Revision as of 01:06, 25 July 2015

Constant Current Sink

Goal: Will sink a constant current independent of the input voltage.

Range: 1mA to 5000mA. Max Vin 30V.

Made the box mostly out of 10mm plywood. It's a quick job. Adding the decal is needed.

The bare PCB and populated PCB.

RV5 (reference voltage adjust) to 5V. Test point at P3.
Set SW_A1 to non-EXT value
RV1 (100mV voltage) to 100mV. Set SW2_B1 to bottom value, Set SW1_B1 to 1x gain (top pin). Use P1 as test point.
RV2 (10mV voltage) to 10mV. Set SW2_B1 next to bottom, Set SW1_B1 to 1x gain. Use P1 as test point.
RV3 (2x gain) to 100mV/10mV. Set SW1_B1 to 2x gain. Adjust until 200mV/20mV on P1

R5 @ 1k8: wrong value. Cannot get 2x gain with 1k8+5k pot. Change to 6k8+5k pot to get 2x gain.
R15 @ 10K: Doesn't give enough current to the LM336Z5.0. Switch to a lower resistance, 3k9 works for 9v input but. If 9v battery is used, voltage will droop till 7v during battery life. R15=1k5 will give stable reference 5V with a Vbat of 9V5 to 7V. Quiescent current does go up, but usable battery life will still be longer.
Board has no mounting holes. These should be added.

Reduce reference voltage to 2V5 or 1V25. 5V is not needed. 5V external input can be divided to get reference voltage.
Investigate the use of a Instrumentation Opamp for voltage voltage comparisons and better controlled gain. INA126's are on hand.
Use a relay (or some such) to switch between 0R1 and 10R resistor shunts, reducing needed MOSFETs and voltage comparison to one.
Use 2 or 3 AA batteries instead of 9V and use boost converter to create needed higher voltages.
Improve the enclosure. Using 3mm wood or sheet plastic. The knobs do not fit very well, but do the job.

@@ Line 32: / Line 32: @@
 * R5 @ 1k8: wrong value. Cannot get 2x gain with 1k8+5k pot. Change to 6k8+5k pot to get 2x gain.
 * R15 @ 10K: Doesn't give enough current to the LM336Z5.0. Switch to a lower resistance, 3k9 works for 9v input but. If 9v battery is used, voltage will droop till 7v during battery life. R15=1k5 will give stable reference 5V with a Vbat of 9V5 to 7V. Quiescent current does go up, but usable battery life will still be longer.
+* Board has no mounting holes. These should be added.
 == Possible improvements for next ref ==