Subject:
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Re: Voltage Sensor
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 19 Sep 2002 17:26:36 GMT
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Viewed:
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706 times
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Hello,
> Rather than use an Op amp to offset the range, use a Zener diode instead of
> the upper resistor of the potential divider described in (A). I'd use a 10V
> Zener which means the you can measure battery voltage in the range
> 10V..15V.
>
> So the circuit is:
> Battery +terminal to Zener +terminal;
> Zener -terminal to resistor-1 terminal A and to resistor-2 terminal
> A
> resistor-1 terminal B to ground
> resistor-2 terminal B to the RCX sensor input
>
> What value for resistor-1? I'd say a couple of thousand ohms. If you use a
> smaller one, you're throwing away more current from the battery. If you use
> a larger one, you get a bigger effect from the input impedance (and pullup)
> of the RCX.
This is a really interesting idea. Make sure to allow a current of roughly
10% of the Z-Diode's maximum rating, even for the lowest battery voltage you
are interested in. Then you will have little effect of the pullup or the
variation of current through the Z-Diode. Select the smallest possible power
rating for the Z-Diode, so these 10% won't eat too much current.
If you have multiple inputs to measure, you will probably use relays to
connect one of them at a time. Whenever you are not measuring, you can also
disconnct all relays to save on battery power.
> But then comes the next worry. If you're charging a car battery then, the
> charger might produce more than 15V (unlikely but better safe than sorry).
> In which case the voltage at the Zener -terminal would rise above 5V. I
> imagine that the RCX sensor input has an internal diode to stop the voltage
> rising above 5V. The battery charger can produce a huge current and you
> don't want it to flow through the RCX sensor input diode. So limit the
> current using resisor-2. Lets say you limit it to 2mA if the charger
> reaches 20V (extremely unlikely!). That implies resistor-2 should be 2.5k
> (2.7k is the nearest you can buy).
>
> Which terminal of the RCX input do you connect to? What if you get it wrong
> and connect to the ground terminal instead of the actual sensor terminal?
> Don't worry, no damage will be done - you wn't measure the battery voltage,
> that's all.
>
> Be careful when you're experimenting. If you connect the 12V battery
> directly to the RCX input, you might damage the RCX. Always have resistor-2
> in place.
>
> I'm no RCX expert - does that sound reasonable to everyone else?
Yes, at a first glance. Never tried it, though ...
Greetings
Horst
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Message is in Reply To:
 | | Re: Voltage Sensor
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| (...) I presume you're using a 12V lead-acid or NiCd battery. As the battery discharges, the output voltage drops, slowly at first but then faster when it gets close to being empty. By the time it reaches, say, 10V, it's time to recharge it. So when (...) (22 years ago, 18-Sep-02, to lugnet.robotics)
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