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TIP120 and arduino current issue Answered

I am using 4 TIP120s to control 4 relays (example in pictures).  When I try to Switch 2 or more TIP120s, they either all switch or none. I am able activate each one separately without any issue. 12 V battery input for transistors to relays, 9 volt on arduino.

Thank You


So, can anyone help me out on the issue? The issue is the arduino does not have enough current to switch the transistors. Is there a way around this?


Here is the Schematic. I can tell you the volts and amps of the batteries if needed.


5 years ago

Are you trying to control all four from one pin? Maybe all four require too much current, it's possible. Have you tried using pwm? What kind of battery do have for the 12v? Is it able to provide enough current? Just some things to look at.
Look at the link in my reply, states you need a diode with inductive loads as steve mentioned.

Current could be a possible problem, for the arduino and the battery. I never thought of it that way.Like when it turns on say pin 5 and 6, nothing happens. Ill post the schematic!

PUT A DIODE ACROSS THE RELAY COIL RIGHT AWAY. You are in grave danger of destroying your computer without a diode placed with its CATHODE (thats the marked end) on the relay end connected to the supply and the ANODE end connected to the other.


Your point ? The integral diode is in the WRONG PLACE TO PROTECT THE TRANSISTOR

not my drawing, I imagine it was supplied by the manufacturer or the supplier.
No need to get defensive, I was not arguing with you. But I guess if you want to argue you could e-mail the company and tell them you know better than they do.

No, there is a body-diode there, but it doesn't do the job I indicated a diode was needed for in the OPs circuit. And my point stands. The diode in the transistor is not a flywheel diode.

A little knowledge is a dangerous thing.

Sorry, I am on an ipad and can't get to a computer until tomorrow. I'll upload the schematic ASAP.

Can't see pictures.
The single circuit looks OK
What is your code.


This is the picture. Replace the lightbulb with a relay in the picture.