Why does a SCR have negative resistance?
Whenever you measure resistance, the resistance itself cannot be negative!When an SCR (Semiconductor Controlled Rectifier) is not conducting, it has a high resistance between its anode and its cathode. When its gate is triggered and the SCR is conducting, it has a low resistance between its anode and its cathode.For more information about SCRs, see the answer to the Related Question (for which a link is shown below) and also the Related Link.