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Thread: Power Supply with Switching Pre-Regulator

  1. #1
    Flashaholic* MrAl's Avatar
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    Default Power Supply with Switching Pre-Regulator

    Hello folks of CPF,

    Here's the start of a power supply which is linear but
    has a switching power supply pre-regulator to reduce
    heat output of the linear. It also uses fairly common
    parts.

    Right now it's still in the preliminary stage, but it
    shouldnt be long now until it's ready.

    Feel free to comment, make suggestions, etc.

    Take care,
    Al


  2. #2
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    Default Re: Power Supply with Switching Pre-Regulator

    Why wouldn't you make it completly switching and off-line so as to avoid using bulky expensive line transformers. Is there some issue with ripple or load reg that would cause you to go with a linear design? MOdern switchers perform quite well- i.e. personal computer supplies uses them exclusively. Heres a 100-240 volt AC input to 12 volts at 1 amp DC output supply that measures only 4"x2"x3/8"

  3. #3
    Flashaholic* PEU's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    yipeee

    can't wait for the final drawing [img]/ubbthreads/images/graemlins/popcorn.gif[/img]

    Pablo

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    Default Re: Power Supply with Switching Pre-Regulator

    Yesss..... finally ! [img]/ubbthreads/images/graemlins/smile.gif[/img]

    Al, just take your time !! [img]/ubbthreads/images/graemlins/popcorn.gif[/img]

    Question, what is the lowest output voltage ? 1.2 V ?


    Hi Tony,

    This project is planned for CPFer with "limited access" for electronic components and the learning process. [img]/ubbthreads/images/graemlins/grin.gif[/img]

    Of course , if you can share how those "isolated" idea can be applied "easily", I think it will be very nice.


    Vic

  5. #5
    Flashaholic* MrAl's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    Hello again,

    Tonyb:
    Yes, that would be a good idea, but i have a feeling
    it would be a bit more dangerous for most folks on CPF here.
    If you'd like to share some design ideas though please
    feel free. We'd like isolation from the main line too.

    Vic:
    I overlooked the fact that this should probably adjust
    down to zero volts. I'll have to correct that.
    Also, i shouldnt have jumped in with the complex
    circuit first...rather, i should have started with the
    simplest configuration and worked up. With this in mind,
    i'll take a step back and post the more simple circuit
    so people can see how simple this can really be...that
    was the main idea anyway. Then later, if anyone wants
    to upgrade to the more advanced design they can simply
    add parts [img]/ubbthreads/images/graemlins/smile.gif[/img]
    The LM317 (and similar) already are capable of voltage
    regulation and current regulation by itself without all
    the extra op amps and voltage regulators, so the first
    design should take advantage of that, which was what
    was supposed to happen anyway. The upgraded version
    will include all the added features.

    I hope to have more on this posted today.

    Take care,
    Al

  6. #6
    Flashaholic* PEU's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    Al,

    does the digital meter affects in any way the voltage or current regulation? It should be measured at the output or in a stage in between?

    Pablo

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    Default Re: Power Supply with Switching Pre-Regulator

    Hi Al,

    Yes, I think adjustability below 1 Volt is handy, especially at observing single cell circuit behaviour on drained cell. [img]/ubbthreads/images/graemlins/smile.gif[/img]

    I agree with you on starting a simple one and advancing step by step for more features, and I really like the knowledge gained during that process. [img]/ubbthreads/images/graemlins/blush.gif[/img]


    Pablo,

    For voltage measurement just connect as usual, parallel with the PS output and it won't affect it since for measuring volt the resistance is very high.
    Even for the el'cheapo DMM, most of them are built based on ICL7106 ic and it needs only 1 pA (typical) and max at 10 pA (pico Amp) to measure voltage. With these pico amp, I don't think it will affects the V-out.

    For current, DO NOT put the DMM in series with the PS since it has quite high resistance, especially on low current ma resolution.
    Just use the sense resistor (the one connected to and below the load symbol) and measure the voltage drop across that resistor to get the current reading.
    If it is 0.1 ohm then it is pretty straight forward using I = V /R formula, if not, you'll need one multiturn potentiometer 1K or 2K parallel with that sense resistor and will need some tuning for the correct reading.

