Measuring voltages

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I've been lurking for a while and now I guess it is time to throw out some
questions. Over the last couple of years I have become very active in
Electric Vehicles which includes the need to charge lots of large batteries.
One of the handiest tools to have available for this is a large variac.

I am now looking to build a variac with some automation. I am looking at
using this.
http://www.fairradio.com/6565.htm

I can handle the motor control, but I need some advice on parts for reading
voltages. I need to be able to to make charging profiles dependent on
either current or voltage levels. This variac is rated at 0-180V and
20amps. I'm assuming the easiest way to measure the current is with a shunt
so that I really only need to be able to read two voltages. It also seems
that by using a precision resistor to read the variacs voltage and picking
my shunt correctly that I really only have to read millivolts. My only
experience with the BS-2 so far is going through the "What are
Microprocessors" material and I have not done much with low powered
electronics lately, so I am not familiar with what chips are available and
easy to use. So what parts will interface well with my BS-2 that can read
this range of voltages, and are there some handy examples of how to use the
two together?

You can see some information about my Electric Motorcycle here if you are
interested. Of course I am always tinkering with it, so it is hard to keep
the information current.
http://www.austinev.org/evalbum/497.html

thanks
damon

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Archiver

<SNIP>

> I need some advice on parts for reading
> voltages. I need to be able to to make charging profiles dependent
on
> either current or voltage levels. This variac is rated at 0-180V
and
> 20amps. I'm assuming the easiest way to measure the current is
with a shunt
> so that I really only need to be able to read two voltages. It
also seems
> that by using a precision resistor to read the variacs voltage and
picking
> my shunt correctly that I really only have to read millivolts.


<Snip>

You have a few ways to go. One is to use an ADC to input an alalogue
value of the thing you are measuring, and then it will change that to
a digial value. For that you will probably need an op-amp to set the
range so your mV input is 0-5v or some such. The ADC's are serial
and will use 3 or 4 pins.

The above would be the highest resolution, getting worse resolution,
but probably with parts you have on-hand......

Another way to go is to use an R/C and RCTime to see a very low
resolution value. This is simple and only uses on input pin for
each voltage or channel.

Another is to use a quad comparitor and some voltage dividers to
generate different trip points. This would only tell you if you are
above a certain voltage. But, if all you really wanted to know was
if you were above or below some voltage this would be the simplest as
the input would be on or off. This would use one pin for each
voltage trip point so 2 channels with 4 points each would be 8 pins.


Dave

plug-in ADC ....
http://www.visualmuses.com/chipcircuit/index.html

Archiver

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Original Message

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From: damon henry [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=zCGLjG_k0KzqSLuqRVVb8TNZgBumpA8DOXDQ5FKzF5II1HHtJXk9SZ5fYyTiys3WTZqnZoJhnxE69tIriRL7GA]damonhenry@h...[/url
Sent: Tuesday, May 25, 2004 11:26 AM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] Measuring voltages

I'm assuming the easiest way to measure the current is with a shunt

Some things to consider.

If you are driving a motor, for example, there will be a momentary surge of
current upon startup, very possibly in excess of 20 Amps. Now the core of
the variac will limit the current somewhat, but surges are common. Be
certain that your circuit can stand the surge. IE is the shunt resistor
large enough in power to handle a surge without heating up and changing
value, and thus causing errors in measurement. Do you have enough bits to
measure the surge, or does the A/D just max out. Do you really need to
measure the surge, or just survive it?

Also, there is the possibility of spikes upon turn on or off in Voltage.
Use limiting transorbs (or zeners) on the wiring between the variac and the
sense circuit with some series limiting resistance.

I have replaced numerous components on boards without these
considerations<G>. It can get ugly, and I'm known for ugly circuits<G>.