Using a higher rated DC transformer

[Connah]

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
work.

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!

Matthew

 

[GregS]

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
work.

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!

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.

greg

 

[Connah]

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:

DC OUTPUT:
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!

 

[Tam/WB2TT]

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
work.

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!

Matthew

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.

Tam

 

[Jon]

Good morning, all.

I have an electronic device that requires a DC Wall transformer rated
for 12VDC at 600 mA.

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.

 

[Connah]

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
regulated?

 

[neon]

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.

 

[Tam/WB2TT]

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
regulated?

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.

Tam

 

[Mike Monett]

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.

Tam

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

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:
http://silversol.freewebpage.org/index.htm
SPICE Analysis of Crystal Oscillators:
http://silversol.freewebpage.org/spice/xtal/clapp.htm
Noise-Rejecting Wideband Sampler:
http://www3.sympatico.ca/add.automation/sampler/intro.htm

 

[Homer J Simpson]

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
regulated?

No. It means it will die if you draw more current than 2.5 A

 

[Tam/WB2TT]

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

Regards,

Mike Monett

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

Tam

 

[Eeyore]

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
work.

Is this true?

Yes.

Graham

 

[Eeyore]

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

DC OUTPUT:
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"
Yes.

which implys (to me) that it CAN be used with lower rated devices. Any
thoughts about that?

Yes.

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.

Graham

 

[Eeyore]

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
regulated?

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

Graham

 

[Eeyore]

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

The dynamic impedance of diodes does indeed vary with current.

Graham

 

[Tom Bruhns]

Mike Monett wrote:
....

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

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
low.

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.

Cheers,
Tom

 

[Mike Monett]

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 low.

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.

Cheers,
Tom

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

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:
http://silversol.freewebpage.org/index.htm
SPICE Analysis of Crystal Oscillators:
http://silversol.freewebpage.org/spice/xtal/clapp.htm
Noise-Rejecting Wideband Sampler:
http://www3.sympatico.ca/add.automation/sampler/intro.htm

 

[Tam/WB2TT]

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.

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.

Tam

 

[Mike Monett]

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?

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.

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:
http://silversol.freewebpage.org/index.htm
SPICE Analysis of Crystal Oscillators:
http://silversol.freewebpage.org/spice/xtal/clapp.htm
Noise-Rejecting Wideband Sampler:
http://www3.sympatico.ca/add.automation/sampler/intro.htm

 

[Tom Bruhns]

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

....

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?

Cheers,
Tom