Related video Introduction TL431 Precision Shunt Regulator: https://www.youtube.com/watch?v=YPyCVvTZDw0 This will be used in a number of circuits. See http://www.bristolwatch.com/ccs/TL431A.htm http://www.bristolwatch.com/ccs/TL431A2.htm http://www.bristolwatch.com/ccs/TL431A3.htm http://www.bristolwatch.com/ccs/TL431A4.htm