Power Supply

[voltair]

I have added a full wave bridge to a plug-in 18VAC transformer. I have also attached a 5K capacitor across the bridge output. The output voltage is now 26V. Why is the output voltage higher than the input voltage?

 

[wizard]

Why don't you try to put a LOAD across your output and then measure the voltage?

 

[saturn1bguy]

I have added a full wave bridge to a plug-in 18VAC transformer. I have also attached a 5K capacitor across the bridge output. The output voltage is now 26V. Why is the output voltage higher than the input voltage?

Because the 18VAC value is given in root mean square (RMS), with a peak value of 18 * √2 = 25.45Vp. Because you added the capacitor, and the fact your circuit has no load, it has charged to ca. 26Vp. Add a load and it will start sinking. The RMS value is used because it approximates the equivalent DC value in a resistive load, say in a heater or a light bulb.

If the voltage in your wall outlet is 120VAC, that's RMS. It has a peak value of circa 170Vp, and a peak-to-peak voltage of 339Vpp.

 

[Leftyretro]

Corey did a good job answering your question. I might add for just general knowlege that the same specific AC voltage can to state to have several different values:

RMS
average
peak
peak to peak

Unless otherwise stated one can usually assume the RMS value applies, however it's best to understand the reasons and differences for having the different voltage values for the same voltage.

From just your example if you were buying the parts what minimum working voltage specification would you want when picking the capacitor? And the answer is not 18 volts

Lefty

 

[Hero999]

If the voltage in your wall outlet is 120VAC, that's RMS. It has a peak value of circa 170Vp, and a peak-to-peak voltage of 339Vpp.

That's assuming he's in the US but he didn't fill his location in so we don't know.

 

[audioguru]

That's assuming he's in the US.

I am not in the US and my mains voltage is 120VAC.
Everybody in the US has a gun. The only people in Canada with a gun are Americans and Jamaicans (same?).

 

[saturn1bguy]

Don't you mean a capacitor?

Aside from the 5k, he said capacitor, and likely meant it as he was measuring 26V.

That's assuming he's in the US but he didn't fill his location in so we don't know.

Nope.

Read my statement carefully. I said "If the voltage in your wall outlet is 120VAC, then blah blah blah..." The example holds regardless of his location, and serves to illustrate an answer to his question about voltage seemingly appearing out of nowhere, which is finely the point, isn't it.

 

[voltair]

Good answer Cory. Thanks to you and the others.

 

[Hero999]

I don't think he's using a 5k capacitor because it would be huge.

 

[audioguru]

Five kilo-farads!

 

[saturn1bguy]

I don't think he's using a 5k capacitor because it would be huge.

I can't explain the 5k, the OP would have to tell us. But he did say capacitor, and he was measuring around 26V. A cap would allow that, it seems a resistor wouldn't.

Mr. Voltair: Capacitors are given in Farads, usually µF (microFarads) for filters in your application. Resistors are given in ohms, often kΩ, like you've sort of done. So, which is it?

(You've sort of told us you put five quarts of gas in your car. Remember, quarts for oil in the engine, and gallons for gasoline in the tank, if you don't mind the provincial example )

 

[Hero999]

I've never heard of a quart before, is it an American measurement?

Unless you're talking about impedance, saying you have a 5kΩ capacitor is like saying you're 75kg tall.

a 5 killo-Farad would be enormous and very expensive so I doubt you have a 5kF capacitor.

 

[audioguru]

America has not gone Metric.
They still use quarts and gallons.
They might even still use inches, feet and Farenheight.

But I am glad they don't weigh things in STONES.
And they don't drive on the wrong side of the road.

 

[Hero999]

I've heard of inches, feet and Farenheight just never quarts.

We're offically a metric country but we still use imperial for some things. People generally still wiegh themselves in stone and I don't see how it's any worse than using pounds. I'd rather we all adopt kg but not everone is going to change their scales. I don't like Celsius either, it's no better than Farenheight, we should use Kelvin because we wouldn't have to deal with negative numbers.

We don't drive on the wrong side of the road, you drive on the wrong side of the road. The left hand side of the road is the best side to drive on because the right eye is slightly stronger in most people which makes avoiding cars coming towards you, on the right hand side of the road, easier.

 

[audioguru]

What happens to a Brit who drives through the tunnel then sees all the Europeans driving on the "wrong" side of the road?

Does the tunnel have a "crossover" at half-way?

 

[saturn1bguy]

Does the tunnel have a "crossover" at half-way?

If you're referring to the "Chunnel," it's for trains only. I wonder who's on the "right" side there?? If I recall the Mission Impossible movie, it seems that the train entering the tunnel was on the left side. The Brits win again

 

[Nigel Goodwin]

I've heard of inches, feet and Farenheight just never quarts.

A quart is two pints (a QUARTer of a gallon) - you're obviously too young

We don't drive on the wrong side of the road, you drive on the wrong side of the road. The left hand side of the road is the best side to drive on because the right eye is slightly stronger in most people which makes avoiding cars coming towards you, on the right hand side of the road, easier.

The reason is far older than that, it's back to horse days - most people are right handed, so riding on the left means you have your sword arm on the right side to defend yourself against an oncoming opponent.

I've asked various times, but no one has ever been able to suggest a sensible reason for driving on the right?.

 

[ericgibbs]

I've asked various times, but no one has ever been able to suggest a sensible reason for driving on the right?.

Here is one explanation.

Why do Americans drive on the Right hand side of the road ? - Yahoo! Answers

Right- and left-hand traffic - Wikipedia, the free encyclopedia

 

[ecerfoglio]

If you're referring to the "Chunnel," it's for trains only. I wonder who's on the "right" side there?? If I recall the Mission Impossible movie, it seems that the train entering the tunnel was on the left side. The Brits win again

The trains do not allways go on the "same side" as the road traffic.

Here in Argentina we allways drive on the right side (at least for the last 60 years or so), but the trains and subway go on the left tracks.

In Eric's Wikipedia post it says that (at least some of) France's trains also uses the left track

 

[Hero999]

The reason is far older than that, it's back to horse days - most people are right handed, so riding on the left means you have your sword arm on the right side to defend yourself against an oncoming opponent.

Yes, I don't think that was the origional reason but Wikipedia suggest that driving on the left might be safer and the right eye being dominant theory makes sense to me.

Research in 1969 by J. J. Leeming showed countries driving on the left have a lower collision rate than countries driving on the right. This research is questioned in Peter Kincaid's book on the rule of the road,[citation needed] but some countries that have switched to driving on the right, such as Sweden), have seen their long-term accident rates increase by more than any increase in traffic volume[citation needed]. It has been suggested, but not proven, that this is partly because it is more common to be right-eye dominant.[12][13][14] Traffic flows in a clockwise direction when driving on the left which enables right eyed people to use the right eye to see oncoming traffic. When overtaking (passing) on a right-side-driving road, the right-eyed driver looks in the wing mirror (side mirror) with the left eye and also views the oncoming traffic with the left eye which is not suited to the majority right-eyed people.