AC-DC adapters and how they function

[Sukhoi]

I have here a AC-DC converter. It takes a 120 VAC 60 Hz signal and converts it to 12 VDC. Does it actually output a solid 12 VCD? Or does it take the 120 VAC sine wave, make the negative parts positive, and squish it down to 12 V amplitude?

The reason I ask this is I need to use the adapter in Europe. To get my 120 VAC I will be using a converter on the 240 VAC over there. As far as I know this converter chops the 240 VAC sine wave off, rather than shrinking it down like a transformer would. Will this cause any difficulty for the AC-DC adapter?

Also will the 50 Hz vs 60 Hz be much of an issue? The AC-DC adapter is low power usage so I don't expect an overheating problem.

 

[TuxDave]

Originally posted by: Sukhoi
I have here a AC-DC converter. It takes a 120 VAC 60 Hz signal and converts it to 12 VDC. Does it actually output a solid 12 VCD? Or does it take the 120 VAC sine wave, make the negative parts positive, and squish it down to 12 V amplitude?

The reason I ask this is I need to use the adapter in Europe. To get my 120 VAC I will be using a converter on the 240 VAC over there. As far as I know this converter chops the 240 VAC sine wave off, rather than shrinking it down like a transformer would. Will this cause any difficulty for the AC-DC adapter?

Also will the 50 Hz vs 60 Hz be much of an issue? The AC-DC adapter is low power usage so I don't expect an overheating problem.

It tries to make it a solid 12V DC source. Textbook examples of AC-DC converters usually aren't affected by a 10Hz signal. As for operating on a clipped sine wave, I'm not really sure what the results would be.

 

[Colt45]

a transformer shrinks the voltage to ~10...11...12v, then it goes through a rectifier circuit that flips the negative hald-cycle postive, and the filters it, usually just capactive filtering on wallwarts.

unless its a SMPS like a computer power supply, thats quite a bit more complicated.



I have no idea what your 240:120 converter is, the way you describe it doesnt make sense to me.

 

[Mark R]

Also will the 50 Hz vs 60 Hz be much of an issue? The AC-DC adapter is low power usage so I don't expect an overheating problem.

Generally not. It should work fine. They work by using a transformer to reduce the voltage (e.g. to 9V AC), then rectifies it (full wave - the negative half is flipped up to join the positive half), then filters it (with smoothing capactiros). None of these are stages are severely affected by a change from 60 to 50 Hz.

I'm not quite sure what you mean about your 240-120 V converter. Usually these are just transformers.

 

[Sukhoi]

Here is how I understand the converter works. Imagine a 240 VAC sine wave. There are two ways to get this to 120 VAC. First way (and best way) would be to simply shrink the amplitude of the wave to 120 VAC. This is what a transformer does. The second way is to cut the 240 VAC sine wave off halfway up the amplitude. So rather than a wave like this /\/\/\/\, you'd get \_/, and so on. I don't know anything about the physics of that, but it's supposedly how a converter works.

Depending how the rectifier and filter work, the chopped sine wave might actually work better for the AC-DC adapter. The chopped wave is going to have a lot more of the wave at 12 VDC before it goes through the filter, so the filter is going to have less work to do to smooth it all out.

 

[raynman68]

Mark R is correct in regards to the 240-120 converter, they are usually just transformers with maybe some type of fuse or circuit breaker built in.

Mark is also correct in how the converter works. In a basic AC/DC converter, a transformer is normally used to step down the voltage. For example, using a 120VAC peak to peak input, a step down transformer would take it down to say 30VAC pk-pk. Next, the 30VAC is passed through a full wave rectifier which in simplest terms, flips the negative portion of the AC voltage positive creating a bumpy +15 pulsed DC voltage which is how the roads in Oklahoma are. At this point a filter capacitor would smooth off the peaks to create your smooth 12VDC signal. I suppose you could just skip the tranformer and go straight to a rectifier but then you would have a 120VDC voltage which would still have to be stepped down using some form of voltage regulator using many more components at a higher cost and has a greater chance of failure than the relatively reliable and simple circuit I described.

As for your frequency concerns, there should be no problems going from 60Hz to 50Hz.

