Need -20v 0.03A circuit

Thread Starter

Gitbizy

Joined Jul 26, 2021
14
I'm trying to restore an Atari 16 bit computer from 1990 (a STACY Laptop).

It has an LCD that requires -20v DC 0.03A

The ORIGINAL input power to the computer from the AC/DC power supply is 16.5VDC 2.5A, which feeds into a DC-DC board which basically outputs 12, -12, 5, gnd, -20v. This board is large and of course, 30 years old. I'd like to replace it and save a lot of internal space in the computer.

I'm leaning towards using a PICO PSU module which has the standard +12,+5, -12, GND

Can someone help me with a circuit that I can use to generate the -20v 0.03a DC using 12v DC as an input?

Thanks in advance!
 

MrChips

Joined Oct 2, 2009
24,134
This is very unusual.
Can you show us the label where it says that the requirement is -20V?
If you take a +20VDC adapter and interchange the wires you will get -20VDC.
 

Thread Starter

Gitbizy

Joined Jul 26, 2021
14
@MrChips

Here are screenshots from the schematics. J16 and J17 are the risers that go between the motherboard and the PSU. You'll see -20v on some of these screenshots. It's known that it is a very unusual value... but the LCD of that era required it.

The bottom-most screenshot is the circuit on the PSU to generate -12v and -20v, but I can't understand it...
the maximum power into the unit is 16.5v DC- from the original power brick.


stacy J17.jpgstacy j16 part 1.jpgStacy DC -20v.jpg
 

BobaMosfet

Joined Jul 1, 2009
1,848
I'm trying to restore an Atari 16 bit computer from 1990 (a STACY Laptop).

It has an LCD that requires -20v DC 0.03A

The ORIGINAL input power to the computer from the AC/DC power supply is 16.5VDC 2.5A, which feeds into a DC-DC board which basically outputs 12, -12, 5, gnd, -20v. This board is large and of course, 30 years old. I'd like to replace it and save a lot of internal space in the computer.

I'm leaning towards using a PICO PSU module which has the standard +12,+5, -12, GND

Can someone help me with a circuit that I can use to generate the -20v 0.03a DC using 12v DC as an input?

Thanks in advance!
Would like to see the actual 'model' information off the unit. A photo.
 

MrChips

Joined Oct 2, 2009
24,134
-20VDC at 30mA is not such a huge demand.
Get a 24VDC wall adapter and use 20V zener with a series resistor for voltage regulation.
 

Thread Starter

Gitbizy

Joined Jul 26, 2021
14
-20VDC at 30mA is not such a huge demand.
Get a 24VDC wall adapter and use 20V zener with a series resistor for voltage regulation.
I would like to stay with a single PICO PSU and try to do the boost to ~24v and then power change to -20v.
If I go the route you suggested I will need two power supplies for the computer and I'm trying to avoid that situation.
 

Thread Starter

Gitbizy

Joined Jul 26, 2021
14
I'm also trying not to blow up the entire motherboard when I do this power supply, and I am not an electrical engineer, that's why I'm reaching out here.
 

Irving

Joined Jan 30, 2016
2,056
A +12v to -20v at approx 1W is pretty straightforward with an inverting boost SMPS, though it might appear to be simpler to go -12v to -20v. Is this something you'd build yourself, or looking for an off-the-shelf module? How much spare current do you have to play with on the +12 and -12 rails?
 

Thread Starter

Gitbizy

Joined Jul 26, 2021
14
A +12v to -20v at approx 1W is pretty straightforward with an inverting boost SMPS, though it might appear to be simpler to go -12v to -20v. Is this something you'd build yourself, or looking for an off-the-shelf module? How much spare current do you have to play with on the +12 and -12 rails?
Thanks for your reply.

Not sure exactly on how much current I have or will use. This is an old 1990 computer. The original AC-DC power brick is 120v 1.5amp AC to 16.5v 2.5amp DC.

I was going to use a PICO PSU ~160watts which I heard was way more than I need (for all the other wires 12, -12, 5 and gnd).

I am willing to build a circuit myself or buy something off the shelf if it is (nearly) a plug and play situation.

Someone recommended to me this https://www.amazon.com/Converter-Ad...egulator&qid=1627389129&s=electronics&sr=1-5&

to use that and a PICO PSU and tie the grounds together.

I just want to make sure I won't get any smoke when I put the power on the mobo.

I am a competent person with a meter, soldering, etc. I just don't know how to design a circuit.
 

Irving

Joined Jan 30, 2016
2,056
The point about a -ve supply is that its output has to be -ve with respect to the input ground. Now that one looks OK, but its a bit overkill. You only want 30mA and that's designed for a lot more, and dual outputs. What you don't know is how stable the -20v will be at low current with no load on the + side. It may be good, or not...

I've had a quick look, but there's nothing on Amazon at the moment that really works. The one you linked to is not currently available.

There are a number of SMPS chips which will do this job, I'll have a look and see if there's an easy one chip solution.
 

Thread Starter

Gitbizy

Joined Jul 26, 2021
14
The point about a -ve supply is that its output has to be -ve with respect to the input ground. Now that one looks OK, but its a bit overkill. You only want 30mA and that's designed for a lot more, and dual outputs. What you don't know is how stable the -20v will be at low current with no load on the + side. It may be good, or not...

I've had a quick look, but there's nothing on Amazon at the moment that really works. The one you linked to is not currently available.

There are a number of SMPS chips which will do this job, I'll have a look and see if there's an easy one chip solution.
Thanks for your help, much appreciated.
 

Irving

Joined Jan 30, 2016
2,056
Well there's nothing on Amazon, eBay or AliExpress that fits the bill.

The circuit below is about as simple as you can get for a build-it-yourself on a piece of perfboard with a little care about the layout. It runs off the -12v rail, generates -20v +/-50mV and is stable from no-load to about 100mA so fits the bill perfectly.

If you fancy building it I'll work out a layout based on parts available from your favourite local supplier...

1627584459392.png
 

Thread Starter

Gitbizy

Joined Jul 26, 2021
14
@Irving , THANK YOU SO MUCH. I have been away from the computer for a few days (taking a break) so I am just looking at this now.
I can get the parts from Digikey or Mouser. And I am willing to build it.
Just for kicks, see the screenshot below. I was able to trace out where the -20vDC actually was used in the computer, and it is the LCD.
On the left, PIN 6 has the -20vDC and that runs through the circuit pictured and then to the LCD itself.
I would really appreciate a layout.
 

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Irving

Joined Jan 30, 2016
2,056
OK. I was a little concerned as to where LCD+ came from in case it wasn't ground referenced, but it appears to be a separate 5v feed derived from the +12V rail. In the original DC-DC converter the main 5v switched mode supply also generates the -20v and -12v rails. A separate switcher generates the +12v rail off which a secondary 5v is taken which appears to be the LCD+ feed. You might consider using a 78L05 to replicate that off the +12v supply using similar parts to the original (IC105, though it doesn't seem to say 78L05!).

I'll work up a layout, including the secondary +5v rail, based on parts from those suppliers - I'll try and keep to one or the other to meet minimum order values for free postage...
 
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Irving

Joined Jan 30, 2016
2,056
Here you go...

1628350506092.png

Board layout on individual pad perf-board. 2.1" x 1.6" (19 x 15 holes, or slightly larger if you want mounting holes)
Thick lines: 20awg tinned copper wire + solder fill,
Thin lines: 24awg tinned copper/component legs
Mouser shopping list attached

1628361917963.png

1628361993089.png

1628357380845.png

1628357791683.png
 

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