AC/DC converter 1

[mielke]

I see a flood of AC/DC converters out there but is there an off the shelf product that does a straight AC to DC conversion at the same voltage (over an AC input range between 0-120V). So I can plug this converter into a wall outlet at 120V AC and then plug something else into this converter (power plug type) and get 120V DC (or at least close to that), but also if the input AC falls to say 40V I still get 40V DC (or at least close to that) outputted. any ideas if there is a specific commercial name for this? Thanks!

 

[IRstuff]

Perhaps if you describe the application, that might help out.

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[mielke]

no application in particular...

got some projects i'm working on that require more and more electronics, thought i'd brush up on my knowledge. and experiementing and hands-on knowledge is the way I learn best. just trying to make a general testing setup for learning.

 

[zeusfaber]

An off-the-shelf solution to that requirement seems a bit unlikely, but it would be easy enough to make, provided you didn't want to draw very much current off the dc side.

I'm picturing a simple rectifier, followed by a smoothing capacitor, followed by a pair of resistors to bring the voltage down to 71% of peak input voltage.

A.

 

[ScottyUK]

Coonect the variac to the input of an autotransformer. Connect the rectifier an appropriate tapping on the autotransformer. Lots more current available, and less heat. Should be easy to find one with an acceptable ratio even if it's not a perfect match.

 

[jraef]

Most AC/DC converters now days are what are called Switched Mode Power Supplies and will include, as an often desirable feature, the specific ability to AVOID the very thing you are looking for, because for the most part, people need to give their electronics a steady regulated DC voltage regardless of swings in the AC feeding it.

But long before the idea of an SMPS existed, we had what are now called "Linear Power Supplies". Linear meant the output voltage could be varied in a linear fashion with the input voltage. That is what you are looking for, and they are still made.

Output voltage of a power supply is always going to be roughly 1.41x the input AC voltage, because what we call the AC "voltage" is the RMS value, not the true peak, but the converter converts at the true peaks of the sine wave. Most power supplies will come with a trimming adjustment to give you a lower level output than peak, however they will also come with "regulation" that will compensate as much as it can to again try to maintain that setting. What you will want to look for is going to be referred to as an "unregulated linear power supply", or at least one where the regulation can be disabled. They are made, most often for the scientific community, but will be more rare than the stuff you can pick up anywhere.

"Will work for (the memory of) salami"

 

[jraef]

Before I accidentally submitted that, I was gong to say that what has already been proposed by zeusfaber and ScottyUK is essentially the same thing, and being that you want 120VAC in and 120VDC out, which is going to be even MORE uncommon (because 120VDC is not used in any common equipment), you may in fact need to "roll your own" on this one as they suggested.

"Will work for (the memory of) salami"

 

[OperaHouse]

That rectifier capacitor combination would work pretty well if you added a 120/120/48V transformer. add one 120V and 48V windings in series to form a 168V primary. Then just rectify the other 120V winding as the secondary. That combination pretty well compensates for the AC peak.

If isn't in demand, it isn't off the shelf. However just about anything you want can be made from off the shelf assemblies with a little modification. Generally you have to limit the operating range. There are the same limitations in the mechanical world. The last ME I worked with thought electronics was a pretty inexact science. He said everytime we had something that didn't work we just added a capacitor to fix it.

 

[VEBill]

"Output voltage of a [linear, unregulated] power supply is always going to be roughly 1.41x the input AC voltage..."

Unless the PS happens to include a transformer. ;-)!!!!

 

[racookpe1978]

On a very similar problem, how do the EE types get a higher DC voltage out of a DC charger than what is available?

For example, if I am plugged into a 12 VDC car-type lead battery, how can I get an 18 VDC appliance to charge up?

My drills are 18Volt, but they recharge from a standard 120 VAC source - as you've written above, that is rectified down very easily.
But going up on the DC side? Surely they don't go
low volt DC-> lower volt square wave AC -> transformer -> higher volt square wave AC -> re-rectify to higher volt DC do they?

 

[xnuke]

It's kind of hijacking the thread, racookpe1978, but I'll explain anyway. We use what is known as a boost converter, a type of DC-DC power converter. They act almost like you said. A simple boost converter has a low-volt dc source connected to develop a magnetic field in an inductor, then the source is switched out and the inductor is switched to connect it to the output circuit. An inductor develops a voltage across it that tries to oppose the change in current through it, and since that voltage V = L(di/dt), the higher the rate of change of current through the coil, the higher the voltage produced by the inductor. This allows the inductor voltage generated by its magnetic field collapsing to be even higher than the dc voltage source that it was originally connected to. This is done many thousands of times per second. The output circuit has filtering to smooth any ripple caused by the switching action. Isolated boost converters do the same thing, but can use the inductance of a transformer instead of an inductor to provide the boost action.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IRstuff]

Wiki has a brief article:

http://en.wikipedia.org/wiki/Boost_converter

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