The current limit of the LT3080 power supply circuit not working  - Page 1
Forum section: electronics / projects

author: sachin11
author_rank: Newbie
timestamp: May 19, 2016, 07:12:15 pm
content: Hello Fellow members,I am putting up the circuit given by dave in his video series on Lab power supply using LT3080. However, during testing i found that the current limit was not working .. the output of the current limit op-amp was not able to pull down the set pin of the LT3080 to ground. I am attaching the circuit herewith, whenever the current limit is reached the output voltage isnt falling down to zero as it should be and the voltage at point A ( Please see the attached figure) moves to -2 V instead of hitting the positive rail +12V , hence the transistor remains OFF. Kindly see the problem and suggest the problems .Thank You,

author: danadak
author_rank: Super Contributor
timestamp: May 19, 2016, 07:25:16 pm
content: The Inv and NI sense connections to the 1 ohm sense R are not reversed in yourwiring ?Also the 1K to ground from the input side of the 1 ohm should be connectedto the NI input of the sense opamp, to make that circuit work correctly as adiff amp.https://www.google.com/search?q=opamp+diff+amp+image&tbm=isch&imgil=txNerwJKbPaGAM%253A%253B_QQ0PLzeV78jWM%253Bhttps%25253A%25252F%25252Fen.wikipedia.org%25252Fwiki%25252FOperational_amplifier_applications&source=iu&pf=m&fir=txNerwJKbPaGAM%253A%252C_QQ0PLzeV78jWM%252C_&usg=__VQfnTpoxsKh6Qvt6AhWLk9GKTq4%3D&biw=1821&bih=870&dpr=0.75&ved=0ahUKEwiRpJe77-bMAhXhx4MKHb3wAYgQyjcINg&ei=pxM-V9G0G-GPjwS94YfACARegards, Dana.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 19, 2016, 08:01:08 pm
content: The resistor at the set input will add an small extra voltage, so the output will not go all the way to zero. Also the transistor has a residual collector emitter voltage (e.g. some 50 mV) so it will not go down al the way for this reason too.Chances are the current regulation will oscillate, because there is a lot of extra gain. The 741 is OPs are not really accurate, so quite some error in the current too. The LT3080 might need a capacitor at the input side to give a stable voltage regulation - so the current limiting will be inherently slow.The LT3080 is usually not a good choice for the output stage of a lab supply. It's a regulator chip for a well behaved load, and fixed internal current limiting. Also power dissipation can be a problem.

author: sachin11
author_rank: Newbie
timestamp: May 20, 2016, 03:04:34 am
content: Thanks Dana,The the 1k resistor from the sense resistor should not go to.ground i just checked that and i corrected it , still the problem of current limit is there, the difference amplifier is working just fine and it is supplying the required signal for current detection.

author: sachin11
author_rank: Newbie
timestamp: May 20, 2016, 03:12:41 am
content: Thanks Kleinstein,Shall i go away with 741 and use may be single supply quad op- amp like LM324? The problem is like this, whenever the load draws more current than the set current , the output of the LT3080 is not going down to zero instead it comes down linearly from some finite value to zero when the set current is changed from.initial value to zero.Like if the set current is 300 ma and load draws 0 ma then the output voltage is at 0 volts when i change the ouptut voltage from 0 to 9 v the load draws 200 ma . Now if i change the load current to say 500 ma the output voltage comes down to 4.5 volt and load current reduces to 400 ma . Now if decrease the current set then the ouput voltage linearly drops from 4.5 to zero.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 20, 2016, 08:47:54 am
content: The 741 is not good, but it is not the main problem. Going from the 741 to a single supply OP like lm324 or TLC277 could save the negative supply, but thats about it.The problem is trying to use the lt3080 to build a lab power supply.You might solve the obvious problem by ganging the NPN transistor to a N-channel mosfet (e.g. 2N7000) and removing the 1 K resistor at the LT3080 set input (pin2).So a better option might be using an LT1577 and an IRF510 or the old L200 instead of the LT3080. These chips are at least made to have an current limit.

author: sachin11
author_rank: Newbie
timestamp: May 20, 2016, 09:40:47 am
content: Okay Kleinstein, Thank youI will try to use MOSFET and remove 1k resistor at set point and report back. Also, LT1577 option that you suggested may not work for me as i am trying to build a regulated power supply from 0-30 V. And if i am not wrong LT1577 works upto 5 V is it?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 20, 2016, 10:44:29 am
content: The LT1577 would work up to about 15 V - so not good for a 30 V supply. But the LT3080 is also troublesome at 30 V, as it can not stand much heat and thus could only provide very little (e.g. 50 mA) current without preregulation. Also a maximum of 36 V at the input is to little for a 30 V output from an unregulated source.

author: sachin11
author_rank: Newbie
timestamp: May 20, 2016, 11:37:58 am
content: So what shall i look for making a 0-30 V regulated 2 A supply, is there any good option available within an integrated package or i'll have to go the tough way of putting up discrete transistors and op -Amp circuit?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 20, 2016, 01:22:32 pm
content: As an integrated chip, there is the L200, but even then power is limited to something like 40 W and current limit is not easy to adjust. For 2 A and 30 V you essentially have to go the discrete power transistors and separate regulator circuit.It is not that much more complicated than trying to add current limiting to the LT3080 or similar. A good basis could be a circuit similar that discussed in a parallel thread:https://www.eevblog.com/forum/projects/linear-power-supply-based-on-hpagilent-e3610a/