Nokia Adapter AC input and DC output

The 150mA is therefore a nominal value, applicable to 100V. It's probably a "buck" power supply inside it (see Wikipedia) to be flexible, so as the supply voltage goes up, the current drawn from the supply will go down. At constant supply voltage, the supply current will change, anyway, as the battery voltage and charge level increases.

Energy conversion always causes losses, reference the Laws of Thermodynamics, though it is small enough not to cause any great worry, perhaps?

First, you're doing the calculations correctly. The numbers provided do imply a very inefficient power supply. The trouble is the imprecise nature of the numbers you've been provided. I suspect that the 150mA input current number is actually the in rush current value of the supply and not the input current drawn while powering the maximum DC load of 4W. I would expect that there is also some harmonic (non-linear) loading factors along with some current phase complications that will make this appear to be a less efficient supply than reality.

Hai

1) Now a days step down transformer not using in cell charger.

2)In AC power calculation power factor need to be considered.

3)Check the in put current once again,It may considered at 100V.

4) Comparatively lower rating AC to DC power convertin has low efficiency.

Hi Jay, I thought the power supplies charging cell phones are switch mode power supplies, this would explain the AC current. Transformers aren't designed to operate over that supply range, they are designed to operate at a specific voltage, if you have a transformer that is designed to operate at 110vac and you connect it to 230vac it will burn out.

The adaptor that you have is a switch mode power supply, that has been designed to operate over a specified voltage range.

If you hold the charger in your hand, you will find that it doesn't have the weight that a transformer of the size required would be, the reason that switch mode power supplies are used are simple; less copper, ability to be assembled by pick and place robots, and are cheaper than traditional transformer regulated power supplies.

That is all that i can add to this discussion

Best Regards

Joe

Well put also but I feel I should add my own caveat that you imply but do not state. With the higher switching frequency of a SMPS it will be easier in many applications to reduce the interference noise fabricated by the supply. However, due to the higher frequencies and harmonics of the square wave like switch there can be easier interfering propagation out of the supply. Also many people forget that the power supply rejection specification of most op-amps roll off from the 60 Hz region so it takes less supply ripple to make an output ripple at 100kHz or higher.

Hey Redfred,

"Also many people forget that the power supply rejection specification of most op-amps roll off from the 60 Hz region so it takes less supply ripple to make an output ripple at 100kHz or higher."

I didn't understand this, if you could explain again in more simpler terms? I had trouble understanding "power supply rejection specification", "roll off", "supply ripple" and "output ripple".

The easiest way for you to understand this is to explain how these measurements are done. First one takes an op-amp circuit with a very high gain design that is powered by a perfectly clean ±12V supply. For the sake of this discussion lets say this circuit has a voltage gain of 1000 for all frequencies, or 60db. If we connect the input of this amplifier to ground we end up with 0*1000=0 for an output voltage. There will be other offset parameters of an op-amp that will not give you exactly 0 for an output but again for the sake of this explanation lets just say that we get 0 output. If you add a 1V 60 Hz oscillation (ripple) to the DC power supply while maintaining your 0 input signal and measure a 10mV 60 Hz output then you have the gain of your circuit (in db) plus the db value attenuation from the supply ripple to output level [ 20*log(1V/0.01V)=40db) so this op-amp has 100 db of power supply ripple @ 60 Hz. This will be an op-amp with a great power supply rejection ratio. Now the point I was making is that this power supply rejection ration is not uniform for all frequencies and few chip manufacturers publish supply rejection values for anything but below 200 HZ values. A rule of thumb is to accept that at 1kHz is 3db less supply rejection (97db)and rolls off now at 20 db per decade so that now at 100kHz of SMPS ripple this op-amp has only 57 db of supply rejection but a gain of 60 db so in this case one will have 3 db of gain from the 100kHz supply ripple to output signal.

I hope that I did not lose you in that explanation.

Hi Andy, yes smps are comlacated little beasts, but I was not saying that smps were simple, What I meant was simply, using smps require little human interaction in the manufacture process once it has been setup, robots do most of the work, I think you know what I mean.

The other advantage is that they can be designed to operate over a wider voltage range and frequency, this making them very universal. For people that travel internationally this can be quite an advantage for them not having to carry additional voltage adaptors.

Cheers

Joe

Correct. Some very valid points!

OK.

It SMPS are more efficient and work on a wide input voltage; normally from 90~250VAC; more constant supply.

