turning a VCR power supply section into a bench power supply? - Page 1
Forum section: electronics / beginners

author: 3n2323
author_rank: Contributor
timestamp: April 04, 2012, 05:57:16 pm
content: i don't have any equipments, not even a bench power supply. got a broken RCA VCR model VR519, took it apart and measured several places on the board, got 37V, +/-30V, 15V, 6v, and 5V at various points on the board. searched the web, couldn't find the schematics for it.could someone point me to a link for its schematics?if possible at all by just looking at the pictures, could some experts roughly explain this power section? i can't even identify the proper output points of these voltages, so that i could do something to them and make a bench power supply out of it.the power rating for the VCR is 18W, guess that's more than enough for what i'll be doing with it, merely playing with small signal BJT amps, and maybe small audio power amps. is turning this power section to a bench power supply a reasonable thing to do for such purpose?any and all comments/suggestions/advices welcome!

author: Unknown
author_rank: Unknown
timestamp: April 04, 2012, 06:28:53 pm
content: This definitely looks like a switching power supply. Modifying the circuit into a lab supply which goes down to (somewhere close to) 0 V and with adjustable voltage and current limits will be non-trivial. At best you may be able to use one of its output voltages as input for a power supply, similar to an external power brick or battery used in Dave's and Richard (amspire)'s designs.If 18 W is from the external label, than that's probably the peak power it can draw, not the amount of DC power it can supply continuously. The efficiency is below 100%, for starters. It will also be divided across the various output voltages, although the majority of the power will probably be on one of the rails (15 V for the motors?).

author: touchh
author_rank: Contributor
timestamp: April 05, 2012, 04:37:38 am
content: They work great as power supplies. I have about 4 bench supplies I've built from power supplies salvaged from dvd players, vcr's, directv receivers.All the ones I've salvaged have been a separate board, so what I'm about to say might be of no use for you but could be of use to someone else.Usually there will be a bunch of outputs 3.3,-5, 5v, 12v, 9v, 6v, etc.. but they will be off. This is because there is an enable line that needs to be pulled high to turn the power supply on. Usually there is a 3v3 or a 5v output that is on all the time, this is what you use to pull the enable line high and turn the rest of the power-supply on. This is because normaly there is a power latch and when you hit the power button on your equipment it is turned on by pulling the enable high.This is not universal, some are always on, some need to be pulled low, so it will just take some fiddling usually before you can use them.Oh, and flatscreen monitor back-light drivers are a nice source of 600v @ a few ma. I want to make an adjustable high voltage power-supply with one of them one day. Pretty dangerous if you don't know how to handle high voltage though. So I don't suggest you mess with this unless you know whats up.

author: amyk
author_rank: Super Contributor
timestamp: April 05, 2012, 11:32:53 am
content: That must've been a really cheap VCR if the PSU is part of the main PCB and the input lead is just soldered in... but looks like they were nice enough to silkscreen where you should cut if you want only the PSUThe outputs will be the traces that cross the line, there will be output rails and ground. Possibly an enable input as well.The PSU circuit itself looks very simple and reminds me of a laptop AC adapter. It's all low-density through-hole construction so tracing the schematic won't be hard. I don't see any controller ICs, so the oscillator is probably made from discretes. Keep in mind this also means there is likely no protections either.

author: 3n2323
author_rank: Contributor
timestamp: April 05, 2012, 05:15:56 pm
content: Quote from: alm on April 04, 2012, 06:28:53 pmthank you very much alm for your reply, i learned about power supply issues/considerations from it!no, i'm newbee and won't be able to do any sophisticated mods to it. the best i could hope for is a very simple, straightforward, and most likely rather sloppy "power supply" with the best voltage range i can get, which would be far from ideal, but still better than nothing for the time being. the power rating is read from the label on the back, other than that there's no information to be found on the VCR in this regard, and i couldn't locate a schmeatic for it.i know this sounds very sloppy, with almost no control over anythng, and i do like the rigor in your apporach. i do wish i could have control over voltage, current, power, efficiency, etc., i'm just not there yet.i'd like to explore it further, and see what possiblilities exist, then either strike a balance between the ideal and having a less than ideal "power supply" availble for use now, or completely rule it out.good advice, thank you!