    Let's wait for Al to finalise the circuit and see how much resistance he decided for that sense resistor.

    Vic

  8. #8
    Flashaholic* PEU's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    ok Vic!

    Yesterday I ordered samples from maxim and included a pair of 7107, the usually don't ask much, so I hope I'll receive them...

    [img]/ubbthreads/images/graemlins/popcorn.gif[/img]


    Pablo

  9. #9
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    Default Re: Power Supply with Switching Pre-Regulator

    Pablo,

    I pray for you... ! Geezzz... I have a feeling that both our PS at final stage will looks like twins ! [img]/ubbthreads/images/graemlins/grin.gif[/img]

    So you also decided to treat your DMM like me too ? ..he...he... [img]/ubbthreads/images/graemlins/grin.gif[/img]



    Al, with all respect, if you find this kind of pic is distracting in this serious discussion, I'll delete it.

    Vic

  10. #10
    Flashaholic* MrAl's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    Hello again,

    Pablo:
    As Vic was pointing out,
    measuring the output voltage wont affect operation, and
    measuring the current output can be accomplished by
    measuring the voltage across the sense resistor, and if you
    like we can use one of the left over op amp sections to
    create a meter amplifier so you can read directly: as 1.000v
    on the meter means 1.000A output.

    Vic:
    Ok then, the more advanced unit will adjust below that, down
    to either 0.000 or very close 0.010 volts.

    The selection of the sense resistor is a bit of a tradeoff,
    i'd like high (1 ohm) resistance but then at 3 amps that eats
    up not only power but available voltage also (3 volts), so,
    we'll probably end up with half that, 0.5 ohms. Of course i'll
    also try 0.1 ohms first and see if the current regulation
    stability AND adjustment checks out ok.

    Actually i like the humor Vic, and i was going to draw some
    cartoon up too but it takes too much time :-(
    Eventually perhaps i'll get to it also.
    I dont know how this affects loading however, when someone tries
    to load the page(s). I like it though [img]/ubbthreads/images/graemlins/smile.gif[/img] I've always liked
    joking around and comedy, and i think your cartoons are really
    a good idea. I think it would be fun to start an Electronics
    Cartoon thread too, just for laughs. You could be on to
    something new.

    Back to PS...

    I hope to have a very simple version posted today.
    This will be very simple to build and very useful
    also, and will take advantage of the already present
    advantages of the chips.

    BTW, Pablo and Vic, i need to know what the max output
    you wanted to get... is it 50v or what?
    That will be one of the advanced units because
    it requires extra parts to get up that high.
    I've noticed that the small electrolytics go up
    in price quite a bit once you get past about 35v ratings.
    Price jump from about $1 to about $3 each.


    Take care,
    Al

  11. #11
    Flashaholic* PEU's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    [ QUOTE ]
    MrAl said:
    BTW, Pablo and Vic, i need to know what the max output
    you wanted to get... is it 50v or what?
    That will be one of the advanced units because
    it requires extra parts to get up that high.
    I've noticed that the small electrolytics go up
    in price quite a bit once you get past about 35v ratings.
    Price jump from about $1 to about $3 each.

    [/ QUOTE ]

    Hi Al / Vic

    In my case I have a 30+30 transformer, so that's the limit for me [img]/ubbthreads/images/graemlins/smile.gif[/img]

    And I guess the magic number should be around 30-40v because as you said prices&problems go quickly up after 40-50v for all the parts.

    [img]/ubbthreads/images/graemlins/popcorn.gif[/img]

    Pablo

  12. #12
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    Default Re: Power Supply with Switching Pre-Regulator

    Greetings Al & Pablo,

    [ QUOTE ]

    Ok then, the more advanced unit will adjust below that, down to either 0.000 or very close 0.010 volts.


    [/ QUOTE ]

    Wow !! [img]/ubbthreads/images/graemlins/ooo.gif[/img] This is better than I expected ! And I'm not complaining ! [img]/ubbthreads/images/graemlins/grin.gif[/img]

    So does it need a precision opamp for that ? Although I don't have problem with that (e.g. OP07 opamp is available here), but I'm thinking if other CPFer might have a problem with it. Just a thought.

