Multiple Solid State Relays

Published:2013/6/25 22:09:00 Author:muriel | Keyword: Multiple Solid State Relays | From:SeekIC

The next diagram shows an inexpensive method of building multiple medium current relays. The circuit uses a ULN2003 - High voltage/High Current Peripheral Driver IC. This device can handle loads of 500 milliamps. The IC is designed to have TTL and CMOS inputs of between 5 and 15 volts can be controlled by any clean input voltage.Two methods of controlling the ULN2003 are shown on the diagram. Method A is preferable and resistor R1 should be sized to pass about 1 milliamp. Method B is OK but requires an extra diode to compensate for the voltage drop across the optoisolator transistor when it is conducting. If this diode was not used the transistor in the ULN2003 would not be able to turn off fully. Resistor R2 would also be sized to pass about 1 milliamp and the voltage drop across the drivers internal resistor will have to be taken into account. As the peripheral drivers in the ULN2003 require only a small control current optoisolators with non darlington transistor outputs such as the 4N35 can be used. There are other devices that are similar to the ULN2003, such as the ULN2803, which has eight drivers in an eighteen pin package. Other devices have higher or lower ratings and/or lack the protection diodes built into the ULN2003 package.



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