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Need help with bi directional constant current source (±100mA)
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duak:
re simulations:  I've been in electronics for almost 50 years - from before SPICE was developed.  It was my hobby and profession.  I'm now retired, and while I've used SPICE here and there, it wasn't how I designed and tested circuits.  I use Linux and tried the Oregano package but couldn't get it to do anything.  Right now I don't have time to fart around with simulations as I have to deal with an ill family member.

I'm asking that the circuits be simulated with stimuli that could show some problems that my experience indicates may happen.  In particular what happens when zero current is commanded after maximum current.  Some of my previous replies address this issue.

BTW, for the designs using a common emitter booster, can anyone predict what could happen when the junction temperatures of the output transistors cause the VBE to drop enough so that the idle current of the opamp is sufficient to provide base current all the time?

Cheers,
iMo:
This is with squarewave input: 0/2.048V, 100ms period, 50/50, 1us rise/fall edges, and 20-140degC step 20degC.
Note: the temperature simulation depends on how the opamps/transistors' models handle the temperature, the resistor's tempco used here is 0ppm/deg. You may define the tempco for your resistors in LTSpice when interested..
SiliconWizard:
Still not sure what your requirements are in terms of rise and fall times at full scale (0 to 100mA for instance), but I'll leave it as an exercise what voltage you'd need to get, say, a 100µs rise time driving a 3.2H inductor from 0 to 100mA... :scared:
Zero999:

--- Quote from: duak on November 18, 2018, 07:24:50 pm ---re simulations:  I've been in electronics for almost 50 years - from before SPICE was developed.  It was my hobby and profession.  I'm now retired, and while I've used SPICE here and there, it wasn't how I designed and tested circuits.  I use Linux and tried the Oregano package but couldn't get it to do anything.  Right now I don't have time to fart around with simulations as I have to deal with an ill family member.

I'm asking that the circuits be simulated with stimuli that could show some problems that my experience indicates may happen.  In particular what happens when zero current is commanded after maximum current.  Some of my previous replies address this issue.
--- End quote ---
Well the maximum di/dt is dependant on the voltage swing of the amplifier and inductance.


--- Quote ---BTW, for the designs using a common emitter booster, can anyone predict what could happen when the junction temperatures of the output transistors cause the VBE to drop enough so that the idle current of the opamp is sufficient to provide base current all the time?

Cheers,

--- End quote ---
Thermal runaway is the most likely scenario, which is why the base-emitter resistors need to be selected to ensure it won't happen. Adding emitter resistors can help to mitigate that risk somewhat, at the expense of a higher saturation voltage.

I think 100R base-emitter resistors should be safe enough with the OP07. I did make an attempt at allowing sufficient headroom. I couldn't find the quiescent current for the OP07 on the data sheet, so I derived it from the maximum power consumption figure of 120mW at a combined power supply voltage of 30V (+/-15V). P = 0.12/15 = 4mA, which would be a voltage drop or 400mV across 100R, which should be too low to bias the BJTs on, unless they're extremely hot.  I also expect the OP07's current consumption will have a negative temperature coefficient, which should make cross conduction less likely.

https://www.analog.com/media/en/technical-documentation/data-sheets/op07.pdf

Glancing at the data sheet again, I've noticed the OP07C has a maximum quiescent power consumption of 150mW, drawing a current of 5mA, so would drop 500mV across 100R, which would be a problem if the transistors got hot. I'd reduce the base-emitter resistor values to something like 82R or even 68R. It's possible to go lower, but going too low, there's an increasing risk the op-amp won't be able to produce enough drive current to turn on the transistors, especially at low temperatures.

Also note that the power and therefore current consumption figures were derived from the worst case data sheet figures, which were specified at 30V. The situation is unlikely to be that bad and at 24V, the current consumption is likely to be lower.
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