Four Quadrant power supply with current measurement - Page 1
Forum section: electronics / projects

author: sandy.holmarc
author_rank: Newbie
timestamp: November 30, 2018, 10:03:49 am
content: Hello,I would like to develop a four quadrant power supply with a ammeter and voltmeter for photo voltaic VI measurement. when a current sensing resistor added in the series with the load in this setup it works fine in the lower voltage .But at voltage above 0.5V the output of LT1970 is oscillates.I need a closed loop circuit to make VI measurement.Is there any other method to develop a Four quadrant power supply with a ammeter and a source meter ? .Any help would be greatly appreciated.Best regards,Sandeep

author: Rerouter
author_rank: Super Contributor
timestamp: November 30, 2018, 10:20:09 am
content: It would appear your sense resistor doesnt make sense? are you sure your schematic is correct, as at the moment the current sense doesn't do anything?Still I thank you for making me aware this IC exists.

author: Kalvin
author_rank: Super Contributor
timestamp: November 30, 2018, 10:28:30 am
content: You may want to check your design against the reference design from Analog Design:"LT1970ACFE | Power Op Amp with Adjustable Current Limit Boosted":https://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/dc453b-b.html#eb-documentationSchematics:https://www.analog.com/media/en/technical-documentation/eval-board-schematic/DC453B-SCH.pdfNote particularly how the feedback capacitor is connected from the LT1970's output to the negative input. You have a capacitor connected from the output to the ground, which is no a good thing as it may cause the op amp circuit oscillate.

author: sandy.holmarc
author_rank: Newbie
timestamp: November 30, 2018, 11:03:27 am
content: hello ,Thank you very much for the quick reply. Apologize for not mentioning about the relays.,The relays S1 and S5 are closed for the high current range (10mA to 1A) ,S3 and S4 closed for the range 0.1ma to 10mA and S2 and S6 closed for the 0.1uA to 0.1mA.

author: sandy.holmarc
author_rank: Newbie
timestamp: November 30, 2018, 11:11:44 am
content: Hello Kalvin ,Thanks for the assistance,I used this circuit as the reference. The capacitor that you mentioned is 330Pf and 220PF ?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 30, 2018, 12:16:36 pm
content: Both the capacitors 220 pF and 330 pf and also the 47 pF have an effect on stability. Especially with a more tricky capacitive load like the solar cell, this can be important.It would be a good idea to check the circuit in LTspice, when changing the feedback.Having the shunts on the high side, like shown, might not be a good idea, as it hampers the voltage control loop.

author: sandy.holmarc
author_rank: Newbie
timestamp: December 01, 2018, 05:29:20 am
content: Hello Kleinstein,This circuit works fine and i checked the voltage from -1V to 1V across the load .I Modified this circuit as closed loop for maintaining the exact voltage across the load(1V to -1V).When I added a series resistor 2.5R for for measuring 10maA to 1A in seies with the load it works fine upto 0.5V.I will check the circuit by replacing the capacitors as you mentioned .

author: Kleinstein
author_rank: Super Contributor
timestamp: December 01, 2018, 08:30:01 am
content: There is no need to use the AMP03 for the voltage feedback: the negative side is already at ground, and it is hard to beat a pice of wire with a unity gain amplifier when it comes to speed, noise, accuracy and price.The AMP03 also need pin 1 to be connected to ground as the reference level for the output - otherwise it works as a times 2 amplifier.The AMP03 is also relatively low impedance (50 K) and would thus causes some error with the current measurement.The 2 µF capacitor directly at the amplifiers output is a bad idea - the series resistor in the original plan is there for a good reason.At least for the larger current measurement resistors one might need to add a capacitor in parallel to help the voltage control loop. Alternatively the 330 pF capacitor should go to the point before S1,S2,S3.P.S: With only a low current (e.g. up to 300-500 mA), there is no need to use the circuit with an external power stage. The much simpler circuit with only the LT1970 should be sufficient and much easier to get stable.

author: sandy.holmarc
author_rank: Newbie
timestamp: December 01, 2018, 10:04:49 am
content: Hello Kleinstein,Thank you very much for the assistance .Please suggest me an op amp equivalent to AMP03 for high precision voltage measurement ,I connected the feedback from AMP03 to -ve input( 8'th pin ) to get an exact input voltage at the output.I would like to develop it with minimum error,If i didn't connect to the voltage measurement output will be equal to the voltage drop across 2.5R(in 10mA to 1A range) resistor + load voltage. Basically i need a source measurement unit (-1 to 1V across the load) ,current measurement (0.1uA to 1A). Any help would be greatly appreciated.