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Using a higher rated DC transformer

Discussion in 'Electronic Design' started by Connah, Nov 6, 2006.

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  1. Connah

    Connah Guest

    Good morning, all.

    I have an electronic device that requires a DC Wall transformer rated
    for 12VDC at 600 mA. Unfortunately, I have lost the power supply and I
    cannot find another one with that exact rating. My friendly Radio Shack
    representative tells me that the device only "pulls" what it needs from
    the transformer, the transformer does not "push" that much to the
    device. He tells me I can use a 12vdc at 1A for the device and it will

    Is this true? I do not want to fry my device by "pushing" 400mA more to
    it than it needs. If he is right, may I use a transformer rated for
    12vdc at 2.5 amps protected by the same principle? You can tell I'm not
    an electrical engineer. :) I would appreciate any thoughts. Thank you!

  2. GregS

    GregS Guest

    Unregulated supplies will have higher voltages if used with lower
    current draw. Your 600 ma divice will see more voltage. Can't
    tell what you have unless you look at with a scope. Some DC supplies
    just have diodes and no filtering. Its likely that a filtered
    DC supply would work in any case. You can buy some 12 volt regulated supplies,
    from Jameco that will put out 12 volts regardless of current. Since you didn't
    say what the device is, we would hope any noise from a switching supply
    would cause interference.

  3. Connah

    Connah Guest

    Its likely that a filtered DC supply would work in any case.

    I don't really know what that means, but the power supply I am thinking
    of using says this:

    12.0VDC 2.0A MAX

    Since it says 2.0 amps MAX, does the MAX imply that a device rated for
    less would be ok? I take it to mean, "the most you can use this
    transformer with is a device that pulls 2.0 amps" which implys (to me)
    that it CAN be used with lower rated devices. Any thoughts about that?
    Thank you!
  4. Tam/WB2TT

    Tam/WB2TT Guest

    If it is a regulated 12V supply, there is no reason you couldn't use a 100
    Amp source. If it is unregulated, the 1 A transformer may put out a slightly
    higher voltage when running at 600 ma. The instrruction book on your
    "device"may specify what the maximum input voltage is. I would buy a
    transformer with a regulated output.

    The RS 273-1667 will work, but you will still have to figure out which
    polarity you need. Should be shown on your device input jack.

  5. Jon

    Jon Guest

    In addition to the cautions in the other replies, make sure that the
    polarity of the new wall wart matches the requirement of your device.
  6. Connah

    Connah Guest

    Thanks all, for your input. Can you tell me if the word "MAX" on my
    12vdc 2.5a transformer (as mentioned above) would imply that it IS
  7. neon


    Oct 21, 2006
    Let me put it to you this way . boulder dam can supply millions of amps and your little tranformer is connected to it trough transission towers. Does it fry when you plug it in no? good then it is safe to put a 50 amp capability to your precious item. final remark put the right voltage and exceed the 500ma. on the other hand do not exceed the 12 volts.
  8. Tam/WB2TT

    Tam/WB2TT Guest

    No. It just means that you can draw up to 2.5 A from it without having the
    thing catch fire.

    Here is what a typical non regulated supplyu might do. I have a 12V, 300ma
    AT&T telephone transformer. At no load it puts out 16V. At 10 ma it puts out
    14V. At 300 ma it puts out 12V.

  9. Mike Monett

    Mike Monett Guest

    Interesting. The internal resistance decreases as the load increases.

    At 10 mA, it's (16 - 14) / 10e-3 = 200 ohms.

    At 300 mA, it's (16 - 12) / 300e-3 = 13.33 ohms.

    You made that up, didn't you:)


    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    SPICE Analysis of Crystal Oscillators:
    Noise-Rejecting Wideband Sampler:
  10. No. It means it will die if you draw more current than 2.5 A
  11. Tam/WB2TT

    Tam/WB2TT Guest

    No, I didn't make it up. I am actually using the thing as a charger for a
    riding lawn mower. The first year I had the mower I destroyed the battery
    because it got left uncharged over the winter. The ATT transformer works
    great, because it will float the battery at about 14V. Regular lead acid
    battery; about the size you would have in a very small car. I don't keep it
    plugged in all the time; mostly when I think about it, which ends up being
    for a day or so every week or two.

    I think the reason the no load voltage is so high is that the diode drops go
    to near 0V

  12. Eeyore

    Eeyore Guest


  13. Eeyore

    Eeyore Guest


    The only caveat is that the 12V rating may be 'nominal' and vary a bit with the
    current drawn but there's precious little you can do about that and these things
    are hardly precicion power supplies in the first place, so get the one the Radio
    Shack guy says will do the job.

  14. Eeyore

    Eeyore Guest

    No such luck I'm afarid. It mean's it'll overheat if you draw more than 2.5A.

  15. Eeyore

    Eeyore Guest

    The dynamic impedance of diodes does indeed vary with current.

  16. Tom Bruhns

    Tom Bruhns Guest

    Mike Monett wrote:
    Gee, that's just what I'd expect. The minimum dynamic resistance
    should be just a bit more than the transformer's secondary DC
    resistance, plus the primary's DC resistance transformed by the square
    of the turns ratio, plus any other resistances in there. But that's
    only at high load, where the diodes are on hard almost all the time.
    Of course, the diode's dynamic resistance is high when the current is

    Interestingly, though, even if the diode switched cleanly between zero
    resistance when forward biased and infinite resistance when reverse
    biased, the output dynamic resistance would vary with load. That's
    because the diodes, when off, disconnect the transformer from the load,
    and the average resistance is inversely proportional to the diode's
    conduction duty cycle. At light loads (assuming a capacitor that holds
    the voltage up), the diodes are off most of the time.

