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DiscoverCircuits.com -- Hobby Corner
"Hobby Circuits for
your Hobby Projects"
Last Updated on:
06/19/2009 02:16:10 PM
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Solid State Relay Circuit Requires
Ultra Low Control Current
designed by David Johnson, P.E. |
| Most solid state relays
require at least 5ma and often more input control current, to fully turn on the device.
But such current levels often force battery powered circuits to use excessively large
batteries. The relay hobby circuit shown below demands only 50uA of input current.
This about 100 times lower than that needed by a typical optically isolated solid state
relays. The circuit uses a combination of a high current triac and a very
sensitive low current SCR, to control up to 600 watts of power to a load, while
providing full isolation and transient protection. |
| At the heart of the
circuit is a Darlington type opto-isolator A1 from NEC. This device needs only
50uA of current through the LED section to activate the Darlington side. A bridge
rectifier and a couple capacitors, strips off a bit of current from the 120vac line,
through the load. A zener diode limits the generated DC voltage to 8v. When
the opto-isolator is turned on, current is routed to the gate of a sensitive SCR.
When turned on, the SCR routes current pulses to the main control triac, through a
bridge rectifier. A 15v zener delays the trigger point of the triac slightly, so a
minimum 30 volts peak to peak is always available to maintain current to the SCR
circuit. |
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Click on Schematic below to view PDF
version of this Circuit |
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