Basic question about power supply

[Lewy]

Hi,
Sorry if this is a stupid question but electronics is not exactly my strong point!
I have a Netgear router with a power supply with the following specs:
Input: AC 100/120V 50/60Hz 0.5A
Output: DC 12.0V 1.5A
I need a 220V power supply and want to use the one that came with my Vonage ATA, the Dlink VDV21-VD. The specs are as follows:
Input: 100-240V 50/60Hz 0.8A
Output: +12V 1.5A
My question is the 0.8A input as opposed to 0.5A a problem?
Also the model number of the Dlink is DSA-18W-12. I assume that means 18 watts. If so is that a problem.
Thanks
P.S. I would appreciate an explanation of the answer so that I could learn a little about this topic.

[Simon (Webmaster)]

Hi,

The replacement power supply you have should be fine, but you need to check the polarity of the output connector, and if it is DC or AC. The input current is of no concern, in the case of the 'new' power supply it will vary depending on the input voltage anyway. It is the output voltage and current that you need to be concerned with, as this is what is powering your equipment (router in this case), and an incorrect voltage could damage it. In general if choosing a replacement power supply:

• The output voltage should be the same as that of the old power supply
• The output type (DC or AC) needs to be the same. In this case your old power supply was DC, check that the new one is too. It may be marked with a symbol of a straight line and a dashed line in parallel (as opposed to a sine wave symbol for AC).
• The output current should be at least as high as the old power supply. This rating is the maximum current the power supply can deliver (the actual current drawn will depend on the equipment it is powering), so needs to be at least as high as the maximum rating of the equipment so as not to overload it. However some power supplies are unregulated, meaning that the output voltage may rise about it's rated output if a low current is drawn. For that reason it is best not to use a power supply that is massively overrated (e.g a 1.5A supply for equipment that only draws 100mA).
• The polarity of the output connector needs to be the same. Reversing the polarity of the supply to the equipment could damage it. Most power supplies connect to the equipment they are powering using a barrel connector (see http://www.electronics2000.co.uk/pin-out/dcpower.php). Normally the centre 'hole' is positive and the outer barrel is negative, but you need to check. Often the equipment and power supply will have a symbol of a partial circle with another circle or a dot inside it. The outer circle represents the barrel and the inner circle the 'hole' each will be marked + or -. Compare the markings on the old and new power supplies, or the new supply and the equipment to check they match.

[Lewy]

Thanks for such a clear answer. I certainly had no idea about the polarity, although I've always seen that + and - symbol I never gave it to much thought. It happens to be that they both are the same, and like you wrote the barrel is negative and the hole positive. Also, the output on both is DC and 1.5A so all seems OK. Thanks again.