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LED Strip lights (2 Amps) Short Fade on / Fade Off effect

ThomsCircuit

Active Member
The basic PWM action works by comparing the control voltage on C3 to the charge / discharge [approximate] sawtooth on the 555 timing cap, using an LM311 (same pinout, different part number) to control the output transistor.
Would you recommend 1/2 watt resistors or can i use the 1/4 watt?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
1/4 watt would be OK for everything, though I'd suggest 1/2W for R7 to avoid it getting too warm, as it will have 12V across it continuously when the light is off.
 

ThomsCircuit

Active Member
Being that im using the TIP122 with a collector current of 5A is it safe to say that this circuit is rated for 5 amps?
All but two of my led lights are under 5 amps where the other 2 draw around 8amps.
if i am correct I plan on having a few boards made to accommodate the larger TIP142 so i can power the 8amp loads
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Being that im using the TIP122 with a collector current of 5A is it safe to say that this circuit is rated for 5 amps?
No..

The transistor will be dissipating some power all the time it is on. With a Darlington, the collector-emitter voltage will never be less than 0.6V to you have at leas 0.6W per amp dissipation. A heatsink would be needed for significant currents.
(That's why mosfets are preferable for such things, a good mosfet may only dissipate a few milliwatts at an amp or two load).

Plus the PCB tracks need to be wide enough to handle the current; I can't remember how they ended up.

With that circuit, you could possibly use an appropriate TO-220 mosfet directly in place of the TIP122, preferably with a lower value series resistor to the gate. It would likely dissipate more power during the fading sequences, but much less when fully on.
 

ThomsCircuit

Active Member
With a Darlington, the collector-emitter voltage will never be less than 0.6V to you have at leas 0.6W per amp dissipation.
So what would the maximum amps be?
Plus the PCB tracks need to be wide enough to handle the current;
the positive rail is 40. Ground pour on top and bottom layers
With that circuit, you could possibly use an appropriate TO-220 mosfet directly in place of the TIP122,
Could you suggest one?

fader.png
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The critical figures are "RDS On", the drain to source resistance when the transistor is switched on by a suitable gate voltage, and the gate voltage needed for that resistance.

The other value that can be critical is the gate capacitance; that can be extremely high with power FETs, which means it takes either a lot of current or some time for the gate voltage to swing between the off and fully on levels. It's not too important with most simple on-off applications, but becomes rather critical with high frequency switching, which is why many high power MOSFET circuits use specialised high current gate drivers.

If the transistor gets too warm during the fade with your device, either C1 or R1 can be increased to reduce the switching frequency.
As long as it's over 100Hz or so it should not have any visible flicker, but the transistor will be fully on or off for a higher percentage of each cycle so dissipate less power. eg. You could try 1uF for C1 and see if that is still flicker-free.
 

ThomsCircuit

Active Member
The critical figures are "RDS On", the drain to source resistance when the transistor is switched on by a suitable gate voltage, and the gate voltage needed for that resistance.
STP80NF55 MOSFET 8 mOhm 80A 55V N-channel TO-220-3
Im using DigiKey to help narrow it down. Then find it at Tayda
this one has an RDS on of 6.5. Your part is 3.9
Trying to figure out the other values are difficult as the names in the specs are different from one another.
If you have no objections i think this one is as close as im gonna get.
If the transistor gets too warm during the fade with your device, either C1 or R1 can be increased to reduce the switching frequency.
ok.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
That looks fine; the worst-case figure is 8mOhm; that works out to 32mW @ 4A, or 200mW @ 5A.

Still a fraction of the darlington at just 1A.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
ps. Your 40 thou tracks are OK for roughly 2.5A in 1 Oz copper.
For 5A, it needs to be 2 Oz; that would give a track temperature around 18'C above ambient.

Calculator here, if you want to try different values:
 

ThomsCircuit

Active Member
ps. Your 40 thou tracks are OK for roughly 2.5A in 1 Oz copper.
For 5A, it needs to be 2 Oz; that would give a track temperature around 18'C above ambient.
This is great news! I was wondering how to figure track thickness. Ya know i noticed at JCLPCB they offer a 2oz copper option. would selecting that option satisfy the track thickness?
1641912299339.png
 

ThomsCircuit

Active Member
That looks fine; the worst-case figure is 8mOhm; that works out to 32mW @ 4A, or 200mW @ 5A.

Still a fraction of the darlington at just 1A.
So the darlington can handle no more than 2 amps. Which is fine. Like i said all but two lights draw more than that. So it would be safe to select the 2oz copper option for all the boards and just get 2-3 STP80NF55 Mosfets and make the resistor and cap changes for those boards (C1,R1,R7&R8)
I plan on bread boarding this. Ill test with just one led then work my way up.
 

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