Designing dual linear power supply - Page 1
Forum section: electronics / projects

author: networkingdude
author_rank: Contributor
timestamp: January 29, 2021, 11:35:15 pm
content: I was hoping someone could give my schematic a once over to see if I've made any terrible mistakes.I used the data sheet as a reference for some of it and other implementations that I thought were good.The design goal was a power supply with low noise, +-15v and should have no issues powering a headphone amplifier or dac.I welcome all criticism good or bad.

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 30, 2021, 08:46:35 am
content: If you want 15V output from LM350, then you should choose a transformer secondary of 18V or more instead of 15V, otherwise the output voltage may not remains regulated at full current output.The output capacitor value of 1500uF is especially large, against the 10uF usual value, which might not be needed or even bad to the overall performance of the regulator.

author: Kibabalu
author_rank: Regular Contributor
timestamp: January 30, 2021, 09:42:44 am
content: D1 and D4 are bypassed by a shortcutTransformer wiring secondary side is wrong. 4 and 5 should be ground and you need just one bridge rectifier. For the negative side you have to use a LM333.

author: ledtester
author_rank: Super Contributor
timestamp: January 30, 2021, 03:53:30 pm
content: Here's a thread discussing a LM317/LM337 design:https://www.eevblog.com/forum/beginners/transformer-voltage-for-dual-power-supply-(12v-12v)/msg1773620/#msg1773620(see the link in the 3rd post for the schematic)You can also get kits on ebay which implement this design -- just search for "LM317 LM337 power supply".

author: fcb
author_rank: Super Contributor
timestamp: January 30, 2021, 04:27:38 pm
content: Ditch D3 and D6. You've got other problems if you can get a reverse polarity on these sections.Your o/p capacitor is unnecessarily large and maybe counterproductive (poor transient response). If it's for audio, you might want to add some lower value capacitors to improve the response at higher frequency, capacitor type might be more important than value - we are straying intocontroversial territory here. Datasheet recommends solid tants, but it's an old ds.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 30, 2021, 04:37:35 pm
content: Hi,looks OK to me. Now provide some adequate heatsinking. The input caps could be a bit larger, but thats it.Have funWG.

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 05:27:57 pm
content: Quote from: eblc1388 on January 30, 2021, 08:46:35 amI was worried about the voltage so making it 18 is not a big change.As for the output cap, the 1uf are all solid tants and the large capacitor idea was on a commercial product so I figured it was helpful.

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 05:30:54 pm
content: Quote from: Kibabalu on January 30, 2021, 09:42:44 amD1 and D4 are like this in all examples of ldo's I could find. Also the idea was to create 2 independent positive regulator power supplies. I know you normally would just use a negative regulator but then you only need one bridge rectifier.This configuration gives lower noise value.

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 05:36:36 pm
content: Quote from: fcb on January 30, 2021, 04:27:38 pmI will remove D3 and D6, I can see how they are redundant. The 1uf caps are solid tants, I will lower the output capacitor.Should I increase or decrease input caps?

author: Shannon
author_rank: Frequent Contributor
timestamp: January 30, 2021, 05:41:32 pm
content: Quote from: Wolfgang on January 30, 2021, 04:37:35 pmstrongly agree

author: tunk
author_rank: Super Contributor
timestamp: January 30, 2021, 05:43:06 pm
content: For the input caps, I think a rule of thumb says ~2000uF/A,which gives a ripple around 4V at full load.

author: Labrat101
author_rank: Unknown
timestamp: January 30, 2021, 05:44:54 pm
content: Hi Your Ground loop is really not a good . and not recommended .Not to Say It won't workYou would be better making the transform into a Center tap arrangement and use just 2 diodes . from a centre tap with a commonground direct to the centre tap will give a full wave rectification . Also it will increase affiance .using LM317 LM337 . The large cap replace with 2 x 480uf . The 1uf at the input of the LMs 0.1ufalso this arrangement with a 15volt transform will work better on full load centre tapedAs someone else mentioned there are dozens of designs on line that are proven .As your circuit stands It will suffer . Theory and Practice don't always work well in real life.Have fun

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 05:45:39 pm
content: Quote from: tunk on January 30, 2021, 05:43:06 pmI will make this change.I have also seen some power supplies with a 2watt or larger resistor with a capacitor across the rails. Should this be considered? Is there a rule of thumb for this?

