Getting + and - 5v

I'm working on a project where I'll need + and - 5v DC on a shield. I'm trying to find the "simplest" way to do this, assuming we have standard Arduino pins available - so, the +5v is easy - just use the Arduino's regulated 5v.

But what about -5v? I have the regulated 5v, plus the unregulated Vin to work with. I'd like to do this with the lowest parts count and physical space possible. Ideas anyone?

It depends on the current you need from the -5V. If it is in the order of 40mA or so then look up "voltage mirrors" or negative voltage generator chips. They work with an capacitor ladder like the Max202. Is it a line driver requirement? If so some driver chips will generate the -5V for you in the same way.

In this thread:

http://www.societyofrobots.com/robotforum/index.php?topic=2734.0

someone essentially suggested using an LM79xx, wired "backwards", to a positive power supply (a battery.) Could that work here too? Use an LM7905, connect it's input and gnd terminals "backwards" (i.e. input to gnd, and gnd to Vin from the arduino)?

Mike, I made my reply before I saw yours.

It's for a pH meter - I need -5v for the op-amps involved. Not sure on the exact current requirement but I can't imagine it's very big.

I've done some limited research into the chips you're talking about, and it seems like they have REALLY small current capacity (like 10mA) and/or their output is not stable. FWIW, other people who have done project similar to mine have used a chip like that, PLUS a negative voltage regulator (i.e. LM7905). I'd like to use a SINGLE IC if possible, plus as few external components as possible.

If it for an op amp then it doesn't have to be particularly stable or accurate. As long as the noise is filtered out it will be fine at any value of voltage from -4.5 to -9, it will not affect the gain or anything like that. So something like the MAX1044 will do you.

someone essentially suggested using an LM79xx, wired "backwards", to a positive power supply (a battery.)

Sorry that is just rubbish. You can use a negative voltage regulator like the LM2991S and wire it up correctly. That is with the + of the battery to ground and the - to the input of the regulator.

However, look at the data sheet of the op amp you are using and see if you can connect the - from the battery directly to the op amp rail, most probably you can.

For currents around 50mA I like the x660 ICs. They are made by a few companies. Only two or three small caps are required for voltage inverting or doubling. I use the Analog Devices ADM660 in one of my kits. 300mil DIP8 package.

(* jcl *)


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