Power supply project - Page 1
Forum section: electronics / projects

author: shebu18
author_rank: Frequent Contributor
timestamp: April 02, 2012, 10:23:09 am
content: Hello, i thought that i should experiment and learn something about power supply's until Dave's design is fixed.I have a 9V-0V-9V 600mA transformer(i don't know if 2x600mA or 2x300mA), 2xLM317 and 2xLM337. I would like to make variable power supply from 1.25V to 16V. It would also be great for me to learn about the ADC and DAC and I2C (never used them).I will use one 317 and one 337 for regulation and the other 2 for current limiting (600mA per voltage regulator) After doing this i would try to intagrate a ADC and DAC for reading the voltage/current and writing the values to the LM's. I think of doing it on a breadboard and after a while maybe evan a small PCB and case. It is only for learning and understanding the functionality. I am learning electronics as a hobby.Is my idea ok? Will it work?Thanks!

author: Rerouter
author_rank: Super Contributor
timestamp: April 02, 2012, 11:40:09 am
content: you would want each coil rated for your desired voltage at 600mA, as your planning on current limiting,you would want a full load voltage of 10.25V or ~ 13W transformer, but you also have to factor in the voltage drop across your current setting potentiometer, but thats for you to decide on,as for if it should work, most definatly, just remeber if you want a dac / adc control loop, make sure the dac only sets the desired voltage and currents with feedback still influencing the regulator ,(you have negative so you can pull the set pin to -1.25V for 0V out on positive and likewise) if you have the loop being completly controlled through digital feedback it gets iffy on its reaction to active loads,also as a tip, you will want to follow the datasheets notes on reducing noise, a lm317 will oscillate if its input capacitance is too far away, (e.g. +10cm away) and this can cause them to heat up very quickly.

author: shebu18
author_rank: Frequent Contributor
timestamp: April 02, 2012, 12:06:48 pm
content: I will take a look at the transformer and creat a schematic for the PSU. I will post it here so the members can comment on it.

author: shebu18
author_rank: Frequent Contributor
timestamp: April 03, 2012, 06:19:22 am
content: On the transformer is only 230V, 50Hz, 600mA, 9V--0V--9V written. I do not know what those 600mA mean, 2x300mA or 600mA for 9V-0V.

author: Rerouter
author_rank: Super Contributor
timestamp: April 03, 2012, 08:45:02 am
content: it would be 600mA for the whole thing, so its good, however the voltage is a bit lacking, as transformers are normally made to reach its rated voltage once loaded down to its rated current,and you will have minimum voltage drops on your regulators,

author: shebu18
author_rank: Frequent Contributor
timestamp: April 04, 2012, 07:52:39 am
content: I have made a model in multisim and with a 18Vrms 50Hz AC supply, a rectifier and the standard LM3*7 schematic i obtained what i wished, and maximum of 20V, -10 to +10). Now i am curios if i can make the adjustment like the one Dave used. 2 DAC's controlled by a arduino which reads the inputs from rotary inputs.

author: Rerouter
author_rank: Super Contributor
timestamp: April 04, 2012, 09:01:30 am
content: yes, and it is quite easy to do, but wouldnt it be 4 dacs?, +- voltage and +- current, in such it would be 2 dual channel dacs, one for positive and one for negative,still the control loop is easy, when you first build the thing up, you calibrate the dac so that you know how far you have to offset it to get your voltages matching what you expect, and then using the adc input add the reading to the dac to offset the leads, e.g. dac outputs 5V (after however your managing your gains) regulator spits out 6.25V, adc sense line measures 6.19V, so dac is increased by 0.06V to make the sense line read correct,as for the encoder that is up to coding alone,

author: shebu18
author_rank: Frequent Contributor
timestamp: April 04, 2012, 09:55:00 am
content: Thank you for your help. I have almost all parts from Dave's PSU and they are all through hole so i can mount them on to a breadboard for testing.What i wounder is how to write a negative voltage via a DAC to the LM337?

author: shebu18
author_rank: Frequent Contributor
timestamp: April 04, 2012, 06:52:16 pm
content: Tried to use a opamp to adjust the lm317, like in the yPSU schematics, but it does not work this way. From searching the Internet i didn't found any usable schematic on how to drive the lm317 via a opamp.

