I don't understand power transformers. Why is converting 220v to 120v so difficult?

fuzzybabybunny

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I have a slim power adapter for a laptop.

It takes in a range of 110-240V AC and converts it to 12V DV @ 6A. Doesn't need a fan and it's pretty compact.

I bought a 220/240V AC to 120V AC converter and its much bigger and has a fan constantly spinning even when the unit is only outputting 30W.

Why is this? Is stepping down AC to AC a lot more involved than stepping down a range of AC to DC?
 

gorcorps

aka Brandon
Jul 18, 2004
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Sounds like your DC power supply is a switch mode power supply (pretty much confirmed given the accepted input voltage is a relatively wide range), which uses more electronics to produce a cleaner power delivery, and doesn't have to use the same bulky parts at a linear power supply. Pretty typical for a good laptop power supply, as the sensitive electronics need a nice clean power delivery.

My guess is your AC converter may not be employing switch mode regulation, and uses more traditional transformer circuitry, which means it'll run hotter and be bulkier.
 

fuzzybabybunny

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Sounds like your DC power supply is a switch mode power supply (pretty much confirmed given the accepted input voltage is a relatively wide range), which uses more electronics to produce a cleaner power delivery, and doesn't have to use the same bulky parts at a linear power supply. Pretty typical for a good laptop power supply, as the sensitive electronics need a nice clean power delivery.

My guess is your AC converter may not be employing switch mode regulation, and uses more traditional transformer circuitry, which means it'll run hotter and be bulkier.
Switch mode power supplies are really really common though right?

Almost all DC power supplies are switch mode because almost all are designed to be used anywhere in the world (100-240V). It's actually very very rare to find a DC power supply that only takes, say, 120V.

Unfortunately in my case I have a device that only uses 120V. I'm in a 230V country and I need an expensive and loud step-down AC converter. If the device had just been designed from the get-go to utilize a ubiquitous and dirt-cheap 100-240V power supply this could all have been avoided.
 

Midwayman

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Just guessing that it is a cheap-ass converter. Just a quick google shows tons that are rated way higher and no larger than a typical laptop charger.
 

VashHT

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Like gorcorps said it's basically because AC to AC step down conversion is done with a transformer and they are pretty large. It's not that it's harder to do than a switcher AC/DC supply (it's a lot more simple really circuit-wise), but that one component has to be pretty large. We have a couple of variable step down transformers I've used here, they're huge (like 30-40 lbs), but you can adjust the ratio of the stepdown and they can handle a lot of power.

Pretty sure there isn't such a thing as a switching AC to AC power supply, doing a quick search online I see people saying you could do an AC to DC then back to AC, but that's so much more complex than just using a transformer I'm not sure anyone would make something like that.
 

Red Squirrel

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Creating DC is easy, creating AC is not. A typical AC DC power supply rectifies the AC to DC, and from there PWMs it to lower the voltage and monitors the output and modifies the PWM based on voltage. It's then smoothed out with capacitors. Well there might be transformers in there too, depends on design. But because it's operating at a high frequency, smaller transformers can be used. The core of the transformer does not need to store as much energy per wave compared to low frequency.

To create AC, you use a similar principal but need H bridge and what not to deal with the negative voltages. Now to get around that if you're simply converting an AC voltage to another then you just use a transformer, but they are rather big and bulky because of the low frequency. When you have power going through a coil you are sorta storing that energy in the core but when it saturates it then acts as a short circuit. So you need a core big enough to avoid it saturating by the time that sine wave is over. That core's energy is then transferred to the other coil and that's how you get your reduced voltage as it will have less turns.

It gets more advanced than that but that's basically the jist of it.

Now one way to do a AC-AC converter without a bulky transformer would essentially be to do a SMPS that then powers an inverter of lower voltage. I'm not sure how common that is though, it's more complicated, but could be made much smaller than a transformer.
 

Ichinisan

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Switch mode power supplies are really really common though right?

Almost all DC power supplies are switch mode because almost all are designed to be used anywhere in the world (100-240V). It's actually very very rare to find a DC power supply that only takes, say, 120V.

Unfortunately in my case I have a device that only uses 120V. I'm in a 230V country and I need an expensive and loud step-down AC converter. If the device had just been designed from the get-go to utilize a ubiquitous and dirt-cheap 100-240V power supply this could all have been avoided.
Is the power cord detachable from this device? If so, you just need to read the specs of its PSU and see what it outputs. Then you can probably replace the power supply with a switching power supply that has the same output specs.
 

fuzzybabybunny

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Is the power cord detachable from this device? If so, you just need to read the specs of its PSU and see what it outputs. Then you can probably replace the power supply with a switching power supply that has the same output specs.

Nope. It's just a wire that goes into the unit.

Well, I took apart the unit hoping that it just had an AC to DC transformer inside of it.

It has a water pump, an LED light panel, LCD control panel, and some circuitry, so I thought it would surely just take in 120V AC and then convert it almost immediately to a usable DC current and split the DC current among the components.

Nope.

The wire goes inside the unit and immediately terminates into a transformer that takes 120V AC and outputs TWO 6V AC rails (first pic).

