Hello-

I made a small circuit which requires a resistor. I noticed after a bit the resistor gets a little warm to the touch, should I raise the resistance or raise the watt rating?

Its a parallel circuit with 6 LEDs. I used a 560 Omh resistor which gives each LED about 21 mA, 9 mA short of their typical forward current.

 

They'll get warm.

An electric range heating element is one big resistor, it gets warm too!

 

lol i know.

I thought I would ask anyways and be extra careful!

 

what is the turn on voltage of the LED and what wattage resistor did you use?

 

Wait, I'm confused. Wouldn't you lower the watts and/or the resistance?

Dan

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nsxxtreme &raquo;</TD></TR><TR><TD CLASS="quote">what is the turn on voltage of the LED and what wattage resistor did you use?</TD></TR></TABLE>

turn on v is a little after 3v


and I used a 1/4 watt resistor

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by LudemanDan &raquo;</TD></TR><TR><TD CLASS="quote">Wait, I'm confused. Wouldn't you lower the watts and/or the resistance?
Dan

</TD></TR></TABLE>

yeh... now that I think about it I may have phrased the question wrong?

 

I imagine you have the resistor going to +14.4 and the diode connected in series to ground.

So one side of the resistor is 14.4V the other side of the resistor is approx 3.0V.

So 14.4-3.0=11.4V that is being dropped across the resistor. Now for the current
(14.4-3.0)/560= 20.3mA

So each LED is getting approx 20.3mA/6 about 3.4mA each. I say approx because the V/I curve will be slightly different for each.

Now for the wattage you take the voltage dropped accross the resistor and multiply by the current through the resistor 11.4*20.3mA=23.1mW A 1/4 watt resistor is capable of 25mW. So it will definetly get hot to the touch but the resistor will be fine. If you want one that doesn't get hot look for one that has a better temperature coefficient. You still want a 1/4W or larger, usually the larger the body the better the temperature coefficient.

 

oh ok thanks

the 6 diodes are connected in parallel, The resistor is before the first anode.

I thought voltage was shared in parallel, and curent was shared in series? I put them in parallel because they are all the same mA rating and that way I just divide my voltage by my resistance and make sure its less than all the diodes' mA added up.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by B18EG6 &raquo;</TD></TR><TR><TD CLASS="quote">oh ok thanks

the 6 diodes are connected in parallel, The resistor is before the first anode.

I thought voltage was shared in parallel, and curent was shared in series? I put them in parallel because they are all the same mA rating and that way I just divide my voltage by my resistance and make sure its less than all the diodes' mA added up.</TD></TR></TABLE>
No, think about what you are saying. If each diode was pulling 21mA you would need 126mA to run all six diodes. This would be 1.43W, all the current has to come through the resistor. The purpose of the resistor is to limit the current throught the diodes.

If you want the LED's brighter you increase the amount of current through them. I would not go past 20mA each for these diodes. You would need a 2 Watt resistor for this. The current through each will not be exactly the same but will be close. If you want a more efficient circuit buy an LED that has a higher turn on voltage. A little under 12V would be ideal then the resistor can be smaller and will not waste as much power.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nsxxtreme &raquo;</TD></TR><TR><TD CLASS="quote">
No, think about what you are saying. If each diode was pulling 21mA you would need 126mA to run all six diodes. This would be 1.43W, all the current has to come through the resistor. The purpose of the resistor is to limit the current throught the diodes.

If you want the LED's brighter you increase the amount of current through them. I would not go past 20mA each for these diodes. You would need a 2 Watt resistor for this. The current through each will not be exactly the same but will be close. If you want a more efficient circuit buy an LED that has a higher turn on voltage. A little under 12V would be ideal then the resistor can be smaller and will not waste as much power.</TD></TR></TABLE>

ahh ok i see what you mean! (i think )

 

I dont want to hi-jack this thread, but WOW. This is the first time I've seen some REALLY GOOD information on Honda-Tech. I want to see some NooB or E-thug regurgitate that information.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Spec R &raquo;</TD></TR><TR><TD CLASS="quote">I dont want to hi-jack this thread, but WOW. This is the first time I've seen some REALLY GOOD information on Honda-Tech. I want to see some NooB or E-thug regurgitate that information. </TD></TR></TABLE> There are many here that contribute good information, there just aren't very many good questions.

The better the question the more likely someone is to get a knowledgeable response. Having a useful title is one way to get more people to read your question. Something like "Help......" doesn't cut it.

 

just be careful running those in series ,one goes out they all go out.