A mains power adapter question

  Batch 14:17 28 Jan 2012

OK, so not really to do with PCs, but there's a variety of knowledge on this forum.

With AC mains power it is quite normal for the supply current availability to exceed the needs of an appliance (e.g. one plugs an appliance that draws 1A into a 13A socket).

Does the same apply for DC current with a mains power adapter. For example, I have a radio that has built in rechargeable batteries. The supplied power adapter (rated at 9VDC 500mA output) has failed. Does the same concept apply as with AC mains power such that I can safely use a 9VDC 1000mA adapter in its place (such that the radio still just draws the current required) or would the 1000mA adapter overload the radio?

  lotvic 14:42 28 Jan 2012

I think the Adapter does the 'reducing' and feeds the appliance with the output so you need to have an Adapter that will reduce current to match the appliance.

I am not an expert, but I wouldn't risk not using the correct rated adapter.

  Fruit Bat /\0/\ 14:48 28 Jan 2012

Does the same concept apply as with AC mains power such that I can safely use a 9VDC 1000mA adapter in its place

Yes this would be OK even preferable a syou would not be overloading the mains charger.

the radio will only draw the current it needs (the voltage must be correct) ad current limiting devices (if fitted dependant on battery type) will be in the radio's battery pack.

  Batch 14:58 28 Jan 2012

Thanks FB, I guessed that would be the case. Particularly as batteries themselves are a DC source and one could readily run a DC appliance from batteries of whatever capacity (but with correct voltage). Just wanted to be on safe side.

  Bris 14:59 28 Jan 2012

As Fruit Bat states, its the voltage that matters i.e. the voltage of the replacement unit must be the same as original one. The ma rating should be at least the same as the original but can be higher. The higher the rating the less stressed the unit will be so its preferable to replace a 500ma unit with a 1000ma unit as its likely to last a lot longer.

  Muergo 15:00 28 Jan 2012

The rating on a power adapter/converter is the maximum safe amount of current that it can provide, you are quite safe using a 1000mA as the radio will only draw what it needs, the adapter will not force any more wattage than 500mA into the unit.

Now if you had a radio needing 1000mA and tried to supply it with a 500mA adapter then the adapter would overheat and likely blow a fuse if there is one.

Note that adapters all heat up as a normal course of events so they should never be covered up, a free flow of air must be provided.

Also note that so called DC adapters are usually half wave rectified (for cheapness) and the output is pulsating DC signified by a logo having a straight solid line with a dotted line underneath.

To charge up batteries the output must be a slightly higher voltage than the batteries or current won't flow, thus if you are charging your radio with an adapter not provided by the radio maker you may find a general transformer needs to be set higher than the nominal voltage, i.e. a 12volt battery needs about 14 volts to charge it. Rechargeable cells like AA or AAA type are 1.2volts and non rechargeable lithium cells are 1.5 volts minimum so 4 non-rechargeables = 6 volts and 5 rechargeables -6volts

  Batch 12:02 29 Jan 2012

As it happens, the battery pack in the radio is a pre-packed 3 x AA NiMH pack, so is 3.6V. But the adapter that came with the radio was 9V and the radio info. label says 9V, so although 9V seems high for the battery, I guess either the battery can cope with 9V input or there is circuitry in the radio to further step the voltage down.

The info on the radio label shows:                ___ DC 9V    - - -   500mA

So I guess is half-wave rectified

  Batch 12:04 29 Jan 2012

That didn't come out as planned. Was trying to get a straight solid line with a dotted line underneath. I'll try again:

 - - -

  Batch 12:05 29 Jan 2012

I give up - the box that is supposed to show how what you type will be actually represented once posted clearly doesn't tell the truth.

  robin_x 12:56 29 Jan 2012

As far as I know solid/dashed DC symbol is not an indication of whether the AC is half or full wave rectified. Just means DC where one terminal is always positive and the other negative.

May be regulated, smoothed, accurate and stable. Or it may be just half a sine wave.

Now that would be a really crappy, cheap PSU and further components would be required in most electronic equipment.

Another important thing to remember is getting the polarity correct. Unless the equipment is protected it will be fried if you connect +/- the wrong way round.

  Batch 13:55 29 Jan 2012

Given that the input voltage on the original adapter was specified as 100-240VAC, I assume it was regulated (surely any adapter that can be used on worldwide voltages is regulated?).

The replacement one I have is similarly labelled (and so again assume to be regulated).

Further research has uncovered that an unregulated adapter would deliver materially higher voltage under little / no load. So an unregulated adapter that has a higher current capacity (e.g. 1000mA in my case v. the original 500mA adapter) would be more prone to delivering a higher voltage to an appliance that was designed to work with a lower current (mA) adapter.

Having checked with a meter, in the case of my 1000mA adapter, the unloaded voltage is 9.23V (i.e. virtually its rated voltage under no load). Which reinforces that it is regulated.

Polarity was first port of call, so no probs there

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