Calculating a needed resistance (18.2v to 15v [AC])

[Brumby]

While we're here, I might like to throw in a couple of other points...

One is "The question".  Sometimes we get asked a question and efforts are made to answer it - but we are doing so blindly.  This thread is a classic example.

You asked a question that seemed obvious to you, but as we found out a bit further down the track, it wasn't the best question for your situation.  Don't feel bad about this - not at all.  We have seen this so often, it's nothing new and even if we don't pursue it, we are usually wondering where this question fits into the actual issue people are trying to resolve.  As your knowledge grows, you will get better at asking questions.

The second here is to look up the datasheets for the devices you are using.  This is, perhaps, a bit more advanced than you might be ready for at the moment, but it would seem appropriate to point you in this direction, considering the basic issue you were trying to resolve.

[hofmann]

Thanks dude. 

I see I may have asked the wrong question. Sorry bout that.

But since I didn't know a simple resistor probably wouldn't work this seemed (To me) like a simple problem to be solved by knowing what resistor to use.
But since it became complicated (Amps/Diodes/Circuit already worked with 18.2vac) it probably seems like I asked a stupid question and or the wrong one.

But since I did And I got a better answer (In the end) we can all move on and enjoy the fact that we have helped someone or completed a beautiful electrical device that has taken months and moths to build.

You know the old saying about asking questions and the type of answers people give...............

Rest easy tonight Brumby  you done good.

[Brumby]

You have summed up the exercise perfectly.   

... and don't apologise for asking the wrong question - you didn't know that until later.  Nor did we, for that matter.


My philosophy is simple - and covers your situation perfectly.....

"The only dumb question is the one you didn't ask."

[hofmann]

You have summed up the exercise perfectly.   


My philosophy is simple - and covers your situation perfectly.....

"The only dumb question is the one you didn't ask."

Eeerm, wouldnt that be the question I asked? The 'dumb' one?

I was actually referring to "There are no stupid questions. Only stupid answers"

Carl Sagan, in his work The Demon-Haunted World: Science as a Candle in the Dark said: "There are naive questions, tedious questions, ill-phrased questions, questions put after inadequate self-criticism. But every question is a cry to understand the world. There is no such thing as a dumb question".

A woman, recounting a story about an old man who used to answer all her "stupid questions", explained "Chica, if you ask a question it makes you look stupid for 5 minutes – but if you don't ask – you stay stupid for fifty years, so always ask questions in your life".

A 1970 Dear Abby column in The Milwaukee Sentinel said: "There is no such thing as a stupid question if it's sincere. Better to ask and risk appearing stupid than to continue on your ignorant way and make a stupid mistake.

Colin Powell says: "there is no such thing as a stupid question, only stupid answers". Presentation Skills That Will Take You to the Top says that within the business world, the adage holds true. The book adds "a question might be uninformed, tangential, or seemingly irrelevant, but, whether the presenter perceives it to be stupid or not, every audience member has every right to ask any sort of question".

This would be directed at the troll. Not you Brumby.

Peace OUT

[Brumby]

Carl Sagan, in his work The Demon-Haunted World: Science as a Candle in the Dark said: "There are naive questions, tedious questions, ill-phrased questions, questions put after inadequate self-criticism. But every question is a cry to understand the world. There is no such thing as a dumb question".

My spin is this ... if you ask the question, then it's not dumb.

I have taken, perhaps, a bit of grammatical licence.  It might be more accurate to say the person is dumb for not asking the question, but that points a finger at the person, which isn't helpful.



Whether here or of others, or simply of yourself - just keep asking questions!

Cheers.

[digsys]

  .... you will need 2 Zeners in SERIES, opposing polarities. 3.3V x 1A = 3.3W ...

OOOPS my mistake, and as pointed out 2.7V Zeners (0.6V is dropped on the other one)

[Zero999]

I need to convert 18.2v AC into 15v AC using a resistor in series.

Is there an online calculator or an App (great business idea )  I'm terrible at maths...

Thank you!

What device are you trying to power?

The voltage drop across a resistor will change with the current.

Diodes can be used but in mind that they are non-linear elements. The voltage across a diode also changes with the current but it's not proportional so the waveform will become distorted. A diode with a 3V drop across it will also not reduce 18VAC to 15VAC.

[danadak]

You have to drop 3.2V, what is the accuracy you need of the drop over temp ?

5W zeners (5W to give you some margin) are min 3.3V, that coupled with a diode
drop of the opposing cycle of AC yields a 4V drop from the 18.2 V.

If thats intolerable then consider using an active control loop to fix the drop at 3.2V.

Example zener circuit (much lower current) attached.


Regards, Dana.

[Cliff Matthews]

Op has his answer, schematic was in the 15th post..
No voltage drop was ever required (within capacity of all 3 regulators, each fed by half-wave source).

[Zero999]

Op has his answer, schematic was in the 15th post..

Which I didn't see when I posted previously because he uploaded it to an external site and I was at work at the time so it was blocked by the firewall.

Moral of the story: don't upload schematics to an external site, enclose them in image tags, and expect everyone to see them!