Reading Current of Solenoids

[Omar Zeyad]

Hi Guys,

I'm having some issues with a circuit that I've created to measure the current going through a solenoid.

Basically, I'm using AD8202 to measure the current being supplied to a load (solenoid),

Here is a small description of the circuit.


The expected current that will go through the solenoid is about 2 Amps! I already tested the circuit with a power resistor and I was able to read the current through PIN 5. but everytime I test the circuit with the real load (the solenoid). the AD8202 gets blows!!

What do you guys think? would this have to do with the back EMF from the solenoid? or is my design have an issue?

Thanks

 

[davenn]

hi there
welcome to EP

Not sure if you have read the datasheet or not ?

http://www.analog.com/media/en/technical-documentation/data-sheets/AD8202.pdf

this section if it should be of interest



Am not sure what the terminal shunt consists of ? maybe it is described elsewhere in the datasheet

Dave

 

[Omar Zeyad]

I was thinking of adding a diode to the solenoid (to drain the back EMF). but I was afraid that it will change the property of the circuit that I'm testing.

Any idea if there is another chip that can handle higher voltages?

 

[Omar Zeyad]

I guess everyone is out watching the new star wars movie!!

 

[GPG]

Am not sure what the terminal shunt consists of ? maybe it is described elsewhere in the datasheet

http://www.allaboutcircuits.com/textbook/direct-current/chpt-8/kelvin-resistance-measurement/

 

[duke37]

Your circuit does not have the clamp diode so excessive voltage is generated on switch off.
Having a higher voltage chip may not work since the solenoid can doubtless produce several hundred volts if turned off fast enough.

 

[Colin Mitchell]

You need to have a completely separated power supply for the solenoid, so the back emf does not enter the chip.

 

[davenn]

You need to have a completely separated power supply for the solenoid, so the back emf does not enter the chip.

his circuit and the datasheet circuit already show that fact Colin !!
a snubber diode is still required tho to stop the large back EMF voltages entering the IC sense input pins


D

 

[Colin Mitchell]

How can a large emf develop across a 100 milli ohm resistor ??????

 

[GPG]

The ground is shared so back emf is impressed on the sense terminals relative to com.

 

[Colin Mitchell]

If the two have completely separate power supplies, no extra protection is needed.

 

[GPG]

If the two have completely separate power supplies, no extra protection is needed.

The AD8202 must have a limit on the common mode difference.
http://www.analog.com/media/en/technical-documentation/data-sheets/AD8202-KGD.pdf

 

[Omar Zeyad]

Thank you guys, I'm going to try adding a clamp diode over the load.

I'm almost certain it's the back EMF, I tested the circuit with power resistors and didn't have an issue. but I did have the same problem when using solenoids.

 

[Minder]

I am wondering why you need to measure the current as if the solenoid is DC the current is predictable based on the resistance of the coil?.
If it is a case of confirming the current is actually present, then you do not need an A/D conv to do this?
M.

 

[Omar Zeyad]

I'm validating a circuit that powers up the solenoids (which are current controlled).

 

[Minder]

If I read that right then all you need is actual current confirmation, not the value itself!
M.

 

[Omar Zeyad]

Well, the current supplied to the solenoid might change.