Ideas to make a Incremental Voltage device

denwag

Im trying the interface the ad5220 to a LM317 to create a testing device that will output voltages from 1 to 15 volts in 1 volt increments. I written the code that Will vari the pot in 16 even increments and pause for a few seconds between changes.

I want to use this to test small circuits for class and send the output them to my computer via rs232 port.

Ive tried to link the wipe resistance directly to the adj of the LM317 and I overheated the AD5220. Ive also tried link the wipe to the negative of a 741 opamp with a 10k resitance from negative to output to create a Non inverting amp. With similar overheating results.

Anybody got any ideas.

Regards
Dennis Waggoner

stamptrol

Dennis,

Without seeing the schematic of what you're doing, I'll take a stab assuming its the standard connection from the LM317 spec sheet.

The ad5220 is probably overheating either due to drawing too much current from it or having it mis-connected so you're accidently shorting it to ground.

What may work for you is to connect the regulator as per the data sheet. Adjust the 5k pot to give the maximum voltage you want. Connect a NPN transistor across the 5K pot ( collector will be where it hooks to the ADJ terminal, emitter goes to ground. Then, control the base of the transistor with the ad5220 output ( through a 1K resistor, to start).

The more the transistor turns on, the lower the output voltage.

Cheers

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Tom Sisk

http://www.siskconsult.com
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denwag

stamptrol said...
Dennis,

Without seeing the schematic of what you're doing, I'll take a stab assuming its the standard connection from the LM317 spec sheet.

The ad5220 is probably overheating either due to drawing too much current from it or having it mis-connected so you're accidently shorting it to ground.

What may work for you is to connect the regulator as per the data sheet. Adjust the 5k pot to give the maximum voltage you want. Connect a NPN transistor across the 5K pot ( collector will be where it hooks to the ADJ terminal, emitter goes to ground. Then, control the base of the transistor with the ad5220 output ( through a 1K resistor, to start).

The more the transistor turns on, the lower the output voltage.

Cheers

Ill give it a try. Just not sure of the tweaking I would have to do. Basically I just need to set up voltage on the LM317 adj pin using the collector voltage on T1 to simulate the voltage divider R1, R2 from ground to output as show on the LM317 data sheet. Just not sure what kind of impedence the collector will see through the adj pin. Would you agree

Regards
Dennis Waggoner

denwag

Just not sure what kind of impedence the collector will see through the adj pin. Got datasheet shows adj pin max current .1ma so I should not have any problems connecting the collector directly to the adj pin.


Regards
Dennis Waggoner

Tracy Allen

The maximum voltage for the AD5220 is 5.5 volts. That is for both the Vdd power and for the voltage on any of the potentiometer terminals. The LM 317 works by servoing its output until the voltage from Vout to Vadj is 1.2 volts. The rest of the voltage appears across the resistor that goes from the adj terminal to ground. When the LM317 is required to put out 15 volts, the voltage from adj to ground will be about 13.8 volts. If that is where your AD5220 potentiometer is hooked up, it will not be happy.

One way to get around that would be to use a different voltage regulator, one that servos to a constant reference with respect to ground instead of with respect to the output. For example, the LT1129 has a reference point of 3.75 volts with respect to ground, so you can use a digital pot with no hassles.

To stick with the LM317, you have to find a way to limit the voltage to the potentiometer. Tom offered one suggestion. Another way to do that with greater accuracy is with a common base cascode circuit. That uses an NPN transistor with the collector to the adj pin, the emitter to the top & wiper of the AD5220 potentiometer, and the base to +5 volts Vdd. The bottom of the potentiometer goes to ground. The cascode circuit holds the top of the potentiometer at +4.4 volts, and passes the variable current from the potentiometer through to LM317 circuit.

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Tracy Allen
www.emesystems.com

denwag

Tracy
Thanks for the info. I believe that I am going to go a different hopefully more elegant route. Perhaps a R/2R ladder circuit to a opamp or perhaps I could easily interface a 74LS90 as a decade counter and apply this directly to a 4 or 8 bit D/A Converter. Then I could control the voltage with simple pulses from the BS2 Chip

Any suggestions for a D/A converter to use. Perhaps a DAC0808
Regards
Dennis Waggoner

Post Edited (denwag) : 1/29/2007 11:21:25 AM GMT

Tracy Allen

Any of those ideas could work. Just keep in mind that the device you use will have to withstand the full 14 volts across its terminals. That rules out many DACs. The R-2R is an interesting idea. Or just a selection of different resistors switched to ground by open drain transistors.

Thinking about it more, the cascode circuit (current sink) I suggested won't work, because the current has to be set by the resistor from Vout to Vadj, which is 1.25 volts/R1.

A circuit using an op-amp could work by controling the voltage at Vadj. You said you tried an inverting op-amp circuit. A noninverting circuit might work better, with the wiper of the pot delivering 0 to 5 volts to the non-inv input, and a gain of x3 delivering 0 to 15 volts to the output, attached to the Vadj terminal of the LM317. That should give 1.25 to 17.25 volts from the LM317 output. The op-amp feedback resistors would be 10k from inv to ground, and 20k from inv to output.

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Tracy Allen
www.emesystems.com