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Need help with bi directional constant current source (±100mA)
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Zero999:

--- Quote from: OM222O on November 11, 2018, 06:56:13 pm ---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?

--- End quote ---
That won't work because Q1 and Q2 will both turn hard on, causing lots of smoke. Look at the DC emitter currents!

Look at Q1 to start with. Its emitter is at -24V and its base is being pulled upwards to -12V via 1k, causing it to turn fully on and the same is going on with Q2.

This one of the traps with simulators: they permit things which would cause fire in real life!


--- Quote from: imo on November 16, 2018, 08:29:33 pm ---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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Yes it will work, but the +/-24V power supply doesn't gain you much. The common emitter follower will not increase the voltage swing of the op-amp, so nothing is gained from using a higher voltage power supply, than what the op-amp can tolerate.

Q1 and Q2 have a voltage gain of just under 1, so if the op-amp's output can only swing from +13V to -13V, the output of Q1 and Q2 won't be any better. There will also be some crossover distortion due to the dead band when neither Q1 and Q2 are on.


--- Quote from: OM222O on November 15, 2018, 11:21:37 pm ---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.
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Snip.

Before going any further. What are your requirements regarding bandwidth and slew rate, precision and offset? Sorry if you've mentioned it somewhere, I haven't read every single word of this thread yet, as I don't have time at the moment.

Have you considered using a bridged output? Doing so would enable the entire circuit to be powered by standard op-amps, with not voltage boosting, only current boosting would be required.

How about an audio amplifier IC?
iMo:
This gives you nice swing (here aprox 45Vpp) on the output.
The Load is L1=3.5H with R6=5ohm in series (no idea what is the Resistance of your actuator).
Current +/-102.4mA into the Load based on input (here 2.048Vpp sine w/ DC offset +1.024V).
Not easy to simulate, however..
Mind it is a simulation only..
Needs some elaboration with R values and final components of choice..
You may add decoupling, output protection diodes, snubbers, etc. (you mess with inductive load)  :)
Zero999:

--- Quote from: imo on November 17, 2018, 10:30:37 am ---This gives you nice swing (here aprox 45Vpp) on the output.
The Load is L1=3.5H with R6=5ohm in series (no idea what is the Resistance of your actuator).
Current +/-102.4mA into the Load based on input (here 2.048Vpp sine w/ DC offset +1.024V).
Not easy to simulate, however..
Mind it is a simulation only..
Needs some elaboration with R values and final components of choice..
You may add decoupling, output protection diodes, snubbers, etc. (you mess with inductive load)  :)

--- End quote ---
That circuit doesn't give a voltage offset of 1.024V, but a current offset of 102.4mA.

It's a good idea but the biasing is a bit off. The quiescent current will be high, resulting in a high power dissipation, even though there's no signal. The DC current through the +/-24V power supplies is nearly 140mA, even with both inputs connected to 0V!

The power amplifier also has far too much gain and there's no frequency compensation network so it will oscillate in real life: something which not all models will show.

I've simulated it in LTSpice. Note how I've improved the presentation, using labels, to make it easier to follow.

Fortunately it can be fixed. See the schematic below.

The circuit is biased into class AB operating using a Q5 which forms a VBE multiplier. RV1 will need to be adjusted to give about 15mA through Q1 and Q2. Start off with the wiper set so the resistance is near zero and increase it until the voltage across R14 or R5 reaches 150mV. Q5 should also be thermally coupled to Q1 ans Q1, so its base voltage varies with the temperature of the output transistors.

Oscillation is avoided by keeping the open loop gain to a minimum, rather than adding capacitors. R6 and R7 provide negative feedback to the power amplifier stage, giving it a gain of just under 3.2, in reality it's 2.1, as R12, R13 and R18, R19 act as a potential divider which reduces the gain further. R16 and R17 reduce the loop gain, without actually altering the gain (figure that one out?).


Zero999:
The most efficient solution is to get rid of the bipolar power supplies and replace them with a single 24V supply and use a push-pull configuration. U1 can be a decent op-amp, with low offset voltage and current, but U2 can be a generic op-amp, such as the old uA741.


duak:
Hero,

I'm not able to simulate yours (or anyone's) circuits.  May I have you run some simulations on your circuits first with a square wave input and then secondly a repeating plus, zero, minus, zero (tri-level) waveform?  The load voltage waveforms should be interesting, especially for the non-bridge designs.

Thx & cheers,


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