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Electronics => Projects, Designs, and Technical Stuff => Topic started by: keitheevblog on February 23, 2015, 03:59:00 am

Title: dealing with out of spec -12v rail on a power supply
Post by: keitheevblog on February 23, 2015, 03:59:00 am
I was trying to use a mini-box picoPSU-120 to power my Commodore Amiga which uses +5v, +12v, and -12v rails. I managed to source the proper square 5-pin DIN connectors, and then I purchased one of these tiny supplies to provide the power.

In preparation of using this new supply, I hooked my amiga up to my lab bench supply, the venerable Rigol DP832.

http://www.techtravels.org/amiga/amigablog/?p=1414 (http://www.techtravels.org/amiga/amigablog/?p=1414)

The main issue is that the picopsu-120 power supply is measuring -13.8v on the -12v rail, and I'm wondering about the best way to deal with the out of spec rail.

http://www.techtravels.org/amiga/amigablog/?p=1419 (http://www.techtravels.org/amiga/amigablog/?p=1419)

I was planning on adding two diodes in series to drop the voltage, but I wonder if this is voltage difference is consistent.

I could add another LDO regulator, but I'm not sure if this is best plan of attack.

I'm welcome to any suggestions.

Thanks
Keith
Title: Re: dealing with out of spec -12v rail on a power supply
Post by: sleemanj on February 23, 2015, 04:15:41 am
A quick google reveals that the original supplies only handled 100mA on the -12v rail, so pretty much anything would be fine I imagine, a 7912 probably the easiest best option.

Title: Re: dealing with out of spec -12v rail on a power supply
Post by: mij59 on February 23, 2015, 05:45:12 am
Download the service manual of the Amiga and check what the -12V rail is use for.
So far I found that the -12V powers the opamp of the audio part, so -13,8V shouldn't be a problem.
The 7912 needs at least 2.5V between in and output to properly regulate.
Title: Re: dealing with out of spec -12v rail on a power supply
Post by: Ice-Tea on February 23, 2015, 06:12:29 am
You might wanna checkout the original PSU as well. Probably a rectified trafo, in which case -13,8 is already more accurate than what was there in the first place ;)
Title: Re: dealing with out of spec -12v rail on a power supply
Post by: Christopher on February 23, 2015, 06:55:18 am
It's probably some kind of a minimum load problem. As the load goes up a bit the voltage should droop back down to 12v. Try a high power resistor across the rail
Title: Re: dealing with out of spec -12v rail on a power supply
Post by: keitheevblog on February 23, 2015, 09:33:30 pm
The -12v rail is used for the opamp of the audio and the RS232 serial drivers. The serial drivers are good to -15v. I can't find the opamp on the schematics, unless it's integrated into one of their custom chips.

The original PSU was right around -12v. It was much better.

Yes, the actual idle draw on the -12v is somewhere around 25ma. Could be higher during use, although 100ma is likely the maximum.

Could I place a 1/2 watt 330 ohm resistor across that rail? It appears that would yield a draw of 36ma, and produce just under 1/2 watt.

I ordered an TI LM2990T-12 which has extremely low dropout, especially at the currents I will be dealing with.

How's this sound?

Thanks