1st Post - advice on project

[Achillies]

Hi All,

I am new to this forum and am also quite a novice with electronics so the following question may be a rudimentary error on my part.
I have built a pretty simple circuit board to carry 5-off relays for use on my cnc milling machine. Everything seems to look correct and also simulates ok on my circuit building program but when I apply the 24V the relay coil immediatley energises.
I guess that this is down to a poor selection of the BC547B transistor as this is 'switch' that should logically stop the flow of electric to the relay coil.
The relay is an Omron G5LA 24VDC item with 15mA rated current and a coil resistance of 1600 ohms.

Any and all guidance would be greatly appreciated but please keep your terminology simple as, like I said, I'm pretty new to this.

thanks ...Richard

 

[ericgibbs]

hi Richard,
Try connecting a 10K from the base to emitter for each BC547 transistor.
The leakage current thru the opto detector could be switching ON the BC547, this is a common problem.

 

[Achillies]

Hi Eric,

Thanks for your prompt reply to my question.
I just added the 10K into the circuit in the simulator which had the effect of not 'throwing' the relay when switched.
I'll go downstairs and add the 10K to the built board and see if that sorts it ...give me an hour!

thanks ...Richard

 

[ericgibbs]

Hi Eric,

Thanks for your prompt reply to my question.
I just added the 10K into the circuit in the simulator which had the effect of not 'throwing' the relay when switched.
I'll go downstairs and add the 10K to the built board and see if that sorts it ...give me an hour!

thanks ...Richard

hi,
It may require a resistor value higher than 10K, try increasing the value in 10l steps, 10k ,, 20k etc.

 

[Achillies]

Tried with the 10K resistor and relay didn't throw at all!
The problem is now that I built the finished board and then grafted on the 10k so it's getting a bit messy.
I will start a-fresh tomorrow and do the sensible thing and build on the breadboard.
Curiously the only relay of the five to throw with the 10k fitted had a rougue coil resistance. Very odd for an Omron massed produced item.

...Richard