-5v

[bacteria]

When I get around to doing the ColecoVision, and NeoGeo, they seem to want -5v as well as +5v and other voltages.

I know you can buy regulators that output -5v, but I was wondering something else - on your multimeter, if you connect the wires in reverse so the negative probe is on the positive battery terminal, and vice versa, you get a minus voltage reading. My question, and logic, is if you had a separate regulator to your main batteries but connected plus to minus and minus to plus, would that give -5v, or is it just a quirk in the multimeter not meaning negative but "wrong way around" as opposed to "negative voltage"?

Anyone know? (electronics is not my strong point!).

[paulie12uk]

dc voltage is polarity conscious so yes, the multimeter will warn you with the - in front of your reading that the meters the wrong way round. You can not get a negative voltage reading because you need a negative and positive to get a voltage reading. Hope that helps ya

[Ivo]

There are no absolute potentials, only relative, so if you switch the cables around on something like a battery you should be able to get -5V. If you swap the cables the electrons are flowing in the opposite sense, and they have to flow in the direction of increasing potential.

[MrPopo]

Voltage is all about potential. Think of it as a hill. You put the ball on the hill and it rolls down to the lowest point. We can call this the 0 point and do all of our measurements can be in reference to this point. So the top of the hill can be +5. Now let's say we dig a well at the bottom of the hill, and the depth of the well equals the height. If we don't move the 0 point we can now call the bottom of the well -5.

So in our example, between the top of the hill and our 0 there is 5 V, and between the 0 and the bottom of the well there is a difference of 5 as well, so it is -5 V (in relation to 0). The difference between the top of the hill and the bottom of the well is 10 V.

So a regulator that deals with both +5V and -5V will still have a designated ground point, which is the 0. This is the part you should always be touching with your black (ground) lead of your multimeter. That way you'll never confuse which is +5V and which is -5V.

[Metalcrack]

An interesting read that may help in your trials http://tangentsoft.net/elec/vgrounds.html

I've used the TLE2426, and it's pretty cool. Basically you take a voltage source and it splits it into two voltages with a "virtual" ground in the middle. Using that point as your ground, you have a +VDC and a -VDC.

An example is a 12VDC source would turn into two 6VDC sources, and using the "virtual ground" as your reference point, you end up with a positive and a negative voltage.

[bacteria]

Thanks for the details. It was over my head but showed that my original idea was incorrect, as I thought it must be.

Saw this on ebay: LINK

Looks like a normal 7805 (which I am used to using), only this one gives a -5v instead of +5v.

Is it ok? No issue if it gets hot, just add a heatsink onto it or a metal plate.

[MrPopo]

Yes, that one should be fine. If you look at the datasheets for both, you see the big difference is which pin is which. This controls whether it outputs the +5 or -5.

7805:
http://www.datasheetpro.com/node/21593

7905:
http://www.datasheetpro.com/node/21677

[bacteria]

Fantastic!

I really love this forum, people are very helpful and friendly!

This is the best forum I have ever had the pleasure to be on.

[MrPopo]

Fantastic!

I really love this forum, people are very helpful and friendly!

This is the best forum I have ever had the pleasure to be on.

Some of us start to get violent if you start indicating that you like dual analog for FPS's, but for the most part we try to help out.