just set the ground to a
>> positive voltage value. Did I dream this?
>>
> ........you can produce a neg voltage by putting the + at earth.. but 
> only
> if the
> power supply and the circuit you are driving wiht it don't already 
> share the
> earth!!!!!!!
>
>> I checked out the 7805/7809 component range and it seems the ones
>> producing neg current have to be supplied with neg current - duh. So
>> how to get the neg current in the first place?
>
> If you have the +-15v supply, then you have a ground (earth) 
> connection,
> which goes to the ground connection of the 7805, and ground of your 
> circuit.
>
> The -15 goes to the input of the 7805.
> Then you are in business.
>
> WARNING: the pins of the 7805 and 7905 are different!!!!!!!!
>
> WARNING: 7805 may want a load to function correctly (a 10K resistor 
> from
> output
> to ground will do).
>
> It is worth readign the National Semiconductor data sheets (on the web,
> just search for LM7805 data)
>>
>> I'm getting more and more confused the more I read...
>
> ..that is a good sign, because at least it means you are starting to 
> know
> what you don't
> know. Unless you know that, you can't learn!!
>
> best, paul perry (self-taught electronic manufacturer)
>