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 05:48:55 pm
content: Quote from: Labrat101 on January 30, 2021, 05:44:54 pmI'm making this with positive voltage regulators only as they appear to have a lower noise. This necessitates building 2 independent power supplies.Could you elaborate on ground loops? I'm still learning the basics.

author: Labrat101
author_rank: Unknown
timestamp: January 30, 2021, 05:57:04 pm
content: Quote from: networkingdude on January 30, 2021, 05:48:55 pmYou Have got the output of ic2 feeding to the negative of the first stage bridge . this will cause a imbalance and will properly makethe second IC2 run very hot and fail . and if it does it will take out your transform as well an the bridge rectifiers.Do not use this type of ground loop its an accident waiting to happen

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 06:11:01 pm
content: Quote from: Labrat101 on January 30, 2021, 05:57:04 pmHow is this different than connecting a 2 channel bench power supply together to get a positive and negative voltage? I'ts connected the same way?

author: Labrat101
author_rank: Unknown
timestamp: January 30, 2021, 06:24:02 pm
content: Quote from: networkingdude on January 30, 2021, 06:11:01 pmA lot first your bench power supply does not have 2 sets of bridge recertifies .Plus your Bench power supply outputs are configured differently to what you are doing.If you read the recommend way of doingthis on the Manufactures site it will explain how to connect as duals .https://www.ti.com/lit/ds/snvs772b/snvs772b.pdf?ts=1612018138744&ref_url=https%253A%252F%252Fwww.google.com%252FThere is all the info you require . But of course remember its your Project so you are free do do what ever you wish .If you ask for guidance fine . your are a Novice for sure . No hard feeling we all started there .But follow the Manufactures recommendation and that way you will learn Why.

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 06:44:10 pm
content: Quote from: Labrat101 on January 30, 2021, 06:24:02 pmThis circuit is the same as 2 different fully independent bench power supplies set up in series. I'm using only positive voltage regulators. I think you are misreading the schematic. Pretend the 2 supplies are not tied to one another. They are independent and isolated from each other.

author: Labrat101
author_rank: Unknown
timestamp: January 30, 2021, 07:40:13 pm
content: Build It . exactly as per your schematic and then see what happens .It will work but there will be an unbalanced as per your arrangement .Put a scope on the finished project and all will be come clear.I have been in electronics for over 60 years done that seen that .IC2 VO is linked back to BR1 AC ~ VO is DC out .. Top minus & Bottom Plusare not true . Your GroundBR1 is coming off the first windingBR2 is the second winding .The chances that the transformer you are using the winding are probable going to be out of phasewith each other . ( Chinese stand i guess And the 2 bridges are identical in all characteristics. yes)The Ground as per your drawing can not & will not be true Zero . To Both .Therefore the top and bottom will not be isolated as per drawingPut up the scope results of the finish project .Good Luck

author: bdunham7
author_rank: Super Contributor
timestamp: January 30, 2021, 07:47:21 pm
content: Quote from: Labrat101 on January 30, 2021, 06:24:02 pmMultichannel linear bench supplies most certainly do have separate bridge rectifiers with isolated separate secondary windings for each channel. What you have here is two completely independent isolated channels that can be connected in series for higher voltage, as bipolar (as the OP is doing) or even in parallel with additional controls. I'm not seeing a 'ground loop' as there is one and only one connection to ground.

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 07:53:15 pm
content: D1 and D4 are in place as per the datasheet. Diode D1 and D4 protects against C4,C5,C9 and C10 discharging into regulator.D2 and D5 protect adjustment pin from C3 and C8.I will be removing D3 and D6 as they appear redundant.

