Is my circuit OK? - Page 1
Forum section: electronics / beginners

author: Grayfox
author_rank: Regular Contributor
timestamp: May 07, 2017, 10:44:17 am
content: I am making a 30v lab bench PSU that can output 10A and I have made some changes to it.I am mainly wondering if I screwed up the connections.Mainly one large filter capacitor instead of the seven in parallel and the addition of a LM7812 to power a fan and the volt meter.This is the circuit I usedhttp://electronics-diy.com/30v-10a-variable-bench-power-supply.phpMy Schematic and PCB layouts as well as the KiCad Project files are attached.

author: Ian.M
author_rank: Super Contributor
timestamp: May 07, 2017, 11:39:29 am
content: Chances are the LM7812 will burn up. For 30V out, you must have at least 34V on your bulk capacitor, and that's 2.2W dissipated in the LM7812 for every 100mA of fan current.A 24V fan would be a far better choice with half the running current and a regulator dissipation of only 1W per 100mA.The voltmeter may require an isolated supply - check carefully if its measurement -Vin is directly connected to its DC power 0V. If so, its OK off a suitable regulator, if not you need to use an isolated DC-DC converter to power it

author: ironmonkey
author_rank: Contributor
timestamp: May 07, 2017, 12:33:39 pm
content: Dave has an excellent explanation about why is a good idea to keep all the parallel capacitors:https://youtu.be/wwANKw36MjwEnviado desde mi MotoG3 mediante Tapatalk

author: newbrain
author_rank: Super Contributor
timestamp: May 07, 2017, 12:38:37 pm
content: What Ian said.But I see a lot of other problems in the schematic you attached.Right hand side of R7 should be connected to positive side of the output, not GND.Your GND is not connected to the bridge rectifier.Vref output (pin 6) is not connected to the divider network, only to the cap (C2).C2 itself is not correctly connected.These are just the result of a cursory look: if the PCB has been generated after this schematic, that's also wrong.

author: Grayfox
author_rank: Regular Contributor
timestamp: May 07, 2017, 01:18:03 pm
content: Quote from: Ian.M on May 07, 2017, 11:39:29 amThe transformer I have is a 2x12v @ 3.33A with the outputs wired in series to give 24v @ 3.33A.This is the volt meter I have.http://www.ebay.com.au/itm/DC-3-5-30V-0-10A-Dual-LED-Digital-Volt-meter-Ammeter-Voltage-AMP-Power-ZXHAPPY-/142276603039?hash=item212058ac9f:g:EMcAAOSwZjJU8YssSo it can run off 24v from the LM7824 but may go for a LM7820 to prevent any voltage drops going below the threshold for the LM7824

author: Ian.M
author_rank: Super Contributor
timestamp: May 07, 2017, 01:38:01 pm
content: So you've effectively got a centertap on the winding feeding the bridge rectifier. If you feed the LM7812 from the secondary center tap, it will get half the DC bus voltage, or about 16V and run fairly fool. It will need a decent sized decoupling cap from the center tap to 0V.I'd be very suspicious of that meter unless it comes with a wiring diagram showing it being powered from the same supply its measuring. If so, its probably OK if powered from the 7812 output, otherwise I'd either use a 12V DC-DC converter to feed it, or add an independent supply for it.You should take a look atHammond Design Guide for Rectifier UseBasically with your transformer configured in series, its only good for 2A DC and at that current you'll be lucky to get over 17V out after the regulator as the DC bus will drop to below 21V. You may not even get 15V @2A without ripple breakthrough.It may be worth considering a series/parallel switch for the secondaries to let you get 4A out below about 7V. It would also need to switch the LM7812 feed from the ex-C.T to the unregulated DC bus..

author: Grayfox
author_rank: Regular Contributor
timestamp: May 10, 2017, 12:52:01 pm
content: Remade it with 7 Input filter caps.Replaced the LM7812 with the LM7824Made a large ground fill zone, rather than relying on traces, any component that needs to have its pad connected to ground will be connected to the plane.The schematic is effectively the same but with 7 caps in parallel instead of the one.

author: BrianHG
author_rank: Super Contributor
timestamp: May 10, 2017, 01:11:05 pm
content: I know this will make the trace a little longer, but, I would swap layers of trace Net Q1-Pad3, going around your supply caps. This way, the GND fill wont be broken between your supply caps. You may also go a lot thicker for the main + power trace between them and the power transistors.

author: rdl
author_rank: Super Contributor
timestamp: May 10, 2017, 01:21:59 pm
content: There's really no need to use seven capacitors for input filtering unless you just want to or there are space considerations. Sometimes multiple parallel capacitors are used for a reason and replacing them with a single one is a bad idea, but the input filtering of this circuit is not one of those times.edit: Due to ESR considerations using more than one filter capacitor is not a bad idea. I will generally use two, maybe four, but seven seems a bit excessive.

author: Grayfox
author_rank: Regular Contributor
timestamp: May 10, 2017, 01:33:01 pm
content: Quote from: BrianHG on May 10, 2017, 01:11:05 pmRouted the trace from Pin 10 from the LM327 down and around C7 and made it a top layer trace, and increased trace width from 1.0mm to 1.25mm.Or should I go thicker?