author: Unknown
author_rank: Unknown
timestamp: April 04, 2012, 07:08:19 pm
content: Did you try the LM317 datasheet? I seem to remember that at least the National LM317 datasheet contains an application hint with an op-amp driving an LM317, probably for current limiting.

author: shebu18
author_rank: Frequent Contributor
timestamp: April 04, 2012, 07:13:35 pm
content: I found this in the datasheet. I think i will change the chip to something else. The LT3080has a set, not a adj pin. This makes the difference.

author: shebu18
author_rank: Frequent Contributor
timestamp: April 06, 2012, 05:27:33 am
content: This is the schematic for the positive part of the variable voltage regulator. 2.048V is the maximum of the DAC output. How could i determine mV/bit? I also would need to verify the voltage after the rectifier. The LM358 has a maximum of 32V as input. Right now i have a gain of 41, as some online application told me.

author: Zero999
author_rank: Super Contributor
timestamp: April 06, 2012, 10:32:45 am
content: If you used the whole transformer a 18V 600mA transformer it will work but you won't be able to get a bipolar PSU.Using an LM317 as a current limiter drops a lot of voltage, between 3.25V to 4.25V, depending on the current drawn, then there's the 2V to 3V lost in the other LM317 used for regulation.How much current are you planning to draw?You should de-rate the transformer to 420mA, it will overheat with 600mA drawn from the rectifier for long periods of time.

author: shebu18
author_rank: Frequent Contributor
timestamp: April 06, 2012, 10:40:50 am
content: Current drawn will be small, it is just for learning. I will use a ATX PSU for the real PSU. I do not need more than 10V.Now those volts lost on the current regulator are not good. I will need to change the regulator and go for something else.

author: shebu18
author_rank: Frequent Contributor
timestamp: June 26, 2012, 10:21:12 am
content: I have changed the topics on this topic. I will use as an input a ATX PSU. I will have at 12V around 30A and at 5V mare then 10A. I will use a 16A relay to switch between 5v and 12V. I will use the schematic from Dave but instead of the LT i will use a LM338 in a schematic from the LM317 datasheet. i will post the schematic when i finish it.

author: shebu18
author_rank: Frequent Contributor
timestamp: July 05, 2012, 08:42:50 am
content: Because of not finding a echivalent of the LM338 for the negative voltage i will try to make a discrete PSu with tranzistors.For the positive part i would use a IRFP240-NPN and for the negative IRFP9140-PNP. They will be driven by a darlington pair and that will be driven by a opamp. So i hope that i will can use 5A on both ends. The psu will be +/- 10.24V and a +/- 5A output. i will poste the schematic and hope for recomandtations/comments/and all that other stuff.LE: Do you know a discrete adjustable negative voltage schematic? And a low side current sense amp with voltage output(max4080 low-side)

author: shebu18
author_rank: Frequent Contributor
timestamp: August 11, 2012, 09:23:39 am
content: Did anybody take a look at the LT1083? Variable 10A regulator. They have a schematic there for a 0 to 35V and 0 to 10A PSU.

author: Rerouter
author_rank: Super Contributor
timestamp: August 11, 2012, 09:45:14 am
content: there is an application note for inverting regulators, however i cannot recall it, e.g. use a positive as a negative or vice versaalso being how your aiming for 5A, perhaps grab a low current negative regulator and use pass elements, pnp for positive, npn for negative,

author: shebu18
author_rank: Frequent Contributor
timestamp: August 11, 2012, 12:42:54 pm
content: I use the LM338 now for the positive part, i also use the psu for a max of 4.096A. What is the minimum difference between input output of the LM338? I mean if i have5V input what is the max output i can get?(5-1.25 maybe?) After this i switch to a 12V line of a PSU for the voltage. above.For the negative part i will develope the psu part and buy a proper transformer to make the PSU.The LT is a interesting part, a bit pricey but interesting. That schematic was interesting for me because the LT can only go down to 1.2V but in that schematic in could go down to 0V, so if i implement that to the LM i can make him also go down to 0V. I will see what i can do.

author: T4P
author_rank: Super Contributor
timestamp: August 11, 2012, 05:18:55 pm
content: They work differently ...

