Electronics > Projects, Designs, and Technical Stuff
Need help with bi directional constant current source (±100mA)
(1/9) > >>
OM222O:
Hello
I need to design a dual channel constant current source which can drive an inductor to ±100mA. the inductance is 3.2H and I want as good of a transient response as possible, so I chose the ADA4522-2ARZ-RL which is a precision differential amplifier that can be driven with upto 60 volts. I will be using a 48V supply which is divided to give ±24V output. I will also use a DAC (LTC2607-1) and a 2.048V reference (ADR4520B) to produce the control voltage. I will also use a voltage divider followed by a buffer amplifier(TLV9001) to produce a 1.024V reference and will connect it to one of the inputs of the differential amplifier. therefore if the output of the ADC is greater than 1.024 I will get -100mA and vice versa. I came up with this circuit which works great:
http://tinyurl.com/y7grgwwp


The only issue is that the differential amplifier can't source or sink that much current (about 14mA at best!) so I somehow need to boost it's output using BJTs or Mosfets. I saw this circuit which was showcased in the datasheet of LT1990 (a single channel differential amplifier) which seemed like the perfect choice as they also use a 10ohm shunt and want to produce 100mA!


after trying to simulate the circuit, it seems to not work for me ... the op amp is still the element that's passing all the current!
http://tinyurl.com/y9ylnbec


as far as I can tell, this is just the basic howland current pump with 2 pass elements. I'm not sure if I'm making a mistake in my simulation or what, but I can't seem to get this circuit to work. please help me fix it.
JackJones:
Your falstad simulation doesn't quite match the circuit in the datasheet. The 1k resistors are supposed to go to the power supply pins of the opamp. I'm not sure if falstad allows to do this though. If not, I'd suggest trying LTspice or some other simulator that allows that.
OM222O:

--- Quote from: JackJones on November 11, 2018, 04:58:09 pm ---Your falstad simulation doesn't quite match the circuit in the datasheet. The 1k resistors are supposed to go to the power supply pins of the opamp. I'm not sure if falstad allows to do this though. If not, I'd suggest trying LTspice or some other simulator that allows that.

--- End quote ---

unfortunately it doesn't allow me to do that, but I also noticed another difference. They have connected the - input to ground and used the "ref" pin instead! the dual channel differential op amp (ADA4522-2ARZ-RL) does not have such a pin. are there any other methods that can provide the boosting effect?
SiliconWizard:
Why not directly use Linear's application schematic instead?
OM222O:
I can't find the same part in the dual op amp package and I'm fairly space limited on the PCB (17.8 x 35mm) and it's mostly populated. I need to change other parts of the circuit if I have to get 2 of these chips. If all else fails, I have to use the same schematic and change my circuit around
________________________________________
EDIT:

I have tried everything I could and gave up on making the ADA4522-2ARZ-RL work. I want to use the same schematic that was provided by the LT1990 and use two of them. now the issue is that I need the response time to be as quick as possible, so the voltage that the op amp can produce is important. the LT1990 can only be used with ± 15v which is nowhere near as quick as I want it to be. it takes about 100ms to go from -100 to 100mA for my inductor.


I tried connecting the transistors to ±24V supply separately but it had no use and there were some strange 1A current draw initially.


at this point I honestly have no idea what to do :( please help.
is it possible to use a low voltage differential amplifier, followed by the ADA4522-2ARZ-RL in unity gain to get the higher voltage range for the desired result?
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod