How to add a current limit circuit to an opamp's output - Page 1
Forum section: electronics / projects

author: RawCode
author_rank: Regular Contributor
timestamp: August 04, 2020, 06:48:02 pm
content: Hello guysI should limit the output current of an opamp, which is buffered by an LT1010.There are a lot of current limiting circuits out there, but i don't know how to implement them in my design, so i tried to develop my own current limiting circuit.This circuit should limit the current between .1uA to several mA. This large current range is given by the current sense method, changing some resistors when needed.But how can i sense the load's current? Considering that the opamp can drive very resistive loads, even GOhm, i think that i can't use a high-side current measuring technique since i think that the opamp's input can be unbalanced considering that one input sees a low impedance and the other can see a variable impedance. Also a low-side current measuring technique is not feasible because i'm already using a transimpedance amplifier to precisely measure the load's current. I could use this current(voltage) information, but this circuit is very slow (switching relays, ADCs and so on) and it can be blind to some "fast" current variations.I'm a student,so expect some stupid and hilarious stuff.My idea is to use a Wilson current mirror in the opamp's feedback loop. I could use then the mirrored current in a faster and less precise transimpedance amplifier. Its output is then compared to a reference voltage.To limit the current i could use a Depletion mode NMOS when the source is positive and an Enanched mode PMOS when it's negative, which both are in series with the load.What do you think about this madness? Can i improve it in some way? There is a better method to limit the circuit in this circuit?I tried this circuit in a breadboard and it works is the same way it does in the simulations. There is though a small noise in X4's output (in the breadboard there isn't that buffer). I managed to suppress it by a bit with a .1uF capacitor between the opamp's output and its inverting input, but it's still present.Here some pictures (sorry for the bad quality). I'm using a 10x probe, so the actual values in the screen must be multiplied by 10.Do you have any suggestions?

author: David Hess
author_rank: Super Contributor
timestamp: August 04, 2020, 11:57:04 pm
content: Current can be limited *without* any current sense resistor using a diode bridge driven by a pair of current sources. Mismatch in the diodes creases an output offset error but this is removed by the operational amplifier's negative feedback. This idea is shown below being used to protect 50 ohm inputs.Somewhere I have also seen this used to make a pulse generator with variable slew rate by driving a capacitor with the current limiter and then buffering the output.

author: Zero999
author_rank: Super Contributor
timestamp: August 05, 2020, 08:08:14 am
content: Are you intersted in just limiting the output current or making a constant currnet source/sink?If it's the latter, then how about the Howland current pump?https://www.ti.com/lit/an/snoa474a/snoa474a.pdf?ts=1596614856141&ref_url=https%253A%252F%252Fwww.google.com%252Fhttps://circuitdigest.com/electronic-circuits/howland-current-pump-circuit

author: RawCode
author_rank: Regular Contributor
timestamp: August 05, 2020, 01:12:15 pm
content: Thank you both for the answersQuote from: David Hess on August 04, 2020, 11:57:04 pmThis option seems quite interesting, but I don't understand it. Referring to "Tektronix 7A29 Input Protection", if pin 8 is the input of the circuit, and 18 is its output, how can a DC signal pass through U100? Is this circuit aimed for a small signal application?Quote from: Zero999 on August 05, 2020, 08:08:14 amI think that I'm interested only in just limiting the current. At given voltage, if the load requires more current from the opamp, there should be a current limit that I can set before the measurement.The uA723 has a current limiting feature like i want, but it's based on a transistor high-side sensing.Also looking at uA723's schematic, I'm not quite sure how i can implement a similar design in my circuit, considering also that the opamp have bidirectional current flow unlike the uA723

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2020, 01:34:51 pm
content: The diode bridge idea works in a way similar to the depletion mode FETs. It can pass current up to a certain limit and if this limit is exceeded will turn high impedance, limiting the current. The diodes only let as much current pass as the 2 external sources provide. It is a nice protection as it also work for higher frequencies, but the 2 source give quite some waste heat.

author: Warhawk
author_rank: Frequent Contributor
timestamp: August 05, 2020, 01:41:38 pm
content: There are opamps with the adjustable current limit. If this would be a solution I find one from LTC for you. Can't recall the part number now.Alternatively, you can turn the opamp into a Howland's current source and close the voltage feedback using an external error amplifier. It works similarly to thecurrent mode control for DC/DC converters. You then need to clamp the output of the error amplifier to limit the current. I actually have design in works that operates this fashion.

author: RawCode
author_rank: Regular Contributor
timestamp: August 05, 2020, 08:46:49 pm
content: Thanks for the repliesQuote from: Kleinstein on August 05, 2020, 01:34:51 pmI tried to simulate one of the circuits suggested before (in particular the Tektronix 7A29 Input Protection), but it doesn't seem to work. What am i doing wrong?Quote from: Warhawk on August 05, 2020, 01:41:38 pmI already saw some current-limited opamps, but the lower limit is in the mA range. I need instead at least 0.1uA limit, since this circuit should have a .1uA-100mA user defined current limit.I tried to simulate a Howland's current source, but the current changes with the load(as you can see in the pictures there are two lines one for 10k load and one for 100k load). Am i doing something wrong?Also how should i close the voltage feedback?

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2020, 09:15:46 am
content: Quote from: RawCode on August 05, 2020, 01:12:15 pmThe bridge is bidirectional from "input" to "output". The currents are balanced but the current source and sink impedances are high so the driven side sees a high impedance from the bridge. The input voltage then controls the voltage at the top and bottom of the bridge which matches the voltage drop of the output side diodes so their junction follows the input. The diode impedance is finite which causes a load dependent mismatch but this is limited by the current through the diodes from top to bottom which controls the forward impedance of the diodes. It could be thought of as the diode version of a 4 transistor diamond buffer except that the transistor version only works in one direction.This circuit is particularly useful where open loop response which cannot be unstable or wide bandwidth is required. Closed loop solutions require frequency compensation.Update: I also like that the current limit can be made variable.Quote from: RawCode on August 05, 2020, 08:46:49 pmThe circuit as shown relies on the operational amplifier supply pins to drive the transistors while most operational amplifier SPICE models do *not* accurately model supply currents which depend on output current. Maybe start by modeling the bridge driven with SPICE current sources.Note that I am not suggesting that the diode bridge current limiter is the only way. A closed loop solution is also feasible but likely more complex unless special parts are used. If a fixed current limit is acceptable, then simple Vbe current limiting could be implemented which is how most bipolar operational amplifiers handle it internally.

author: ConKbot
author_rank: Super Contributor
timestamp: August 07, 2020, 01:35:34 am
content: J111 jfet and source-gate series resistor to set the current limit. 1 resistor with a jfet on either side if you want bipolar limiting. Won't be particularly precise without calibration or trimming the resistor value, and it may be a little "soft" on the V-I curve, but for current limiting for the purposes of protection, iit's a nice combo. I like a J113 for current limiting LEDs that may get 3,5 or <24V battery voltage applied. (35V part, so it will be fine with the 15V symmetric supplies.)