WCCA is all about reliability for stress factors that increase with Temp or V or I and functional requirements with design specs like brightness and max hotspot.

In this case, it is simple with only a few parts.

e.g.

Note that for 1206 R's derating is the same as Rja ( junction to ambient temp resistance ) thus \$R_{ja(1206)}= (155-70)['C])/0.5W\$ ['C/W] above max internal ambient temp, then prudent design criteria might say <100'C max. even if tolerance to 155'C

You may find your current and temp. rise is too high in both R and LED.

Repeat for all hot parts including LED and ABS. MAX. means going above affects lifespan significantly. Do you feel lucky?

You learn by computing temp. rise. Then you remember this in future so it does not have to be repeated every time and remember what margin to use.

Note that the rating of 20mA is recommended and not 30 and Vf=2.0V @ 20mA +20%/-5% unless just experimenting. Otherwise, you must consider what copper area you have to dissipate heat under a tiny.

Time savers include simulations, and modelling each part to remember in future like 170'C/W rise for a 1206 R. Ask what is the acceptable worst case hot spot for this design (from all sources of heat and component tolerances for you to estimate) and ask what is the spec for brightness [mcd] as there is a large range per mA in chips.

These LEDs are Rja=+400 'C/W or [K/W] or 500'C/W depending on part.

When many random parts are involved, statistics like Monte Carlo can play a role or worst case in design tools to do this automatically.

WCCA is all about reliability for stress factors that increase with Temp or V or I and functional requirements with design specs like brightness and max hotspot.

In this case, it is simple with only a few parts.

Calc. Imax using V+max. Vf min, Rmin and 0.2V for transistor Vce.

Approx. for Yellow, @ Vmax, Imax=( 16.5V-2V-0.2V ) / 1.2k = 11.9mA so operating at 12/20=60 % of rated current so even 20% tolerance on these parts is ok.

Calc Imin @ Vmin, Imin=(7.5V-3.4V(Green)-0.2V)/1.2k= 3.25 mA

When approaching max raing, you need to compute temp rise.

Since there is so much margin WCCA is not warranted, but you may want to define your acceptance criteria.

other

e.g.

Note that for 1206 R's derating is the same as Rja ( junction to ambient temp resistance ) thus Rja(1206)= (155-70'C)/0.5W ['C/W] above max internal ambient temp, then prudent design criteria might say <100'C max. You will find your current and temp is too high.

Repeat for all hot parts including LED and ABS. MAX. means going above affects lifespan significantly. Do you feel lucky?

You learn by computing temp. rise. Then you remember this in future so it does not have to be repeated every time and remember what margin to use.

Note that the rating of 20mA is recommended and not 30 and Vf=2.0V @ 20mA +20%/-5% unless just experimenting. Otherwise, you must consider what copper area you have to dissipate heat under a tiny part in sq.cm/W.

Time savers include simulations, and modelling each part to remember in future like 170'C/W rise for a 1206 R. Ask what is the acceptable worst case hot spot for this design (from all sources of heat and component tolerances for you to estimate).

It gets easy once you learn this and how to model the equations for each part.

Like a Yellow LED might be Vf = 1.7V+If*Rs for Rs=15 +20%/-5% then add other parts to the loop current equation at worst case cold temp knowing the tempco. (temperature coefficient, like -4mV/'C for Vf).

It depends how rigorous, you need to be for WCCA.

• Consumer, automotive, corporate, military, space, all have different requirements.

OK I'll do it once for you.

\$R_{tot.} = {(Rs_{LED} + Rce_{SOT363}+ R_{1206} \$

Pd(R)=If^2R= = 12mA* 1.2k

\$Vf_{LED}=1.7_{[V]}+If_{[A]}*15_{[Ω]} \$ for Vt~1.7 for Yellow ( 2.0V nom @ 20mA) , ~2.8V for Green at 1mA 3.1V nom @ 20mA for this size, approx.

WCCA is all about reliability for stress factors that increase with Temp or V or I and functional requirements with design specs like brightness and max hotspot.

In this case, it is simple with only a few parts.

e.g.

Note that for 1206 R's derating is the same as Rja ( junction to ambient temp resistance ) thus Rja(1206)= (155-70'C)/0.5W ['C/W] above max internal ambient temp, then prudent design criteria might say <100'C max. even if tolerance to 155'C

You may find your current and temp. rise is too high in both R and LED.

Repeat for all hot parts including LED and ABS. MAX. means going above affects lifespan significantly. Do you feel lucky?

You learn by computing temp. rise. Then you remember this in future so it does not have to be repeated every time and remember what margin to use.

Note that the rating of 20mA is recommended and not 30 and Vf=2.0V @ 20mA +20%/-5% unless just experimenting. Otherwise, you must consider what copper area you have to dissipate heat under a tiny.

Time savers include simulations, and modelling each part to remember in future like 170'C/W rise for a 1206 R. Ask what is the acceptable worst case hot spot for this design (from all sources of heat and component tolerances for you to estimate) and ask what is the spec for brightness [mcd] as there is a large range per mA in chips.

When many random parts are involved, statistics like Monte Carlo can play a role or worst case in design tools to do this automatically.

WCCA is all about reliability for stress factors that increase with Temp or V or I and functional requirements with design specs like brightness and max hotspot.

In this case, it is simple with only a few parts.

e.g.

Note that for 1206 R's derating is the same as Rja ( junction to ambient temp resistance ) thus \$R_{ja(1206)}= (155-70)['C])/0.5W\$ ['C/W] above max internal ambient temp, then prudent design criteria might say <100'C max. even if tolerance to 155'C

You may find your current and temp. rise is too high in both R and LED.

Repeat for all hot parts including LED and ABS. MAX. means going above affects lifespan significantly. Do you feel lucky?

You learn by computing temp. rise. Then you remember this in future so it does not have to be repeated every time and remember what margin to use.

Note that the rating of 20mA is recommended and not 30 and Vf=2.0V @ 20mA +20%/-5% unless just experimenting. Otherwise, you must consider what copper area you have to dissipate heat under a tiny.

Time savers include simulations, and modelling each part to remember in future like 170'C/W rise for a 1206 R. Ask what is the acceptable worst case hot spot for this design (from all sources of heat and component tolerances for you to estimate) and ask what is the spec for brightness [mcd] as there is a large range per mA in chips.

These LEDs are Rja=+400 'C/W or [K/W] or 500'C/W depending on part.

When many random parts are involved, statistics like Monte Carlo can play a role or worst case in design tools to do this automatically.