relay found in switch mode power supply

Posted on Feb 6, 2014

In the example above a low voltage (12Vdc) is used to turn on a relay to switch on a 240V ac main circuit. Please note that there is no electrical connection inside the relay between the two circuits, the link is only magnetic and mechanical. If there is Zero (0) volt at the Base (B) of transistor Q1. The transistor is open and therefore current is not able to flow from the 12V rail

relay found in switch mode power supply
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to the Ground and hence the relay contacts are open and the 240V ac Bulb is off. Now if a Voltage is applied at the Base (B) of transistor Q1. The transistor will close(ON) and the current will start flowing from the 12V rail through the relay coil to the Collector of the Q1 down to the Ground. When the current flow through the coil a magnetic field is created around the wire and this magnet (electro) will attract the contacts of the relay. Once the contacts closes then the other circuit is switched on and the Bulb light. When the Television is first switched on a signal is send from the TV micro processor to the base of Q973 just for a moment (2 seconds). When this signal reaches the base of transistor Q973 the transistor is turned ON and the 12 Volts DC moves to the ground via relay coil. When current flow through the relay coil it behaves like a magnet and relay switch (contacts) is closed. When the relay switch is closed the current flow through the relay on the 240V ac into the Degaussing Coil and the screen is cleaned. Coil: Use your meter set to diode test and you should get a reading if the coil is okay. If no reading then the coil is open and therefore the relay is bad. Contacts: To test if the contacts are working you need to apply some Voltage (5V dc) to the two pins of the coil and if the contact is working then you should hear a click sound from the relay.

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