PWM Chip substitution

Discussion in 'General Electronics Chat' started by R!f@@, Mar 24, 2010.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
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    Hey guys.
    I was abt to repair a PSU when I came up with a dead PWM controller
    Find the attached data sheet.
    I have replace MC3842 with 3843 no problem.
    But I do not have 3845.

    I checked the specs, note the second paragraph.
    I am having doubts on swapping 45 with 42 or 43.
    Will it work or do you like to see me blow the PSU bridge and FET. :D
     
  2. R!f@@

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    Apr 2, 2009
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    oh well!!!
    it seems this is also my only reply thread.
    No matter... it works.
    u can swap 42, 43, 44, and 45.
    Cheers
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, they're different IC's for a reason.

    They have different thresholds and different maximum duty cycles. The '45 has a low threshold and a low duty cycle limit, while the '42 has a high threshold and can go to 100%.

    It may work for awhile, but it would take a thorough analysis of the original circuit to determine whether or not such a replacement is acceptable or not. It may work OK in your circuit for a while, but under a heavy load, you may see smoke and flames coming from your supply.

    The designer probably had a good reason for using the '45 vs the others.
     
  4. R!f@@

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    That is what I like to know, the duty cycle thing.
    I compared the data sheets and every thing else is almost the same.

    I know what duty cycle is, but the thing is when it comes to analysis I go berserk.
    What I like to know is that it says that it will work with in 100% duty cycle, and other one says with in 50% duty cycle.

    The duty cycle is the mark space ration of a PWM, the thing is "with in", I am having a hard time grabbing what it means, I think it's because my native is not English, and somethings are hard to understand for me, even though I may seem pretty confident in English.

    But the good news is the supply is working and I have replaced the power switch and Vcc resistor plus Isense resistor in accordance with the data.
    I tried to quickly power up and down and now still it is powered and loaded and seems fine. So I guess we'll now when the max load occurs how well it can handle. right?

    Thanks Sgt
     
  5. R!f@@

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    Well Sgt.
    I know you'll be smiling after reading this.
    you know I could just ignore but I am not that type, If I made a boo boo I will admit it .
    and then I will learn. And then my chest will come up...well!! .u know what I mean.
    So here go's.

    The Bloody PSU smoked at the customers hand.
    Returned and checked and found the FET, PWM, All the low R's ..kaput....gone....Pffffffffft!! :eek:

    But ...nooooo!!! I ain't gonna stop. Wait till I get my hands on some new FET's and then we'll see who has the last laugh :cool:.

    This is not the End...It is the beginning.............................................................. more to come
     
    Last edited: Mar 28, 2010
  6. SgtWookie

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    Jul 17, 2007
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    I find no joy or satisfaction in failing to convince you to install the correct replacement part.

    I hope you took some photos of the board while you were working on it; at least that will make the re-build much easier.
     
  7. R!f@@

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    It's not that you get joy... I was just humoring....u know me...
    OK let's see photo's ....coming up.
    I went to buy FET's... no luck.
     
  8. R!f@@

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    Apr 2, 2009
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    Here's the pic.
    What do you have in mind
     
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