Power Supply Review - Page 1
Forum section: electronics / beginners

author: cyb3rfly3r
author_rank: Contributor
timestamp: January 02, 2021, 08:12:14 pm
content: Hello,I will need a power supply to power a low THD Wien Bridge oscillator that I am working on. Being based on the TL072 Op. Amp. it will need a dual power supply +/- 15 V.I came up with the attached (simple) design. However, I have never made a power supply before, so I am wondering if this is too simplistic. Requirements:1. +15 V, -15 V regulated output2. I would prefer a linear regulated output, to minimize noise. It is for generating a test signal for Audio applications.3. Current: not sure, but should be about 250 mA max from what I can measure now using the bench power supply4. Standard temperature range for consumer electronics.Here is the schematic:Comments and suggestions from the experts would be very much appreciated. I would especially appreciate suggestions to improve any of safety, longevity, or reliability of the power supply, without going nuts. It is a hobby project, not a commercial product.Thanks in advance!What else does it need to be safe?

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 02, 2021, 08:32:41 pm
content: Think about the output of the bridge rectifier - it's not DC. So, you need something else to smooth out the rectifier's output so it is always above the level needed by the regulators. i.e., smoothing capacitors.Seehttps://www.electronics-tutorials.ws/diode/diode_6.html

author: cyb3rfly3r
author_rank: Contributor
timestamp: January 02, 2021, 08:49:58 pm
content: Quote from: JohnnyMalaria on January 02, 2021, 08:32:41 pmYes, I was wondering about smoothing capacitors. I could add one for each rail, e.g. 500uF or so? I was not sure whether it was necessary since there are regulators already. The notes you linked are not using regulators, but I guess smoothing capacitors cannot hurt

author: MarkF
author_rank: Super Contributor
timestamp: January 02, 2021, 09:11:56 pm
content: Here is a 12V version of your supply.Just use your 15V regulators and increase the current limit resistors for the LEDs.The LEDs provide the minimum current requirements for the regulators.

author: tunk
author_rank: Super Contributor
timestamp: January 02, 2021, 09:16:54 pm
content: I think a rule of thumb is to use around 2000uF/A.

author: cyb3rfly3r
author_rank: Contributor
timestamp: January 02, 2021, 09:24:39 pm
content: Quote from: MarkF on January 02, 2021, 09:11:56 pmThank you. So, this design includes smoothing capacitors, protection diodes to discharge the output capacitors, protection diodes at the output to protect against reverse polarity, and LEDs to provide an indication that each output rail is working, correct?

author: MarkF
author_rank: Super Contributor
timestamp: January 02, 2021, 10:59:18 pm
content: Quote from: cyb3rfly3r on January 02, 2021, 09:24:39 pmNot exactly.- It has reverse polarity output protection diodes.- It has over voltage output protection diodes.- It has LEDsprimarilyto provide the minimum load current for each regulator.As I remember, each regulator requires a minimum 5mA load for proper regulation.Some brands of regulators seem to function without it. But, it doesn't hurt to have it and it does provide an indication that it's on and working.The first two are there to protect the power supply if what you are trying to power already has some voltage on its terminals.The LEDs will slowly discharge the output capacitors (not their main function).There is no discharge for the input capacitors.The diodes are for protection only. Not capacitor discharge.

author: MarkF
author_rank: Super Contributor
timestamp: January 02, 2021, 11:06:17 pm
content: You will also need heatsinks for the regulators.For example these:https://www.mouser.com/ProductDetail/Ohmite/EA-T220-51E/?qs=%2Fha2pyFadui4dO%252BO0vDXtEu6%2Fm4Tu5bpsSSUd7lIhHaMlhsrSfB9UQ%3D%3D

author: cyb3rfly3r
author_rank: Contributor
timestamp: January 03, 2021, 12:06:47 am
content: @Markf: Thanks for your help and clarificationsHere is rev2 trying to incorporate as much of the suggestions as I can:Good point about heatsinks, thanks: will add to the BOM. I have also increased the output voltage of the secondary to 48V, because, with the rectifier diodes drop, it seemed to be too close to the min input of 18V of the regulators.I also checked the datasheet, you are correct, the min load current for regulation is 5 mA for those models.Very much appreciated. When I build it (and make it work) I will post pictures.

author: Circlotron
author_rank: Super Contributor
timestamp: January 03, 2021, 12:18:25 am
content: I'd be inclined to simplify it a bit. Use a tranny with a single 15V winding feeding a voltage doubler with upper and lower 4700uf 25V caps. Peak voltage on the caps will be approx 21V and the ripple at 250mA will be approx 20x0.25=5V p/p with 1000uF so about 1V p/p give or take.