Here is a basic example I found on the web, of how a rectifier circuit works prior to being filtered by a capacitor.

 

[Mark R]

For example, using a 120VAC peak to peak input, a step down transformer would take it down to say 30VAC pk-pk. Next, the 30VAC is passed through a full wave rectifier which in simplest terms, flips the negative portion of the AC voltage positive creating a bumpy +15 pulsed DC voltage which is how the roads in Oklahoma are. At this point a filter capacitor would smooth off the peaks to create your smooth 12VDC signal.

Some dodgy numbers there, I'm afraid.

Mains voltage in the US is 120 VAC rms (340 VAC peak-to-peak). In a 12V DC adapter, this 120 VAC is stepped down with a transformer to 9VAC rms (25 Vp-p) this is then rectified to 8 VDC average (12.5V peak), which is then filtered to about 12 VDC. (The filter is a reservoir that charges when the input voltage reaches a peak. When the input voltage is below the voltage in the reservoir, no power flows into the reservoir, however, power flows out from the reservoir into the load).


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I've thought a bit more about electronic voltage converters. I've never come across one myself - only ever seen the transformer ones. However, thinking about it some more, most devices used by travellers fall into 2 groups - low power electronics (mobile phones, battery chargers, etc) and high power heaters (kettle, hair dryer).

There is potentially a very simple way to convert 240V into a form suitable for heaters. (Simply 'cutting-off the top' of the peaks is actually a difficult thing to do - so I can't believe that this is what happens.)
Ohm's law states that V=IR, and therefore Power = (V ^ 2) * (1 / R). Because heaters are purely passive devices, you could safely provide double the voltage, for 1/4 the time.

I suspect that cheap high power voltage converters simply do this - they connect the device to 240V for 0.01 s, then switch it off for 0.03 s, on for 0.01 s, off 0.03 s, and so on. The circuit to do this is trivially simple, and dirt cheap.

This is fine for heaters - but for electronics this would be a disaster - because the full 240 V will be supplied to the device. Even though the 240 V will only be present for a short time, that's all that is required to toast the various components.

 

[Sukhoi]

Interesting stuff there guys, thanks for all the help. My 240->120 converter does have a switch for 50W and 600W (or something like that) usage, so it might be doing what you say Mark R. It would probably be much more expensive to build a transformer that can handle 600W compared to one that does 50W.

 

[Calin]

I don't think a 240 to 120 V voltage converter works by cutting the power amplitude at half - while this would reduce the voltage on the output at bearable levels, the power received would be alot more than what a proper 120V current (well, voltage) will give you. While the proper 120V AC sine wave will give you as much power as a 120V DC current (but AC will have voltage peaks at 120V * sqrt(2) ), the voltage with the tops cut down will give you about twice as much power. Not a good idea if you want to use a lightbulb.

 

[Megamixman]

What about the new solid state Adapters. I have an adaptor that is about as thick as a CD case. There is almost no way a transformer could be put in that.

Active PFC is the interesting thing though. If you look at your power supply, it will either be Active or Passive PFC. How can you tell? If you have to change a switch it is passive PFC. Active PFC will automatically detect 240V and convert it to 120V before sending it to the transformer. How? Active PFC means that the PSU has a circuit between the transformer and Plug. This circuit is meant to keep the mains clean by acting as a resistive load and then feeding that power to the transformer. Because you have a circuit between the Transformer and the plug, it is easy to make detect the voltage and send the power through a bypass transformer when the voltage exceeds a certain amount. So if it is as 120V the PFC will send the power straight to the transformer, but if it detects 240V, it will send the power through a bypass inductor to halve the voltage and then it goes the transformer.

 

[AbsolutDealage]

Originally posted by: Megamixman
What about the new solid state Adapters. I have an adaptor that is about as thick as a CD case. There is almost no way a transformer could be put in that.