    About the sense resistor selection , if we use low resistance such as 0.1 Ohm , what's the catch ? I mean worst case scenario ?





    For me, I don't have any problem with high voltage as you can see at the above picture. "But", is it possible let say other CPFer only has access on lower voltage one , say 15VAC out transformer ?

    Realistically, at least for me, I think the highest voltage I used in the past was only to simulate car battery 12V, but looking what I got here, I really like to have the one that can put out at least 24 Volt (truck battery) or 30 Volts ..such testing series of leds ! [img]/ubbthreads/images/graemlins/grin.gif[/img]

    Again, as agreed to make it simple 1st, is it possible that your design to accomodate this difference requirement on the max. V out "without" major re-designing. e.g.: only change some resistor or reference ?

    Another questions about the transformer and the rectifier part :

    - Does it require a centre tapped transformer ? I got both though...psstt..only $2 a pop [img]/ubbthreads/images/graemlins/grin.gif[/img]

    - In the circuit drawing at the rectifier part I see you use 2 types, two (1N5402) for high amp connected to switcher, the other two (1N4003) for the opamp supply. My rectifier is the one at top of left transformer ? It is rated 1000V at 35 Amps [img]/ubbthreads/images/graemlins/cool.gif[/img] , should be fine right ?

    Hey .... its cost me only $40 cents ! I know it is overkill, but for that price, I can not bear not to buy it ! [img]/ubbthreads/images/graemlins/smile.gif[/img]

    Thanks & regards,
    Vic

  13. #13
    Flashaholic* MrAl's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    Hello again,

    The simplest circuit is now here...




    Vic:
    It doesnt really need a precision op amp, although that would be nice,
    but it has to be fast. I'd like even faster than LF347 but that might do.
    OP07 will be too slow. OP27 if you feel like spending the money.
    Now you see what i've been dreaming about, and how many many options
    there are for a circuit like this. So many choices, it's hard to
    decide [img]/ubbthreads/images/graemlins/smile.gif[/img]

    Worst case for 0.1 ohm is that the response to a current overload
    step is too slow or rings too much. Im shooting for 1us max with
    maybe 10% overload. I'll take 2us, but i think 100us is too slow.
    Also as i was saying, a left over section of the op amp can be used
    to scale the voltage meter to read directly in Amps with something like
    two additional standard resistors.

    Transformer doesnt have to be center tapped, but for the advanced
    unit some means of supplying a negative bias has to be employed, so
    that means either center tapped or an additional wall wart or small
    transformer (100ma only).

    The main reason for the simpler design is so that people who dont
    want to have to connect lots of parts can still build one and get
    very good performance and versatility. This means two parts lists
    more than anything else, but the connections are a little bit
    different too for voltage mode and current mode operation.
    See the schematic for a better explanation.

    The reason for two types of rectifier diodes is mainly because the
    negative supply only has to supply maybe 100ma or less while the
    positive supply has to provide maybe 3 amps. This means smaller
    diodes can be used for the negative supply, but of course this
    isnt mandatory so if you want to use a bridge rect that's rated
    for the max positive supply (or more) that's ok too.
    Note however that if you use a single winding transformer
    (no center tap) you need four 1N5402 diodes (as well as
    another transformer).


    Take care,
    Al

  14. #14
    Flashaholic* PEU's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    Hi!

    adding to what I said, I want to start from almost 0v to 30v

    I don't understand the 2 different loads, this means that if I want to limit the current supplied to a circuit I have to choose a diferent output? or I'm totally wrong? [img]/ubbthreads/images/graemlins/smile.gif[/img]

    Pablo

  15. #15
    Flashaholic* MrAl's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    Hi again,

    Pablo:
    This 'simplest' version (001) only adjusts from 1.2v up.
    The more advanced version will adjust from 0.00v and up.