author: shebu18
author_rank: Frequent Contributor
timestamp: August 13, 2012, 07:05:16 am
content: Ok, so i will use the LM338. How can i calculate de disipated power of the LM? Is it (Vin-Vout)xI? I want to use a LM3475 for lowering the Vin-Vout voltage. The switch controler supports a 12V input and drives a PNp transistor, so the current is limited by the PNP transistor. I can now use a digital pot do modify the voltage divider on the FB pin to set the output voltage 1,25V greater then the output on the power supply.What would be the maximum output if the input is 5V? 5V-1,25V?Thank you for your help. I learned many things developing this supply.

author: T4P
author_rank: Super Contributor
timestamp: August 13, 2012, 01:27:40 pm
content: Quote from: shebu18 on August 13, 2012, 07:05:16 amYes in ALL linear regs the difference between the two Voltages * the current drawnAnd then there's the dropout voltage ...

author: shebu18
author_rank: Frequent Contributor
timestamp: August 25, 2012, 06:52:01 am
content: Hello, my schematic is almost ready. I still have some small problems. I wnat to ask someone to take a look at the circuit attached and tell me if it is ok. It is a buk converter, i used the LM25085 for it and U22 is a digital pot for changing the output of the buck(like rev C of uPSU).I still ahve a dilema with the 3 outputs of the rev b schematic. OUT+ it the output from the regulator, Sense is the negative terminal where i measure the voltage output, shouldn't i do it on the OUT+? OUT- is used for the uCurrent. If i connect it normaly to out- the current flows through the IRL to GND. If i switch Q3 on i measure uCurrrents.What should i use for my normal - terminal?Should't i measure the output voltage on the OUT+ line?

author: shebu18
author_rank: Frequent Contributor
timestamp: August 28, 2012, 04:22:04 am
content: Anybody?

author: shebu18
author_rank: Frequent Contributor
timestamp: August 29, 2012, 10:10:44 am
content: So i changed the design of the first schematic.I developed a PSU starting from Dave's PSU rev B. I have a max output of 16.384V and 4.096A. I use LM25085 as a switching regulator before the linear regulator, a LM338. I have 5(or more) memory locations, i use a INA138 for the high side current monitor and the DAC and ADC are the same. As voltage source i use a 4.096V reference.I will make it OSHW after i tested the design and code. If someone wants to see the design or modify it please feel free to ask me. I used DipTrace 2.2.9 beta for it.I will update the post with the schematic and the design. The design is still under construction but i want to make it 10x10cm and make it at seeedstudio. I will post some pics with it and some oscilloscope ripple pics after i write the code.

author: Rerouter
author_rank: Super Contributor
timestamp: August 29, 2012, 10:19:33 am
content: you face an issue with the INA138, its input range starts at 2.7V, that means to measure with any level of accuracy, you need your input voltage at 2.7V when your supply is outputting 0V, though a bit more expensive what about a LT6105?equally interested in your approach, my own design is facing some limitations and hoping others may have though up a unique approach,

author: shebu18
author_rank: Frequent Contributor
timestamp: August 29, 2012, 12:41:23 pm
content: I go down to 1,25 with the LM338, the LM25085 can not go more than 1.25V down, so i will use 3,4 or even 4V when i want 1.25V on the output. I do not need less than 1,25V on the output. For me this is ok.

author: shebu18
author_rank: Frequent Contributor
timestamp: August 29, 2012, 04:33:28 pm
content: So these is the schematic and the buck converter. I also have the uCurrent but it is not displayed.If you have any comments please feel free to comment.

author: shebu18
author_rank: Frequent Contributor
timestamp: September 02, 2012, 06:05:05 am
content: I also finished the PCB layout. Now i need to order it at seeedstudio.Yellow-TOPOrange-BOTTOMedit: What are good cap values for filtering spikes? 100n or 10u or both?

author: T4P
author_rank: Super Contributor
timestamp: September 02, 2012, 01:57:53 pm
content: you can try both but 100nF is for higher speed spikes and 10u for slower spikes ( but obviously it can take more )So yeah

author: shebu18
author_rank: Frequent Contributor
timestamp: September 02, 2012, 02:25:32 pm
content: I will use both in an 0805 package.What can i use instead of a SPST NC relay? I want to connect a pin to GND when the PSU starts, then using the MCU "free" the pin. It is about the RT pin on the LM25085. Tie it to gnd to enter shutdown and leve it unconnected for normal operation. So i want to start in shutdown mode and after i set the values the chip should exit that state.

author: ptricks
author_rank: Frequent Contributor
timestamp: September 03, 2012, 01:38:00 pm
content: You can use a transistor latchable switch .

