# 1st time picking components and resistors - look over please?

I'm trying to figure out how to use this forum.  How can I attach a picture?

Joined September 21, 2021      4
Tuesday at 10:28 PM

Joined September 19, 2021      4
Sunday at 04:40 PM

Thanks!!

I found this forum with a search for; electronics forum for beginners.

This is my first project where I'm trying to determine the components I want to use as well as needed resister, etc.

Up till now I've just taken info online and hoped that it works out. I enjoy this hobby so it's time to start learning.

Below is a description of my project and how I've come up with resistor values. So I'm hopeing to get confirmation of my choices and calulations. You can't hurt my feeling because I admit I know nothing.

Green LED
Arduino to TLP621-1 to TIP120 to 12V 320 mA solenoid. (See schematic)

TLP621 data sheet;
Input voltage - typical 5V, so no resistor here?
Forward current - typical 16 mA, so .016 X 75 ohm = 1.2V

TIP120 data sheet
Dase current 120mA = 1.2V (the forward voltage from above)

Green LED;
2.1V 20 mA using onlie resistor calculator 145 ohms.
I only have a 150.

Here's the schematic.

Joined September 21, 2021      4
Tuesday at 10:28 PM

Problems -

1. The 5V of TLP621 is reverse blocking voltage of the LED. Use a series resistor minimum 100R / maximum 680R.

2. The Solenoid freewheeling diode is in forward bias. It will conduct current and the TIP120 will be destroyed when the Solenoid get closed. Change the diode direction. Use 1N4007 which is commonly available.

3. I am not sure why you choosed such high power Darlington pair transistor. It will work but it is not for what you have intended to use it. Try proper switching transistor with atleast 1A current rating.

4. If you choose to use TIP120, having 75 ohms base resistor could provide [12V / 75R = 160mA of current if the TLP phototransistor is ignored] a lot currnt to the transistor base. Use atleast 180R to 680R resistor if the current consumption is not a problem. Otherwise, use a Mosfet which will be cheaper and heat dissipation will be lower than high power transistor. If cost of the component doesnot matter, use low RDSon resistance in miliohms rating.

Joined February 12, 2018      696
Monday at 02:11 PM

1. & 2. taken care of. Thanks

3. I picked the TIP120 because Adafruit suggested it with the solenoid. (Remember I know Nothing.) Would this be an appropriate switching transistor?

# TIP47 at Digi-Key

4 "If cost of the component does not matter, use low RDSon resistance in miliohms rating. Could you recommend one of these?  I really don't know what to look for.

Joined September 21, 2021      4
Tuesday at 10:28 PM

1 & 2 points taken care of, thanks.

3. Would this be a proper switching transistor.

4 Could you recommend a low RDSon?  I really don't know what to look for.

Is the TLP621 even needed?

Joined September 21, 2021      4
Tuesday at 10:28 PM

You could try STP90NF03L. You would not need a higher size of heatsink.

Joined February 12, 2018      696
Monday at 02:11 PM