Reading 2708 EPROM, need help with +5V, -5V, +12V & voltage regulator circuit - Page 1
Forum section: electronics / beginners

author: RogerMc
author_rank: Contributor
timestamp: May 22, 2014, 07:13:11 pm
content: 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!

author: retrolefty
author_rank: Super Contributor
timestamp: May 22, 2014, 07:24:03 pm
content: 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.

author: Kappes Buur
author_rank: Regular Contributor
timestamp: May 22, 2014, 07:39:11 pm
content: http://www.goldpt.com/virtual_ground_circuit.html

author: RogerMc
author_rank: Contributor
timestamp: May 22, 2014, 08:15:18 pm
content: 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.

author: Kjelt
author_rank: Super Contributor
timestamp: May 22, 2014, 08:26:42 pm
content: 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.

author: Thilo78
author_rank: Regular Contributor
timestamp: May 22, 2014, 08:29:12 pm
content: 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?

author: mariush
author_rank: Super Contributor
timestamp: May 22, 2014, 08:45:35 pm
content: Just get a MC34063 plus the required parts (inductor, diode, resistors + capacitors) and configure it in inverting mode.http://sourceforge.net/projects/mc34063uc/

author: RogerMc
author_rank: Contributor
timestamp: May 22, 2014, 08:58:22 pm
content: Quote from: Thilo78 on May 22, 2014, 08:29:12 pmThilo78'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!