  17. Mike Monett

    Mike Monett Guest

    I can see the internal resistance changing with the diode
    conductance angle, but that amount seems excessive. I tried modeling
    it in SPICE but could not find any combination of leakage
    inductance, source voltage and resistance, and filter cap that
    produced those voltages. Maybe one of the data points is in error.

    Here - you try:)

    Version 4
    SHEET 1 880 680
    WIRE -592 144 -672 144
    WIRE -464 144 -512 144
    WIRE -336 144 -384 144
    WIRE -272 144 -336 144
    WIRE -208 144 -272 144
    WIRE -32 144 -144 144
    WIRE 16 144 -32 144
    WIRE 192 144 16 144
    WIRE -672 176 -672 144
    WIRE 16 176 16 144
    WIRE 192 176 192 144
    WIRE -336 208 -336 144
    WIRE -272 240 -272 144
    WIRE -208 240 -272 240
    WIRE -96 240 -144 240
    WIRE -96 256 -96 240
    WIRE 16 288 16 240
    WIRE 192 288 192 256
    WIRE -672 336 -672 256
    WIRE -336 336 -336 272
    WIRE -336 336 -672 336
    WIRE -272 336 -336 336
    WIRE -208 336 -272 336
    WIRE -32 336 -32 144
    WIRE -32 336 -144 336
    WIRE -272 432 -272 336
    WIRE -208 432 -272 432
    WIRE -96 432 -144 432
    WIRE -96 448 -96 432
    FLAG 16 288 0
    FLAG 192 144 Vout
    FLAG 192 288 0
    FLAG -96 256 0
    FLAG -96 448 0
    SYMBOL cap 0 176 R0
    SYMATTR InstName C1
    SYMATTR Value 10000µ
    SYMBOL voltage -672 160 R0
    WINDOW 123 24 134 Left 0
    WINDOW 3 9 108 Left 0
    SYMATTR InstName V1
    SYMATTR Value SINE(0 17 60)
    SYMBOL diode -208 128 M90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName D1
    SYMATTR Value MURS120
    SYMBOL CURRENT 192 176 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName I1
    SYMATTR Value 300m
    SYMBOL diode -208 320 M90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName D2
    SYMATTR Value MURS120
    SYMBOL diode -144 224 R90
    WINDOW 0 -9 34 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName D3
    SYMATTR Value MURS120
    SYMBOL diode -144 416 R90
    WINDOW 0 -9 34 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName D4
    SYMATTR Value MURS120
    SYMBOL ind -480 160 R270
    WINDOW 0 32 56 VTop 0
    WINDOW 3 5 56 VBottom 0
    SYMATTR InstName L1
    SYMATTR Value 5mh
    SYMBOL cap -352 208 R0
    SYMATTR InstName C2
    SYMATTR Value 2n
    SYMBOL res -496 128 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R1
    SYMATTR Value 2.2
    TEXT -320 24 Left 0 ;'Full Wave Bridge Rectifier
    TEXT -288 64 Left 0 !.tran 0 20 0 200u


    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    SPICE Analysis of Crystal Oscillators:
    Noise-Rejecting Wideband Sampler:
  18. Tam/WB2TT

    Tam/WB2TT Guest

    OK, there is more to it than that. It has a REAL US made ATT transformer. I
    believe these things had to withstand operating into a short circuit without
    self destructing. Reluctance protected?

    Also, had measured a Nokia 3.6V cellphone charger. That put out around 7V at
    no load.

  19. Mike Monett

    Mike Monett Guest

    I think that would be leakage inductance. It doesn't seem to help.

    The problem appears to be the 14 volt, 10mA data point. It's easy to get
    16V with 0.16uA drain (a 10 meg dvm), and 12V at 300mA. But this gives an
    output voltage of about 15.37V at 10mA. This is an internal resistance of
    (16 - 15.37) / 10e-3 = 63 ohms, which seems much more realistic.

    I tried adding an internal bleed resistor, but that didn't help.

    It would be nice if you were drawing a lot more current at 14V.


    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    SPICE Analysis of Crystal Oscillators:
    Noise-Rejecting Wideband Sampler:
  20. Tom Bruhns

    Tom Bruhns Guest


    I agree that for a common wall-wart type supply, the 10mA voltage
    sounds too low, given the open-circuit and 300mA load output voltages.
    16V OC, ~15.5V @ 10mA, and 12V @ 300mA seems more like what I'd expect.
    I can think of a couple possible ways that you could see such
    voltages, but they aren't all that plausible, given that it's a simple
    wall-wart. One way is that the supply uses a choke input filter. Then
    you get an open-circuit output voltage nearly as high as the sine input
    peak voltage, and the output at full load is considerably less: diode
    drops and I*R drops lower than the average of the absolute value of a
    sine, which is 2/pi times the peak voltage, or sqrt(8)/pi = 0.9*Vrms.
    The output voltage at 1/30 full load could well be mid-way between the
    full load and open circuit values in that case. A second way is to put
    an NTC thermistor in the output path: it's a moderately high
    resistance at 10mA, but at 300mA it heats up and drops to a low
    resistance. -- Hey, I did say it's not very plausible, didn't I??
    ;-) Another thing to consider: the line voltage may not be very
    sinusoidal. I've seen some pretty ugly ones, but probably not ugly
    enough to give results like that.

    So--inquiring minds would like to know: just what's inside that
    particular wall-wart, to give results like that?

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