Subject:
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Re: What AC adapter to use for RIS 1.0?
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 23 Feb 2001 04:34:01 GMT
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Viewed:
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594 times
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On Thu, 22 Feb 2001 05:51:10 GMT, "Cyborg" <cyborg@euroseek.com>
wrote:
> I just got a second RIS, this one is the RIS 1.0 with the power adapter
> input. I tried to find that AC adapter on S@H , but couldn't find it.
> There is not even any info about it in the RIS 1.0 itself.
> All the info I found was, that I need a 10V 7VA adapter for it.
> Where can I find such an adapter?
> RadioShack sells universal PA, but they have no 10V setting and most
> adapters I can see have a mA or a A rating, e.g. 12V 500mA or a 7V 1.8A
> Would a 9V adapter work? and what are 7VA in mA or A (I guess 1000mA=1A)?
> And in case I could use a universal PA, what side is the positive (inside or
> outside)?
Interesting point. I haven't run the RCX with an external adapter yet
and hadn't even noticed that it didn't come with one. The
documentation doesn't specify the input requirements, nor does the RCX
itself. I was alway under the impression that this was a legal
requirement in Canada and probably the U.S. as well. If you check any
other device requiring an AC adapter, either the adapther, the device
using the adapter, or the user's manual will specify the power
requirements (voltage, current and polarity [if applicable]) for the
device. This is to prevent electrical damage and/or fires that may
result from improperly matched power supplies. If this is in fact a
legal requirement, it's interesting to note that Lego missed it.
Anyway, here's the scoop, but I should point out first that I haven't
taken the RCX apart, nor have I seen any schematics for it to confirm
my educated guesses.
A 10V, 7VA specification translates as follows:
The VA is significant - it refers to Volt-Amperes, as mentioned in a
previous post, but it has a specific meaning. If you are semi on top
of your electrical theory, you might be saying to yourself that Volts
* Amperes = Watts (the unit of power). This is true when the voltage
and current are in-phase as with a DC power supply, but it is not
necessarily true in the case of AC power. The fact that the supply is
rated at 7VA indicates that the 10V is 10V AC(rms). The VA designation
also implies that the input current is being passed through a
full-wave rectifier - most likely a bridge rectifier - and as such, it
won't matter if you use a 10V AC(rms) or 10V DC power supply. The
rectifier will place two diodes in the current path providing a
voltage drop of about 1.4V and hence about 8.6V to the RCX. If you're
thinking this is less than the 6 1.5V batteries (9V total), keep in
mind that the 1.5V rating of the batteries is under no-load conditions
- it drops as the current demand increases and as the batteries age.
You'll only ever get the full 9V with fresh, warm batteries under
light loads.
I know for certain the MPU doesn't run at 8V - it's likely a 5V or
3.3V part so there will be an internal voltage regulator. The
electronics (MPU, display, IR port, etc. [as opposed to the electrics
(motor outputs)]) do not draw much current and hence you could
probably get away with a 12V DC power supply from Radio Shack.
The big question concerns the motor outputs: does anyone know if they
are regulated? If they are, a 12V supply could cause a problem,
particularly under stall conditions. One would have to know what the
power dissipation ratings are for the voltage regulator(s) to
determine what the maximum safe input supply voltage would be.
Finally, since a full-wave rectifier is involved, the polarity of a DC
adapter will be irrelevant.
As stated at the top of my reply, I don't have any technical
documentation on the RCX so if someone has anything that indicates
what I've said isn't true, Please let us know.
Matthias Jetleb
VA3-MWJ
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Message is in Reply To:
 | | What AC adapter to use for RIS 1.0?
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| I just got a second RIS, this one is the RIS 1.0 with the power adapter input. I tried to find that AC adapter on S@H , but couldn't find it. There is not even any info about it in the RIS 1.0 itself. All the info I found was, that I need a 10V 7VA (...) (24 years ago, 22-Feb-01, to lugnet.robotics)
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