author: julian1
author_rank: Frequent Contributor
timestamp: January 30, 2021, 08:17:42 pm
content: It *looks* like a really interesting tradeoff. Add another rectifier, but gain the benefit of using identical positive regulators on both top/bottom, with more component choice.A question, more than a comment. Is there a difference in the ripple phase, between this and the more conventional single rectifier approach? Here, the ripple would be 180 out of phase - which is desirable I think - for stuff like powering bipolar supply ops. eg. the common mode is cancelled.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 30, 2021, 08:24:18 pm
content: Your diodes are all OK. I also have D3 and D6 to protect the circuit against outside voltages when power is accidentially switched off.The design with separate transformer windings and rectifiers as you have it has advantages:- The outputs can be used as independent power supplies if needed- NPN-Type (positive) regulators are better, cheaper and more robust than negative voltage parts. LM317 is better than LM337, and so on ...With a tapped transformer winding and just one rectifier bridge, you must use negative regulators. In your design, you can avoid that.Otherwise, go ahead.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 08:48:40 pm
content: Quote from: networkingdude on January 30, 2021, 05:48:55 pmThere is no need to change schematics to centertap. Everything is fine.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 08:51:47 pm
content: Quote from: networkingdude on January 30, 2021, 05:36:36 pmDon't get rid of them. It's more reliable with them.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 08:57:41 pm
content: Quote from: Labrat101 on January 30, 2021, 07:40:13 pmNo. It won't be unbalanced.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 08:59:38 pm
content: Quote from: networkingdude on January 29, 2021, 11:35:15 pmI don't see any critical mistakes. In general, it is ok.An only possible problem: It was mentioned yet that 15 VAC is not enough to have a well-regulated 15 VDC. So you need at least 18 VAC secondaries.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 09:01:29 pm
content: Quote from: networkingdude on January 29, 2021, 11:35:15 pmLow noise design is a bit another and deeper thing. It is a bit lazy for me to start one more topic here about how to do low noise regulated power supply (I'm sorry for that).

author: bdunham7
author_rank: Super Contributor
timestamp: January 30, 2021, 09:04:12 pm
content: Quote from: networkingdude on January 30, 2021, 05:45:39 pmWhat output current are you aiming for? If you need 0.5A or less, your input caps are plenty big and the transformer will likely be fine. If you need more current, you will have to be more detailed as to the transformer's specifications. If the datasheet is showing 1uF tantalums, I would not follow that. For your output, I think a 10uF electrolytic and a 0.1uF film cap would be good. For the input, a low-ESR electrolytic over 1000uF plus a 0.1uF film cap will work as well.I'm not sure what you mean about the resistor across the rails. Perhaps this is to give the regulator a minimum load? Can you explain this better or draw it?

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 09:14:35 pm
content: Quote from: julian1 on January 30, 2021, 08:17:42 pmThe LM350 has no ideal negative counterpart (That I could locate anyway) so this appeared as the best solution that gives the most flexibility with lower noise I hope. Being out of phase and it helping is over my head at this point

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 09:17:57 pm
content: Quote from: bdunham7 on January 30, 2021, 09:04:12 pmLike thisI'm aiming for 2A output x 2. I understand I need to increase transformer voltage to 18V if I want regulated 15v out.The data-sheet said 0.1μF ceramic or 1uF tantalum on the input that's why I chose it. The 5600uF was close enough to 6000uF for rule of thumb of 2000uF per amp for ripple rejection.EDIT: Transformer is 35VA 18V Talema 70074K I think is what I will use. I have not ordered parts or anything.

author: iMo
author_rank: Super Contributor
timestamp: January 30, 2021, 09:30:20 pm
content: I built myself a 2x15V/50mA and 2x5V/100mA fixed voltage lab power supply on a single 10x10cm pcb. With two potted pcb transformers on it, each with 2 secondary windings, with 4 dil8 diode bridges in total. 2xLM317 for 5V and 2xLM723 for 15v (with 50mA foldback). All 4 sources are "galvanic" isolated, thus I have 4 voltages with 4 grounds. You may interconnect those sources as you wish (imagine 4 separate batteries with 15V, 15V, 5V, 5V). With a lab voltage source there is always the problem how to limit the current such you do not smoke your chips when experimenting. With bigger pcb I would use 4x 723, or similar, in order to provide a current limiting with foldback at each voltage source..

