Reading 2708 EPROM, need help with +5V, -5V, +12V & voltage regulator circuit

[RogerMc]

The IC has a single ground and pins for +5, -5, +12.

I have a single PSU set to 14V.

Successfully generating +12V and +5V with voltage regulators, no trouble there.

But, I can't get -5V from the 7905.  When wired I get +9V which is taking 5V off the 14V supply.

Yes, I understand that the pin outs of 7805 & 7905 are different.

I also understand that ground is a relative concept and not an absolute one.

So how can I generate -5V using 7905 and sharing the same ground and PSU as the +5V & +12V?

Apologies for demonstrating my ignorance.

Thanks!

[retrolefty]

Well a 7905 needs a negative voltage source to regulate down from (like -8 to -12) and you didn't mentioning having such a negative voltage source? If you wired a positive voltage to the input of a 7905 you may well have damaged it.

[Kappes Buur]

http://www.goldpt.com/virtual_ground_circuit.html

[RogerMc]

Thank you Kappes.  I can't say that I completely understand, but the link does seem to describe one of my attempts.  Let me clarify.

I can generate +5V & -5V using the 7805 and 7905 using the concept of virtual ground which results from using a voltage divided 14V before input to the voltage regulators.  Trouble is that I can't then also get 12V from the 7812 seemingly because the PSU ground is not the same as this virtual ground created by the voltage divider.

So I have two scenarios which work.
1. I can generate +5 & +12, but not -5.
2. I can generate +5 & -5, but not +12.

But I need +5, +12, -5 all with the same ground and using only a single PSU.  How?

I'll keep studying and hope it becomes more clear to me.

[Kjelt]

Is this for a one time reading out the ic or do you want to build a programmer so you can use it more often? If it is for one time only you could use (rechargeable) batteries for the negative voltage.

[Thilo78]

Let me have a noob's go on this one:
Why don't you use batteries for the +5V and -5V?

As I understand, you are going for a temporary solution, right?

If so, you could use your PSU for the +12 V, and two 9V batteries in a center ground config, effectively giving you +/- 9V.
Those rails could be used for +/- 5 V DC with the 7805 and the 7905...
Voltages are floating, so you could connect center ground with the PSUs negative for proper reference.

Another option:
Use an old ATX power supply, which supplies you with +/- 12 V DC and +/- 5 V DC.
Shouldn't that work to read that EPROM? 

[mariush]

Just get a MC34063 plus the required parts (inductor, diode, resistors + capacitors) and configure it in inverting mode.

http://sourceforge.net/projects/mc34063uc/

[RogerMc]

Voltages are floating, so you could connect center ground with the PSUs negative for proper reference.

Thilo78's response helped me.  I am now working with 12V from PSU (no 7812).  Grabbed the center of two 9V batteries in series and connected to PSU ground.  Grabbed +/- from the 9V batteries and fed into 7805 & 7905.  Everything checks out.  Yes, this is just a quick & dirty read.

Thanks everyone!