    Yes, this 'simplest' version has the load applied to
    a different place in the circuit depending on whether or
    not you want to run constant current or constant voltage
    mode. Again, the more advanced version will have the load
    connected the same for either and will have both simultaneously,
    so that the current is also limited by the set point you
    choose, not by the device itself (note that the simple
    version 001 current limit is set by the device itself,
    unless you're operating in constant current mode).

    The more advanced version will go from 0.00 up to
    whatever your building for, and current limit and set point
    from probably around 1ma up to full current.

    Does this make more sense?

    The simplest version (001) is there to minimize
    parts count yet still get some of the benefits of
    doing this in the first place, that's all.

    Vic:
    I guess you want to go 0 to 30v too then?

    0-30v seems like a reasonable range.


    Take care,
    Al

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    Default Re: Power Supply with Switching Pre-Regulator

    [ QUOTE ]
    The more advanced version will go from 0.00 up to
    whatever your building for, and current limit and set point
    from probably around 1ma up to full current.

    Does this make more sense?

    [/ QUOTE ]

    YES!



    [img]/ubbthreads/images/graemlins/popcorn.gif[/img]

    Pablo

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    Default Re: Power Supply with Switching Pre-Regulator

    Hi Al,

    Ok, since you asked for it, here are my wish lists [img]/ubbthreads/images/graemlins/grin.gif[/img]

    1. Adj. 0 - 30 Volt (with 0.1 volt or better resolution)
    2. Adj. 0 - 3 Amp (with 1 ma or better resolution) ... stage I
    3. Adj. 0 - 5 Amp (same as above) .................. stage II
    4. Output voltage that will not overshoot during switching off or on of the PS main supply (during off caused by collapsing negative supply)
    5. Low ripple , dunno how low yet, to be tested with various cap and/or LC filter as you describe OR double caps & inductors

    Planning to separate switcher and linear board (modular), stage I will be 3 Amp switcher and stage II 5 Amp switcher. Linear part already 5 Amp capable.

    Another bell & whistles : [img]/ubbthreads/images/graemlins/smile.gif[/img]

    1. Constant Current mode indicator (either LED and/or switchable mini buzzer ) just in case it got shorted by accident or trouble shooting circuit without turning our head to see if the CC mode was activated ..... [img]/ubbthreads/images/graemlins/grin.gif[/img]

    "Enhancements" which I'm planning to do it my self later :

    2. Coarse and Fine (10% of coarse value) potentiometer for both Volt & Amp adjusment
    3. Digital meter with BIG 7 seg. LED display (size 3.5 X 2.5 cm / digit) for Volt & Amp
    4. Ripple meter using bar graph IC -> LM3914 indicator utilizing your ripple detect circuit, wondering if this is necessary once we got a low ripple, if the ripple is consistent then I don't need it
    5. High temperature activated low noise & long life DC fan (Panaflo), just ON or OFF only, no variable speed. Adjustable temperature threshold.
    6. Output disconnect & connect switch, possibly driven by semiconductor based (MOSFET) rather than mechanical swicth or relay. (need to discuss with you "later" for this switch to avoid disrupting the regulation loop)

    These enhancements will added step by step if I got time for it. [img]/ubbthreads/images/graemlins/grin.gif[/img]



    Ok, did I miss anything ? Hope this wish list is not too much ! [img]/ubbthreads/images/graemlins/smile.gif[/img]

    Vic

  18. #18
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    Default Re: Power Supply with Switching Pre-Regulator

    Hello again,

    Pablo:
    Funny cartoon [img]/ubbthreads/images/graemlins/smile.gif[/img]

    Vic:
    0-30 volt should be possible with high enough input and HV devices.
    0-5 amp can only be had with a higher power switcher and the 5 amp linear reg.

    I'll keep an eye on the negative supply so it stays active longer
    than the positive supply.

    We'll have to see what kind of ripple we get, but it should be pretty
    good because we'll have the linear reg on the output.

    Im going to build my switcher separate too...for one thing, the switcher
    itself will be very handy to have for some regulation needs too, even
    without the linear on the output.

    Constant current move indicator shouldnt be a problem.

