LM324 Power Supply with variable voltage and current - Page 1
Forum section: electronics / beginners

author: mike_mike
author_rank: Super Contributor
timestamp: March 08, 2019, 04:17:50 pm
content: Hello,I have a few questions about the attached schematic of power supply.1. Is the schematic functional ? If not, what are the mistakes in the schematic ?2. Is the layout correct ? If not, how it can be improved ? The DRC does not return any error, but I am wondering about ground connection and other stuff like this...3. Is the minimum output current really 0A ?4. I will use R3=680R, R4=2K2, R7=12K, R11=120K, because I did not found the specified values at the local electronics shop. On the VT4 footprint I will solder the wires that go to the power transistors that will be on another PCB.Here you can find the schematic image:Thank you in advance.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2019, 05:49:18 pm
content: The regulation circuit does not look good. Chances are it could oscillate and react rather slow. The transistors VT2+VT3 add quite some nonlinear gain to the loop. With VT3 the output stage is more like current controlling and thus usually needs an accurate frequency compensation to get acceptable performance.The way the current limit is done is also not good - the LM324 output level is not well defined when in saturation - so voltage regulation it would not be really temperature stable. Worst case, with the output voltage set to zero and a positive offset of OP1A the current limitation may not work as the voltage would only go down to the offset value.So before thinking about the layout, one should get a working circuit. A simulation could be a good idea to get a check, as a lab supply is not such a simple circuit. Stability can be quite tricky with possible highly capacitive loads.

author: mike_mike
author_rank: Super Contributor
timestamp: March 08, 2019, 06:48:21 pm
content: Thank you for the reply.Could you please recommend me a schematic wich is stable and have variable current (0-5A) and voltage (0-25 or 30V) and which can accept input voltage up to 45Vdc ?I know that I can find on the internet lots of schematic, but I simply do not know wich of them are good...

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2019, 07:02:19 pm
content: With up to 30 V and 5 A out, a simple linear regulator would produce up to some 180 W of heat worst case. At that power level it would really help, if there is some transformer tap switching or similar.For 30 V DC out the right transformer would be more like 28 or 30 V. Is there already a transformer in mind or at hand ?For more than about 25 V, my personal favorite would be the floating regulator with a small 2 nd transformer / winding to supply the regulator and display. Here using a transformer with center tap could allow for a relatively simple tap "switching" in the electron way (more like a class H amplifier). For a 30 V output this would be something like 2x15 V AC.

author: mike_mike
author_rank: Super Contributor
timestamp: March 08, 2019, 10:18:54 pm
content: Yes, I have: 24V/160VA and 30V/250VA transformers which I could use for power supply.I found the attached schematic. It is a good schematic ? I would like to build the 0-30V version.

author: anbudroid
author_rank: Newbie
timestamp: March 09, 2019, 06:57:44 am
content: can we input 12v SMPS as its input and also may I swap IRLZ44N instead of 2N3055. what happened if I do this

author: not1xor1
author_rank: Frequent Contributor
timestamp: March 09, 2019, 08:14:10 am
content: Quote from: mike_mike on March 08, 2019, 10:18:54 pmat those power levels (approximately corresponding to usable 100W and 150W max out DC power) you would need a preregulator or multitap switching unless you want to further increase CO2levelsIn past I postedan alternative schematic(yet untested for lack of space-time, I've to get closer to the event horizon).Here is another proof of concept (inspired by this circuit suggestion from 0999) . The drawbacks is that current reference needs to be buffered by an opamp and to get proper current regulation (i.e. with little dependence on output voltage) you would need 4 0.1% or better tolerance resistors).

author: Kleinstein
author_rank: Super Contributor
timestamp: March 09, 2019, 10:04:06 am
content: Quote from: mike_mike on March 08, 2019, 10:18:54 pmThe circuit looks reasonable and might work. So it could be a good start for a simulation. It is a little similar to the common kit circuit that in principle might work but not with so much voltage. The circuit here adds an output stage with a voltage gain of 2.There are still a few slightly odd points, that might be done better / check in the simulation:The current through R22 is also flowing through the shunt. It should probably better go to the other side of the shunt.No need for R23: R12+R22 do the Job as well.The current regulation could be rather slow to react: the LM324 is not really fast and may take quite some time to get down from some 20 V back to zero. So an additional fast acting crude current limit would be a good idea. It could also be a good idea to limit the voltage how high OP3 (current regulation) can go - no need to go beyond some 10 V.C3 is rather small compared to C4.The current regulation may not work in the initial phase before the negative supply comes up.

author: mike_mike
author_rank: Super Contributor
timestamp: March 13, 2019, 08:46:28 am
content: It is possible that the power transistors (2N3055 in the schematic at reply #4, but I used TIP35C in my project) to be destroyed if I make a short circuit on the output of the power supply ?I am talking about the schematic at reply #4 and I am talking about the situation when I use a properly large heatsink for the power transistors...

author: Kleinstein
author_rank: Super Contributor
timestamp: March 13, 2019, 03:54:54 pm
content: With a suddenen short from a high voltage there is a slight chance to damage the output transistor(s). As the current limit takes some time to react (the OP has to slew down from around 22 V to slightly below 0 and this can take some 50 µs for the LM324 alone), there can be quite some peak current. The compensation can add to this delay.The circuit from post #4 has a high current gain and could thus go quite high (possibly 100 A) if the filter cap is low ESR.Ideally one would have another fast acting current limit in one way of the other.

author: mike_mike
author_rank: Super Contributor
timestamp: March 13, 2019, 04:18:36 pm
content: What should I do to solve this problem ?Sorry for bothering you and others with such problems but I searched the internet from top to bottom and I did not found a good power supply schematic. This year I built about 5 power supply each with different schematic and all of them had problems...

