If you have an ac adapter for a device that uses 30 watts can you use it on a device that uses 18 watts?

Don't count on it. Your 18 watt device should be matched with the proper AC adapter. You cannot rely on the device to resist and limit a supply voltage more than 50% beyond its rated capacity.

You wouldn't try to run a 110 volt electric shaver from a 220 volt receptacle, would you? If you try to purchase a fixed or universal (adjustable voltage) adapter you must also be concerned to match the polarity where the two meet. Some electronic devices are sensitive to mismatched polarity and will fail.

Your best bet is to contact the manufacturer of the device to purchase a replacement ac adapter Answer outside of improper polarity (assuming dc here, ac doesn't matter), if the VOLTAGE OUTPUT of the adaptor matches the INPUT VOLTAGE of the device it is powering, all is ok. As long as the current (milliampre rating) of the adaptor is equal to or exceeds the current rating of the device, all is ok. Wattage is equal to voltage X amperes ... hence one cannot use the wattage rating of an ac adaptor to determine if it is is correct to use on a given device Answer Perhaps.

If the two devices accept the same range of voltages, the polarity is correct (although AC circuits do not care about polarity, reversed polarity is a serious safety hazard), and the power adapter to be used can provide at least the amount of current required by the device in question, then yes If the adapter is not rated to sufficient current, the device may or may not function -- the current will still be provided, but the supply voltage will drop. The power supply will generate additional heat A voltage mismatch may destroy the device Mismatched polarity for a device requiring DC power will not work and may damage the device. Mismatched polarity for a device requiring AC power will work, but may present a shock hazard, as many cases are connected to the neutral wire.

Reversing the polarity will cause the case to be wired to 120V.

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