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Unread 06-02-2003, 23:11
bigqueue bigqueue is offline
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Re: confusion!

Quote:
Originally posted by CHSguard72
Hey bigqueue, thanks for the help, but being a novice at electronics, I can understand BASIC schematics, but complicated ones overwhelm me. The 2nd schematic, I guess of the custom circuit itself, is a little intimidating. Could you explain what's goin on in plain english, and exactly what parts (ie. resistors, capacitors, input types) i would need to build this circuit. Once again, thanks for your help.
I'm not sure I understand the question. (sorry) Let me try to take a crack at giving more detail.

1) The circuit pictured is a difference amplifier made with a single OP-AMP. The voltage gain is given by the ratio of R1 and R1 (and R7 and R9 which are the same values 12.1K and 100K respectively) I will not have enough space here to explain exactly how an OP-AMP works, but surfice it to say that this circuit gives you a voltage output that is 8.26 times the difference of the voltage difference between MS1+ and MS1-.

2) Given that this voltage is measured across the circuit breaker, and that we measured the 30A breaker to have a .006 ohm resistance, the output of this circuit when it measures across the 30A breaker is 49.4 millivolts per amphere flowing through the breaker. (it actually measures the voltage across the breaker, and assumes that ohms law actually works) I=V/R.

3) The zener diode at the output is used to clamp the output which is capable ot going up to 12v. (since the circuit is powered off 12v) I added this top protect the A/D inputs......(I'm not sure it is needed, especially given the series resistor and potential clamp diode in the A/D input...but I thought it was better to be safe than sorry)

4) The resistor and capacitor at the output (R51 and C10) are used to filter the output. Don't forget that the motor is controlled via a PWM, so the voltage to the motor is pulsed on and off...andthis will show up in the motor current....and this RC filters all that switching out into a steady DC voltage which is fed to the analog to digital converters of the robot interface.

5) Given that the input of the A/D is 0-5v, and that it is a
n 8 bit converter (256 counts), each count is 19.5 millivolts of input....or about .4Amps per count. (for a total of 101Amps full scale)

Sorry...but as I read this, it looks basically like what I wrote in my paper.
I'm not sure this is any help.

Please give me some more specific questions and I will try to answer them.

Please note: picking the OP-AMP for this applicaiton is important. Its input characteristics must be specific to allow it to operate in this environment. (high common mode inout voltage...that is, both inputs must be able to sense at or above the power supply voltage of the OP-AMP.)

Ask me more and I will reply.

-Quentin
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