author: Kleinstein
author_rank: Super Contributor
timestamp: March 13, 2019, 05:11:38 pm
content: To add a fast current limit one could add a current limit to T2: add a small resistor at the emitter and than take a small PNP transistor to take away the base current to T2. To use a smaller emitter resistor one might be able to use the middle of R4 for the base.An alternative way could be a NPN transistor turning on with more than 0.6 V over the shunt and pulling down the base of T1.A diode in parallel to R21 could speed up time for reaction with gross over-current. It could also help to have a slightly faster OP for IC1/3 and maybe IC1/4 - so maybe a TLE2022 for those 2. The other 2 OPs could be still a LM358.I would in addition do a simulation to check if the output stage is stable under all loads. If not, it might help to more the right side of R12 to the left side of R15 or similar - thus take the inner loop feedback from before the emitter resistor. This way the inner loop would have the emitter resistor as part of it's load and would be thus less sensitive to a highly capacitive load.It would probably also help to have a protection to only enable the output if the negative supply is at at least - 3 V or so. Other wise there might be a peak during turn on.

author: mike_mike
author_rank: Super Contributor
timestamp: March 13, 2019, 05:30:36 pm
content: Can you please draw a schematic for the fast protection using a PNP transistor ?1. I am attaching a schematic, but I think that it is not good ...2. Is the second attached schematic better ? Does it solves the problems of the first schematic ?

author: iMo
author_rank: Super Contributor
timestamp: March 13, 2019, 08:59:23 pm
content: I would make the Fast CC (current limit) this way (an example) - see below.When the CV goes into CC (ie when shorting the output) there is usually 10-50us delay the information from the shunt resistor propagates to the pass transistors. During that time the pass transistors create a current pulse, 10-50us long (its duration depends on the opapms types used and on their compensation), pushing the high current into the load. The "fast current limit" limits it. Set the fast current limit to say 4.5A when the max "slow" limit is for example 4A.

author: mike_mike
author_rank: Super Contributor
timestamp: March 14, 2019, 05:57:50 am
content: @imo Please have a look at the attached schematic and tell me what you think. It is better to connect the collector of BC547 in the B of BD244 ? I think that there needs to be a resistor to limit the current through BC547. I used 0.18R resistor (rated at 5w) to limit the current at about 0.6V/0.18ohm=3.33A, while the maximum output current set from the potentiometer will be 3A.

author: mike_mike
author_rank: Super Contributor
timestamp: March 14, 2019, 09:18:51 am
content: I uploaded a new schematic.Please have a look and tell me if the schematic is ok.Thank you.

author: xavier60
author_rank: Super Contributor
timestamp: March 14, 2019, 09:36:14 am
content: Quote from: mike_mike on March 14, 2019, 09:18:51 amIf there is an overload, the protection transistor will drive T2 more and the whole output stage will latch up. I can't see a simple way of using the voltage drop across the sharing resistors for current limiting with that sort of output stage. It will work with a Darlington output stage, and where the drive current is limited to some safe low amount.

author: iMo
author_rank: Super Contributor
timestamp: March 14, 2019, 09:53:06 am
content: I would make it this way.The FastCC transistor has to be with 1A max collector current one, as its Ice pulse could be several hundreds of mAmps (it depends on several factors).The FastCC transistor fully opens itself when the voltage at those 0.16ohm resistor(s) (R13/14/15) will be around 0.7V, thus the Fast CC limit will be aprox 3x4.4Amp.I would use higher 3055 emitter resistors, for example 0.33ohm, the Fast CC limit will be 3x2.2Amp=6.6Amp then, and so on.

author: iMo
author_rank: Super Contributor
timestamp: March 14, 2019, 11:27:20 am
content: And a simulation of Mike_Mike's FCC:Code:R1 Q1_3055_Ie Q1+Q2+Q3Ohm Ampere Ampere0.1 4.73 14.190.16 4.54 13.620.22 3.94 11.820.33 3.14 9.420.47 2.50 7.500.56 2.20 6.600.68 1.90 5.70

author: mike_mike
author_rank: Super Contributor
timestamp: March 14, 2019, 11:42:58 am
content: Thank you @imo.If I want to use 4 or more 2N3055 or TIP35C transistors, the schematic will be the same (the schematic at FAST CC 2.PNG), only the number of the power transistors will increase ?

author: iMo
author_rank: Super Contributor
timestamp: March 14, 2019, 11:48:31 am
content: Yes, with N pass transistors the total current will be N*3055_I(R1).Mind the BD244 may limit the total current (it drives those N*3055 with N*3055_Ibase, which is for example 990mA in the above simulation for 3*3055).It could be you have to drive the BD244 with larger Ib current then.

author: mike_mike
author_rank: Super Contributor
timestamp: March 14, 2019, 11:52:29 am
content: Can I use a higher Ic transistor instead of BC639-16, if I can't find BC639-16 at the local shop ? For example can I use BD135 or BD139 ?

author: iMo
author_rank: Super Contributor
timestamp: March 14, 2019, 11:53:15 am
content: Yes, BD139 would be ok. (best BD139-16)

author: mike_mike
author_rank: Super Contributor
timestamp: March 14, 2019, 12:02:22 pm
content: Does R10 needs to be connected between the C of BC337 and B of BD244 or it can be zero ohms, as it is in the attached schematic ?

author: iMo
author_rank: Super Contributor
timestamp: March 14, 2019, 12:14:30 pm
content: The 1k R10 in my simulation is there to simulate the input current.Do not use my R10 in your schematics.Btw, the fast current limit currents depend on the BD244 drive current a bit too, for example here is the simulation with BD244 Ibase=25mA (R10=100ohm) and Ic=1.5A in the pulse.You have to measure it and adjust at the end of the day. Simulation only here..