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Need help with bi directional constant current source (±100mA)
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OM222O:
that made things even worse! I honestly can't explain this behavior!


Can you please run a simulation yourself and check what I'm doing wrong?
duak:
OM, I didn't look too closely at what you'd presented previously.  Sorry about that.

You need another current sense resistor after or to the right of the current booster.  The INA's feedback resistors are to be connected to the new resistor, not the original R1.  R1 develops a voltage that turns on the booster transistors when more current is needed.

I can't do a sim or schematic.  I have Linux and can't seem to get the design package working.

I'm just about to head out for a few hours - observe & cogitate; I think you'll get it.

Cheers,
OM222O:
The LT1990 does not have a feedback output  :-// they instead went with a x10 gain functionality which is useless  :palm: I instead chose to use the AD8277 which is a 2 channel difference amplifier with a gain of 1. It's the perfect fit for the job. The only thing is it uses 40K trimmed resistors instead of 1Meg ... I'm not sure what issues if any will that cause (some advice would be nice). I think they will create an error in the output voltage which is why it's recommended to use another op amp in unity gain configuration to drive the "ref" connection.


also after running the simulations, it seems that the external voltage supply will have no effect unless the output of the op amp can reach that voltage (i.e. if I use ±12V to drive the op amp but connect the transistors to a ±48V supply, the transient response would be exactly the same!) any comments on how I can improve that?

I also ditched the idea of using transistors as they don't seem to have any SOA rating  :-//  so I went with mosfets.
also at the beginning of the transient, the voltage goes about 1.5V above the supply rails of the op amps ... can that cause an issue? if yes, is there a way to fix it?
here is a link to the falstad simulation:
http://tinyurl.com/y7294uw7
and here are the things that I was asking about
duak:
OM,

1.) You are probably opening a can of worms by using FETs for the current booster.  In particular, high frequency stability and possible damage to the FETs by exceeding their gate to source voltage limits.  I'm surprised the bipolar transistors do not have a graph of the Safe Operating Area.  What devices were you using?

2.) Without going into great detail, I'd say that by not buffering the + divider, you'll get a current error in absolute terms equal to Vload / 80K.  This is because the divider is diverting some of the current pump's output to circuit common.  If you want 100 mA at 20 V, you'll get 99.75 mA.  If you want 1 mA at 20 V, you'll get 0.75 mA.  If this is used in a closed loop servo, this is most likely acceptable.  IOW, the non-linearity just gets lumped into the stiction of the actuator.  Another way to look at it is that current pumps should have infinite output impedances, ie., the current is independant of the voltage.  This will put an 80 K resistor in parallel with the current pump.

3.) if you decide to use an opamp to buffer the + divider, good design practice is to not exceed its input to supply rail voltage and current specs.  It'd probably  be acceptable to use a resistor of say, 10 K to 100 K in series with the + input of the buffer to limit current in case a greater voltage is applied.  This will introduce another error due to an offset voltage developed across this resistor by the input bias current of the opamp.  (see Final Note)

4.) If the INA is powered by +/-12 V, there won't be any real difference in the transient response.  An inductance generates a counter voltage across itself equal to the rate of change of current thru it.  Therefore, if you want to change the current faster, you need to apply a greater voltage to overcome counter voltage.  Using +/- 40 V on the current booster does not change the fact that the INA output is still limited to +/- 12 V.  The current booster has a voltage gain of almost 1 but a current gain related to the beta of the transistors.

BTW, I've been using the term INA for Instrumentation Amplifier.  It's probably not completely accurate as these parts are maybe better described as Opamps with Integrated Differencing Divider Networks - OIDDN - or something like that.

Final note: As a Jewish friend said, "Oy Gevalt!"  These things are like a game of Whack-a-mole.

Hope this makes sense and helps in some way (at least)

Cheers,
iMo:
This works somehow.. Parts chosen are from my junkbox, use yours if any, and do elaborate.. It gives you +/-102.4mA into R6 or L1 (the load)..
The signal generator is 2.048Vpp sine with DC offset +1.024V.


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