555 (TLC555) Relay Driver Circuit

Posted on Feb 4, 2014

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

 555 (TLC555) Relay Driver Circuit
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200mA. This is ideal for driving my 133mA relay coil. 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. This is a 40A automotive relay (contactor) that I selected for this application. Since its coil current exceeds the TLC555 output sink current rating, it is a good candidate. Manufacturer or part number is unknown. What is the difference between a relay and a contactor There is no clear difference other than perhaps current rating and/or application to me, relays are anything from signal devices to approx 20A. Anything rated at 40A or larger, or is used in power applications is a contactor. The trigger input (pin 2) and the threshold input (pin 6) pins are tied together ”this is commonly done. With a 12V supply, the upper threshold is 8V and the lower is 4V. The two voltage levels, being far apart, make a great Schmitt trigger. This may be driven directly by 4000series CMOS logic that is also powered via 12V. To make it compatible with TTL logic levels, simply add the two resistors that are shown in the schematic. This loads down the internal divider to a lower voltage. The calculated levels...

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