Zener Voltage Divider Circuit

[plasmahunt3r]

I am using a Zener along with 2 Resistors to form a trigger circuit for a transistor but I cannot find a formula to use.  The circuit uses a 13v Zener along with a 120 Ohm resistor and a 2k Ohm resistor, and at 13.4 volts, the voltage output to turn on the transistor is .6 volts.  This is the result I want.  I can find a regular voltage divider formula, but I cannot find a formula that applies when using a Zener in the middle.  This circuit effectively modifies a 13 volt Zener into a 12.8 volt Zener.

Does anyone have a formula for this circuit??

I have seen this technique used in motorcycle voltage regulator circuits.

[OperaHouse]

Zeners are rather imprecise.  I bought a hundred 12V zeners and they were all around 1.5V.  You can do the same thing with a TL431.  Even a generic FET will have a sharper transition than a transistor.  I suggest always using a resistor in series with the transistor to limit base current.

[tanner0441]

Hi

What is wrong with a 7812 voltage regulator 15 to 30 V in 12V 1A out. if you want more current put a 4007 in the ground leg of the 7812 and feed it into a darlington buffer..

Brian

[plasmahunt3r]

I replaced the 2000 Ohm resistor with a 1K pot.  This gives me flexibility to dial in a desired trip voltage.  At 100 Ohms, the trip voltage was upped to 14.4v while still using the 13v Zener.  At 20 Ohms, it upped the trip voltage to 17.2v while still using the 13v Zener.

Using a Zener with a resistor voltage divider does allow limited capability to dial in a specific voltage.  Just use a Zener that is close to desired trip voltage.  There has got to be a formula for this, but without one, I can just adjust the pot until I get the desired voltage.

[JW]

Try this 

https://www.allaboutcircuits.com/tools

There are 4 pages of calculator's

[OperaHouse]

Is this is a continuation of the current limiter circuit.  A transistor will have a certain gain.  The current thru the base will be multiplied by that.  This will be how much current can be sunk by the collector. That will vary with the voltage somewhat, especially once a zener is introduced into the divider.  You would be better off using a TL431 to drive a PNP on the high side as the on off. That transistor could drive the base of the power transistor.  The current change from 12.5 to 14.5V wouldn't  be that much. Ideal would be extra circuitry to make a constant current source with current sensing.

[plasmahunt3r]

Yes OperaHouse.  This is an extension of the current limiter circuit.  In the boat, I have a VSR that kicks in at 13.2v, that in turn allows the battery to battery charging. 

What I am doing with this version, is to eliminate the VSR, but combine it's voltage trigger function with the current limiter circuit to keep a house battery in my truck.  I have the circuit working like I want.  There are a lot of smart people out there, and I was hoping some one would know the formula for Zener with Voltage Divider.  I can get close with R1/R2 + Zener Voltage when R2 less than 1K, but that isn't totally right.

My Truck battery at rest is around 12.8v.  When I start the truck, the alternator kicks in at around 14.3v, and winds down shortly afterwords to 14.1v.  So I set a trigger voltage of 13.4v, which will kick in when I start my truck.  This will charge my house battery.  When the truck is off, the start battery voltage will drop off below 13.4v and turn off the circuit.  I don't want there to be a connection between the start battery and the house battery unless the truck is running.  I don't want to drain the start battery while the truck is not running.  And since I am not constantly using the house battery, I want to limit current and voltage to the house battery to keep from destroying it.  The house battery is more like a standby battery.  Little used but always charged and ready.

There is a commercial towed battery to battery charger for $55 that I could buy:  https://rvibrake.com/products/towed-battery-charger?variant=8804388741&_vsrefdom=adwords&gclid=EAIaIQobChMItK7vvIDY3gIVxbXACh0WqAdmEAQYASABEgJ1YfD_BwE

But it is more fun to make one that works.