Wall warts

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5440
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Wall warts

Post by 5440 » Sun Jun 08, 2003 12:12 am

Would it be possible to use a class transformer, wall adapter with a 240 V AC input even though it is rated for 120 V? I need a cheap, quick source of about 24 V DC, 100 mA.

bodgy
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Re: Wall warts

Post by bodgy » Sun Jun 08, 2003 1:42 am

Puff of blue smoke time with additional bang!<p>I can't quite make out what you mean. Am I right in thinking you are saying that you have a 120V AC tranformer that has a 24V AC secondary, and you want to connect it to 240V AC?<p>If however you are lucky that there are two primary windings (4 pins) then the transformer is probably a 0-120 0-120 wound so connecting the two primaries in series will be fine. Only one winding, back to my first comment.<p>Colin
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russlk
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Re: Wall warts

Post by russlk » Sun Jun 08, 2003 6:32 am

You can operate a transformer at lower voltage than it's rating, but higher voltage will cause it to overheat. The cheaper the transformer, the less overvoltage it will tolerate. A wall-wart is the cheapest, so will probably not work very long at double the voltage. BTW, for 240 to 24 volts the transformer will provide 12 volts at 120 volts input. Many new wall-warts are switching supplies which may or may not tolerate double the input but will not double the output.

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Re: Wall warts

Post by analogee » Sun Jun 08, 2003 12:49 pm

I have no direct experience with Russ's comment that operating at a higher voltage will make it overheat, so I'm curious as to the reason.<p>I'm thinking that (in all of this, I'm assuming no load on the secondary; so we're talking about losses, not power delivered to a load), leakage inductance is independent of the input voltage, so essentially the thing will draw twice as much current with a 240 V input as it would with a 120 V input. Using I^2*R, that implies it would dissipate 4 times the power.<p>Anybody else confirm this?<p>Regards,
Todd
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Chris Smith
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Re: Wall warts

Post by Chris Smith » Sun Jun 08, 2003 1:22 pm

Impedance is everything. It’s a working factor of design, saturation, winding length, loss, and magnetic coupling to mention just a few. Much like a resistor, you cant substitute a value by half and expect it to work.

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Re: Wall warts

Post by Dimbulb » Sun Jun 08, 2003 3:58 pm

I have wondered about this as some warts can select 120 or 220. In the states we are mostly familiar with 120 and 220 is usually a big kitchen appliance or clothes dryer or welder on a heavy duty circuit.<p>So giving advise in general would need to include the rating, how well it was made possibly there is a resistor scheme that will make up for a cheap wart. I am guessing it will poof otherwise.

5440
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Re: Wall warts

Post by 5440 » Sun Jun 08, 2003 4:46 pm

What I am trying to do is hook up a watt-hour meter to my clothes dryer. The meter requires 6-50V DC. I pretty much figured that the wall wart will blow. Impedance and possible insulation issues, I quessed. I have millions of the little bastards and thought someone might have tried it. A lot cheaper and quicker than fabbing up a cheap circuit.<p>I have a larger power supply that has a switch for either 120/220 V operation. Most likey a switcher?? I will look into that. I just had another idea. I would now like to add a moisture sensor to the dryer exhaust. This way the dryer will turn off when the clothes are dry even though the timer is still on. Like the new dryers work. Anyone know of a source for affordable moisure sensors that will take the heat? Perferably for a dryer applications. I could slap an 8 pin PIC with the ADC option to monitor and control.<p>Jason

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Chris Smith
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Re: Wall warts

Post by Chris Smith » Sun Jun 08, 2003 4:59 pm

220 volts is the standard in England and Australia, and thats why the manufactures make them with a switch. If you need to run one off 220, just parallel two transformers together in a series wiring configuration, and you don’t even have to use the second transformer. It will act like a resistor, [imped] dropping the voltage down, but not use up a whole lot of current at idle because of no usage or load.

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Re: Wall warts

Post by josmith » Sun Jun 08, 2003 5:59 pm

YOu could put two similar 120 v transformers in series and run them on 220 as long as the secondaries were in series as well. Two 120v warts with 12v outputs would do it.

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Re: Wall warts

Post by Calcbert » Sun Jun 08, 2003 10:35 pm

From my informal understanding, dryers don't so much detect a drop in the moisture of the exhaust as they detect a sharp rise in the air temperature, indicating that the majority of the water has been taken from the clothes. This would also seem to overcome the possibility that the ambient humidity would confuse the sensor. This means you're probably going to be looking for a thermistor or thermocouple instead.

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Re: Wall warts

Post by rshayes » Sun Jun 08, 2003 11:42 pm

The type of transformer that you are looking for is often called a control transformer. These are used to power control circuits in electronic equipment and are available with primary voltage ratings of 110, 208, 220, and 440 volts. The secondary is usually either 12 or 24 volts, with 24 volts being the most common. These are available from Allied or Newark, or possibly Mouser. They are also available from local electrical distributors. The cost is probably in the $10 range.<p>The control circuits in the dryer are probably powered by a control transformer. Running the control circuits on a transformer isolated supply is much simpler than providing the insulation required for operating curcuits on 220 volts.<p>The standard residential power circuit in the United States is 110 volts at 20 amps. This is what the three prong wall outlet will provide. If more power is required, for appliences such as an electric range, clothes dryer, or air conditioner, a 220 volt circuit will be used. The 220 volt circuit is usually also capable of much higher currents as well, such as 25, 50 or 100 amps. The result of the higher circuit capacity is that mistakes are much more exciting and cause much more damage before the fuse or circuit breaker opens. Wall warts are not designed for use on this type of circuit.

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