author: shebu18
author_rank: Frequent Contributor
timestamp: September 03, 2012, 01:43:53 pm
content: i thought about that but no, i need it to be pulled to gnd right at the moment when i push ON. I could tie the base of the transistor to 5V, than after i turned the MCU on, set the rheostat and the output to pull the base to gnd, turn the transistor off and start the LM25085. Right now i had a relay with one NC and one NO contact, so i used this for the solving of the problem.I will test the transistor method on the breadboard, maybe it would be better.

author: shebu18
author_rank: Frequent Contributor
timestamp: September 03, 2012, 05:46:37 pm
content: I could use something like that in the picture. Q1 is connected to RT pin and GND and the base from Q2 is connected to the MCU pin. If i turn the psu on Q1 pulls RT pin low and the LM is in shutdown mode, is i saturate Q2 then he pulls the base of Q1 low and RT pin works normaly. So the LM is in the working state. I will try to add this method to the one with the relay.

author: NewBeginner
author_rank: Regular Contributor
timestamp: September 05, 2012, 12:57:52 pm
content: Quote from: shebu18 on August 29, 2012, 04:33:28 pm@shebu18I may be wrong but I don't think that the current limiting method used in Dave's design will work well with the LM338 (or other regulators that don't go down to 0V). Especially at low set voltages/set currents.More details:https://www.eevblog.com/forum/projects-designs-and-technical-stuff/general-purpose-power-supply-design-7488/msg143536/#msg143536MaybeDavecan shed some light on this.For a higher current you could use the LT3080 with an external pass transistor (as shown in the attached image - extracted from the datasheet).But in that case I think that care must be taken when selecting the filtering capacitors. A very low ESR capacitor can cause the output to oscillate (at least that's what my LTSpice simulation shows).That way you can use the LT3080 to set the output voltage and the external pass transistor to carry the large current, leaving only a few 10s of mAmps for the LT3080 to handle.I did not test this setup so I may be wrong. I only did some simulations in LTSPice for this.Thank youEdit:I have attached the LTSpice simulation. The TIP147 model used is from:http://www.onsemi.com/pub_link/Collateral/TIP147.LIB

author: shebu18
author_rank: Frequent Contributor
timestamp: September 05, 2012, 01:18:01 pm
content: Than i change the LM338 with a power Transistor and do the powersupply the old way. i will test the LM338 theory.

author: NewBeginner
author_rank: Regular Contributor
timestamp: September 07, 2012, 09:01:41 am
content: @shebu18Have you tested the LM338 theory? What results did you get?Thank you

author: shebu18
author_rank: Frequent Contributor
timestamp: September 07, 2012, 09:40:52 am
content: Still not, i didn't found the time. i hope i will have the time this weekend.

author: shebu18
author_rank: Frequent Contributor
timestamp: September 12, 2012, 09:27:44 am
content: I tested the LM338. I had 5V on Vin, 1k1 on Vout to GND and a 100R on the adjust pin. The output was 3,75 with adj pin in the air. If i tied adj low the output fell to 1,25V and the current did not increase. As a test i tied for a short tiem vout to gnd and got to 3amps. I do not know what happens if you tie adj directly to gnd, but i think the lm will go to 1,25V

author: ptricks
author_rank: Frequent Contributor
timestamp: September 12, 2012, 01:29:55 pm
content: The way to make these type regulators go lower is to feed them a -V on the ground pin. Of course you need a dual supply voltage, but those are pretty trivial for the load the regulator will present and designs are everywhere for how to convert a positive to a negative.

author: NewBeginner
author_rank: Regular Contributor
timestamp: September 12, 2012, 01:42:43 pm
content: So basicallythiscurrent limiting method won't work properly with the LM338 (or other voltage regulators) if it won't go down to 0V.I may be wrong but consider this case:If you set the output voltage to be ~1.25V (Vout=~1.25V) and, say, you limit the current to 100mA, then when you short the output to ground you will get a current which is much higher than 100mA.So the current is not limited in this case.If I understand this correctly this is because the current limiting circuitry won't be able to lower the output voltage (by pulling the ADJ pin low) because the output is already at its lowest possible value (~1.25V).Thank you

author: shebu18
author_rank: Frequent Contributor
timestamp: September 12, 2012, 02:20:15 pm
content: Theoretically yes, i will need to test this particularly case when i get the boards.