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Running on 12VDC

PostPosted: Sep Fri 03, 2010 11:42 am
by vincemulhollon
Good Morning Fellow Retrocomputing Enthusiasts

I have my altair 8800 micro running on my workbench off my lab power supply at 8 VDC. Seems to draw "about half an amp" when I manipulate it into turning on all/most of the LEDs using the front panel switches. I believe the inside of the case is signed #30, or its thirty something anyway. Nice kit, immensely enjoyed building it, no problemo. Switch bodies are the kind that will melt if kept at soldering temp too long, gotta be careful.

Anywall, all good except all the ham and computer gear I own runs off 12VDC, you know, Anderson Power Poles and the associated distribution bus devices, etc.

So, I have been looking for a 12 VDC to 8 VDC solution to get this dude off my lab bench supply and onto my work desk.

Mouser was hopeless, unless I want a non-stocked minimum order of quantity fifty DCDC conv at $75 each and enough amps to arc weld thin sheet metal.

Digikey does not stock, but will sell, a nice line of TI linear dropin replacements (in other words, works like an old fashioned 7808 but is actually a 90+% efficient switcher). Looks like about $20 which won't break the bank. The TI number is PT78HT200. Outputs two amps if well enough heatsunk.

Even a linear would only be dumping 3 watts or so worst case. So maybe I should just go ole fashioned linear bolted to the metal back panel? A TO-220 bolted to the back panel Should be happy at a typical loading and not get too toasty even at peak loading. I've certainly dumped more watts into smaller heatsinks in the past.

On the other hand, lazyness being a virtue, I could simply insert eight more "reverse protection rectifiers" and at .7 volts drop a piece, I would be OK. Of course if the "12V" battery charger ever drops, I'll be on battery power, and say, 11.5 volts - (0.7*8) = a hair under 6 volts down she goes. Yet I can't just use fewer diodes, as 7 dropping rectifiers at full charge 13.8 volts in would make a toasty 9 volts into the Altair8800micro. Hmm I guess thats why us EEs invented regulator circuits...

So, just idly wondering if anyone else has come up with an adequate solution, etc.

Re: Running on 12VDC

PostPosted: Sep Fri 03, 2010 2:43 pm
by jsissom
I have been thinking about buying a few of these:

http://cgi.ebay.com/3-35V-1-2-33V-3A-LM ... 3a5e6482da

They are only $7 each (plus shipping) and will step down 12V to 5V and handle up to 2A (more with a heat sink). I would think that you could remove the 5V regulator chip from the Altair board and replace it with this (after you have adjusted the output to 5V) and it should work.

I haven't actually tried this yet though.

Jay
W9IUF

Re: Running on 12VDC

PostPosted: Sep Fri 03, 2010 9:51 pm
by vbriel
You are #30 :)

If you want to build your own circuit the 7808 at Jameco is well under $1. On the input side put a cap around 50-100uF and on the output side put about 1-10uF (both electrolytic). The connectors are 2.1mm and are common. Here's the link for jameco's 7808

https://www.jameco.com/webapp/wcs/store ... llDownView

These are 2.2A so you should be fine. If you do this circuit, try and post your work to help others out who might want to do this.

Vince

Re: Running on 12VDC

PostPosted: Nov Sat 19, 2011 11:50 am
by vincemulhollon
Ahh it only took me 14 months to get around to it, but I finally did it late last night

https://plus.google.com/111619035631991 ... XhftkaAjsu

I did the 7808 linear reg thing instead of a bunch of series diodes. Runs cool as a cucumber, can barely feel it warm up when bolted to the giant metal back panel. The tab of a 7808 is grounded... The G+ post has a pic of how I laid out the components and wiring. I know, decoupling caps would have been a good idea, ring terms instead of tinned wire would be nice, it was late and I was tired...

I built the reg on the back panel using a barrier strip and a short jumper from the main board to the barrier strip, so I can separate the back panel from the power if/when necessary.

Fun fun fun!

Re: Running on 12VDC

PostPosted: Nov Mon 05, 2012 10:55 am
by jscheef
I purchased one of these http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 1015751519 "AC 100V-240V Converter Adapter DC 9V 1.5A Power Supply US 5.5mm x 2.1mm 1500mA" All of my power issues disappeared and the wall wart I was using went back to powering my PockeTerm. The old wall wart made the 8800 reluctant to boot up the first few times. Seems to be made exactly to Vince's specs. For five bucks delivered to your mailbox, this ain't bad and you're helping to lift some Chinese family out of poverty.

Jim