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Witchy
October 8th, 2016, 05:18 PM
Hi folks,

Theoretical question here since I haven't had a need to do this yet, but given a standard PET 4116 needs +5, -5 and +12 I could use 3 bench supplies to provide each voltage, yes? How would the ground/0V/commons be connected? Obviously in the real world these 3 voltages come from the same single brick which has a common ground and single line back to AC mains but putting 3 separate boxes together seems to be messing with my head even though ultimately they all go back to the same common ground/earth connection.

The more I read the more I'm puzzled.

Any help appreciated.

W

Chuck(G)
October 8th, 2016, 06:01 PM
Depends on how well the outputs are isolated, but nominally, you'd simply tie the grounds together as one. That assumes, of course, that your -5 bench supply really does supply -5 with respect to the ground terminal and you're simply not swapping the red and black leads to get a negative voltage.

Witchy
October 9th, 2016, 01:57 AM
Like I say, I haven't done it yet but the -5V question is one I was reading late into last night. Instinct would tell me that swapping the red and black would be OK but it's not a true -5 and I'd have to build something to do it.

Very much a learning process :) Building circuits with 5V Arduinos and 3.3V RasPis is helping a lot with the common ground reference side of things though.

Chuck(G)
October 9th, 2016, 07:09 AM
Again, it depends on how well your black terminals are isolated. Most PSUs leave them completely floating, but a notorious few tie them to chassis and mains ground--those you want to avoid. Since the -5 power draw is on the order of a milliamp or two, you could even use a battery to supply it--it's basically substrate bias. Or use a low-cost DC-DC inverter IC to supply it.

g4ugm
October 9th, 2016, 08:21 AM
One issue with separate PSU's is that if I remember properly the spec for the DRAM chips says that rails must not cross at power on. So the +5 must never be less that the +12. So if you +12 is slow to come up to voltage, because it has a big capacitor, or a small transformer you can pop the chips. Not sure if simple diodes are good enough to fix this

Chuck(G)
October 9th, 2016, 08:53 AM
So the +5 must never be less that the +12.

I know what you're trying to say, but this statement would have some people scratching their dandruff. :)

Power sequencing is important. Personally, I'd probably just grab an old PC PSU and use that rather than trying to coordinate three bench supplies.

Witchy
October 9th, 2016, 02:07 PM
Power sequencing is important. Personally, I'd probably just grab an old PC PSU and use that rather than trying to coordinate three bench supplies.

Funnily enough I've just this afternoon robbed parts from an old PC board so I CAN do that in the future, and there's people selling little interface boards on ebah that let you do the same thing...

daver2
October 12th, 2016, 11:23 AM
>>> So the +5 must never be less that the +12.

I scratched my dandruff over that one for a while. Just for readers coming along later - should that be "the +5V must never be higher than the +12V"?

You can arrange for this by using diodes between the +5V and +12V rail. Be sure to get the diodes the right way round though otherwise you will be replacing power supply units rather frequently! This was the way Nascom arranged for their power supply units to work with the DRAM and other devices (see http://www.nascomhomepage.com/pdf/Nascom2.pdf drawing 022-405).

Or - as a bench test - just switch the power supplies on in the right order...

Dave

g4ugm
October 12th, 2016, 01:54 PM
sorry, brain fade... you are right, but its true for all rails at all times. So -ve < Gnd < +5V < +12v at all times but especially during start-up and shut down.
So at switch off as well. If you are using multiple PSU's you need a meter on the +5v and at switch it off first and only switch off the 12v when the +5v has dropped to zero....

Chuck(G)
October 12th, 2016, 04:59 PM
Just a little addition to the 3-rail supply dilemma. There are 16K DRAMs that are pin-compatible to the 4116, but are +5V only. A couple such are the MB8118 (Fujitsu) or MK4516 (Mostek).

MikeS
October 16th, 2016, 04:58 PM
Just a little addition to the 3-rail supply dilemma. There are 16K DRAMs that are pin-compatible to the 4116, but are +5V only. A couple such are the MB8118 (Fujitsu) or MK4516 (Mostek).If you're going to replace the RAMs anyway you can also use easier-to-find 4164s; lots of articles out there. If you don't need the 9V for the cassette you can power the whole thing from 5VDC, or use a common 12V & 5V supply (still need to power the display).

Another approach is to modify an AT/ATX supply to output +/- 8 & 15V, which is basically what the dynamic RAM PET supply uses.

m