250W resistor?

Hi there,

I'm looking to power some LEDs via mains and have come across this guide: this guide to hooking them up.
In the circuit a resistor is required, however the author mentions it as a

...220W, ¼-watt resistor

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I've never heard of resistors being measured in "W" before, could it be that he means to use ohms or kohms?
The thing is he does it a couple of times throughout the para, so I thought I might be missing something...

He also says he gets the value by

...using the R=P/I² i.e. 0.25W/(33mA)² = 229W (closest available is 220W)

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This is the whole section:

"The 220W, ¼-watt resistor serves three purposes. The first is to act as a fuse should the capacitor fail. The second is to cater for harmonics and other imperfections of the mains waveform that are prevalent on modern-day mains supplies. The third to to buffer the LED against transients and the like. The value is chosen using the R=P/I² i.e. 0.25W/(33mA)² = 229W (closest available is 220W)"

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Thanks for clarifying this for me

In some texts, the symbol Ω is printed as W, why I do not know but I have seen it a few times.
Very confusing but nothing to get excited about once you know what is going on.

So in this case you just want 220 ohm ¼ watt resistors.

JimB

On edit:

I have just looked at the article in your link, and for me the Ω symbol displays as an Ω symbol.
There may be something a bit odd about the set-up of the PC you are using.

Wow, great service round here

Thanks very much for clearing that up so quickly.

Just the luck of the draw.

JimB

JimB said:

I have just looked at the article in your link, and for me the Ω symbol displays as an Ω symbol.
There may be something a bit odd about the set-up of the PC you are using.

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Really? Hmm, that's odd. Using a newish HP dm3t notebook... never had anything like this before (unless I haven't noticed).

Sounds like 'ALT' character generator problem on some computers. Mine shows proper ohms symbol. It may depend on what default font you have selected for that application being used to view (or print) document. Some fonts may not supply the same ALT font mapping.

Not an expert on hypertext or Postscript docs, but I think there is embedded commands to allow changing fonts on the fly. Your computer might not have that font and did a substitution with one that it had.

Another question about the circuit if I may:

Although the author uses a 470nF cap and a 220ohm resistor, he recommends for better reliability to decrease the cap size and subsequently increase the resistor to an appropriate value.

The lower the value of resistor, the less resilient the LED will be to noise, impulses, and harmonics on the mains. The more resilient circuit would be a 'super-bright' LED and a smaller capacitor as opposed to a normal LED and a larger cap (one achieves the same brilliance for lower current with the added advantage of higher reliability). If you're stuck having to feed the LED with a higher current, then consider increasing the resistor ensuring the power handling capacity is not exceeded (power dissipated is I² x R)

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An example he gives is reducing the cap to 150nF and increasing the resistor to 2.2kohm
I'd use the example values if I could, but I can only get a hold of a 220nF cap - and am not sure what to increase the resistor value to. This is how he describes it:

If you want the LED less bright, decrease the size of capacitor (do not increase the resistor to decrease the brightness). Then adjust the resistor accordingly using the formula 0.25/(I²). An example: A 150nF (0.15µF) will run the LED at about 10mA (enough for most applications, especially using super-brights). Increase the resistor to 2.2k.

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I'm not sure how to use this formula to get the right value, so if somebody could calculate it for me or explain it then that would be great.

EDIT: With regard to the ohm symbol thing, the language/dictionary I use in my Firefox browser is a downloaded British dictionary to replace the stock US one (damn American spelling ). I guess its missing an ohms symbol.

revans said:

EDIT: With regard to the ohm symbol thing, the language/dictionary I use in my Firefox browser is a downloaded British dictionary to replace the stock US one (damn American spelling ). I guess its missing an ohms symbol.

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It's nothing to do with the dictionary (that's not its job). The page is written using an obsolete way of displaying symbols: it's using <font face="symbol">W</font> in an effort to show an Ohms symbol. This approach was a semi-useful workaround in the olden days but there are much better ways to display symbols now (entities and UTF-8, for instance) which all modern browsers support. Perhaps the page was written 10 years ago.

One could write a Greasemonkey script for Firefox which would translate that old markup into Ohms symbol entities (&Omega; or Ω when displayed).

So don't worry--if you can see the omega symbol in the last paragraph, then the problem isn't on your end.


Cheers,

Torben

Oh ok, thanks Torben. Good to know there's nothing up with my shiny new notebok for uni

On another note, any help with my second question just above?

revans said:

Oh ok, thanks Torben. Good to know there's nothing up with my shiny new notebok for uni

On another note, any help with my second question just above?

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Sorry, nope--I'm here to learn electronics, just like you. But I am a professional programmer.

I've done some linear regulators and preamps and basic logic stuff etc, but I haven't yet had a reason to look too deeply into the maths behind running LEDs off of AC.


Regards,

Torben

revans said:

Another question about the circuit if I may:

Although the author uses a 470nF cap and a 220ohm resistor, he recommends for better reliability to decrease the cap size and subsequently increase the resistor to an appropriate value. An example he gives is reducing the cap to 150nF and increasing the resistor to 2.2kohm
I'd use the example values if I could, but I can only get a hold of a 220nF cap - and am not sure what to increase the resistor value to. This is how he describes it:

I'm not sure how to use this formula to get the right value, so if somebody could calculate it for me or explain it then that would be great.

