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6V supply for digital circuit?

P

Paul Slocum

Jan 1, 1970
0
I have this computer forune teller machine that I've been modifying
and repairing, and I thought it was odd that it was running on 6V, so
I traced through the power supply thinking there was a bad component
or something. I traced it back to the regulator and then I noticed it
was a 7806, so I guess it's supposed to put out 6V (converted from
30V).

Is there some reason this device would be running at 6V? It has a LCD
display, 68705 microcontroller, serial buffer chips, etc. All this
stuff is designed to run at 5V. Might they have done it just to
reduce the heat on the regulator? Is it safe to run all this stuff at
6V?

-Paul
 
H

Howard Henry Schlunder

Jan 1, 1970
0
Maybe one of the fortune's told is that the fortune teller is going to break
down soon. That way, at least one fortune will be true and nobody will sue
the manufactures for spewing completely fallacious information.

I'm wondering if the LCD has a fluorescent backlight driven by the 6V
supply. Although 30V would be the best source, 6V (versus 5V) is
significantly better for converting to the 400V or so that cold cathode
fluorescent lightbulbs need. Also, you mention serial buffer chips. Does
that imply that an RS232 interface is present someplace? The EIA232
standard is supposed to specify signaling voltages for serial ports of a
minimum of -3V (logical high) and +3V (logical low). That 6V swing
requirement might be why 6V is used.

I kind of doubt that reducing heat on the regulator was the goal of choosing
6V. Most CMOS chips have a tendency to draw a fair amount more current
whenever you increase the voltage, so by running at 6V versus 5V, most
likely the regulator sees more current draw and in turn gets hotter than a
5V one would. Although it certainly seems undesireable running 5V chips at
6V, I don't think it would violate any absolute maximum specifications for
the components you've mentioned, so it strikes me as adequately safe for a
commercial application used for non-critical recreational purposes.
Besides, the sooner your product breaks, the sooner you can sell your
customer a new one... or something like that :).

Howard Henry Schlunder

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J

Joe Legris

Jan 1, 1970
0
Paul said:
I have this computer forune teller machine that I've been modifying
and repairing, and I thought it was odd that it was running on 6V, so
I traced through the power supply thinking there was a bad component
or something. I traced it back to the regulator and then I noticed it
was a 7806, so I guess it's supposed to put out 6V (converted from
30V).

Is there some reason this device would be running at 6V? It has a LCD
display, 68705 microcontroller, serial buffer chips, etc. All this
stuff is designed to run at 5V. Might they have done it just to
reduce the heat on the regulator? Is it safe to run all this stuff at
6V?

-Paul

You would have to consult the data sheets for the parts to see whether
6V is kosher. But before you go an a wild goose chase, check the ground
connections at the chips - "ground" may be at 1 Volt. This is one way of
getting a "negative" supply (i.e. ground), perhaps for the LCD.
 
P

Paul Slocum

Jan 1, 1970
0
I'm wondering if the LCD has a fluorescent backlight driven by the 6V

It is. Maybe that's the reason for the 6V, although most inverters
I've seen expect 5V. The EL backlight is actually something I'm
trying to fix since it's very dim.
...Also, you mention serial buffer chips. Does
that imply that an RS232 interface is present someplace?

Nah, my description wasn't very good. They're "latched shift
registers", used to drive a lot of LEDs off few microcontroller ports.
Although it certainly seems undesireable running 5V chips at
6V, I don't think it would violate any absolute maximum specifications for
the components you've mentioned, so it strikes me as adequately safe for a
commercial application used for non-critical recreational purposes.

Okay. Well the machine was built in 1987 so I guess it's probably
alright. But I'll likely convert it to 5V just to make things a
little more safe. I'm going to have to get a new inverter when I
replace the LCD backlight, so if that's the reason for the 6V, then
that's no problem.

Thanks for your help,
-Paul
 
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