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 09:40:31 pm
content: Quote from: imo on January 30, 2021, 09:30:20 pmVery nice, I like the flexibility a design like that offers.

author: tunk
author_rank: Super Contributor
timestamp: January 30, 2021, 09:49:08 pm
content: I guess your 35VA 18V transformer will give you ~1+1A, not 2+2A.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 10:02:27 pm
content: Quote from: julian1 on January 30, 2021, 08:17:42 pmThe ripple is the same with two diode bridges. There can't be any ripple phase difference because they both are full-wave bridge rectifiers.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 10:05:16 pm
content: Quote from: networkingdude on January 30, 2021, 09:17:57 pmYou need those caps near the IC regulator if main rectifier caps are far from the IC regulator. 'Far' I mean more than 7-10 cm.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 30, 2021, 10:06:55 pm
content: Quote from: tunk on January 30, 2021, 09:49:08 pmYes. 1A AC, which is about 0.7 A DC.

author: julian1
author_rank: Frequent Contributor
timestamp: January 30, 2021, 10:11:23 pm
content: Quote from: Vovk_Z on January 30, 2021, 10:02:27 pmThanks. I was (incorrectly) thinking it was input freq, rather than 2x input freq, fully rectified.

author: bdunham7
author_rank: Super Contributor
timestamp: January 30, 2021, 10:24:14 pm
content: Quote from: networkingdude on January 30, 2021, 09:17:57 pmI'm glad I asked. Your transformer is way too small (by a factor of 4 or 5) and you will need very large heatsinks on the regulators.

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 10:44:56 pm
content: Quote from: bdunham7 on January 30, 2021, 10:24:14 pmLargest I can go is 50VA but you guys are right, my transformer is way too small.

author: bdunham7
author_rank: Super Contributor
timestamp: January 30, 2021, 11:03:09 pm
content: Quote from: networkingdude on January 30, 2021, 10:44:56 pmWhy do you need so much power for a headphone amp? I would have thought 500mA was plenty.And to answer your other question, the resistors are there to bleed off the capacitors when the power is off. They don't have to be large, just use a resistance high enough that the power dissipation is low.20V/2.2K = 9.1mA and 182mW. So a 2.2K 1/4 watt would be fine if you wanted this feature--which isn't a bad idea.

author: networkingdude
author_rank: Contributor
timestamp: January 30, 2021, 11:08:24 pm
content: Quote from: bdunham7 on January 30, 2021, 11:03:09 pmI absolutely don't need that just for a headphone amp, my thoughts were to make it larger in case I want to connect something larger in the future.

author: fcb
author_rank: Super Contributor
timestamp: January 30, 2021, 11:53:52 pm
content: Quote from: Vovk_Z on January 30, 2021, 08:51:47 pmD3/D6 protect against reverse polarity. This is an unlikely or potentially impossible scenario (only the OP knows for sure what’s connected, but it’s described as an audio amplifier). In that case it would technically be less reliable if fitted.As for the size of the input capacitors, this isn’t so straightforward. With the circuit as is, the datasheet reckons you’ll achieve something like 86dB of rejection (quite a lot, but datasheets can fib), so any ripple you get will be attenuated by 86dB (this will drop hugely as the frequency goes up). Then you’ve got the CMRR of the amplifier, which might be perhaps 40-70dB, which will mean that you’ll be reducing 50/60Hz ripple by perhaps 130+dB, far in excess of what you’ll hear or be able to measure.So going mad with the input capacitors will likely not help. Reducing the HF component of the noise (from the diode bridge, transformer noise, mains borne interference) will be as (or more) important - so adding an LC filter after the each rectifier might be a better use of space/BOM cost. Look up the rejection graphs at higher frequencies.As for the topology and ‘ground loops’ - nothing wrong with the design, you effectively have two separate floating supplies, commoned at a single point. In some senses it’s a better design as you’ll get the same performance out of each (not that a LM317/LM337 complementary supply is has wildly different +ve / -ve performance).

author: Kibabalu
author_rank: Regular Contributor
timestamp: January 31, 2021, 04:59:43 pm
content: I have to repeat it: You bypassed the diodes D1 and D4 and the voltage controllers IC1 and IC2 by a shortcut! Examine your schematic exactly!And follow the link in #3!

author: bdunham7
author_rank: Super Contributor
timestamp: January 31, 2021, 05:19:30 pm
content: Quote from: Kibabalu on January 31, 2021, 04:59:43 pmCould you draw the short on the schematic and post it? I don't see any short across those components.

author: tunk
author_rank: Super Contributor
timestamp: January 31, 2021, 05:21:11 pm
content: QuoteIgnorant question: is this a rule of thumb, or is it a calculation?

author: networkingdude
author_rank: Contributor
timestamp: January 31, 2021, 05:26:13 pm
content: Quote from: Kibabalu on January 31, 2021, 04:59:43 pmI can not see where there is a problem. Most of this schematic is directly from data-sheet. I will check the link again.EDIT: Are you aware that this is a dual secondary transformer and these power supplies are both positive voltage regulators only as they are fully isolated from one another? I get what your saying is true when using a center tapped transformer with a negative regulator but this is not the case here.

