Home > Circuit Bricks > Latching Driver - Thyristor
UPDATED:.21:06 21 October 2013

  • A latching driver will power an output device once triggered, and stay on even when the trigger is removed.
  • An ideal component for this is the Thyristor (SCR) since it can drive devices directly and latch also.
This latching driver uses a device called a Thyristor.

When the "SigIn" rises above 1.5 volts, the Thyristor will conduct driving the output. It will continue to conduct even if the gate voltage is removed - thus, latching.

Once latched, there are two ways to switch it off:

  1. Disconnect the power supply
  2. Short out the Anode and Cathode.

The type shown in the diagram 2N5061 can drive up to 800mA, but other types can drive several amps.

Generally the gate current is around 40mA.

Remember that the output sinks current, and thus should be connected to the negative connection of any output device, with the positive connected to the supply.

Equivalent circuit using a switch. The transistor acts as an electronic switch.


By changing the thyristor types you can increase the current capability of the drivers.

Some type available are:

  • C106D - 4A - (gate current = 200mA)
  • BT151 - 12A
  • TYN825 - 25A
Transistor Case Styles
(C106D, BT151, TYN825)

If you are driving high current devices you may find the thyristor gets hot. You can attach a heatsink to the tab of the thyristor to help cool it.

Heat transfer can be aided by the use of a 'thermal transfer compund' paste and can improve the efficiency of the heatsink.

TO220 heatsinkThermal Transfer Paste
Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers