tbu in that voltage will be developed across j 1 due to the current which if sufficient will turn on m 4 that in turn will effectively connect i e short the gate of m 2 to the source or drain of j 1 which in turn will turn off m 2 in this way it is possible to provide tbu current protection capability without the use of depletion mode nmos devices if the protected terminal of the tbu is connected to a high resistance protected device then current flow may not suffice to trip the tbu in this case the protected terminal will also be dragged positive towards v if the rate of change of voltage is relatively slow then when the source of m 2 approaches v r 4 will hold the gate of m 2 at v thus causing m 2 to turn off thus the tbu is also turned off by any surge which forces the tbu above the positive rail in practice m 2 is usually a fairly large nmos and thus has significant gate capacitance in this case because of the long rc time constant formed by the gate capacitance and r 4 the tbu will only turn off as a result of slow moving surges but may not be fast enough to react to say 60 hz mains ac which is a prime requirement for protection devices to increase turn off speed the circuit can be modified by the use of two addition small enhancement mode devices m 5 and m 6 devices the resulting circuit is shown in fig5 here m 5 and m 6 act as a simple comparator in this circuit m.