author: Kibabalu
author_rank: Regular Contributor
timestamp: January 31, 2021, 05:53:28 pm
content: jahimmelhergottsakra(a bavarian curse ;-)

author: Kibabalu
author_rank: Regular Contributor
timestamp: January 31, 2021, 05:54:57 pm
content: And follow link in post #3!You'll find a design suitable for the combination LM350/LM333.Or use the design attached to this post (excuse the poor quality of the picture). It works well and outputs a nice smooth output voltage, even if you feed it from a switch-mode power supply. If you need 3A, then replace LM317/LM337 with LM350/LM333.

author: networkingdude
author_rank: Contributor
timestamp: January 31, 2021, 08:46:07 pm
content: Quote from: Kibabalu on January 31, 2021, 05:54:57 pmI must reiterate that I do not wish to use a negative regulator in this design.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 31, 2021, 08:55:22 pm
content: Quote from: Kibabalu on January 31, 2021, 05:53:28 pmLOL I heard similar stuff growing up! From my Dad when he was tooling around his power tool repairs and projects...

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 31, 2021, 09:20:05 pm
content: Quote from: networkingdude on January 31, 2021, 08:46:07 pmAnd that's perfectly fine. It's not different than two positive lab supplies hooked together to make +/-. You've just put them in one box and freaked some people out.The problem is that your schematic shows a line under your diodes (D1 and D4) , those dots are showing a connection to that green line under the diode symbols. You may not have intended that, so you don't see it, but other people will point it out but you will now be talking at cross purposes. If you ever make a netlist from this schematic to go to a PCB, the short circuit WILL end up on the PCB unless you notice it. Then you've got problems.The diodes are there to protect the regulators because usually when Vout>Vin+.7V, they fry. The diodes prevent that, they are cheap.I would also reduce that 1500uF output cap. Maybe check the transformer specs at max current, and the filter cap.The only thing is I'd get rid of is the connection between the two supplies, and make that connection externally, for future flexibility. Banana jacks and cables are cheap. And if you are going to put this thing in a box, maybe some input protection on the AC side like a fuse and a common mode filter, with a connection for earth to chassis.You've basically built my student project from college, a dual linear supply with a Hammond transfo, and a sheet metal box. The thing is robust and I still use it today.Of course I am old and lazy and if I wanted a new supply, I would buy an already made supply, if only for the transformer and input is already taken care of.https://www.ebay.ca/itm/International-Power-IHBAA-40W-100-240V-Input-5VDC-Output-Power-Supply/274523214819?epid=1933579616&hash=item3feadbd3e3:g:qP0AAOSwewdffNlLThese are cheap and great. I have one of those, and if you were in Montreal we could arrange a pick up since I rarely use it. 20$?You could then use the chassis to hack away to your heart's content, or just use it as-is... 20$ doesn't even get you a decent transformer IMO.

author: networkingdude
author_rank: Contributor
timestamp: January 31, 2021, 10:50:51 pm
content: I now see this line through the diodes and have removed it, thanks for that!I will be reducing output capacitor to100uF, increasing input filter caps to 10000uF, adding 2.2K 1/2 watt bleed resistor next to filter capacitor, removing 1uF output capacitor as the 100uF is good by itself. Not pictured is the IEC input which will be filtered for noise and all 1uf, 10uf and 150uf caps will all be aluminum polymer for lower ESR.Also the transformer will be upgraded to 18v 50va pcb mount.I will also separate the connection to make it flexible.Thank you everyone for the suggestions!

author: bdunham7
author_rank: Super Contributor
timestamp: January 31, 2021, 11:12:08 pm
content: Quote from: networkingdude on January 31, 2021, 10:50:51 pmWhoa! Where did that come from? I have large, very powerful audio amplifiers (for speakers, not headphones) that have filter caps that are smaller than that!

author: networkingdude
author_rank: Contributor
timestamp: January 31, 2021, 11:14:16 pm
content: Quote from: bdunham7 on January 31, 2021, 11:12:08 pmRule of thumb is 2000uF per amp, I have 3a so 6000uF works, so I figure 10,000uF even better! I'm probably way overkill here. I figure extra capacitance removes even more ripple?

