protectiontl431triacvoltage
So I'm trying to create a OVP using TL431 an a triac, i have checked the datasheet but since i suck at math i don't really get how to set the ref voltage..
datasheet: http://www.ti.com/lit/ds/symlink/tl431b-q1.pdf
My example circuit: 
I'm having a 19v power supply, and i want to make it so that if the supply for some reason should reach over 20v, it blows the fuse.. Any tips? Thanks!
To scale your PWM output voltage (0 - 5 V) up to 0 - 12 V you'll just need a an op-amp in the standard non-inverting configuration with gain of 2.4. You could do this with just about any commodity op-amp. You can also add low-pass filtering to this circuit to (further) reduce the ripple from your PWM setup.
For this to give 0-12 V output from the '317, Vcc needs to be 14-15 V (to bias the LM4041 properly at 10.8 V out), and you also need a negative rail low to let the op-amp generate -1.2 V when you want the LM317 output at 0 V.
You also want to remember the LM317 can only source current, not sink. So if your load will require some current to be drawn out of it to reach 0 V, you'll need either some kind of pre-loading (a pull-down resistor from the output pin to the negative supply rail)
I think no capacitor at all is appropriate. The graph indicates it's stable at any anode current with no capacitor.
If you did put a capacitor on there, for your stated reason, I would definitely not put it where it is shown, I'd put it across the gate-MT1, and no more than a few nF.
In the shown position, if the power is applied suddenly, the triac may trigger via the cap, causing the crowbar to short the supply.
Best Answer
That might work, but try flipping the triac so MT2 is grounded and MT1 connects to the input voltage, and connect the 'cathode' of the TL431 ONLY to the gate.
Calculate the gate resistors as @Tony suggests, so for about 19.5V you could use 2.49K and 16.9K, standard E48 values.
This circuit won't work with an SCR because SCRs don't generally come in the opposite polarity.
Edit: Like the below
simulate this circuit – Schematic created using CircuitLab
When the voltage at the REF input of the TL431 exceeds ~2.5V the gate current increases until the triac triggers and shorts out the supply, blowing the fuse. The triac is operated in quadrant III (gate and MT2 both negative wrt MT1), which is always acceptable. Below the trigger voltage the TL431 conducts < 1mA which should not trigger most triacs. If it is a problem (or just to be sure), you can connect a 100 ohm resistor from MT1 to gate.