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Elektor Wireless RS232 link and current consumption

Discussion in 'Electronic Basics' started by Mike, Jul 15, 2004.

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  1. Mike

    Mike Guest

    I've built the Wireless RS232 link as described in Elektor Electronics
    Magazine (December 2003) - and I've even done it on their own
    (extortionately expensive) board.

    When powered up, the 7805 1A REG gets VERY hot - enough to remove the
    skin on my finger, and I've traced the high current consumption to the
    MAX232 chip. I don't believe this can be right.

    However on previous things I've built where I've used a MAX232, I've
    also noticed that the voltage regs run a lot hotter than I expect,
    though not enough to burn my fingers.

    At the moment, the Elektor PCB just has the PSU bits, and the MAX232
    chip + caps, and the voltage reg starts getting hot (not just warming
    up) after 30 secs or so.

    Has anyone out there built one of these things ? So I expect this ? I
    don't want to risk the £20 ER400TRS if it's going to blow up !
  2. The power requirements of the MAX232 can be read from the datasheet. When it
    uses (much) more, you know there's something wrong. So if nothing but your
    7805 regulator becomes that hot, it may be defective itself.

    BTW what is the voltage of the regulators input? It is the power (I*U*t)
    that's warming up the regulator. You may need a heatsink.

    petrus bitbyter
  3. Ryan Wheeler

    Ryan Wheeler Guest

    if you only have the max232 connected to the +5V DC output
    of the regulator and the regulator get hot you got something wrong.
    I dont get the Elektor mag so dont know how that is wired.
    1. check the reg input range, I am used to +12VDC to +18Vdc range.
    2. most common mistake are capacitor connections on the MAX232 chip.
  4. John Jardine

    John Jardine Guest

    Make sure those 'lytic caps are polarised correctly. I recently spotted some
    wrong on an expensive commercial design that was just ready to go into
  5. The maximum power dissipation of the max232 is (depending on package)
    between 450mW and 880mW. That means something like 170mA max current,
    assuming the higher number and a 16 pin DIP case.

    However, the 7805 is a linear regulator, meaning it'll dissipate more
    power with input voltage. If you are powering the circuit with a 12V
    supply, for example, and drawing 880mW with the MAX232, then the 7805
    will want to dissipate 7/5 of that, or 1.25W, which will generate
    about 80C rise above ambient. Thats toasty, but not too hot.

    Bob Monsen
  6. Mike

    Mike Guest

    Input voltage is 7.5V. It then goes through a diode for input voltage
    reversal protection, so there's only 6.9V hitting the voltage reg.

    I've checked the elctrolytics. They agree with the printed legend, and
    that agrees with the circuit diagram. I've checked the pinout on the MAX232.

    There's nothing else on the board to check !

    My only other thought is that this 7805 is over 25 years old. I bought
    a lot and I'm just reaching the end. It's actually marked up
    UA7805/UC7803. It does regulate to 5V, I'm just wondering, if somehow
    the spec for these things has changed over the years. But it's pretty
    much a standard building block isn't it, 7805, can't have changed ?

    To summarise then, with the voltage reg, it's associated capacitors,
    and a small LED/resitor to indicate power, it all runs perfectly.

    Add MAX232 powered from the regulator, the 4 voltage doubler
    electrolytics, and the voltage reg starts cooking.
  7. Not certain how your circuit is working, but I know that if at any time,
    the output voltage and input voltage show a reversal, a simple 7805 will
    start doing odd things because it is trying to protect itself from
    It can go into thermal shutdown, or in some cases can be tricked into
    going into reverse conduction. I have a metal cased one that literally blew
    a pin right through the top of the TO-3 case when this happened. Like a
    welding arc- a perfect little round hole, pin vanished right out of the
    glass opening from the underside and left a perforation in the case so you
    could look right through it.
    Have a look at some of the protection schemes to be employed with 7805
    style regulators so that this will not occur. They are present in the data
    sheet or if you have the old style linear data book, it should also be


    Sir Charles W. Shults III, K. B. B.
    Xenotech Research
  8. Ryan Wheeler

    Ryan Wheeler Guest

    and this is probaly your trouble. A 7805 needs at least 8VDC, better
    9Vdc on the input to give you a reliable 5VDC output.

  9. There you go. I bet your 7805 is oscillating like an alarm clock. 6.9V is to
    low for an ordinary 7805 and the diode makes things worse. I advise to
    remove the diode and rise either the input voltage to at least 8V or to
    replace the 7805 by a low drop regulator.

    petrus bitbyter
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