6.3VAC to 5.2VDC @ 5A - Page 1
Forum section: electronics / general

author: zitt
author_rank: Regular Contributor
timestamp: May 18, 2020, 07:03:58 am
content: Need some advice. I think I'm over complicating this but I think I'm lost in the forest.Simply put; I'm looking to use a uC to rainbow a set of WS2813 strips inside my pinball machine. Specifically; a Williams MonsterBash. I want to run the powersupply of the WS2813 strips at near their maximum voltage at 5.2VDC so the LEDs at the end of a 30-ish inch run still have enough margin to operate. Here's the specs as I've come up with.Voltage input:6.8VAC "General Illumination" circuit which meased with scope at 8.5Vp-p 60Hz.Desired Output:5.2VDC at 5A max.What I've tried is a MC33063A based buck controller which works... but it has two problems:1) on the bench; I don't get the full load unless the input voltage is at 12VDC (IIRC). I believe this mostly due to voltage drop across in the input bridge "diodes". I'm measuring about 1.5V drop due to the diodes and fets.2) The PCB is dissipating quiet a bit of heat. So much that it's melting the solder at Q3/Q7 if not "heatsinked" to a aluminum slug.3) I'm concerned that even with the capacitance shown; this many LEDs driven by a 120Hz DC supply will still require re-programming the strip at each "zero cross" because the voltage is drooping too much. IE not enough capacitance.Schematic attached.I think I'm over complicating the design which means I'm spending too much money and time on a solution.Really; I think I only need a transistor Zener series vreg which would limit to 5.2VDC on the top cycles of the rectified 8.5Vp-p wave. My concern is that the Zener is going to need to be driven by a Darlington pair at a minimum which just puts me near non-regulation when Vdrop of bridge diodes and the Vsat of the darlington pair are taken into account.Again; my concern with any of these is that at 5Amps; I'm going to need a good heatsink to prevent overheating. Something I didn't want in the design.

author: Styno
author_rank: Regular Contributor
timestamp: May 18, 2020, 09:18:31 am
content: The transistor/zener thing sounds easy to simulate so you can get a better idea of the dissipation and voltages. TI’s Tina or KiCAD or any other Spice program will do.

author: MarkF
author_rank: Super Contributor
timestamp: May 18, 2020, 01:49:25 pm
content: A few questions:It may just be cheaper to buy a LED Strip power supply.This is the first I saw on Amazon. 5V@15A for $26Can the 6.8VAC "General Illumination" circuit provide the necessary current?Is it possible to divide the strip into two pieces? You can have two power supplies (with common ground) to divide the needed current.Do you really need a regulated power supply? Maybe an unregulated supply with power resistors to limit the current.Just don't push the current handling capability of strip to account for some voltage fluctuation.

author: bdunham7
author_rank: Super Contributor
timestamp: May 19, 2020, 03:34:27 pm
content: Can you scope SW-E, pin 2 of the controller? I'm just curious what it's doing. Also, your 6.8 VAC should be 19.2 Vp-p, can you check that??If you aren't too attached to your current design and just want a solution, maybe look at the new Monolithic MP2338.https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP2238/document_id/5006/You have to calculate the dropout voltage since it isn't explicitly listed or charted, but the efficiency chart shows better than 95% efficiency with Vin = 7VDC, Vout=5VDC, so it looks ideal for your application. And, if you just want to get the job done, you could just buy the eval board for $25.https://www.monolithicpower.com/en/catalog/product/view/id/3129/s/ev2238-d-00a/category/295/

author: themadhippy
author_rank: Super Contributor
timestamp: May 19, 2020, 04:04:55 pm
content: Quotepower the leds from either end.

author: zitt
author_rank: Regular Contributor
timestamp: May 19, 2020, 08:09:28 pm
content: These are addressable WS2813 LEDs... unregulated supply... or at least a supply with too high of voltage will burn the internal ICs up. 5.2 to maybe 5.5volts is the max they can withstand, as I understand it.The 6.8VAC supply rail is designed for incandescent lamps. I haven't specifically checked; but it should handle 10s of amps.Yes, I measured at a lamp socket and saw 8.5ish Vp-p. I'll repeat the experiment again soon to confirm.

author: ogden
author_rank: Super Contributor
timestamp: May 19, 2020, 08:33:32 pm
content: Quote from: zitt on May 18, 2020, 07:03:58 amIs it so that mains voltage in your house is 6.8VAC?Usually it is much higher voltage. Then solution is easy: buy powerful enough off-the shelf 5V power supply. Voltage drop in LED stip is no concern because strip can be powered from more than single point.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: May 20, 2020, 01:35:16 am
content: Quote from: zitt on May 19, 2020, 08:09:28 pmI assume you mean 8.5 volts peak. not peak to peak. A sine wave should produce 6.8*sqrt(2)*2 for peak to peak.

author: zitt
author_rank: Regular Contributor
timestamp: May 21, 2020, 01:01:04 am
content: If I wanted an AC adapter in the pinball machine; I would have just done it and never posted.It makes nearly zero sense to put yet another power supply in a machine that already has dozens.Sure it's easy; but if everything were easy - we'd never innovate.Quote from: ahbushnell on May 20, 2020, 01:35:16 amCorrect. WMS lists the GI as 6.8VAC... in the schematics but everything I know says it's really 6.3VAC.6.3VAC makes more sense and is in better lint to the 8.5v peak measured.