author: 3n2323
author_rank: Contributor
timestamp: April 05, 2012, 05:27:17 pm
content: Quote from: touchh on April 05, 2012, 04:37:38 amno, touchh, this is very helpful and useful to me, thank you very much, really appreciate all that info!when measuring, i just plugged it in without pushing the power button, and got that bunch of voltages. guess this means the power is always on? advantage of cheap VCR'syou have 4? could i have one?do you have the circuit design, physical design, the building of them, and maybe some notes, warnings, etc., documented somewhere on the web?thank you for the high voltage warning too, that's what scares me away form vaccum tubes for now, but they do look cool.

author: 3n2323
author_rank: Contributor
timestamp: April 05, 2012, 05:55:33 pm
content: Quote from: amyk on April 05, 2012, 11:32:53 amhaha, cheap it must be. it has only one large board, and most of it is empty, with wire jumpers all over.that's my thoughts when seeing those white silkscreen lines. good thing i completed the Advanced PCB Sawing course!Quotethank you for the pointer! i looked at the board a little closer, and realized the power section within the white line box is surrounded by fat ground trace, and only 5 thinner traces come out of this surrounded area. alright! the 37V, 14V(not 15V, i made a mistake in the original post), and 5V are all there. +/-30V don't seem to come out of this area. there are 2 jumpers jumping out of this area, marked F+ and F-, i don't know what they are. 6V shows up in some another area, i don't understand where it gets the 6V from.Quotethere are only 4 parts in this section with the IC/transistor look, please see the picture.S1WB: bridge rectifier.C4533: 3 legs, the best matching number i could find on the web is 2SC4533, npn power transistor.D1548: 3 legs, best match 2SD1548, npn low frequency amplifier.2501: 4 legs, couldn't find anything that makes sense from the web.no crystals, only one packaged up gray thing that might be an inductor, and there are some parts i can't identify. i'm a newbee, sorry.switching power supply can't be without some sort of oscillator or IC controller, correct? and what does "protection" mean then, if you don't mind explaining that a little?

author: baljemmett
author_rank: Unknown
timestamp: April 05, 2012, 10:12:12 pm
content: Quote from: 3n2323 on April 05, 2012, 05:55:33 pmFrom the photo it looks like an optocoupler / optoisolator -- "NEC 2501 optocoupler" turns up hits on the web that might be worth investigating. You can see the fat white line nearby that separates the two sides of the power supply (control and grunt, basically), and a slot in the board under the component; a four-legged component that straddles such a split is a reasonably good bet to be an isolation component in the supply's feedback loop. Basically it allows the control side of the supply to see what's happening in the power side without having a direct connection that would break the isolation.Or at least, that would be my understanding, but I could be mistaken since I'm pretty much a newbie at this sort of thing too

author: amyk
author_rank: Super Contributor
timestamp: April 06, 2012, 09:13:21 am
content: That looks like a classic Ringing Choke Converter / self-oscillating flyback circuit. Very simple schematic, but difficult to analyse its operation. Search for an article named "Analysis and Design of Self-Oscillating Flyback Converter" for the details. Trace a schematic for your PSU and the primary side will probably be very similar. The C4533 is the switching transistor and D1548 regulates it via the optocoupler (2501) feedback.Quote from: 3n2323The oscillator is made from discrete parts and relies on the inductance of the transformer to oscillate. Look up "blocking oscillator" to learn how it works. The RCC is just an adaptation of it, with an extra transistor inserted for regulation. By protection I mean overcurrent/overvoltage/short circuit/etc., SMPS with ICs usually have some sort of overload protection built in but this PSU probably doesn't --- it'll just keep supplying as much current as it can until something fails.6V is probably because there are other regulators on the board somewhere else. Not sure what the F- and F+ are but my first guess is filament voltage for a VFD --- trace where on the PSU it comes from.

