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Lab power supply from microwave transformer project.

Discussion in 'Power Electronics' started by Damien, Jan 28, 2015.


    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Damien, a couple of notes here.
    CAT3 - CAT5 UTP cable is cheap. A 1ft. length of it will produce more breadboard jumpers than yon a you'll use in a year. Each conductor is the proper gauge for these breadboards too. If you use solid core that's too large you will stress the breadboard contacts to the point of premature contact failure or intermittent connection. Nothing is less fun than an intermittent problem.

    Regarding analog vs. switch mode charging regulators:

    The charging regulators found in automobiles have always been a form of switch mode regulator. Prior to Solid State they were comprised of adjustable "pull-in" and "dropout" electromechanical relays.


    EDIT: Never "EVER" use tinned wire with your breadboard! The contacts will shave the solder every time you pull out a jumper. They will also leave flux deposits behind.
  2. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    Aug 27, 2013
    YES. I mean no disrespect, but you should absolutely "go back to low voltage stuff". Regulatory laws, rules, codes and ordinances are in place to protect life and property, circumventing those safeguards without certain knowledge of the ramifications is a fools errand with potentially dire consequences.....Most modern power generating stations output in excess of 1 Mega Watt and typical residential mains transformers will supply 30kW or more of that power without hesitation....the latter is roughly equivalent to instantaneously igniting an entire gallon of gasoline under ideal conditions...while this same gallon of gasoline is relatively safe to ignite a few mg at a time in an internal combustion engine, it could also be weaponized to have a kill radius of > 250m^2....just because gasoline and electricity are readily available and commonly used does not imply they are inherently safe or that they are to be handled casually. I would encourage you to learn about electricity and electronics behind the safety of galvanic isolation or from a less potentially lethal source than mains power....As far as working with 240V long as what you do meets or exceeds standard safety codes then by all means carry on, but when what you are doing is well outside of safety protocols, borders on outright danger and you repeatedly demonstrate you do not understand the general concepts in play, then I think it might be a good time to "go back". I will add, that as long as you are only rewinding the secondary of the transformer, (and you observe insulation and environmental concerns) you should be relatively safe....Typical microwave transformers are less than 1500VA and while not a trivial amount of power, typically not lethal.

    As far as recharging your batteries while camping, I would suggest you consider a marine battery charger designed for safely charging lead-acid batteries from 120Vac/220Vac (either mains or generator supplied). While marine battery chargers are considerably more expensive than automotive battery chargers they are designed to be safe in wet environments....a big plus when camping. You might also consider simply purchasing a generator specifically designed for charging 12V batteries...they are typically light-weight, fairly inexpensive, very quiet and as an added bonus they are really safe :) At any rate, if you do continue with your microwave oven transformer retro-fit, you should consider the fact that a 12V lead-acid battery nominally rated @ 12Vdc will typically require a 14V or higher charging voltage....And you should be aware that typical AC voltage readings are RMS values...the no-load rectified DC voltage of a 12Vac output will be ~ 17Vdc....and just to wrap things up, a properly wound 1200VA transformer with a 12V secondary should provide roughly 100A....which is suitable for welding....any electronics you are considering should be designed with this in mind....FWIW, a microwave transformer will have a very short life if tasked @ even 50A for any length of time, the heat build up in the secondary and core will be tremendous....remember, heat losses = I^2 R, so @ 50A even 100mOhms of resistance ==> 50^2 * 0.1 = 250 Watts in heat from the secondary alone. Thermodynamics is not really my strong suit, but I think it fair to say that failure would be eminent ....I will leave any haggling about the ETA to others......

  3. Damien


    Jan 28, 2015
    Oh I see yep that makes perfect sense. Ridiculous though isn't it?
    I think people should be held responsible for their own actions.

    Just to give you a basic idea of where I'm at -
    I did a couple of months of a sustainability/p.v course but didn't get further than ohms law and dropped out because I couldn't keep up with the maths as it was never a strong point for me.
    I worked at macgen for a while (Profile pic I chose for this forum is at macgen) working on small generators (I was more interested in the mechanical side of things at the time) but picked up a little experience with the other end (Alternator) & went on to start my own business running a small engine repair business and had a contract with kipor/bunnings to repair all their warranty item generators. *Imagines the expressions of horror from people reading this* Nothing too hard just diagnostics, a.v.r/inverter replacement etc. (trained monkey stuff) Also played around with 12v basic circuits/automotive alternator hacks etc but yeah just always been scared of transistors & itty bitty circuits so some exposure therapy is in the works. :) I do still consider myself to be a beginner though as it may seem apparent to you that I am only just brushing the surface.

    Somebody suggested I look into 12v purpose built generators for my camping application. -

    I was making and selling these as well for a while -

    but decided that it's much more practical for charging batteries to generate the power at a distance away and then step down the voltage (transformer) at the camp site.
  4. Damien


    Jan 28, 2015
    Well in the interests of liability and stress levels, , lets just assume that I'm talking about low voltage stuff from now on.

    I'm curious about transistors.
    I've been salvaging old components from scrap circuit boards and
    have a method of determining whether a tranny is n.p.n or p.n.p but is there a simple method to determine what kind of transistor it is?
  5. Damien


    Jan 28, 2015
    Cddrive - Thanks for that cat5 utp cable trick for breadboard jumpers I've got a bit of that lying around somewhere I think! :)

    Fishforfun - Ok I hear ya. =) I'll be extra careful and only "experiment" with low voltage stuff until I know more. Thank-you for highlighting the dangers.

    Where is everyone from just out of interest?
    Last edited: Feb 8, 2015
  6. davenn

    davenn Moderator

    Sep 5, 2009
    I'm in Sydney, Oz, an ex Kiwi

    some have they locations below their avatar pics others like to keep more private ;)


    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Well it used to be included under my avatar. This the first time I've noticed that it's gone. Another senior moment I guess. Anyway, My QTH is South Florida USA.

    Welcome Aboard!

  8. Damien


    Jan 28, 2015
    Awesome I'm in Melbourne Australia. =)

    What's everyone currently working on in the electronics world?
  9. BobK


    Jan 5, 2010
    Say hello to Phryne Fisher for me! Or would that be G'day?

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