Computer Controlled Sorensen Hack

7 10 2009

Sweet! Just got the high current (120 Amp) Sorensen power supply working with computer control (to power the superconducting magnet).

Here is a video:

Details after the jump.

The Sorensen SRL 10-100 allows for remote control of voltage and current. For current control you simply remove the jumper between terminals 7 and 8 and install a resistor to set a current. This model uses 10 ohms per amp – so a 10 ohm resister would set the current to 1 amp. From the manual:

To adapt unit for current mode, resistance programming operation, proceed as follows:
a. Close POWER switch, adjust output voltage to desired compliance setting.
b. Open POWER switch and rotate CURRENT controls fully counter-clockwise.
c. Remove link between terminals TB2—7 and TB2·8, and replace with programming resistor.
CAUTION
Operating unit with open programming leads or links will result in high output current with possible crossover to voltage mode. If ·step changes in output current are to be made by abrupt resistance alterations, use make·before-break switching.

To adapt unit for current mode, resistance programming operation, proceed as follows:

a. Close POWER switch, adjust output voltage to desired compliance setting.

b. Open POWER switch and rotate CURRENT controls fully counter-clockwise.

c. Remove link between terminals TB2—7 and TB2·8, and replace with programming resistor.

CAUTION

Operating unit with open programming leads or links will result in high output current with possible crossover to voltage mode. If ·step changes in output current are to be made by abrupt resistance alterations, use make·before-break switching.

d. Run load to terminals TB3·1 and ‘I‘B3·3, observlng polarity.

e. Set POWER switch to ON. Unit supplies regulated programmed current to the load.

NOTE Any rotatlon of the CURRENT control alters the output current. Dangerous output currents may result. The panel current knobs may be removed lf desired.


So easy enough. But I still need some way to change the resistance from the computer. Enter the digital potentiometer.

This is the specific digital potentiometer chip (MCP4021 – 202E/SN) I used:

IMG_4138

Pretty tiny. I had to switch my soldering tip:
IMG_4101

Here it is with tiny leads soldered to it:
IMG_4141

Very fragile, so I expoxied it to some prototype PCB:

IMG_4147

Schematic:
chip

Wired up to the DAQ, and the fluke multimeter set to measure ohms:

IMG_4153

Here is an overview of the protocol for wiping the potentiometer:

A 2-wire synchronous serial protocol is used to increment or decrement the digital potentiometer’s wiper terminal. The Increment/Decrement (U/D) protocol utilizes the CS and U/D input pins. Both inputs are tolerant of signals up to 12.5V without damaging the device. The CS pin can differenciate between two high-voltage levels, VIH and VIHH. This enables additional commands without requiring additional input pins. The high-voltage commands (VIHH on the CS pin) are similar to the standard commands, except that they control (enable, disable, …) the state of the non-volatile WiperLock technolgy feature. The simple U/D protocol uses the state of the U/D pin at the falling edge of the CS pin to determine if Increment or Decrement mode is desired. Subsequent rising edges of the U/D pin move the wiper. The wiper value will not underflow or overflow. The new wiper setting can be saved to EEPROM, if desired, by selecting the state of the U/D pin during the rising edge of the CS pin.

So I wrote a small ruby program using the DAQmx library to slowly ramp up and down.

Short circuit the power supply for current mode operation:

IMG_4155

Finally hook up the assembly to the power supply:

IMG_4208

SUCCESS as you can see from the video at the top of the post!

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