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555 (TLC555) Relay Driver Circuit

2015-03-01 07:39  
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555 (TLC555) Relay Driver Circuit
Many integrated circuits have undocumented features or abilities. This is one of them. The TLC555 output (pin 3) can sink a 100mA load to 1.28V. The open drain transistor reset (pin 7) can sink 100mA to 1V. Tying both lines together is permissible because they are logically the same polarity and this potentially doubles the sink current ability to 200mA. This is ideal for driving my 133mA relay coil.The input also has the undocumented, but better known, feature of a Schmitt trigger that provides positive switching. 555 Relay Driver Schematic Output current rating (pin 3): TLC555 vs. NE555 (Vcc = 15V) TLC555 NE555 Source current 10mA 200mA Source mode saturation voltage Vcc-0.8V Vcc-2.5V Sink current 100mA 200mA Sink mode saturation voltage 1.28V 2.5V While it can be seen that the NE555 has the higher current rating, its saturation voltage is grossly inferior and this is a detriment in driving loads without excessive voltage drop. Also it can be seen that the TLC555 is much like TTL in that its sourcing ability is far less than its sinking ability. However for driving a relay, we are interested only in current sinking properties.