Thursday, May 12, 2011

Relay found in switch mode power supply (s.m.p.s)

Relay found in switch mode power supply (s.m.p.s)

Sometimes in CRT Television s.m.p.s you will find relay in the power supply. In this lesson I would like to have an overview of this component.

First what is a relay? Basically a relay is a switch which is operated electrically.

 The internal structure of a relay is composed of:

1. Coil – work with low current
2. Contacts-Work with high current

Basic relay Circuit

A relay allows one circuit to switch ON a second circuit which is completely separate from the first.

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 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.

With that basic knowledge let us now move on to see how relay are used in switch mode power supply (S.M.P.S) to degauss the CRT tube.

Below is the actual circuit of relay in a JVC Television power supply 

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.

The picture of the actual relay used in this supply

Testing a relay:

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.

Thanks you guys
Let meet in our next class.


    ok wellcomback to weblog .
    also this article like other articles is full of golden points for me

  2. whoah this help me with my problem, my tv wont power

  3. Hi Humphrey,my problem is with a Panasonic EVO 54 CRT tv in that the relay switching transistor Q803 and diode D802 are completely burned out so I do not know their values and I can't seem to find the schematic diagram