author: touchh
author_rank: Contributor
timestamp: April 06, 2012, 12:55:22 pm
content: Quote from: 3n2323 on April 05, 2012, 05:27:17 pmYeah I guess the cheap VCR's have all power enabled all the time.Sadly I don't document most of the stuff I do, something I should start doing more often... Next time I find a power-supply I'll and mess with it I'll try to document it.

author: 3n2323
author_rank: Contributor
timestamp: April 07, 2012, 02:52:40 pm
content: Quote from: baljemmett on April 05, 2012, 10:12:12 pmthank you baljemmett for your great observation, and information on optocoupler! the two legs on the outside of the slot go to some capacitors at the output end of hte board, so you must be right!

author: 3n2323
author_rank: Contributor
timestamp: April 07, 2012, 02:57:56 pm
content: Quote from: touchh on April 06, 2012, 12:55:22 pmso what did you do to those 4 power modules?did you just take one of the many rails as the input to some regulating/limiting circuit?are there ways of utilizing all the power capacity of such modules/sections?

author: T4P
author_rank: Super Contributor
timestamp: April 07, 2012, 05:00:41 pm
content: You can use the 5V for micros , 15V for opamps ( Sadly no negative voltage so you have to use a virtual ground )+/-30 i don't really know what they are useful for , 37(or rather 35) neither do i know what they are useful for .High voltage opamps maybe ?

author: amyk
author_rank: Super Contributor
timestamp: April 08, 2012, 11:38:26 am
content: It might be an anode voltage for a VFD, or for motor drivers.

author: 3n2323
author_rank: Contributor
timestamp: April 09, 2012, 01:48:21 am
content: Quote from: Dave.S on April 07, 2012, 05:00:41 pmthank you Dave, that way i could get more use out of it.just realized this reading your reply, the waste is not really in using one of the rails only per se, but in not utilizing the power sectin's full power capacity, is that right? if so, are there ways of getting only one output voltage out of it, with its full power capacity, and then make a variable power supply from there, to get all the other voltrages? does such a question even make sense?

author: amyk
author_rank: Super Contributor
timestamp: April 09, 2012, 08:19:44 am
content: You would have to rewind the transformer to do that.

author: T4P
author_rank: Super Contributor
timestamp: April 09, 2012, 12:56:03 pm
content: You can be okay with ahttp://www.bristolwatch.com/ele/lm317.htmand a small heatsink , i don't think there's much power anyway .But a decently sized 6C/W should be okay .

author: 3n2323
author_rank: Contributor
timestamp: April 21, 2012, 12:06:37 am
content: Quote from: amyk on April 09, 2012, 08:19:44 amthat's beyond me right now, and take power the starting point in learning makes me nervous, so has to be some other way

author: 3n2323
author_rank: Contributor
timestamp: April 21, 2012, 12:14:20 am
content: Quote from: Dave.S on April 09, 2012, 12:56:03 pmthank you very much Dave.S for the link and the advice!would you mind explaining the transistor in this circuit a bit, what does it do?also, if want to add a voltmeter and an ampmeter in there, how should i do it? how should i include some protection circuit to these meters?for now, i don't need much power at all, just a simple and easy one to start me with small signal transistors and smalll audio power amps will do.

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 21, 2012, 12:32:58 am
content: The transistor in that circuit is what is commonly called a series-pass transistor. It is doing all the "heavy lifting" in that circuit. It is passing the majority of the current. The LM317 is really just acting as a voltage reference.This works due to the fact that there is always a (more or less) fixed drop from the base to the emitter (this is called Vbe) when a transistor is on. So the LM317 outputs a variable voltage based on the setting of the pot and the transistor outputs that voltage minus the constant Vbedrop.You lose the LM317's current limiting and thermal protection features by doing this but can get higher output current capability.