Both 6V AC rails go into another board that runs the microcontroller and LCD panel which then connects to a third board that runs the pump.

Not sure why they would design it like this. Thoughta? Is there such a thing that takes 100-240V AC and outputs 6V AC?
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Red Squirrel

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This unit might be designed to do 240 or 120 but set at the factory depending on a few wire changes. Might actually be the thing of just switching a wire around, check on the PCB where the two 6v rails end to see if there's any signs of unpopulated jumpers or anything and any markings that would indicate 120 or 240 usage. Maybe you can actually run it on 120 but have to put the two 6v rails in series or something. Just a wild guess though. Do they get split up at all throughout the circuit or just get combined? Could also be that the device needs a negative voltage (for LCD maybe?) so it would be using multiple taps for that purpose as it can set ground on the centre part then you get 6v and -6v.
 

Mayne

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Mayne

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fuzzybabybunny

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This unit might be designed to do 240 or 120 but set at the factory depending on a few wire changes. Might actually be the thing of just switching a wire around, check on the PCB where the two 6v rails end to see if there's any signs of unpopulated jumpers or anything and any markings that would indicate 120 or 240 usage. Maybe you can actually run it on 120 but have to put the two 6v rails in series or something. Just a wild guess though. Do they get split up at all throughout the circuit or just get combined? Could also be that the device needs a negative voltage (for LCD maybe?) so it would be using multiple taps for that purpose as it can set ground on the centre part then you get 6v and -6v.
No, there doesn't seem to be any jumpers or things I can easily set.

One of the boards has this printed on it:

240V (empty checkbox)
120V (checkbox that has been checked by a marker)

So it looks like they make two versions of the board, one that's 240V and another that's 120V but are otherwise not easily visually distinguishable.
 

Red Squirrel

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No, there doesn't seem to be any jumpers or things I can easily set.

One of the boards has this printed on it:

240V (empty checkbox)
120V (checkbox that has been checked by a marker)

So it looks like they make two versions of the board, one that's 240V and another that's 120V but are otherwise not easily visually distinguishable.


Hmm that's good to know, it most likely means it's reconfigurable. Though it could also be that there are different components used in each version... so yeah it's hard to tell without fully analyzing the circuitry. Where that 6 pin connector goes with the 4 wires, does it look like there's traces going to the unpopulated pins?

Basically you kinda want to reverse engineer the circuit, we know there are two 6v taps coming out of the transformer so need to see if they are using that in a way to support 120/240 via switching a few things around.
 

Scurra

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Jul 27, 2018
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Nope. It's just a wire that goes into the unit.

Well, I took apart the unit hoping that it just had an AC to DC transformer inside of it.

It has a water pump, an LED light panel, LCD control panel, and some circuitry, so I thought it would surely just take in 120V AC and then convert it almost immediately to a usable DC current and split the DC current among the components.

Nope.

The wire goes inside the unit and immediately terminates into a transformer that takes 120V AC and outputs TWO 6V AC rails (first pic).

Both 6V AC rails go into another board that runs the microcontroller and LCD panel which then connects to a third board that runs the pump.

Not sure why they would design it like this. Thoughta? Is there such a thing that takes 100-240V AC and outputs 6V AC?

Hard to say for sure without access to a schematic or a better look at the device, but looking at the second picture, it looks like the 6 VAC outputs are being fed into a bridge rectifier (D1 through D4) to give you unregulated DC, then a filter to provide stable regulated DC (the capacitor and possibly some combo of the resistors/other diodes). The device pictured is the original power supply I assume? If so, AC to unregulated DC to filtered DC is pretty standard progression.

I'm not sure about the third picture, without being able to trace the wiring I can't really comment.
 

Red Squirrel

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That package in the last picture with the heat sink is probably a 5v regulator too. Though it is kinda odd that they have the transformer set to output 6 volts I would expect a bit more just to provide some leg room for the regulator.
 

PowerEngineer

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As others have already suggested, it is easier to rectify AC into something that can be filtered down to what is acceptably close to DC for most electronic devices. It is much harder to take AC at one voltage and change it to another voltage because you still need a fairly clean (i.e. no off-nominal frequency noise) AC voltage wave as the output. The easiest, most straightforward way of doing this is with a transformer (where the ratio of winding counts for the input and output coils determine the ration of the input to output voltages).

I gather that the device you opened up is the device you now wish could accept a 240 volt input rather than 120 volt. You may find that the transformer has a label with some rating information for its windings. Most likely you will see one 120 volt winding and two six volt windings, but it is possible that it has two 120 volt windings. It may be that these two windings are wired in parallel for 120 volt input but can be wired in series for 240 volt input. If I were designing a device for markets requiring both input levels, that might be the easiest way to do it (rather than stocking two different transformers). If so, then it might be possible to covert your device to 240 volt input by changing the winding connections.

I DO NOT ADVISE YOU TO TRY THIS UNLESS THE TRANSFORMER WINDINGS ARE CLEARLY MARKED AND YOU REALLY KNOW WHAT YOU ARE DOING! YOU COULD OTHERWISE PRETTY EASILY FRY THE DEVICE AND/OR YOURSELF.