    Take care,
    Al

  19. #19
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    Default Re: Power Supply with Switching Pre-Regulator

    Hi Al,

    Great, you've thought of the neg supply ! [img]/ubbthreads/images/graemlins/smile.gif[/img]

    [ QUOTE ]

    Im going to build my switcher separate too...for one thing, the switcher itself will be very handy to have for some regulation needs too, even without the linear on the output.


    [/ QUOTE ]

    Are you saying that you're planning to make the switcher part detachable and as independent PS ? That is great idea !

    Vic

  20. #20
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    Default Re: Power Supply with Switching Pre-Regulator

    Hi

    can this IC be used for the negative part of the PS ?

    it transforms a positive voltage to negative.

    And again, I may be totally wrong [img]/ubbthreads/images/graemlins/smile.gif[/img]


    Pablo

  21. #21
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    Default Re: Power Supply with Switching Pre-Regulator

    Hello again,

    Vic:
    Yes, i think it would be good to have a decent switcher
    circuit to work with things like wall warts and automobile
    supples (for Luxeon and similar). It would be nice
    if it didnt get too warm. As you know, a linear will get
    warm powering a luxeon from the car 12v battery.

    Pablo:
    Yes, that could work, but i think something a little more
    high powered would be better (100ma or more).
    That would help relax the need for two windings, but then
    the person building would have to order that part too,
    so it's a tradeoff.

    Take care,
    Al

  22. #22
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    Default Re: Power Supply with Switching Pre-Regulator

    Hi folks,

    Pablo,
    That ICL7660 is capable only at 10ma or below, I got it too, and its only suitable for low current needs below 10ma such as your ICL7107 DMM display. I tried before and even driving LEDs at 20ma the Vout dropped about 3 volts ! [img]/ubbthreads/images/graemlins/frown.gif[/img]

    Remember, you'll need another positive regulator for it since the max Vin only at 10 Volts !

    Al,

    I know you already provided the way of supplying neg supply but it is from transformer with center tapped.
    Please take a look at this pic below, is this ok (and safe) for generating negative supply ?

    Any recommendation for the D1,D2 and C2,C3 value especially for 100ma or more ? Or maybe you have other better solution ?




    About inductor you put 200uH, could you explain where or how did you get this value ? Because from the datasheet there is only a formula for fixed output (for Adj. version). I'm lost how to use those formula for this kind of circuit since the output can be starting very low to high with very low current to high too ? (Adjustable Volt and Amp)

    Your idea separating those modules is very handy too, think of other CPFer that interest or can afford building the linear part only ! [img]/ubbthreads/images/graemlins/grin.gif[/img]

    Regards,
    Vic

  23. #23
    Flashaholic* MrAl's Avatar
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    Default Re: Power Supply with Switching Pre-Regulator

    Hi Vic,

    I'll take a look at your idea of generating the minus
    supply and let you know what i find ASAP.
    I'll also try to explain the inductor value selection.
    I should have this info a little later today ok?

    In the mean time, here's the next version of the PS,
    so you can take a look and see what you might find.
    Note that i'll still be looking at replacing the neg
    supply shown here with your idea Vic.

    Version 002:


    Take care,
    Al

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    Default Re: Power Supply with Switching Pre-Regulator

    Hi Al,

    Again, take your time ! [img]/ubbthreads/images/graemlins/smile.gif[/img]

    This is interesting, I noticed something new here, what are those D6 (1N5226), 2N4401 transistor and the 10K resistor used for ?

    The linear C-in, how big for 5 Amps ? and are those Cout for switcher still the same 3 X 300uF even for 3 or 5 Amps ?

    Regards,
    Vic

  25. #25
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    Default Re: Power Supply with Switching Pre-Regulator

    Hi again,

    Vic:
    Your idea seems to work very well, so i included it on
    the new schematic for the new version...(see below).

    The new parts (incldg. 2N4401) is to swamp the FB pin in
    case the neg supply goes away before the pos supply as
    you mentioned earlier. This will kill the output voltage
    in case input power goes down while powering something
    critical, so the output wont jump up unexpectedly.

    Actually, the 3x300uf might be an overkill, probably
    one 1000uf would be ok, but it will probably end up
    being 1000uf for every amp output, so that's
    3 x 1000uf for 3 amp output, etc.
    I'll double check this too.

