current limiting doesnt go above 1.2A? - Page 1
Forum section: electronics / projects

author: xReM1x
author_rank: Regular Contributor
timestamp: July 31, 2015, 09:25:01 pm
content: I'm using the LT1083 as alternative to LM317.so it does limit the current that is below 1.2V but when I try more then 1.2V it just stuck at 1.2Vschematic:

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 31, 2015, 10:48:26 pm
content: You have two control loops simutaneously trying to decide the output voltage. Disconnect one of them. The collector of Q1 and the output of U2 cannot be connected together.

author: xReM1x
author_rank: Regular Contributor
timestamp: July 31, 2015, 11:38:12 pm
content: Quote from: ElectroIrradiator on July 31, 2015, 10:48:26 pmso when Q1 turns on I need other transistor to disconnect the output of the op amp or something like that?

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 01, 2015, 12:08:57 am
content: Quote from: xReM1x on July 31, 2015, 11:38:12 pmYou have two control loops. Disconnect the current loop for now, and debug the voltage control loop to your satisfaction.Once you have voltage control working, you need to decide what should happen with the current sense limit is reached. Do you want the maximum voltage to top out such that the set current is never exceeded? If yes, then you need to put on your thinking cap, because you cannot just switch off the output (even if the direct connection between Q1 and U2 worked), because doing so would cause an oscillation.1) Current limit is reached, and Q1 trips, shutting down the output of U1.2) Input current to U1 is now reduced to close to zero, causing Q1 to stop conducting.3) Output voltage of U1 races back up to its set value, causing the load to draw too much current again.4) Goto 1).

author: xReM1x
author_rank: Regular Contributor
timestamp: August 01, 2015, 12:32:29 am
content: Quote from: ElectroIrradiator on August 01, 2015, 12:08:57 amoh well. will try all of those the next day. thanks for the replay.

author: void_error
author_rank: Frequent Contributor
timestamp: August 01, 2015, 09:14:30 am
content: Shorting the output of U2 to ground with Q1 doesn't look like a good idea...Maybe add a resistor (10k?) from your Voltage + pin to U2 IN+ and move the collector of Q1 to IN+ of U2.You can actually get rid of Q1 and use a diode to pull your voltage reference down (10k resistor in place ---> Voltage + pin to U2 IN+) and swap U4 IN+ and IN-.

author: mij59
author_rank: Frequent Contributor
timestamp: August 01, 2015, 09:54:09 am
content: The maximum output current of the LTC6101 is 1 mA, so with R6 being 1k the maximum output voltage is 1 V

author: xReM1x
author_rank: Regular Contributor
timestamp: August 01, 2015, 03:02:29 pm
content: Quote from: void_error on August 01, 2015, 09:14:30 amQuote from: mij59 on August 01, 2015, 09:54:09 amstill the same thing.I did not understand how to get rid of Q1. what do you mean by using the diode?

author: void_error
author_rank: Frequent Contributor
timestamp: August 01, 2015, 05:38:53 pm
content: Swap the inputs of U4 and use a 1n4148 from U2 in+ to U4 out.

author: xReM1x
author_rank: Regular Contributor
timestamp: August 01, 2015, 06:17:34 pm
content: Quote from: void_error on August 01, 2015, 05:38:53 pmthat is what you meant? if yes then it still doesnt work

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2015, 06:22:32 pm
content: Even if the control pin of the LT1083 is set to GND, the regulator will still regulate to about 1.2 V at the output. So to really limit the current the control pin would need to go to something like -1.5 V.The 1 mA limit of the LTC6101 will also cause trouble at more that about 1 A in this setup.Just using Q1 to short out the output of the OP to GND is working - the output current of most OPs is limited to something like about 20 mA. You should perferrably have ohne with not to high a max current.However using a low drop regulator as a basis for a lab supply is usually a bad idea. The LT1013 is likely to slow to keep the voltage regulation stable. Low drop regulators usually don't have much phase reserve left over for the OP-amp - they are sometimes tricky to get stable on there own.

author: xReM1x
author_rank: Regular Contributor
timestamp: August 01, 2015, 07:55:58 pm
content: Quote from: Kleinstein on August 01, 2015, 06:22:32 pmI will not use LT1083 for the final schematic.Anyways I was playing around and like you said if the regulator will need to go to voltage less then 1.2V to keep the current limit then it cant because its on 0V not negative supply..so what I need is an op amp that will change it output voltage to meet the current limit voltage.the only problem is that im trying to do this but it doesnt seems to work. the op amp doesnt have a feedback - the + of the op amp goes to the set current and the negative to the current sense output so the op amp is supposd to try and make those 2 the same and output negative voltage (it has negative supply connected) so both of the input are the same but it doesnt work..

author: void_error
author_rank: Frequent Contributor
timestamp: August 02, 2015, 06:58:56 am
content: You have the diode backwardsDo you actually understand how the circuit works?Also RTFM. For EVERY component. Then use brain to figure out if it works or not.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2015, 07:54:08 am
content: I think there are a few basics missing:In a regulating circuit the feedback for the OP is usually separate for DC and AC. The low frequency and DC part goes all the way through the system (here output voltage). The high frequency feedback is through a lokal feedback - usually as a capacitor (missing in the shown circuit) that sets the speed of the regulator.With a current limiting volatge regulator you need some kind of a circuit to output the minimum of two voltages: the output is controlled by the minimum of the control from the voltage loop or the control from the current loop. This is often done with a diodes at the output of the controlling OPs, so they can only lower the voltage.Just have a look at a few older curcuits for lab supplies - this is nothing new.

author: xReM1x
author_rank: Regular Contributor
timestamp: August 02, 2015, 12:15:09 pm
content: Quote from: void_error on August 02, 2015, 06:58:56 amwell in a magic it works now, Expect 1 thing - it oscillate.here I set the current to 0.01A.also it still doesnt let me more then 1.2A. but at least now it can go to lower voltages.

author: mij59
author_rank: Frequent Contributor
timestamp: August 02, 2015, 12:59:03 pm
content: The current is limited by the LT1083 due to the high voltage difference between the input and the output of the regulator, it's in the data sheet on page 7.