Sizing of LED current matching resistors.

[treez]

Hello,

We have three parallel strings of Advanced Power TopLED LEDs (LB G6SP).

Each string has eight LEDs.
There is a 10R 1206 resistor in series with each string (our design contractors put these there for current matching)

LB G6SP datasheet:
**broken link removed**

Each led carries 150mA of current, or at least it should , if matched.

Thye entire thing is fed by a single constant current source of 450mA.


As you can see, even with perfectly matched currents, the 1206 resistors dissipate 225mW…..so they are undersized……but what power rating should we make them?


….given that Osram will not give us statistical spreads of LED Vf variability.

 

[BradtheRad]

I believe you have said previously the strings will get about 32V. Each LED draws 4V.

Conventional engineer's wisdom says to rate the resistor for twice what you expect it must carry.

For normal circumstances this would suggest using 1/2 W.

However, what if one LED wants to draw 3V instead of 4V?

The resistor must take on an extra 1 V.

At 150 mA current flow, that adds 150 mW to the normal 225 mW.

So a 1/2W resistor will not be overburdened. Flow through the string will increase a small amount, around 3 percent (or 1/32 of previous).

If an LED draws 2 V, however, then a 1/2 W resistor will be at its limit of endurance.

Current through the string will rise 6 percent.

Of course you would like to know how much variance to expect in the run of LED's.

In any case, it depends on which components you wish to protect.

Will an LED string continue working if one LED goes short-circuit? Then it's a question of: Will failure of one LED allow the product to continue being used? Or will it render it totally unusable?

If you want the product to continue working then you should increase the watt rating of the safety resistor. If not then you have to decide how to handle one LED failure, in that you don't want it to chain-reaction to other components.

 

[tpetar]

You can use stronger resistors case 1218 or 1210 or like this 2W

**broken link removed**

But first is good to make lower input voltage.

 

[treez]

Thanks

tpetar:
They look great...Do you have a part number for them?, i cant find them anywhere

 

[tpetar]

www.tme.eu


https://www.tme.eu/en/katalog/?f_zamowienie=1#id_category=112311&page=1&s_field=artykul&s_order=ASC

Click on Cancel for Logon.

 

[treez]

Thanks!

I never knew they existed

 

[tpetar]

O yes there is even stronger then that in SMD, in SMD also you have fuses, NTC, thermistors, inductors,...

I mean on 1206, 1210, 1218,.. cases.