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 ?

 

Probably. What are the design constraints and voltages are available?

 

Can this be done simpel ?

Define simple. If I understand your needs correctly, I would use a comparator with a small amount of hysteresis. There are other options also. If you already know what that means, it will be simple.

 

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.

 

You just convinced me to put the laptop down and go outside. Sunny and warm here, which is rare for the season.

 

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.

 

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.

 

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

 

I believe a TL431 voltage monitor circuit would work for you. Here's one example using an LED, which you would replace with your relay coil.

 

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 ?

 

What transistor? The TL431 has one inside it that should be adequate for switching your relay.

 

Define simple. If I understand your needs correctly, I would use a comparator with a small amount of hysteresis. There are other options also. If you already know what that means, it will be simple.

or an op amp with a zener diode as a reference voltage

 

Something like this, ?
Will it work, ? use lamp as relay....

Have calulatet 10k and 2k4 gives a 12,916 v, and that is fine,

 

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.

 

So something like this will deffintly do the job, as i understand the TL431

 

Zener diode(s) in series with the base.

ak

 

So something like this will deffintly do the job, as i understand the TL431

That should work.

 

I believe a TL431 voltage monitor circuit would work for you. Here's one example using an LED, which you would replace with your relay coil.

Does the relay draw 100 ma?

 

It only draws 12v/ 390 ohm coil = 30 mA

 

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 ?