author: David Hess
author_rank: Super Contributor
timestamp: January 03, 2021, 12:28:31 am
content: Peak voltage will be about 27 volts. Subtracting the output voltage and dropout voltage yields 9 volts maximum ripple. Limiting ripple to half that or 4.5 volts and using the rule of 8200 microfarad-volts/amp and 0.250 amps yields 455 microfarads minimum with a 100% overdesign factor. Limiting input ripple to 2 volts as in your example would yield a minimum capacitance of 1025 microfarads which is the result you got.Note that power dissipation in each regulator will be about 1.5 watts at 250 milliamps which is just barely within the capability of a TO-220 part without a heat sink, so I would at least use a small heat sink.For C3 and C4 I would use 47 to 100 microfarads per amp or 10 to 22 microfarads. I would *not* use a ceramic output capacitor; ceramic decoupling capacitors belong close to the load and not at the output of the regulator.For an audio application consider placing like a 0.001 microfarad (1) ceramic capacitor across each of the rectifier diodes to suppress noise from reverse recovery. Alternatively use soft recovery rectifiers.(1) Value depends on diode characteristics and it is hit or miss whether any particular diode has this problem. 220 picofarads is usually sufficient for a 1N4001 type diode but 1000 picofarads should cover all cases. I usually experiment to find the minimum value and then double it.

author: MarkF
author_rank: Super Contributor
timestamp: January 03, 2021, 12:33:40 am
content: Quote from: cyb3rfly3r on January 03, 2021, 12:06:47 am48VAC is excessive and will generate a lot of unnecessary heat.Stick with the 20VAC you originally had.Remember that 20VAC RMS will be 20V * √2 = 28Vpbefore the rectifier.Also, your capacitors seem small.Unless you intend to pull very little current, I would at least use 3300μF on the input.And have 10μF to 100μF capacitors on the output. Especially since you are doing audio.Maybe two capacitors on each output (a 1μF and a 100μF in parallel).The PCB I shown was for a generic power supply capable of supplying the max output power.If you have minimal power requirements, you can downsize the input capacitors and the heatsinks.There is a 38mm height version of the same heatsinks._________________________________________________________________________If this is a dedicated power supply just for this purpose, you probably don't need any of the protection diodes.You won't be connecting and disconnecting the power supply.Nor would you need the LEDs unless the load will be less than 5mA. The load will always be connected.

author: cyb3rfly3r
author_rank: Contributor
timestamp: January 03, 2021, 12:38:24 am
content: Quote from: Circlotron on January 03, 2021, 12:18:25 amIf I understood correctly, you are suggesting to replace the split secondary with a single winding and a voltage doubler to feed the regulators. Am I close?In that case, one limitation would be the current provided to the load; probably OK in my case, but how do you estimate the current that a voltage doubler can provide? Do you just go with discharging the capacitors and calculate an average current for one period T = 1/120 s?

author: cyb3rfly3r
author_rank: Contributor
timestamp: January 03, 2021, 12:53:34 am
content: Quote from: MarkF on January 03, 2021, 12:33:40 amYes, it will be a dedicated power supply, but I appreciate the comments and suggestions because one of the main purposes of doing any of this is to learn more about power supplies. In that spirit, how does one get to 3300 uF for the input? Is it experience, simulation, or some other more precise calculation I have not learned yet?Regarding the transformer, perhaps I misunderstood the specifications. When it says that the secondary has 20V output, does it mean 20V RMS or 20V peak? I took it to be peak but on second thought it would make sense to be RMS since that is also the spec of the primary (115V). I wish the datasheet was clearer on this, but I guess it is a long-established convention.I will add more capacitors at the output; 1uF and 100uF in parallel seem more than reasonable. I initially went with the regulator datasheet suggestion, not knowing any better. I have not gotten to the PCB stage yet. That will be next once I trick myself into believing that I understood this design entirely.Thank you again.

author: MarkF
author_rank: Super Contributor
timestamp: January 03, 2021, 02:24:18 am
content: The 3300μF value is more of a gut feel.It all depends on how confident you are in your current estimates for the final product.A little larger capacitor just gives you a little more headroom (i.e. voltage above regulator dropout) at higher current draws.Although, I think my input voltage was closer to the regulator dropout than you have. You can stand to have more ripple than I did.The 6800μF I used is about the minimum needed when testing my general purpose supply pulling 1A andmonitoring the voltage drop at the input of the regulators.The two different size capacitors on the output are for different frequency responses.The smaller value for higher frequency response and the larger for a lower frequency response to changes in current demands by your circuit.You could put the smaller 1μF capacitor on the PCB with your Wien Bridge oscillator (Assuming two PCBs).

author: Circlotron
author_rank: Super Contributor
timestamp: January 03, 2021, 04:50:59 am
content: Quote from: cyb3rfly3r on January 03, 2021, 12:38:24 amYep, you got it right. For a voltage doubler the transformer has to supply twice as much current but seeing the winding is only half the voltage the total amount of copper in the winding is the same in both cases. If you use individual rectifier diodes you only need two instead of four. And single winding transformers are possibly cheaper and more common than centre tapped ones. Maybe. 15VAC should be heaps with 2x4700uF 25V caps for 250mA.