Attached is the spec sheet for the digital pot and my schematic.

I am taking the output of three mechanical pots and based on certain conditions I am either adding or subtracting voltage from the mechanical pots to get a voltage output that represents a different position of the mechanical pots.

My problem is I am smoking the digital pot. The max voltage on one leg of the digital pot is 3v. It fails when I input the max 3v into the digital pot. So I thought to myself, what the hell I will read the spec sheet for the pot, maybe I am exceeding the max amperage tolerance of the digital pot. Here is a line from the spec sheet
AX–BX, AX–WX, BX–WX . . . . . . . . . . . . . . . . . . . . . . ±20 mA

So if I have 3v, I need a 150 ohm current limiting resistor on the leg getting the 3v. So I lift up the pin and place a SMT resistor in between the SMT pad and the pin. It failed again.

So am I reading the spec sheet correct? Maybe I am just so close to the max amperage I need to go with a higher value current limiting resistor.

On the schematic the digital pot is on the bottom left corner

Thanks
Toby

 

What is the value of your voltage Vdd?

hgmjr

 

5v is the VDD

 

Is opamp OA2d being power by +5V and ground?

hgmjr

 

12v for the power.

 

That means that the output of the opamp can drive one end of the digital pot to a voltage greater than the digital pot's 5V power supply. Am I correct?

hgmjr

 

Take a look at pin 12 of the op-amp. It has 4.33 volts on it constantly. Pin 13 has the output of one of the mechanical pots with a voltage range of 4.33-1.33 volts. I am using the op-amp to output on pin 14 the difference between the two pins so pin 14 is 0 to 3v. This 0 to 3v is then sent to A1 of the digital pot.

I used 12v to power the op-amp so my 3v wasn't even close to the rail value.
Hope this helps

 

Perhaps the output of the opamp is spiking to a voltage greater than 5V for a very brief period. If it did, that would be enough to latch up the digital pot and destroy it.

Do you have a 5V tranzsorb that you could put on the output of the opamp so that it would clamp it to 5V?

hgmjr

 

So R29 needs to be canned? I am pretty new to the op-amp deal. This project has been a hell of a learning experience. I am using the schematic in the following link as my baseline. When I test the outputs everything seems to be fine until I get to the full span of the mechanical pot.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar6.html#c1

 

Whats a tranzsor?

Am I interpreting the spec sheet correctly and need the current limiting resistor? I am going to put a 1k resistor between the op-amp and the digital pot.

 

So R29 needs to be canned?

No, R29 is needed. I think the reference voltage on the positive terminal does need to be at 2.15V.

hgmjr

 

Whats a tranzsor?

Am I interpreting the spec sheet correctly and need the current limiting resistor? I am going to put a 1k resistor between the op-amp and the digital pot.

A transorb is like a zener except that it can react to voltages higher than its voltage rating in nanoseconds.

hgmjr

 

Ill see if Radio Shack has them tomorrow or I will order some to try too
Thanks

A transorb is like a zener except that it can react to voltages higher than its voltage rating in nanoseconds.

hgmjr

 

Take a look at part number SA5.0CA. I think Mouser or Digikey has it.

hgmjr

 

Thanks for your input. I have some ES2D switching rectifiers. Would that work?

 

Thanks for your input. I have some ES2D switching rectifiers. Would that work?

Nope, those are just fast recovery diodes. A 5V transzorb is what I am suggesting. The name TransZorb is a trade name for a specific manufacturer's device. The generic name of the part is a "transient suppressor".

hgmjr