Active PFC is the interesting thing though. If you look at your power supply, it will either be Active or Passive PFC. How can you tell? If you have to change a switch it is passive PFC. Active PFC will automatically detect 240V and convert it to 120V before sending it to the transformer. How? Active PFC means that the PSU has a circuit between the transformer and Plug. This circuit is meant to keep the mains clean by acting as a resistive load and then feeding that power to the transformer. Because you have a circuit between the Transformer and the plug, it is easy to make detect the voltage and send the power through a bypass transformer when the voltage exceeds a certain amount. So if it is as 120V the PFC will send the power straight to the transformer, but if it detects 240V, it will send the power through a bypass inductor to halve the voltage and then it goes the transformer.

Actually, this is not the case. Active/Passive PFC on a power supply refers to the power factor correction used on the supply in order to increase efficiency.

Power factor (the PF part of PFC) is the ratio of the real power to apparent power. Apparent power is the "traditional" measure of power, being the product of the voltage and current rms values. Real power is the product of the voltage and current magnitudes multiplied by the angle between them. Take the real power over the apparent power, and you will end up with a number that is less than one for any load which is not completely resistive (from the perspective of the supply).

The inherently inductive load attached to the power supply causes the voltage and current waveforms to become out of sync. This causes angle measurement used in the "real power" calculation to be non-zero, resulting in a power factor that is less than 1.

When the waves are out of sync, the supply is working harder than it has to... it is consuming more power than it is actually supplying to the load. Adding a PFC unit to the supply simply attempts to add the correct the load such that the waveforms are again in sync.

A passive PFC adds a fixed capacitance that is a "best guess" solution. Active PFC involves a variable capacitance being added to the load depending on the charactaristics of that load.

 

[Mark R]

What about the new solid state Adapters. I have an adaptor that is about as thick as a CD case. There is almost no way a transformer could be put in that.

They use switched-mode PSUs - the same as your PC PSU. By operating at very high frequency, the transformer can be much smaller, cheaper, lighter and more efficient. Essentially, high voltage AC is rectified to high voltage DC which is then electronically switched on and off, to produce a very high frequency (usually around 100,000 Hz) pseudo-AC. This AC goes into a transformer, where it is stepped down. Then it is rectified and smoothed.

Because you have a circuit between the Transformer and the plug, it is easy to make detect the voltage and send the power through a bypass transformer when the voltage exceeds a certain amount. So if it is as 120V the PFC will send the power straight to the transformer, but if it detects 240V, it will send the power through a bypass inductor to halve the voltage and then it goes the transformer.

Nope. That's not at all how active PFC works.

In a conventional PC PSU, the mains voltage is rectified with a set of diodes which feed into a bank of smoothing capacitors. When the switch in in the 240V position - the diodes are configured as a normal full-wave rectifier. When the switch is set to 120 V the diodes are rearranged into a voltage doubling rectifier circuit. In both cases, the smoothed rectified DC is about 340 V.

Because of the capacitors at the front of the PSU - the PSU has poor power factor (the current is out of sync with the voltage). In a passive PFC PSU, an additional inductor is added to compensate for the capacitance of the PSU. The inductor is selected when the PSU is designed to give a good level of correction under normal operating conditions (but worse when idle or at max load).

The other problem is the interaction of the rectifiers and the caps - together they form a non-linear system. This means the simply adding inductors or capacitors cannot actually get the power factor back to unity.

In an active PFC PSU there is an additional circuit between the diodes (arranged as a normal full-wave bridge rectifier) and the smoothing capacitors. This circuit monitors the current flow and makes sure that the current flow through the rectifiers is in sync with the voltage. The circuit also acts as a step-up voltage regulator which is set to produce about 360-370V. Because it is a regulator, it will automatically detect the input voltage and step it up to an appropriate level. This smoothed, regulated DC then goes into the rest of the PSU. There is a big advantage to active PFC because the PSU gets regulated DC - this makes it easier to design the rest of the PSU, and allows you to use a much more efficient design.

 

[Marcuf]

:hmm: So complication. I didn't know that.This is one case where more isn't always better. Replacing a 5V 100mA adapter with a 5V 2A wall wort may in fact feed more like 8-9V in to your equipment at the lower current draw you actually use. This can easily cause damage if the device isn't designed to account for possible higher input voltages. isit right?

 

[bobdole369]

holy necro thread, and "it depends"

 

[Sukhoi]

I don't even know why I was asking that question.