Start transistor when voltage is correct

Thread Starter

FroceMaster

Joined Jan 28, 2012
683
Hi
Need to do a circuit that will set volt to a transistor when the supply is more than 13v and shut off again when under 13v

Can this be done simpel ?
 

#12

Joined Nov 30, 2010
18,224
My dental appointment got cancelled today, so I'm bored enough to do useless stuff like answer the question as it was asked instead of playing 20 questions with the T.S.
 

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wayneh

Joined Sep 9, 2010
17,189
You just convinced me to put the laptop down and go outside. Sunny and warm here, which is rare for the season.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
683
HAve now drawn a little sketch,

When voltages goes over 13v the transistor should open fully and "pull" the load.
When voltages is below 13v the transistor should completly close.
 

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wayneh

Joined Sep 9, 2010
17,189
Is the voltage supplying the load the same as the one you want to control the switch, or are they independent?

You may be able to use a simple resistive voltage divider to bias the transistor.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
683
Is the voltage supplying the load the same as the one you want to control the switch, or are they independent?

You may be able to use a simple resistive voltage divider to bias the transistor.
Source is the same but need to control the load with max 10mA and the load is 10A by a relay. So the transistor only need to draw 100 mA
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
683
Sounds a little difficult . All that calculate to find out resistor and will it be able to draw the relay ? Can i drop the transistor ?
 

wayneh

Joined Sep 9, 2010
17,189
I don't believe you need R3 in parallel with the relay although I'm not certain. It is in the example circuits to prevent the LED from glowing dimly while the TL431 is "off". That's not relevant to your relay. Again, I'm not 100% certain and anyway it wouldn't waste much current.

But you should add a reversed-biased diode there. It will not conduct in normal operation, but will allow the magnetic field in the relay to collapse when it is switched off. This will protect the TL431 from a potentially high voltage spike. Something like 1N4007 or any other silicon diode rated to at least 2X the relay voltage and same or more current. Very cheap insurance.
 
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Thread Starter

FroceMaster

Joined Jan 28, 2012
683
The project is now running, but some om them gives a problem,
When voltage drops from ex 14v down to 12,9 the relay cuts off. ( like it should)
but then the voltages stabalize on battery and raise a little bit, and relay draws again.( wrong )
flixkering very much..

I think the best is to say start at 13.0 V and cuts off again at 12,8-12,9 V.
How should i do this ?
Or any other soulutions ?
 
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