RE: [CAD_CAM_EDM_DRO] Electrical Help

I suspect you may be looking at "servo" control from a black and white
perspective, so I'll first see if I may help you qualify your end objective
a little. A few simple questions will get to the heart of the matter
quickly.

If you apply one polarity of DC voltage when your error signal is at 120 VDC
+ 3 VDC (123 VDC) and then nothing until the error signal is at 120 VDC - 3
VDC (127 VDC) where you apply a reversed polarity voltage, you might end up
with a mechanical oscilator, if the energy and mecanical initeria is much,
or the electrical or mechanical delays are significant. In other words, if
you are thinking of applying +100 VDC, nothing, and -100 VDC to a motor, you
might find that it goes back and forth with more initeria and over-shoot
than is reasonable or tolerable.

If you have all that considered, then there are transistor (or Triak)
solutions to your problem. You desire a 6 Volt "dead-band" and want to
operate up to 280 VDC. The current required by the motor is a large
consideration, as well as the fact that the motor is an inductive load.
Solid state devices are very sensitive to the excessive voltage found in
switching inductive loads. This is solved by the type of solid state device
used (certain Power MOSFETS for example) and with Voltage clamping devices.

All this may mean as little as 4 large transistors, a few small opto
issolators, some Zener diodes, and a few misc. parts, or it goes up from
there when you want to do more than just switch large amounts of voltage
into a motor.

Temperature has an effect on the accuracy of electrical components. SO like
tolerance in machining, what sort of tolerance over temperature must you
hold +- 3VDC to?

When tolerance and precision is not much of an issue, a few diodes and
transistors make a switch which is rated by the type of transistor used
(easily over 400 VDC). If you want to control much over 1/4 Amp, the parts
begin to get larger, more expensive, and require heat sinks and so on.

There are many here who can draw a circuit diagram on a napkin. I am only
offering a few of the many issues which may surround your project, and you
may have thought out your application beyond this for all I know. You'll a
little thin on the technical requirments (electrically) to give you a
meaninful reply.

-----Original Message-----
From: Ejay Hire [mailto:ejayhire@...]
Sent: Friday, August 04, 2000 10:48 AM
To: CAD_CAM_EDM_DRO@...
Subject: [CAD_CAM_EDM_DRO] Electrical Help


Let me preface this by saying I don't know of any op amps that work over 40
VDC.

The normal way to do what you want to do is with a special op-amp circuit
called a comparator. You compare the actual voltage to a "reference
voltage" and te op-amp amplifies the difference. An analog filter would
give you the +/- 3V dead zone, and an H-bridge would allow you to turn the
motor.

On second thought, This would be a great project for a 2$ PIC
microcontroller and an A/D Converter. Less Chips + Less Cost = More Cool!



----Original Message Follows----
From: STAN MCDONALD <SMCDONAL@...>
Reply-To: CAD_CAM_EDM_DRO@egroups.com
To: "CAD_CAM_EDM_DRO@egroups.com" <CAD_CAM_EDM_DRO@egroups.com>,
modeleng-list <modeleng-list@...>
Subject: [CAD_CAM_EDM_DRO] electrical help please???
Date: Fri, 04 Aug 2000 09:18:42 -0700

Hello list.
I am trying to come up with a device that I can feed 100-280 volts DC
into and then if the voltage is less then a preset voltage (around 120
DC) then it would inturn connect a separate DC power source in one
direction to a DC motor, If on the other hand the voltage was higher
then the preset 120 DC it would connect the same separate DC source to
the motor in the opposite direction.

This would inturn drive the dc motor clockwise if voltage is to high and
counterclockwise if the voltage was to low. If the voltage is the 120
volts +/- 3 volts nothing happens.

I tried this idea with relays but it didn't work out because the relay
would turn on at one voltage and turn off at a different voltage.

Does anyone here have a good way to make such a device?
Thanks
Stan



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