author: james_s
author_rank: Super Contributor
timestamp: May 21, 2020, 04:26:35 am
content: The machine has dozens of power supplies in it?You can easily rectify and regulate 6.3VAC down to 5VDC, the voltage drop should not be a problem, as someone else mentioned you can always power the strip from both ends.I never could figure out why so many pinball guys are into modifying vintage machines instead of keeping them original. I've seen so many nice machines ruined by stupid mods, I just immediately lose interest in anything that's not original. I've even seen LED lights in 70s EM machines, it looks ridiculous IMO. Virtually all retrofit LEDs strobe, it's visible with the moving ball and drives me nuts.

author: zitt
author_rank: Regular Contributor
timestamp: May 21, 2020, 04:47:27 am
content: Quote from: james_s on May 21, 2020, 04:26:35 amAn excageration... but there is +5, +12, +12unregulated, +20 unregulated, +50unregulated. All derived from several large windings in the main transformer. The 6.3VAC lines are very beefy because they use to run Lamps.Quote from: james_s on May 21, 2020, 04:26:35 amWhy do people do anything? Because they can.I personally do it because I enjoy doing it. Like it or not; LEDs are here to stay.I do tend to agree tho... For controlled inserts; you need to be careful that you have a ghosting patch - then the LEDs look ok.For controlled GI; you need OCD GI which basically PWMs the LEDs to prevent flickering.I tend to do all kinds of mods; usually they are tasteful. Here's an example:http://pinball.click/sstPopcapsTBH; "vintage" machines do not age well when you have a collection of modern machines with all LEDs from the factory side by side against a 1970's Bally with real #44 bulbs. The 1970's machines look outdated and dim. That said; I still have my STNG machine with Lamp based GI because I prefer that look to LED bulbs and the afore mentioned LEDs.I'm considering reconverting this Monster bash's GI back to Lamp because it strobes bad during some game modes.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: May 21, 2020, 05:28:46 am
content: Quote from: zitt on May 21, 2020, 04:47:27 amThe local arcade here deals with it by splitting the room in two. Vintage machines in one half, modern in the other. Oh, and the vintage half has all the good beer too.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 21, 2020, 07:28:33 am
content: The bridge rectifier is the biggest loss element.Choose low-drop schottky diodes for it. Something down to about Vf=0.25V is available, a huge improvement from 0.7V (per diode; for a full bridge, double that). Look at the reverse leakage spec, though (it depends on temperature and reverse voltage).Note that paralleling diodes is difficult or impossible. Pick a larger current rating than 5A, if at all possible, and cool the diodes well.Active FET-based rectifiers, further reducing the voltage drop and loss, are available but quite expensive.You need quite a lot of filter capacitance to filter the 120Hz ripple (provide power to the buck when the rectified line is below the minimum buck input voltage), but this can be made of cheap-ass electrolytics.

author: james_s
author_rank: Super Contributor
timestamp: May 22, 2020, 06:08:29 am
content: Quote from: zitt on May 21, 2020, 04:47:27 amThey are outdated, that's kind of the point. The EM pins are from another era, playing one is a bit like a trip back in time. I'm mostly into vector arcade games but I do have a Gottlieb Sure Shot EM pin I restored, it has its own charm. I don't really think it's fair to compare it to modern pins, it's a whole different type of game with a different feel.My favorite pin is still the Time Machine my friend has, it's from the best era IMHO, mid generation solid state, it has all the fun features before playfields got so cluttered with toys and gimmicks.

author: zitt
author_rank: Regular Contributor
timestamp: May 22, 2020, 06:37:20 am
content: Quote from: james_s on May 22, 2020, 06:08:29 amit isn't fair... but human nature says differently.If you look at an for-profit arcade which doesn't specialize in any specific genre... most flock to late gen Modern pins from Stern or Jersy Jack. I don't think EMs get much love because I don't see them in arcades often. It's either an A-list WMS or a modern Stern.I haven't spent a lot of times on EMs... mainly because I grew up in the Pacman generation. I like classic videos; but repetition is boring so I moved to a specific set of Pinball machines. RFM and Monster Bash are my two non-Star Trek pins.QuoteNot sure I've played that one. They don't make it to the pinball shows I frequent often.Data East are fun... but they can be hit and miss when compared to Williams machines.Back on topic; I found the following article interesting:https://www.maximintegrated.com/en/design/technical-documents/app-notes/1/1853.htmlAnd I'm already using low-drop schottky diodes in the full wave bridge. The problem is to get the current I was using some beefy ones that could handle the 8A switching currents from the Buck converter.I think what I'm going to do is try and simulate the app note design with an pass transistor designed for 3A max to see what it does. Hopefully I can get the voltage drop down to a manageable level with that circuit topology and diodes reduced to handle 3A max. My hope is designing for smaller current and doubling (both ends) them up like suggested above that I can get the circuit in a stable region.