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problem with relay connection..

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Deexith Hasan

Advanced Member level 4
Oct 24, 2014
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i'm building a voltage regulator and using kt-909 relay 12v..when the relay is off the supply voltage is 15.3v when the relay is on the supply voltage gets reduced to 13v y??

i want to main constant voltage when relay is either on or off how can i do it?

my supply voltage is 15.3v & relay is 12v and coil resistance is 170 ohm do i need to connect a resistance in series with relay ??

Try adding a fixed 150R resistor across the output terminals of the power supply.

Re-measure the Vout with relay energised and de-energised, let us know what you measure.

Use a 2W to 5W resistor.
The problem is the regulation of the 15V supply. This is because when it has to deliver an extra 70 mA (12/170) its output falls from 15v to 12V. So the design of the power supply must be investigated or it should be followed by a 12V regulator IC. This will maintain the output at 12 V providing the input to it is greater then 12.5V .

Your supply seems unregulated type, which requires a preload of 5~10% to reduce the Vpk towards Vavg.

You can also reduce coil current if necessary.

Show schematic and ratings.

this the schematic diagram...i think this circuit will not draw more than 100ma and i'm using 500ma transformer

R1, R2, R3 = 2K2,

P1and P2 = 10K preset,

C1 = 220uF/25V

All diodes are = 1N4007,

T1 = BC557,

Relay = 12 V, 400Ohms, SPDT,

op amps = 2 opamps from IC LM 324

Zeners = 4.7 volts, 400mW,

Transformer = 12V, 500mA

You have an unregulated supply and two Voltage comparators to define when the Relay should be enabled.

To regulate better you can use an LDO or power Zener to give a dynamic load.
Without more details on final goal, perhaps a ~13V 2W Zener will suit your needs to limit the no load voltage.

Another cheapo way is to drive a second transistor via a 1K resistor from the collector of T1, this transistor has a load resistor with the same value as your relay. So when your relay is not consuming current the new transistor is on and its load resistor is drawing the same current. This keeps the load on the Vcc line constant, its actual value will though wander up and down with the incoming mains voltage.

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