ESP8266 Community Forum

- Wed Jan 13, 2016 10:48 am #38685 I'm using this formula for diode of 1W

Ohm = (Vcc - Vcz) : (30 + mA) x 1000

... but in my opinion the problem is that i'm tring to find a resistor value , that is common in the voltage 9, 12,24.
So considering your formula :
24v-6,8=17,2 --> 17,2V/0,06A=286ohm

ok .. i install a 286ohm resistor.
What's happened when the voltage is 12?
12-6,8=5,2
5,2/x=286 ---> 0,018A , i need 60mA!

and 9V?
9-6,8=2,2/x=286 --> 0,007A, i always need 60mA!!

So my exercise is to find a value that can provide 60mA at 9V ... and more with 12 and 24.
Considering the same formula it could be 36ohm, in this case i have:
9v ---> 60mA , 200mW
12v ---> 145mA , 900 mW
24 ---> 475mA , 8,7W !!! <----- this is my problem i suppose.
Maybe my exercise is without reason, but my goal is to obtain minimum 60mA with 3 different source voltage 9,12,24v

Thanks
Mirko

- Wed Jan 13, 2016 11:02 am #38687 If you need to have a circuit compatible with all those voltages at the same time, then you don't have other choice to use a voltage regulator or, even better, a DC-DC converter

- Wed Jan 13, 2016 11:07 am #38688 Yes ... it is without reason.

But i have a last question:
if my resistor can provide me 475mA with a lot of hot problem (8W!), if the l293d use ONLY 60mA so 12% of totla mA provided, the power dissipation will be 8Wx12%?
in this case i have more or less 1W of dissipation, and could be work (?)

- Wed Jan 13, 2016 11:12 am #38692 All three components, L293D, the zener and the power resistor will dissipate heat.
Of course, this is not efficient.