author: BrianHG
author_rank: Super Contributor
timestamp: May 10, 2017, 01:46:57 pm
content: When I say thicker trace, I mean the + trace on all the caps to your diode bridge rectifier.I would use 2-4mm. You have the room without need to move anything else.Keep the moved trace (net Q1-Pad3) the original size.

author: BrianHG
author_rank: Super Contributor
timestamp: May 10, 2017, 01:55:25 pm
content: Like this... These changes will ensure a clean low impedance power source.

author: Grayfox
author_rank: Regular Contributor
timestamp: May 10, 2017, 02:07:29 pm
content: Quote from: BrianHG on May 10, 2017, 01:46:57 pmMade them 2.25mmThe traces are getting close so some pads, Unless I reroutre some areas to avoid the traces touching the pads I cant go higher

author: BrianHG
author_rank: Super Contributor
timestamp: May 10, 2017, 02:15:19 pm
content: One place was too close for comfort. Just copy my Yellow trace recommendation.

author: Grayfox
author_rank: Regular Contributor
timestamp: May 10, 2017, 10:08:50 pm
content: Fixed it.Saw it last night before bed.

author: StillTrying
author_rank: Super Contributor
timestamp: May 10, 2017, 10:47:12 pm
content: Random suggestions.C9 the FC cap could be moved to the other end of the LM723 to greatly reduce the track lengths, it's usually a good idea to get input and feedback components as close as possible to the IC pins.If the input really was 40 VAC I think that would be too much for the LM723.I'm surprised there's no pull down resistor on the emitter of Q1/bases of Q2-5, to help them turn offslightlyfaster, even 18k to gnd would help.If a heatsink is going along Q2-Q5, treble check that the mounting tabs are on the correct side.

author: Grayfox
author_rank: Regular Contributor
timestamp: May 13, 2017, 12:05:38 pm
content: Quote from: StillTrying on May 10, 2017, 10:47:12 pmDoneQuote from: StillTrying on May 10, 2017, 10:47:12 pmhttp://au.mouser.com/ProductDetail/Texas-Instruments/LM723CN-NOPB/?qs=sGAEpiMZZMvu8NZDyZ4K0fnoEHnTJ%252bgFMany data sheets state 40V is max.I doubt I will ever need to ever use 40v.Quote from: StillTrying on May 10, 2017, 10:47:12 pmThe 3D view shows the backside on all of them is facing the same direction, but will double check using the data sheet that the pinouts are correctQuote from: StillTrying on May 10, 2017, 10:47:12 pmWould this be how to add pull down resistors?

author: rdl
author_rank: Super Contributor
timestamp: May 13, 2017, 06:25:17 pm
content: Quote from: Grayfox on May 13, 2017, 12:05:38 pmThe maximum input for the LM723 is 40 volts DCIf you connect 40 volts AC from a mains connected power transformer, the input to the LM723 will be well over 50 volts DC

author: StillTrying
author_rank: Super Contributor
timestamp: May 13, 2017, 09:57:11 pm
content: Quote from: Grayfox on May 13, 2017, 12:05:38 pmNope, just 1 resistor from the emitter of Q1 to GND, if you're using a big one you might as well make it a bit lower in value, a 10k would only dissipate 100mW at 30V.There's at least one other thread on this PSU on heresomewhere.

author: Grayfox
author_rank: Regular Contributor
timestamp: May 13, 2017, 10:07:33 pm
content: Quote from: rdl on May 13, 2017, 06:25:17 pmWhich would be 56v Peak as the 40v is RMS.I am aware of this.

author: rdl
author_rank: Super Contributor
timestamp: May 13, 2017, 10:25:07 pm
content: Quote from: Grayfox on May 13, 2017, 10:07:33 pmSince you indicate on the board that 40VAC is okay, it did not seem as if you were.A more reasonable maximum AC input would be 28 VAC rms, but even that is pushing it.

author: Ian.M
author_rank: Super Contributor
timestamp: May 13, 2017, 10:27:41 pm
content: I don't know why you are using both emitter resistors for current sharing and a separate sense resistor. You can simply use the voltage drop across one of the emitter resistors for the current sense, which will decrease the headroom required for regulation.You'll also need to implement 'Figure 22. Positive Floating Regulator' from the LM723 datasheet, with R5 and the Zener replaced by a floating auxiliary supply so your LM723 never 'sees' the 56V max voltage on your unregulated rail. Might as well make it a 12V supply so you can also use it for cooling fans.

author: Grayfox
author_rank: Regular Contributor
timestamp: May 14, 2017, 12:50:31 pm
content: Quote from: Ian.M on May 13, 2017, 10:27:41 pmI am not, I miss read the drop down resistor reply.As for adding that 1ohm resistor between pins 1 and 10 and the zener between pin 7 and pin 12.I cant find a datasheet for the 1N1426 or that part on mouser or element 14.Is their a direct replacement for the 1N1426?

author: Ian.M
author_rank: Super Contributor
timestamp: May 14, 2017, 01:11:12 pm
content: No, I said*REPLACE*R5 and the Zener with a 12V floating supply. If you keep it as drawn, when the output is at 0V and the input is 56V, you'll have 20V across a 200R resistor, which isn't too bad, 1/2W dissipation, but the Zener will be dissipating about 3.5W. If you *MUST* use it, use any small 36V Zener wired base-collector of a well heatsinked power BJT. That will give you a boosted Zener that can easily handle the dissipation.