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Topic: ADC and a zener  (Read 1 time) previous topic - next topic

LandonW

Might be <=75mV between the inputs, but your diagram shown them connected to the +side of the motor.
Opamp inputs shouldn't normally have more than the supply voltage of the opamp on them.

Understood, but not needed, and creating more problems.
Forgot to add that R9 also must go, and R2 also must connect to 3.3volt.
R2 can actually be removed if the Trinket has internal pull up enabled.
Leo..
ok, i see what your saying now.... the shut math is 1mV = 75 amps.
how would you suggest that i get the amp reading for my calculations??

this is my first attempt at high voltage project....
if you could find the time to draw a schematic that you would use i would appreciate it.

this project is for a EMD 40 -2  locomotive.    the AC circuit is for RPM sensing

The main gen of this locomotive has an alternator in the same housing.  this gen has no need to maintain Hz. its basically more RPM = more power.


LandonW

question about the opAmp….  since the opAmp isn't connected at + and -  the potential difference isn't 1000vdc?   since the potential difference between the shunt is indeed smaller than the potential difference on the supply

Wawa

A shunt (galvanic connection) will only work of shunt voltage is within the supply range of the opamp,
although some chips can measure somewhat outside that range.

So not a big problem if the shunt was in the ground wire of the motor, but impossible on the +600volt side.
I remember you once posted that the shunt was part of the instruments of the loc.

Not sure how to solve it, but a clip-on DC hall sensor (on any of the motor wires) could work.
You might not even need the opamp for that.
Leo..

LandonW

A shunt (galvanic connection) will only work of shunt voltage is within the supply range of the opamp,
although some chips can measure somewhat outside that range.

So not a big problem if the shunt was in the ground wire of the motor, but impossible on the +600volt side.
I remember you once posted that the shunt was part of the instruments of the loc.

Not sure how to solve it, but a clip-on DC hall sensor (on any of the motor wires) could work.
You might not even need the opamp for that.
Leo..

yes this has been an on again and off again project.
the normal procedure for finding the horse power with these test points is  voltage between TP1 and TP2 which is usually around 600vdc,  Then take a voltage reading at TP3 and TP4. (it will show in mV) 
I guess i need some education on why it would be 600+ volts for the OpAmp and not for a DMM?? 
Im half tempted to "go for smoke"

Wawa

Your DMM is not connected to any ground.

But the opamp is.

Assuming you have connected the Arduino to loc ground, for safety, and to measure that 600volt.
Leo..

LandonW

The only ground that is on the op amp is battery negative.  Same with the DMM. (When taking readings from TP3 and TP4)

LandonW

My project does not share ground with the locomotive.

polymorph

Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

LandonW

It's in a plastic case and I plan to fill it with that insulation gel/appoxy shtuff and remove any of the unneeded solder points on the PCB

Wawa

If you're going to use the Trinket high-side (near the 600volt supply), then you can't measure voltage.
Current will also be a problem, because you also have to power the Trinket high-side.

I would forget about that loc shunt for the Trinket (only use it for your DMM).
Much better/safer to keep the Trinket grounded to the loc, and use a clip-on or pass-through hall sensor based DC current sensor with 4-20mA output.

And the string of resistors I recommended for voltage.

What sort of current are we actually talking about (Amps), and is it bi-directional (loc forward/reverse)
Leo..

LandonW

It's 3000 horse power. So the amps through the locomotive load (wheel motors or resistor grid) there are four wheel motors in parallel. When the locomotive is under heavy load it can reach 1500 amps measured on one wheel motor via shunt resistor in that circuit. There are two wires in that shiny (one on each side) that go straight to an analog meter on the operator control stand.
To answer your question about current, it is all one direction out of the generator and through the shunt shown on my schematic. There is switch gear to control forward or reverse but the current at the given test points is always the same (under regular conditions)
In regards to the hall sensor that you mentioned. How big is it and how does an ADC know current output of that sensor? The ADC only recognizes voltage change correct? If the hall sensor is a "clip on" like an amp clamp is then it defeats the design and intended functionality of the calculator.

I was intending to have the calculator plug int to these test points via male banana prongs on the back of the calculator. All of these test points on the locomotive are centralized. I was planning to take advantage of that.

If you would kindly post a link to the hall sensor that you mentioned and or a schematic of how you would design my project, it would be greatly appreciated.

LandonW

Also the locomotive has NO chassis ground in its electrical system except for high voltage ground sensing relay that will shut down load if tripped.

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