author: Labrat101
author_rank: Unknown
timestamp: January 31, 2021, 11:27:50 pm
content: Doctor Frankenstein I presume .A lighting Bolt will Have a better effect .A Tad more Power ..PS .. I have some 100,000 UF caps they weigh about a KG ..Interested .

author: networkingdude
author_rank: Contributor
timestamp: January 31, 2021, 11:32:46 pm
content: Quote from: Labrat101 on January 31, 2021, 11:27:50 pmYes very much so.

author: networkingdude
author_rank: Contributor
timestamp: January 31, 2021, 11:33:48 pm
content: Is there a better rule to follow here? Should I be sizing the caps at 2000uf per amp?

author: Labrat101
author_rank: Unknown
timestamp: January 31, 2021, 11:37:39 pm
content: A big heat sink and a concrete Bunker .O you might think of doubling up those LM,s to take the Extra PoWerI have some 100,000 UF caps they weigh about a KG ..But it wont help .. but it will be fun to watch

author: networkingdude
author_rank: Contributor
timestamp: January 31, 2021, 11:43:32 pm
content: Quote from: Labrat101 on January 31, 2021, 11:37:39 pmMight take some time to charge up eh.

author: Labrat101
author_rank: Unknown
timestamp: January 31, 2021, 11:49:36 pm
content: Nop . I can charge them up within 10ns .But it still wont solve Your ProblemI really Suggest you take up A new hobby or Read some good books on Basic Electronics .

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 31, 2021, 11:52:14 pm
content: Quote from: tunk on January 31, 2021, 05:21:11 pmA fast calc ( +- 10...20%).

author: networkingdude
author_rank: Contributor
timestamp: January 31, 2021, 11:55:15 pm
content: Quote from: Labrat101 on January 31, 2021, 11:49:36 pmSure, can you recommend some good ones?

author: bdunham7
author_rank: Super Contributor
timestamp: January 31, 2021, 11:55:33 pm
content: Quote from: networkingdude on January 31, 2021, 11:14:16 pmWhere are you getting 3 amperes? You have 25VA per winding, a power factor of probably 0.6 under load, I think you'll struggle to go much over one amp.Yes, extra capacitance removes more ripple, but the VR will be very good about blocking 120Hz input ripple so I wouldn't worry about that. I think the rule of thumb for capacitors ahead of VRs is more like 1000uF/A, but if you want to double it and use 2200uF, all the better. 10mF would be a lot. Go ahead if you like, I don't think it will burn anything up on turn-on. Use a robust bridge rectifier.

author: networkingdude
author_rank: Contributor
timestamp: January 31, 2021, 11:58:29 pm
content: Quote from: bdunham7 on January 31, 2021, 11:55:33 pmI was going by the size of the regulator and I had mentioned earlier that I would be making the transformer larger. 50va should get me up to 1.5 amp ish, so then would 3000uf be good enough?

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 31, 2021, 11:59:48 pm
content: Quote from: networkingdude on January 31, 2021, 10:50:51 pmThere is no need to use polymer caps here. Their impedance is too low for IC regulator output. And they won't really help at the IC input.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 01, 2021, 12:01:52 am
content: Quote from: networkingdude on January 31, 2021, 11:58:29 pmYou may use any value you like, or any value you can fit into the place. Either 3000 uF or 30 000 uF.3000 uF is only a minimum.

author: networkingdude
author_rank: Contributor
timestamp: February 01, 2021, 12:04:45 am
content: Quote from: Vovk_Z on January 31, 2021, 11:59:48 pmI was misunderstanding the datasheet I guess, it says to use between 1uf to 1000uf tantalum or electrolytic.

author: bdunham7
author_rank: Super Contributor
timestamp: February 01, 2021, 12:07:21 am
content: Quote from: networkingdude on January 31, 2021, 11:58:29 pmSure. 3300uF will be easier to find.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 01, 2021, 12:27:33 am
content: Quote from: networkingdude on February 01, 2021, 12:04:45 amDays of tantalum caps are gone. You may use them if you have them in your hands yet and don't know where to use them, or you have some temperature requirements (tants are more stable with temperature). Otherwise in all similar circuits just use usual aluminum caps. That's it.

