# Digital Pot max amp question

Discussion in 'General Electronics Chat' started by Tobias, Oct 18, 2009.

1. ### Tobias Thread Starter Active Member

May 19, 2008
158
0
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

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2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
What is the value of your voltage Vdd?

hgmjr

3. ### Tobias Thread Starter Active Member

May 19, 2008
158
0
5v is the VDD

4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Is opamp OA2d being power by +5V and ground?

hgmjr

5. ### Tobias Thread Starter Active Member

May 19, 2008
158
0
12v for the power.

6. ### hgmjr Moderator

Jan 28, 2005
9,030
214
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

7. ### Tobias Thread Starter Active Member

May 19, 2008
158
0
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

8. ### hgmjr Moderator

Jan 28, 2005
9,030
214
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

9. ### Tobias Thread Starter Active Member

May 19, 2008
158
0
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

10. ### Tobias Thread Starter Active Member

May 19, 2008
158
0
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.

11. ### hgmjr Moderator

Jan 28, 2005
9,030
214
No, R29 is needed. I think the reference voltage on the positive terminal does need to be at 2.15V.

hgmjr

12. ### hgmjr Moderator

Jan 28, 2005
9,030
214
A transorb is like a zener except that it can react to voltages higher than its voltage rating in nanoseconds.

hgmjr

13. ### Tobias Thread Starter Active Member

May 19, 2008
158
0
Ill see if Radio Shack has them tomorrow or I will order some to try too
Thanks

14. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Take a look at part number SA5.0CA. I think Mouser or Digikey has it.

hgmjr

15. ### Tobias Thread Starter Active Member

May 19, 2008
158
0
Thanks for your input. I have some ES2D switching rectifiers. Would that work?

16. ### hgmjr Moderator

Jan 28, 2005
9,030
214
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