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Topic: LED Ramping with DAC problem (Read 441 times) previous topic - next topic

ThreeDMD

Hi Everyone,

So I'm attempting to ramp and LED from a 8-bit MCP4901 Digital-to-Analog IC. I have it running between 0-5 V with .06 V intervals. I am then reading the voltage right before the device and having it print to the serial window. I am able to achieve a ramped out. I am not trying to control brightness, simply apply voltage and measure the current.

The problem i'm seeing is that depending on device/component I plug in, my voltage will cap off well before ever reaching 5V. I'm wondering why this is happening and what I might do to negate this problem. Could this be a problem with the current coming from the DAC?

Thank you in advance.

CrossRoads

LED has a forward voltage drop, say 2.2V for Red as an example.
Once the DAC voltage hits 2.2V, the LED turns and will conduct all the current you can supply, until it overheats and burns out with enough current.
The MCP4901 appears to have some current limit built in, I see 15-24mA Short Circuit on page 4, 17mA on page 6.
Once the output reaches Vf and the LED turns on, the voltage won't go any higher and you see the result of the short circuit current limit, until the DAC fails.
http://www.digikey.com/product-detail/en/MCP4901-E%2FP/MCP4901-E%2FP-ND/2332819
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

ThreeDMD

Thank you!

I'm looking around now for a DAC that does 100 mA or considering using an emitter follower configuration with a separate voltage source. The more minimal i can get the better off i am. Perhaps a LED driver would also work...

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