Problem with relay driver as the LEDs of relay are always on

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auto_mitch

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i have built a pcb with an atmel tiny2313 which drives a few relays. the mcu drives the relays through the 2N3904 transistor. parallel with the relay's coil a 1N4148 diode and a 1k resistor with a led are connected. the power supply of my circuit is 24V. The problem is that, without the mcu on board the leds of each relay are always on. what is going wrong?
 

Re: relay driver problem

Bases of transistors are floating and can pick-up 'something' ..
Try to add 10-100kΩ between the Bases and GND (Emitters) ..

Regards,
IanP
 

Re: relay driver problem

I agree, the base of the transistor is not completely off when the MCU is not applying power.
 

relay driver problem

what about using some simple circuit. See this circuit
**broken link removed**

just one resistor, no problem and a commonly used one

Bibin John
www.bibinjohn.tk
 

Re: relay driver problem

you can drive relay by ULN2003 , cost is also very low and easy to interface with microcontroller
 

Re: relay driver problem

If your relay load requirements are not too heavy (over 1/2 Amp on the contacts) I can recommend the Fairchild HSR312 which I use in several projects. Opto-isolated, up to 400V for the load, super-low "contact" resistance, and bi-directional (AC capable) on the output.

http://www.fairchildsemi.com/ds/HS/HSR312.pdf

**broken link removed**
 

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