Additional you need not to by a 220V adopter if ever you go to or if you are in 220V area and go to 110V one.

Thaks Joe =)

"if you have a transformer that is designed to operate at 110vac and you connect it to 230vac it will burn out"

But connecting a 110 V ac to a tranformer designed to operate at 240 V ac wouldn't burn it..or would it?

Hi Jay, connecting 110vac to a 230vac transformer would not burn out but it would output a lesser voltage, lets say the output from the 240v transformer is 12v when operated on the 240v supply; now if you operate the same transformer at 110v your output = 12 ÷ 240 x 110 = 5.5v. I am not sure but the VA ratting may also be reduced when operating at a lower voltage.

Cheers

Joe

Even if 36 W input with 4 W output is the worst case of the adapter (efficiency of 11%), I don't know if it has a very good efficiency even at the best. Where does the excess energy get spent? Does the remaining go as heat?

After all, energy is never lost (or gained), right?

Jay, with a transformer, you have 2 types of losses. 1. Iron losses this loss is constant, this is the energy required to saturate the transformer. 2. I²R losses this loss is dependant on the current drawn. Both of these losses are reflected as heat.

Energy is never lost or gained, it is simply transfered to a different form; IE heat, light,sound to name a few.

Cheers

Joe

Yes, this is the worst case scenario but you're missing the point that 36W is just the initial surge power number, not the operating power number. For an imprecise analogy, think of your car's fuel economy when you start the car on a cold day. Before everything comes up to temperature the throttle stays open a little and most of the fuel energy goes only to getting internal parts moving and heating the parts up. Before you leave your drive way your getting 0 MPG during this time.

Quick answer here: if the input was really 36W into the power supply unit (PSU), and the output load only drew 4W, then you've got it right - the "disappeared" 32W would heat the power supply.

The temperature the PSU box reached would depend on the thermal characteristics (°/W) of the enclosure. For an idea, think of a 30W car headlamp bulb sealed in a little black box - or (not too far off) think of grabbing hold of a mains 40W incandescent light bulb that's been on for a while.

If its a reasonably designed SMPS, its very efficient, so it simply only takes a tiny bit more energy than its giving out. This still causes slight warming, but only slight.

SMPSs can be better than 90% efficient!!!

Most Linear supplies simply waste any energy not required as heat and need cooling.....usually a "hot" regulator chip, that needs a heatsink or similar.....

Jay, going back to your OP you said that the mains current draw was 150ma, this would be correct if the adaptor was in-fact a switch mode power supply, the reason I say this is that switch mode power supply's do not draw current over the entire sine wave, they just take little bites at the sine wave. So at a moment in time when it does draw power it would be drawing 150ma, the power is converted into dc and stored in capacitors for later use.

I would doubt that a transformer would be the inefficient, so it must be a switch mode power supply.

Note; switch mode power supplies when used in high numbers of a power supply can cause harmonics, this problem can cause several problems in buildings that house lots of personal computers, because each computer has a switch mode power supply, power utility companies don't like them either because of the way that they chop up the sine wave it means sometimes bigger transformers and cables are required to supply these buildings.

Cheers

Joe

I don't think that your worries are still accurate today, at least not in quality made SM Power supplies.....

I have many different units in our house/workshop with SMPSs (I like to NOT buy linear supplies for anything!), no problems as you describe....though I am sure the possibility exists with less than good quality units......has anyone here had any experiences as Joe Sparky describes? At Wiki there is a mention of such possible problems from cheap or badly designed units at:-

http://en.wikipedia.org/wiki/Switched-mode_power_supply

I personally have not even seen any interference on any of our TVs or on our radios, though we mostly hear FM which might reject interference better, I must try with a medium wave transmitter. All of which are usually susceptible to foreign frequencies.....But simply no problems even when driving stepper motors at various frequencies on SMPS supplies...maybe I am just lucky!!

Thanks guys..I think your replies have cleared some of my doubts. Just to recap:

1. The power I calculated (and the V and I mentioned) are not operating values, so the efficiency is not as low.

2. The losses are given of in form of heat - the iron losses and I2R losses.

3. The adapter makes use of SMPS instead of transformers. I will learn something about SMPS from google or other sources and get back if I have anything questions.

Thanks.

There is quite a good explanation here:-

http://en.wikipedia.org/wiki/Switched-mode_power_supply

I did not see any glaring errors.....