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I need to choose a Transistor to cope with the power requirements of a pair of 12V 710mA 8W LED cluster lamps. The circuit I plan to build is the classic PWM LED dimmer circuit from the EEVBlog. Can anyone point me to a table from which to pick one for the job as the BD136 is not rated for the current needed. This could be a job for a MOSFET but I wouldn't know where to start in identify the parameters to make a choice. Any help and pointers would be greatly appreciated. Before and after optimised circuit.

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  • \$\begingroup\$ It's not often I've seen a 555 used in what someone describes as a classic circuit. \$\endgroup\$ – Andy aka Sep 10 '17 at 18:34
  • \$\begingroup\$ I guess I've been staring at 555 circuits too long ;-) It is a very common place circuit :- inventable.eu/2013/04/18/63_led_dimmer_555 talkingelectronics.com/te_interactive_index.html \$\endgroup\$ – Alan Metcalfe Sep 10 '17 at 19:04
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    \$\begingroup\$ but I wouldn't know where to start in identify the parameters to make a choice. Yet you do know that the BD136 doesn't cut it. The BD136 has a stronger brother, the BD236 which is rated for 2 A. But a MOSFET would be a better choice, I'd choose one with a Id,max of at least 3A. Find them on websites like digikey, farnell, mouser. \$\endgroup\$ – Bimpelrekkie Sep 10 '17 at 19:37
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Short Answer: Drive it harder with up to 10% load current.

The important thing to remember about BJT switches is Vce(sat) is rated at Ic/Ib=10 and is much less than hFE , (often 10% hFE for good saturation)

Your BD136 will work fine with Vce=0.28V @ 1A and Vce=0.225 @ 710 mA but only if Ib=10% of Ic. Thus using the figure for Vbe(sat)=0.9V vs Ic @710mA and the actual Vol of your 555 at 70mA compute the lower Rs needed to get ~70mA base current. 470 ohm is too big.

This may or may not work with a CMOS 555 rated for 50mA out.

But at 70mA out Vout low=2.5V + 0.9V Vbe leaves 12-3.4=8.6V then /70mA = 123 Ohms @ 602mW so try 180 ohms 1/2W then 150 or 123 with 1W if it is not bright enough.

The BD136 from above is equivalent to (0.28-0.225V)/(1A-0.7)= 0.183 ohm. An alternative is to choose a FET with RdsOn<<0.1 ohm. The smaller, the cooler.

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The outputing power you re looking for is simply about 1500ma at 12v. You dont have to just get the same current permitting mosfet you should allow enough current for tolerance so a 3 to 5 amp transistor should do bearing in my that the l e d s do have a current limiting resistor or the transistor or mosfet should have a current limiting resistor . This will ensure that too much current do not burn the fragile l e d s . The p w m should be able to regulate the voltage supply irrespective of the decreasing voltage supply during utility and we can say bobs your uncle

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