EDIT: With regard to the ohm symbol thing, the language/dictionary I use in my Firefox browser is a downloaded British dictionary to replace the stock US one (damn American spelling ). I guess its missing an ohms symbol.

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I'm not sure if this explaination is correct, someone please double check me.

First off, I don't know why the author came up with the original 220Ω value resistor the way he did. From what I can tell, it looks like he used the resistor's power rating of ¼ W to calculate what he thought the resistance should be. A 220Ω ¼ W resistor doesn't mean it will dissipate ¼ W, it just means it is capable of dissipating up to ¼ W. Maybe he knows that but it wasn't clear to me from reading the text.

By using ¼ W in the equation to obtain the resistance value, the resitor will be dissipating it's rated power, and generally it's bad practice to use components at their maximum ratings. Someone correct me if I'm wrong but I think the ¼ W is RMS power and that the peak power with be around 330 mW. Maybe that's OK and it's just the RMS power the resistor cares about. I'm not sure.

If all you have is a 220nF capacitor and you want to limit the current to 10 mA as the author suggests, you'll need a resistor of about 6.8 kΩ (at a mains frequency of 50 Hz) but the power dissipated across the resistor will be much higher, about 1W.

It's going to end up being a trade between how much power the resistor dissipates and how much current the LED gets. If you use a 1.2 or 1.3kΩ resistor with the 220nF cap, the LED will see about 15 mA peak (which is fine for most standard LEDs) and the resistor will dissipate about ¼ W RMS. For this I would use a ½ W resistor and not the ¼ W talked about in the text.

I don't have a ton of experience with AC circuits so someone please jump in and shoot me down if I'm out in left field on this. Thanks.

Thanks for the explanation, I'm definitely not going to be shooting you down about it

I don't think I'd want to use a 6.8kΩ 1W resistor - might be a bit hefty - especially when I could use 1.3kΩ 1/2W instead. 15mA peak will suit me fine, as the LEDs I'm using are 10k mcd super-brights that have a DC forward current of 30mA.

So again thanks for the explanation... unless someone does want to shoot him down

If you're going to be using many LEDs, then I'd recommend connecting as many in series as possible because you can use a resistor with a lower power rating and it saves energy.

I'm only going to be using 2, so they'll just be going in series together.

I take it that you have no qualms with vne's suggestion then?

Also, if anyone's interested, this is being used to light acrylic in a project of mine that's based off this cool LCD monitor mod

thanks for this thead

The simple circuit is banned from discussion on some electronic chat forums because it is extremely dangerous for a nOOb to make and use.
Electrocution or burning down your home is not nice.

The voltage rating of the capacitor must be higher than the peak voltage of your electrical mains.
The resistor does nothing most of the time so its power rating is low. It is used to briefly limit the charging current of the capacitor the moment the circuit is turned on when the mains is at its peak voltage.

I forgot to say that it's also a good idea to connect a 1M bleeder resistor across the capacitor so it discharged quickly when disconnected from the mains and doesn't give you a shock.

audioguru said:

The voltage rating of the capacitor must be higher than the peak voltage of your electrical mains.
The resistor does nothing most of the time so its power rating is low. It is used to briefly limit the charging current of the capacitor the moment the circuit is turned on when the mains is at its peak voltage.

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I may be a noob, but I'm not a stupid noob... I think...

I read through the author's guide fully, and have made especially sure that the 220nF cap I have is rated at 630V (that's why I couldn't use a 150nF one, it was only rated at 100V).

Hero999 said:

I forgot to say that it's also a good idea to connect a 1M bleeder resistor across the capacitor so it discharged quickly when disconnected from the mains and doesn't give you a shock.

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I must admit I wasn't going to bother with this (the author notes it), as I won't be unplugging it very often. However, no harm in spending half a minute putting in some extra reliability/safety I suppose.

mgha.98 said:

thanks for this thead

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I agree. Thanks guys, this has been very helpful. Now... to the electronics store!

audioguru said:

The simple circuit is banned from discussion on some electronic chat forums because it is extremely dangerous for a nOOb to make and use.

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I think it's silly to ban a circuit from discussion just because it has the potential to be dangerous. But in the spirit of Audioguru's statement the first thing I should have done with my reply was to caution you that the voltage levels you're working with here are potentially LETHAL. You probably know that already and hopefully respect it. Just make sure you set up the circuit completely before you apply power and make sure you have a way to quickly and safetly disconnect power should something not work properly.

One more thing just to double-check, the 220 nF capacitor is unpolarized, correct?

Let us know how everything turns out.

vne147 said:

Just make sure you set up the circuit completely before you apply power and make sure you have a way to quickly and safetly disconnect power should something not work properly.

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Triple-checked the circuit and hooked it up to a power board with individual power on/off switches... kept my finger on the trigger

vne147 said:

One more thing just to double-check, the 220 nF capacitor is unpolarized, correct?

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Indeed it is.

vne147 said:

Let us know how everything turns out.

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**broken link removed**

The upper circuit is the one this thread's about, however it's with the stock values of 470nF and 220Ω.
The lower circuit is actually one from a thread I found on this site; but it turned out pretty weedy and dim compared to Marc's deisgn.
I need to go get some more 1N4148/1N914 diodes before I can knock up another of Marc's design with the adjusted cap and resistor values to compare.