# Start transistor when voltage is correct

Discussion in 'General Electronics Chat' started by FroceMaster, Oct 18, 2016.

1. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
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 ?

2. ### dl324 Distinguished Member

Mar 30, 2015
3,377
651
Probably. What are the design constraints and voltages are available?

3. ### wayneh Expert

Sep 9, 2010
12,379
3,232
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.

4. ### #12 Expert

Nov 30, 2010
16,670
7,314
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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5. ### wayneh Expert

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

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6. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
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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7. ### wayneh Expert

Sep 9, 2010
12,379
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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.

8. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
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

9. ### wayneh Expert

Sep 9, 2010
12,379
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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.

10. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
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 ?

11. ### wayneh Expert

Sep 9, 2010
12,379
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What transistor? The TL431 has one inside it that should be adequate for switching your relay.

12. ### renemonte New Member

Oct 7, 2016
13
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or an op amp with a zener diode as a reference voltage

13. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
Something like this, ?
Will it work, ? use lamp as relay....

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

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14. ### wayneh Expert

Sep 9, 2010
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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.

Last edited: Oct 20, 2016
15. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
So something like this will deffintly do the job, as i understand the TL431

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16. ### AnalogKid Distinguished Member

Aug 1, 2013
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Zener diode(s) in series with the base.

ak

17. ### #12 Expert

Nov 30, 2010
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That should work.

18. ### ronv AAC Fanatic!

Nov 12, 2008
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Does the relay draw 100 ma?

19. ### FroceMaster Thread Starter Member

Jan 28, 2012
402
4
It only draws 12v/ 390 ohm coil = 30 mA