User:Chotee/constantCurrentSink
Contents
Constant Current Sink
Device goal: Will sink a constant current independent of the input voltage.
Range: 1mA to 5000mA. Max Vin 30V.
Device is floating (battery powered), so can be used both high and lowside. However, if external set-voltages are used, these must be within 0-5V of the GND input
Build goal: Learn to think and reason about electronics at a more internalized level.
Build Notes
Update 2015 07 23
- Added a jellybean BJT transistor for the 1-50mA ranges.
- Connected all the leads to the board.
- The device is _completely_ out of whack. No idea what it is yet. Reference voltages still check out when no load is connected.
Update 2015 07 15
Made the box mostly out of 10mm plywood. It's a quick job. Adding the decal is needed.
Update 2015 06 20
- The bare PCB and populated PCB. The boards were manufactured by http://dirtypcbs.com/
- Did the first adjustments. The 5v reference is bang-on 5.000v :D.
Adjustment
- 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
Errata
- 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
- Reduce reference voltage to 2V5, 1V25 or 1V. 5V reference is not needed. The 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 switching boost converter to create needed higher voltages for the opamps.
- Improve the enclosure. Using 3mm wood or sheet plastic. The knobs do not fit very well, but do the job.
- It'd be interesting to expand this to do Constant Current, Resistance, Voltage & Power in one device and be able to switch between these functions. This would be at least an order magnitude more complex then the current device.
Links
- LM336z5.0: http://www.ti.com/lit/ds/symlink/lm136-5.0.pdf
- 9v battery characteristics: http://ww2.duracell.com/media/en-US/pdf/gtcl/Product_Data_Sheet/NA_DATASHEETS/MN1604_6LR61_US_CT.pdf
- Jellybeans opamp used: LM324. http://www.ti.com/product/lm324-n
