Semi-OT: mix and match plug packs - what is allowable in terms ofVolts and Amps?

Discussion in 'PC Hardware' started by spodosaurus, Jan 19, 2008.

  1. spodosaurus

    spodosaurus Guest

    How does one determine when a left over powerpack from one dead device
    can be used on another device whose plug pack has returned to the wild
    to join the feral plug packs? What are the rules with volts, amps, and
    watts to prevent ‘bad things’ happening? For instance, I have to ADSL
    modems. One is dead, but with a 10V,1A power pack. The other is alive,
    but needs a 7.5V,1A plug pack. A friend of mine wants to relieve me of
    my modem and plug pack, and I want to know if ‘bad things’ are going to
    happen with this compination? (such as fire, release of magic smoke from
    the working modem, etc)

    I ask here because I know we have a handful of resident experts in this
    area :)


    spammage trappage: remove the underscores to reply
    Many people around the world are waiting for a marrow transplant. Please
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    spodosaurus, Jan 19, 2008
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  2. spodosaurus

    davy Guest

    In simple terms you wanna connect a 10V 1A power pack to a modem that
    requires 7.5V at 1 amp.

    It's 'risky' to say the least although it may well work* but certainly
    not advisable*, what happens if the thing catches fire... you'll be for
    the 'high jump' simply because the power pack wasn't made for that
    particular modem.

    It would be quite ok to use a 7.5V 3 amp power pack on the modem...
    because it's the modem what determines what power is drawn and would run
    cooler than the 1 amp version at full load.

    The power in Watts is derived from the Voltage x the Current consumed,
    so the modem consumes a maximum of 7.5 Watts, with the 10V power unit
    this would be 10 Watts... flee power of a difference I know but what if
    a fault develops ?? Keep on the safe side I say.

    davy, Jan 19, 2008
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  3. spodosaurus

    JAD Guest

    that would be ok as long as the POLARITY is the same....he 'may' find that it wont
    function as should be fine
    JAD, Jan 19, 2008
  4. spodosaurus

    spodosaurus Guest

    Thanks guys, that's what I suspected. :/


    spammage trappage: remove the underscores to reply
    Many people around the world are waiting for a marrow transplant. Please
    volunteer to be a marrow donor and literally save someone's life:
    spodosaurus, Jan 21, 2008
  5. spodosaurus

    Paul Guest

    I can give a practical example that happened to me.

    I bought a label maker (LCD display, keyboard, motor etc), got
    it home and discovered it runs on batteries. Well, I hate that
    kind of shit. I checked and there was a barrel connector for
    an adapter.

    I went back to the store, and they wanted $39.95 for an
    adapter. As I'm a cheapstake, I decided I could do better
    than that.

    One of the requirements for barrel connectors, is that they
    at least be labeled for "center pin positive" or "center pin
    negative". So I had that to go on. The adapter itself was
    7VDC at 1.2A at the store. That is all it said.

    I bought a 7VDC adapter with more than 1.2A rating. The
    adapter I bought was *regulated*, which means the output
    will never change from 7V. It will always be exactly that
    voltage, from zero amps, to at least 2 amps load.

    I plugged in the adapter. The label maker LCD display lit up,
    but as soon as the motor engaged, the thing shut down.

    I ended up buying that $39.95 adapter. It turned out, the
    proper adapter was *unregulated* DC. With my multimeter,
    I got 9.4V when zero amps are being drawn. The adapter
    output voltage would drop, as more current is drawn. I
    suspect if I were to draw exactly 1.2 amps from the
    adapter, I'd have seen 7V as the result. (The label on
    the adapter says 7V 1.2A, and yet under no load, I got

    I don't think they are required to list whether the
    adapter output is regulated or unregulated. They may
    distinguish AC versus DC (as some adapters may choose
    to output AC, such as a transformer based one). If a
    bridge rectifier and filter cap are added to the
    transformer, that makes it an unregulated DC adapter
    (with voltage falling as more current is drawn). The
    third kind (the kind I tried to run the label maker
    with) was a switching adapter. A switching adapter
    converts AC to regulated DC, just as an ATX supply
    would. But since the voltage didn't "pop up" to over
    9 volts, that seemed to cause an issue with the label

    So I was as careful as I could be, to match what I
    thought the requirements were, but it still didn't work.
    Since the polarity and type (DC versus AC) was right,
    there was no damage.

    If you had a device with a 5V input spec, it is possible
    the device has no internal regulation at all. In that case,
    the input is likely expected to be tightly regulated.
    Not too much deviation would be tolerated in that case.
    Even the friendly "unregulated" adapter, could blow
    such a design into the weeds, due to the "voltage popup"
    under no load. (Different logic families have different
    tolerances to excess voltage. For example, there used to be
    a CMOS logic that could take 5V to 15V, and for that,
    virtually any DC adapter in that range, would work.
    But that is decades old technology. Newer stuff is less

    With some internal dissection and investigation, maybe you'd
    have better luck matching them up. I got screwed out of
    $40 on my attempt, just to be taught a lesson.

    Paul, Jan 21, 2008
  6. spodosaurus

    davy Guest

    Further to Paul's comment... you only need to get the +/- polarity wrong
    and thats enough.

    I just had a look at a palm sized TV and dvd player, total scrap at 14
    months old, the guy had wired the polarity wrong, there were big holes
    in a couple of transistors making them unrecognizable, and a load of
    electrolytic capacitors blown to pieces.

    A lot of damaged could be prevented to all portable equipment by
    fitting a diode in series with the power jack, if connected the wrong
    way.. nothing would happen...

    ... better still a bridge rectifier directly from the power jack to a
    regulator and then to the circuit.... this would enable the device to be
    connected any way round with a lesser restriction on the input voltage,
    but problem may arise when used on batteries, there will be a slight
    voltage drop because of the diodes usually about 0.6V per diode, could
    always use Schottky diodes the drop would be around 0.3V better than a
    load of blown silicon.

    In spodosaurus case it's a risky thing to do in case of any legal

    davy, Jan 21, 2008
  7. spodosaurus

    spodosaurus Guest

    I checked that first :) It's printed on both the modem and on the 10V/1A
    power plug I tested it with (and posted here with).

    I've found a 1A 7.5V (multi volt power plug) at Altronics for $25AUD
    that'll do what I need (and has polarity set by how you connect the plug
    to the plug pack's cord).


    spammage trappage: remove the underscores to reply
    Many people around the world are waiting for a marrow transplant. Please
    volunteer to be a marrow donor and literally save someone's life:
    spodosaurus, Jan 22, 2008
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