is always turned 34 off 34 at least a few microseconds before transistor 11 can be turned 34 on 34 after the transistor 13 is turned 34 off 34 its inter electrode capacitance is charged at a safe rate low dv dt through resistor 29 this pulls up the collector voltage of transistor 13 to the dc source voltage by charging the inter electrode capacitance through resistor 29 prior to transistor 11 turn on thus the collector of transistor 13 is not subjected to a positive step change of voltage when transistor 11 turns 34 on 34 and transistor 13 remains 34 off 34 isolation diode 27 prevents unwanted charging current flow into the load when transistor 11 is turned 34 off 34 in the up mode the inter electrode capacitance of transistor 11 is charged through resistor 31 by pulling its emitter voltage down to the negative source voltage so that when transistor 13 is switched 34 on 34 a high step voltage is not applied to transistor 11 causing it to momentarily turn on and short the power source the isolation diode 25 prevents unwanted charging current flow from the source through the load when transistor 13 is turned 34 off 34 transistor 11 is always turned 34 off 34 at least a few microseconds before transistor 13 can be turned 34 on 34 to allow charging of the inter electrode capacitance of transistor 11 thus it will be seen that an improved up and down chopper circuit employing the many advantages of darlington transistor switches has been provided with very.