What happened to my thread?

[oharag]

I had a question on this forum concerning a SM resistor in a LED circuit. It has since disappeared? There was a poster that posted an admonishment (to whom I do not know), and I checked today and it has since disappeared. Was there a violation in posting rules?

 

[PHovnanian]

Keith,
Its probably the shilling that got me tossed. Why they deleted the original post I can't figure. The guy seemed to have a legit question, even if it was one of terminology and not straight engineering.

I find it hard to believe that they remove posts based on IP addresses manually. That's a lot of work for something so easily automated.

 

[IRstuff]

Everything is on a case-by-case basis.

Most likely, the OP was found out to be a student.

TTFN

FAQ731-376

 

[itsmoked]

I believe it is automated but requires individual confirmation. It's not just a specific one it's adjacent ones and that's why it's not automatic.

For instance a school might have hundreds of IPs that are in a block. Sometimes a banned individual tries to come back on all the adjacent IPs. Or you could have a shill working from the same corporate block as the original spammer.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[oharag]

I'm back

I got my boards in and they work fine.

Though I have one last question:

I designed in 11 SM resistors for each series chain to drop 1.6 volts. Did I make a mistake?

Why wouldn't I have just one SM resistor at the start of the circuit to drop 1.6 volts and the deliver the remaining 22.4 volts down stream in a series parallel design? I designed the circuit as I see depicted on the web.

Wouldn't it be the same if I just used one resistor and disapated .25 W * 1 device instead of .25W * 22 devices?

Again I am a Mechanical designing LED circuits. I really do not have an Electrical of knowledge on board. Hopefully this will change.

 

[ScottyUK]

Your could use one resistor but it would need to dissipate the total wattage of the 22 resistors. You might not be able to use a SMD type. Or maybe you could - depends on the resistor and the wattage.


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If we learn from our mistakes I'm getting a great education!

 

[oharag]

@ScottyUK

Because I have to disappate 3.3 A * 1.6 VDC instead of .150 A * 1.6 VDC, right? Now I get it

Thanks for the answer.

 

[ScottyUK]

One thing to bear in mind is that with a single drop resistor shared among many LEDs you may not get equal sharing of the current because of slight variations between the individual LEDS. Others here know a lot more about LEDs than I do and will probably comment on how big a problem this would be in a practical design. The one-resistor-per-LED solution avoids this possible complication.


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If we learn from our mistakes I'm getting a great education!

 

[PHovnanian]

What ScottyUK said.

The LEDs in each series string will pass equal current, by definition. But between strings, differences in the sums of the LEDs' Vf drops could result in each string drawing more or less current. In most cases, differences between individual LED Vfs will average out within each string.

Another problem to consider is that your current regulating resistor is dropping only 1.6V out of a total string voltage of 24 Volts. The 22.6 V LED drop assumes some nominal operating temperature. As the ambient temp and/or the LED temperatures change, so will their Vf. And since the regulation slope of the resistor (the inverse of its resistance ) is pretty steep, a small change in the strings Vf will result in a large change in current.

Not knowing the device parameters (temp coefficients) and the thermal operating characteristics, I can't say whether this will be important. But this is the sort of stuff that people with advanced degrees in LED strings have to deal with every day.