author: networkingdude
author_rank: Contributor
timestamp: February 01, 2021, 12:47:22 am
content: Revised schematic

author: Wolfgang
author_rank: Super Contributor
timestamp: February 01, 2021, 02:02:36 am
content: Quote from: networkingdude on February 01, 2021, 12:47:22 amSome comments:- LM350 is capable of delivering 3A. At 18V winding, we have 3*18*1.4 = 75VA per winding.- I dont see a current limiting mechanism except the limiting in the LM350 (plus overtemp shutdown).- For 3A the cap is way too small.I guess you had 1A in mind. Then:- Total transformer rating would be 2*18*1*1.4 = 50VA. 1.4 is loading factor for full bridge rectification.- Filter cap is OK for 18V secondaries.- You should have a current limiter.RegardsWolfgang

author: networkingdude
author_rank: Contributor
timestamp: February 01, 2021, 02:10:40 am
content: Quote from: Wolfgang on February 01, 2021, 02:02:36 amThe capacitor was much larger but I was advised it was too large given the 1.4 amp limit of the transformer. I've not put much thought into current limiting as of yet.The transformer is the largest size I could find that was pcb mounted on digikey. 2 or 3 amps would be nice but I don't need that much power. lm317 could have been used but this gives good margin and much lower noise.

author: bdunham7
author_rank: Super Contributor
timestamp: February 01, 2021, 02:32:57 am
content: Quote from: Wolfgang on February 01, 2021, 02:02:36 amWhy? The 3A 'capability' of the regulator is meaningless and there's no more issue oversizing the regulator than there is oversizing the rectifier or the wires. If there is a concern about shorts or overloads, a fuse would be a sufficient answer.

author: Wolfgang
author_rank: Super Contributor
timestamp: February 01, 2021, 03:40:41 am
content: Quote from: networkingdude on February 01, 2021, 02:10:40 amFor 1A the component values are OK. If you dont want to think about current limiting use a fuse.´ or a "weaker" regulator chip.If you need better regulation, less noise, ... you could go for a bit more complex design using op amps or a 723.

author: networkingdude
author_rank: Contributor
timestamp: February 01, 2021, 04:03:04 am
content: I would like to do a power supply with op amps in the future. Interestingly the LM723 appears to have a worse noise figure than the LM350 I am using here. I'm sure I'm missing something important here.

author: bdunham7
author_rank: Super Contributor
timestamp: February 01, 2021, 05:03:50 am
content: Quote from: Wolfgang on February 01, 2021, 03:40:41 amYou have to consider all the factors. A smaller regulator will need a larger heatsink, for example. Also, the circuit as drawn may outperform more 'sophisticated' systems unless they are very carefully designed and tested. Some pretty high end test equipment uses prepackaged regulators in the power supply section, including long-scale DMMs.

author: Kibabalu
author_rank: Regular Contributor
timestamp: February 01, 2021, 06:40:13 am
content: Regarding needed space for big capacitors on the PCB it could be beneficial, to use a 'capacitance multiplier' (I don't like this label, because it doesn't multiply any capacitances, but this the name, especially in the english spoken community) like the one inside the design shown in post #48.With that the capacitors don't have to deliver any big current, because the current is is supplied by the emitter follower bypassing the capacitances. The input voltage of the voltage controller is now very smooth, resp. ripple and noise suppressed are very good, just like you would have used very big capacitors.

author: feihong519
author_rank: Newbie
timestamp: February 03, 2021, 07:21:03 am
content: 你好，虽然IC最大支持3A，但实际工作中达不到3A电流。如果实际输出电流为2A，你应该考虑一下IC的散热的问题。Hello, although the IC supports a maximum of 3A, it cannot reach 3A in actual work. If the actual output current is 2A, you should consider the heat dissipation of the IC.

author: David Hess
author_rank: Super Contributor
timestamp: February 11, 2021, 02:31:59 am
content: Quote from: networkingdude on January 30, 2021, 05:27:57 pmAn output capacitor size of 10 to 100 microfarads per amp is typical. Small RF bypass capacitors might be added at the external terminals to chassis ground.Quote from: networkingdude on January 30, 2021, 07:53:15 pmI would not remove D3 and D6; they are commonly included in dual bipolar power supplies. They protect the regulators against an output short from the positive to negative output where one regulator will try to drive a reverse voltage into the other regulator.