Crowbar for Aeroplane

Hi everone, nice to be here
Problem overload protection for an Aeroplane for radios and transponder min 10volts max 16v

Crowbar circuit

Scr + 15 volt zener>>>>>> problem as temp goes up the zener`s accuracy goes down ie now 17 volts meaning a fried radio.

So a 5.1 volt 1watt zener with a 1/4w resistor.

scr data www.ortodoxism.ro/datasheets/stmicroelectronics/7477.pdf

Thanks in advance

And an early happy new year to everyone

Use a 5.6V zener and a potential divider. 5.6V zeners are more stable the avalanche and zenner effects cancel each other.

https://en.wikipedia.org/wiki/Zener_Diode

Hero999 said:

Use a 5.6V zener and a potential divider. 5.6V zeners are more stable the avalanche and zenner effects cancel each other.

https://en.wikipedia.org/wiki/Zener_Diode

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Interesting Wikipedia says 5.6V, I've always understood it to be 5.1V?, presumably Aeroplane has as well, hence his choice. I feel a long and heated discussion to follow!

You could be right. I wonder why 5.1V appears to be a common value than 5.6V especially when the latter is an E12 value and the former is an E24 value. Perhaps it's because it's a suitable voltage for powering digital logic. There again, casting my mind back to college I think 5.6V was mentioned as being the optimim voltage.

If you want stability try a **broken link removed** or a **broken link removed**.

I seem to recall that 5.1V has a zero temperature coefficient, below that it's negative, and above that positive (or the other way round).

A 5.6V zener has about +2.1mV/C tempco.

Therefore one can make a very good and stable zener by putting in series a silicon diode which has a tempco of about -2.1mv/C.

Now the total voltage becomes 6.2V. One can buy a zener diode with the forward diode built-in and they are called temperature compensated zener diode. They are the 1N82X series.

https://www.answers.com/topic/zener-diode

The effect of temperature on the breakdown voltage can be nulled by having a second forward-biased junction, which has a small negative temperature coefficient, in series with the Zener junction. Such a device is called compensated Zener and has a breakdown voltage of 6.2 V rather than the normal 5.6 V (for the smallest possible temperature coefficient)

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Maybe adding a normal diode in series with a 5.6V zener might help?

Wikipedia isn't ALWAYS right... Sad really...

Well it is in this instance.

From Motorola, I think the 5.1V wins on the attached graph.
This is what I was taught many years ago.
Some types of zener have wider tolerance than others, in critical applications (mainly military) it used to be necessary to measure them before fitting to board. Thankfully there are better and more suitable devices now but modern manufacturing has made for a tighter spec. Imagine what we paid for 1% resistors years ago, they cost just pennies (cents) now!

Les

First up thanks for the replies.

2nd up everyone here has much more knowledge about all this, so I need a bit of hand holding.

3 Yes I could use a ic to ref the voltage.
3a 5.6 vs 5.1
3b IN82x temperature compensated zener diode ,not at Dick Smith or Jaycar


4 The idea is to make a website with easy to follow instruction for aviation nuts, with no electonics experience.

5 I had made a prototype with a scr case anode soldered to a spade connector making it truly small with no pcb etc and able to be plugged into a 10amp resetable circuit breaker (range avai 3,5,10,15) >>>>> I just lookup the resetable breaker not good news 500% of rating 1.5 seconds hmmmm 50amp short, so the new idea is one crowbar per 3 or 5amp breaker ...




Thanks again I have to go through the info again