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  1. #1
    Thanks for the info john, I’ve got a few TT's laying about that I had planned to use as a bench supply for testing bits and bob's.

    What I would really like, is something that would allow me to turn a knob and change the voltage output to different amounts.

    As and when I need to for supplying and testing different things, can you please recommended what it is I should use, what kind of "knob" I need to be looking at.

    Panel mount voltage regulator?, I would more than likely use a little digital screen of sorts so I can fine tune the “knob” to just the right volt output each time.

    Thanks guys !
    .Me

  2. #2
    I'v just had a quick look on ebay and found this:

    Link: http://cgi.ebay.co.uk/50Vdc-BLUE-LED...1%7C240%3A1318

    This is the kind of thing i was thinking, i would prefer to change the output via a front panel using a knob, over poping inside the unit to change the dip switches everytime i need a new output amount.
    .Me

  3. Quote Originally Posted by Lee Roberts View Post
    I'v just had a quick look on ebay and found this:

    Link: http://cgi.ebay.co.uk/50Vdc-BLUE-LED...1%7C240%3A1318

    This is the kind of thing i was thinking, i would prefer to change the output via a front panel using a knob, over poping inside the unit to change the dip switches everytime i need a new output amount.
    Well its a pretty voltmeter, but its not a voltage regulator...the way its described is misleading. It has a built-in regulator allowing it to be powered from a 5 - 15v supply - while reading that supply... so it can be used as a supply monitor.

    Its not autoranging tho, so not much use for monitoring a truely variable power supply.

    So lets start by listing what you want...

    A power unit with an adjustable output voltage from 0 to 25v is a good range for hobby stuff, switched in 2.5 volt steps maybe ?

    Output current at 5A maximum, current limited. Maybe a switched limiter, for .1,.5,1 and 5A?

    Output voltage and current monitoring, current limit warning (LED, buzzer?)

    What else?

    What transformers etc have you got to start the ball rolling?

    Alternatively...look here
    Last edited by irving2008; 24-11-2008 at 09:29 PM.

  4. #4
    Quote Originally Posted by Kip View Post
    Try a Variac Lee.
    Good idea but YOU MUST use an isolating transformer downstream of the Variac as these are at mains potentional.

    Here's a Variac
    http://cgi.ebay.co.uk/CLAUDE-LYONS-V...mZ110313909476

    and here are some isolating transformers that can handle 110 volt.

    http://cgi.ebay.co.uk/12-x-BBH-Isola...mZ350125103969

    John s.

  5. A variac will give variable AC volts, but that's not what Lee was looking for if I understood his original post correctly - he wanted a bench supply for trying out bits and bobs.. I read that as a DC supply.. Lee?

  6. #6
    What sort of voltage and current range are you looking for?

    What I think you need is something like an ordinary lab bench supply. These often come up on ebay for a reasonable price.

    Alternatively you can build one easily enough. I have three or four variable voltage bench supplies that I've built, the most recent being one to power my home made capacitive discharge battery tab resistance welding machine (see pic below - it's the silver box on the left with the digital voltage readout).



    Jeremy

  7. #7
    For my DC suds pump speed control I used an LM338T, just google for LM338 and get the data sheet with circuit diagrams.

    Up to 5 Amps, needs a few extra components, but with a bit of jiggery pokery you can solder them on to the pins and enclose with shrink wrap.

    Bit of an industry standard, costs a quid.

  8. Quote Originally Posted by Robin Hewitt View Post
    For my DC suds pump speed control I used an LM338T, just google for LM338 and get the data sheet with circuit diagrams.

    Up to 5 Amps, needs a few extra components, but with a bit of jiggery pokery you can solder them on to the pins and enclose with shrink wrap.

    Bit of an industry standard, costs a quid.
    LM338 is good device for a fixed supply, but not ideal for a variable unit unless you have a big heatsink. if you want 0 - 25v off a 30v supply at 5A then you are outside its thermal characteristics - it can dissipate 25W max so thats 5A at 25v output, dropping to 1A at 5v output. Has the advantage of simplicity tho with only 5 other parts needed (excluding bridge rec and smoothing cap)

    Although slightly more complex (as in needing 6 more parts) a switch mode supply based on LT series devices needs no heatsink and gives variable current limit as well as voltage.

    A similar spec bench supply on ebay is about 60 so by the time you bought the bits and made it up and boxed it for safety its marginal to make them really... except for the fun/satisfaction of knowing you did it yourself.

  9. #9
    Quote Originally Posted by irving2008 View Post
    A variac will give variable AC volts, but that's not what Lee was looking for if I understood his original post correctly - he wanted a bench supply for trying out bits and bobs.. I read that as a DC supply.. Lee?
    Yes it DC i am after !

    capacitive discharge battery tab resistance welding machine
    Ermmm...lmao...O one of them , that unit looks like a nice little thing to have, thats the kind of thing i was thinking about making.

    The TT i have are:
    Primary x2 120 Volt
    Secondary 2 x 15 Volt

    I think they maybe a bit big for what i want tho, given how small you can get them and given i only want to test things in the 0 - 30v range maybe.

    i also just whanted to make a little gadget as well for somthing to do one night, i cant remember what the ampage is on the TT's but i think it was somthing like 160va, right now i'v got one hooked upto two (big) caps:

    http://www.mycncuk.com/forums/showthread.php?t=261

    as youll read that little box is nolonger going to be used and the contents going to waste, so i just wanted to make use if it all really, maybe you guys could point me the a direction to go with so as to make the most of what i'm hoping could be a very nice bence supply ? in terms of being able to go from a low out put, to a high out put both in DC and in amps should i need to.

    I had a look on ebay and for 15 a ready made one dose look appealing, although i would rather design and make a home made system, i'v got leads BR and so on so all i would need really is an enclosure, display and maybe a few other bits.

    I dont know, ill leave it with you guys for ideas and thoughts on the above please.

    Cheers,
    Last edited by Lee Roberts; 28-11-2008 at 02:20 AM.
    .Me

  10. Quote Originally Posted by Lee Roberts View Post
    Yes it DC i am after !

    The TT i have are:
    Primary x2 120 Volt
    Secondary 2 x 15 Volt

    ...
    Cheers,
    Well you have options... wire secondaries in parallel 15v AC 10A will give a 0 - 20v 10A supply, or in series will give 30v 5A will give a 0 - 40v 5A supply.

    To make it variable you need to go down the switched mode supply route, else the heatsinking arrangements will be extensive (ask if you want explanation)... this will mean milling a PCB, you can't bird nest a switcher (any more than you can a high current stepper driver, indeed the two are very similar). A suitable fully-integrated chip for this will cost around 10 or a cheaper one with external MOSFET around 3 (again same issues as steppers) with about 10 worth external parts. You might be able to get the chip as a sample tho...

    If you want we can do this as a design/learning exercise and I'll take you through it - then you'll have the satisfaction of both building it and designing it. And of course others can chip in with their views...

    regards,
    Irving...

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