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turning a VCR power supply section into a bench power supply?

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3n2323:
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!

alm:
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?).

touchh:
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.

amyk:
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 PSU :)

The 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.

3n2323:

--- Quote from: alm on April 04, 2012, 06:28:53 pm ---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?).

--- End quote ---

thank 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!     

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