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Thread: Did I screw up? (MAX787)

  1. 06-30-2002, 02:59 AM #1
    masch
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    Did I screw up? (MAX787)

    I built a power supply using the MAX787 schematic given in MAXIM's site. The +12 came from an LM7812 circuit.

    I tested the supply out from a DC adapter plugged into the wall socket and the +12V rail was giving me 11.82 V and the +5V rail was giving me +5.03V.

    So I decided to plug it in my cigarette lighter. Well not more than 10 seconds after plugging it in I saw smoke coming up from the circuit I built. I took it out and tried to track where the smoke was coming from, it seemed to come from 2 sources, 1 was the MAX787.

    Well I took the circuit back inside and soldered back some joints that seemed to have been disconnected. I tested it again with the DC wall adapter and found out that the +12V rail was giving me 11.82V still but my +5V didnt work.

    Pin 1 was giving me 14.82V (input)
    Pin 2 was giving me 14.82V
    Pin 3 was giving me 0V (ground)
    Pin 4 was giving me 6.12V
    Pin 5 was giving me 1.01V (output)

    I checked the circuits and I'm sure they're connected properly. I dont know if the diode or the MAX787 or maybe the insides of the capacitors are already damaged though.

    Is anyone familiar with this? I'm thinking of giving up building the supply and go the inverter way.

    Parts are expensive in Malaysia. The MAX787 itself took 3 weeks to ship from the States. I hope that's not the problem.


    Thanks in advance!
    masCh
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  2. 06-30-2002, 07:22 AM #2
    Casper
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    In theory, you should never connect a switching power circuit to a supply if you dont attatch a load to it first. This could have been the cause of your problem, especially due to the fact that it was just the switching power circuit (787) that was damaged. Your going to need to replace the MAX787 I think.

    Also, I think you'll find that when connecting your 7812 to a load, the voltage will dip below that of the tolerances of your motherboard... You should look into building the MAX1771 circuit, it gives reasonable power from a varying input.
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  3. 06-30-2002, 08:31 AM #3
    Paladin
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    hey masch!

    Which part of Malaysia are u from? Im from KL. For your info, u can get the Max787 from Farnell for around RM69. Ingram Micro is the distributor Maxim ics in Malaysia but so far they are not much help. Im building the sproggy power supply myself. So far the only parts thats impossible to get are the Max788. Drop me a pm or something...maybe we can help each other out



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  4. 07-01-2002, 08:11 AM #4
    THEMP3KID
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    Did I screw up?
    Sounds good to me... I assume you had the parts heatsinked, and isolated from the heatsink, and that you had a fuse inline with it when you tested it in the car, and that hmm... something else.. lol If not, I am not sure why you are asking this, BUT I AM ALWAYS WILLING TO HELP!!!!!!!!!
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  5. 07-04-2002, 10:24 PM #5
    masch
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    .

    I just read somewhere recently that I was meant to connect the zener diode with the silver ring facing the more -ve side, is that true? Because it was connected the other way around in my circuit. Could that be the cause of my problems?

    Do you think my MAX could probably still function if I change the diode back the right way? I wont have the circuit with me for a couple of weeks or else I would test this myself.
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  6. 07-05-2002, 12:53 AM #6
    Paladin
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    i cant say masch.....i'll have to see the whole circuit.

    Paladin
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