    Here's the new version with the simpler neg ps:


    Take care,
    Al

  26. #26
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    Default Re: Power Supply with Switching Pre-Regulator

    Hi Al,

    Great ! Actually those are not my design, I'm sorta copy it from my other circuit. (the other Adj. PS with so many-many discrete components) [img]/ubbthreads/images/graemlins/grin.gif[/img]
    Finally after hours of staring those new parts, I finally understand how it works when the neg. supply collaped and how it shutoff the whole thing !

    It is so simple and pardon for my "noobeeness", another great thing I've learned today ! [img]/ubbthreads/images/graemlins/bowdown.gif[/img] -> Mr.Al

    Hi Pablo,

    You can save those ICL7660 ICs now [img]/ubbthreads/images/graemlins/grin.gif[/img] , trust me, you'll need it for those ICL7107 based DMM for good accuracy and consistent measurements.

    Regards,
    Vic

  27. #27
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    Default Re: Power Supply with Switching Pre-Regulator

    Hi Al/Vic

    yeah! I ordered 2 sets of samples from maxim, and both of them included icl7107/7660 ics [img]/ubbthreads/images/graemlins/smile.gif[/img]

    They will go to friends there and then forwarded via usmail here.

    Also ordered some lm338 from national this way also [img]/ubbthreads/images/graemlins/smile.gif[/img] hope they deliver this time (but I'll use the lm350 for the PS)

    So, as you can see, I'm in no hurry, and learning a little every day it passes [img]/ubbthreads/images/graemlins/smile.gif[/img]

    I wait for the single output version

    [img]/ubbthreads/images/graemlins/popcorn.gif[/img]

    Pablo

  28. #28
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    Default Re: Power Supply with Switching Pre-Regulator

    Should the 50uf electro for the -ve supply should be a non-polarised one?. The current polarised one will have reverse charge on it for some time which may lead to premature failure.

    Chris

  29. #29
    Flashaholic* MrAl's Avatar
    Join Date
    Sep 2001
    Location
    New Jersey
    Posts
    3,144

    Default Re: Power Supply with Switching Pre-Regulator

    Hello again,

    Vic:
    I hope you like the way the power is shut down if the neg supply goes
    away first. It's interesting that with light load, the thing could
    put out power for some time without it.
    We're all learning here :-)

    Pablo:
    So i guess you are making your own volt meters? What are you
    going to use for the display?
    Were you able to get the 338's as samples or no?

    Burnt_Retinas:
    Thanks for the input!
    Well, when the ac input goes negative at the top (+) of the 50uf cap
    diode D4 conducts, and with enough current flowing to the Cin caps
    it might drop 1v or so, which means the top (+) of the 50uf cap sees
    about -1 volt, and since the second 1N4003 diode drops about 0.8 volts
    or so, the difference is -0.2 volts (approx). This means the 50uf
    cap will at most, see about -0.2 volts. Since it's rated at 50uf
    i think it can take this small neg voltage.
    Did i miss something or does this seem ok? I'll double check too.
    [LATER]
    I've found that there could be 1/4 to 1/2 cycle reverse
    voltage across the 50uf cap, if the input power is turned
    on during the negative half cycle of the 60Hz input.
    Changing this to 500uf reduces the voltage to about
    1 volt or so, so i guess we'll just do that.
    This change makes the 50uf cap 500uf instead.


    Take care,
    Al

  30. #30
    Flashaholic* PEU's Avatar
    Join Date
    Feb 2004
    Location
    Buenos Aires / Argentina (I like ribs)
    Posts
    3,598

    Default Re: Power Supply with Switching Pre-Regulator

    [ QUOTE ]
    MrAl said:
    Pablo:
    So i guess you are making your own volt meters? What are you
    going to use for the display?
    Were you able to get the 338's as samples or no?



    [/ QUOTE ]

    I already have the voltmeter, but is nice to receive free stuff and I may end building a second one for current reading [img]/ubbthreads/images/graemlins/smile.gif[/img]

    Pablo

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