MOSFET reverse polarity protection + crowbar circuit combined

Thread Starter

Bantha Poodoo

Joined Jun 8, 2018
3
I'm trying to combine two well known circuits, one is reverse polarity protection with MOSFET, another is crowbar circuit against overvoltage. Before trying to make the real circuit on the breadboard, I've decided to simulate it in TINA. However, I can't make it working. Separately both snippets work. Below is schematic and simulation result. It seems that SCR is always on for the reason I cannot explain. I've changed SCR to TRIAC (it was a wild guess), and now circuits works as expected. Zeners are rated 15V. In real application ammeter would be replaced by a 1.5A fuse. The power would be supplied by a off the shelf AC/DC 12V power supply, so protection is required in a situation when user accidentally plugs in wrong type of supply (24V, or reverse polarity). The load is actually a quite complicated electronic device with MCU, joystick, buttons, LCD screen, some BT transmitters and a couple of stepper motors.
S5.png S6.png
Why first circuit isn't working as I expect in simulator? And will the second one work in a real application?
 

crutschow

Joined Mar 14, 2008
24,404
It would appear that both circuits should work but you probably should put a small resistor in series with the SCR/TRIAC gate to limit the peak gate trigger current.

Why are you using large and expensive 50W zeners?
They are dissipating no power under normal operation.

Put the MOSFET reverse voltage protection circuit in front of the crowbar circuit and you won't need D1.

You don't need Z1 unless the supply voltage is more than 20V.
 

Thread Starter

Bantha Poodoo

Joined Jun 8, 2018
3
It would appear that both circuits should work but you probably should put a small resistor in series with the SCR/TRIAC gate to limit the peak gate trigger current.

Why are you using large and expensive 50W zeners?
They are dissipating no power under normal operation.

Put the MOSFET reverse voltage protection circuit in front of the crowbar circuit and you won't need D1.

You don't need Z1 unless the supply voltage is more than 20V.
Thanks! I was advised that I cannot put reverse voltage protection MOSFET first, because in case of overvoltage the crowbar current will flow through MOSFET first, but since source will be close to ground, MOSFET will be off, and all that current will flow through intrinsic diode, which will blow in no time.
Z1 is used right because input voltage can be 24V - some equipment which works together with my device is using such voltage. Mixing power supplies can be done easily, unfortunately.
Zeners in real application of course will be small ones, 1N2804 are just added by default in TINA TI.

The question is, why SCR isn't simulated correctly?
 

AlbertHall

Joined Jun 4, 2014
9,333
Zeners don't have a sharp right angle knee. They will start to conduct before they reach the specified voltage. As you are using such large zeners that current will be greater than with a small zener.
The SCR will likely have a more sensitive gate than the triac and so the zener current may be enough to trigger the SCR but not enough to trigger the triac.
 
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