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AC vs DC input power

falter

falter

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I'm working on a Netronics keyboard and terminal I got recently and am at the point where I need to power it up. It did not come with a power adapter, but the case seems to indicate 8-10vac. IIRC Netronics for whatever reason preferred AC rather than DC power supplies for their equipment.

Just as a precaution.. is there anything I need to be wary of with AC vs DC? I have a 10vac 1200ma adapter here.. I was going to remove the barrel connector (the keyboard requires a 3.5mm), and temporarily wire it directly in.
 
No schematic that I've found so far but here is a pic of the board.

On the left side is the power connectors.. they are marked simply "AC in" and "ground'.. so I'm assuming it's as simple as that. That big grey cap has broken off its pads though so I have to fix that before i connect anything.

I have a creative labs adapter, it says 10VAC 1200ma 12VA for output.

Don't know why I'm so weirded out by AC vs DC.
 
I see a filter cap, some diodes and a voltage regulator. It's not clear if the diodes from a full-wave bridge, but DC input can be used. If this is just a half-wave rectification, you may need to reverse the polarity on your DC supply.
 
Would there be any reason not to use the AC? I looked at some of my other Netronics equipment and the power supplies all appear to put between 8-10VAC to the boards rather than DC. Not sure what the advantage or reason is.
 
The advantage would be that the power adapter need be nothing more than a simple transformer. And if you want to use it somewhere with different mains voltage, you just use a different transformer.
 
A couple of reasons for using AC instead of DC, when one considers the time period. An AC adapter, as mentioned, is just a simple step-down transformer. Inexpensive and the output is symmetrical (that is, it doesn't matter which output lead is connected to the tip or barrel on a barrel connector). Most of all, a DC supply using a transformer usually involves a set of diodes as a rectifier, one or two electrolytic capacitors and usually a linear voltage regulator--all in a closed box. Eventually, the heat will boil the capacitors to death and the adapter, being sealed, is pretty much not repairable.
 
Ahhh. Thanks Chuck, that makes eminent sense. I wonder why that practice wasn't more common?

So are you saying it doesn't matter which wire I connect to ground or AC on the board if I cut off the barrel connector on this supply and wire it in directly?
 
Depends on the rectifier configuration used on the keyboard. If it's a full-wave bridge, then it doesn't matter which wire goes where. If it's half-wave, then you might have to reverse the wires. Use a voltmeter on the regulator side of the circuit to verify.
 
It might also be 2 half wave rectifiers to give you plus and minus voltages. If that's the case then DC wont work.
can't tell without a zoom in to the power supply area of the PCB.

joe
 
That's a valid point--but looking at the various part numbers (M2716, COM8017, 2114, CRT96364B) are all single-rail +5 supply--the rest appears to be LSTTL.

So probably good with DC. The original FASTVID 64 board didn't have the rectifiers and regulators; it required +5 and -12V to be supplied. But this is a Rev C board, so it looks to be different.

Of course, there may be a surprise lurking in this thing, so proceed cautiously.
 
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I didn't see any LSI parts that need minus voltage either. But it does have a UART so RS-232 (sort of) or for the keyboard?

joe
 
I didn't see any circuitry associated with level conversion. I suspect that the interface is strictly TTL--and meant to be used with a separate CPU board. But I'm guessing about that part.
 
Looks like the only place that the -12 is used is in the RS232 output, unless I missed something (the hand-drawn schematic is pretty hard to read).

But the positive voltage is clearly derived from a half-wave rectified AC, which means that a DC input would be polarity-sensitive.

So, easiest to use an AC supply. :(
 
So here is a 10VAC adapter I propose to use with this Netronics setup. What does the dashed line represent? I note on the Netronics board it has two wires connected.. 'AC in' and Ground. Would the non dashed wire be the ground?
 
It doesn't matter. AC is bipolar--and AC transformer supply secondaries are, as a rule, ungrounded. It seems to this observer that 10VAC is a little light, given that this thing expects to deliver -12VDC from it. However, given that the -12V just supplies RS232 "mark" level, it probably doesn't matter--many later laptops with serial ports were pikers on those levels, some supplying less that -5V for mark. All of the ICs take +5 only; you're not going to burn anything out if the supply doesn't deliver the full -12V.
 
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