Breakwire alarm (squib) utilising a transformer

Question

Following my yesterday post which was a break wire alarm based on logic gates to drive a siren for the protection of a camp site, I have decided to work on a second concept that uses completely different circuit based on a simple transformer design. Considering that the main uninvited visitors of the camp site are stray dogs and wild boars, I've decided to use squib (firecracker) as the main load simply because I've found it extremely more effective than siren, which doesn't seem to scare them anymore.

In the following circuit the primary of T1 is energised upon closure of switch 1. The role of R2 is to limit the current coming from a 9v battery. When the wire is broken by animals, the sudden collapse of primary will induce a transient in secondary, which in turn, heats up the extremely thin nichrome wire (0.02mm) of the squib. The switch 2 is a safety measure which is closed at first to bypass any transient when the circuit is powered up first and then it's opened by the operator. My question is regarding the transformer itself, what type of transformer would work best? My assumption is step down type with a very high number of thin wire turns at primary and a secondary with thicker and fewer turns to provide sufficient current for the squib. Please share your suggestions.

Answer 1

I'd suggest you need to invert your architecture and get rid of the transformer altogether.
Perhaps something like this:

^{simulate this circuit – Schematic created using CircuitLab}

With this you'd have 90 uA loop current and plenty of energy to fire the squib even with only a small 9 V battery. I've included both your 'safety' switch and a loop test switch so you know that the break loop is ok before you toggle the safety.

Answer 2

You need electrical data on the squib before continuing. In particular, you need to know how much voltage and current must be supplied to the squib for how long to set it off.

I expect the total energy required would be significantly more than what can be stored in the magnetic field of a reasonable transformer. You're also only allowing 90 µA to flow thru the transformer. That's going to require a very large primary inductance to store a reasonable amount of energy.

Your circuit is also very wasteful of the limited battery energy because the energy has to be maintained in the transformer core all the time, and most of the time it won't be used. It would be much better to use a transistor to invert the current on/off in the loop. The squib would be actively driven directly from the battery whenever the loop is opened.

Again though, start with the squib specs. Making a circuit to drive it when a connection is opened should be trivial.

Answer 3

Simpler to do something like this.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 4

I'm a hobbyist but here's my view on the matter:

Using a transformer to match up to the very low impedance of your squib may seem like a good idea (and it is), but ignoring the details of your primary circuit for a moment (it's not relevant to me for now) the problem will be in transferring sufficient energy to guarantee an all-fire specification for your squib.

You've provided zero specifications for the squib. Estes provides very clear all-fire specifications for their rocket engine squibs, by comparison. You have to deliver a certain number of Joules (about \$\frac{1}{2}\:\ J\$) within a certain period of time (\$50\:\textrm{ms}\$) in order to meet their all-fire specification. Their specification is actually rather difficult to meet with a transformer arranged as you have it.

Review this answer I provided earlier regarding the basic energy equations for capacitors and inductors.

A capacitor's energy is \$\frac{1}{2} C V^2\$ and is stored as lines of electric force in vacuum regardless of the dielectric itself (which actually acts to counter the electric force in dielectrics), but it's easiest to imagine that once you've selected your capacitor the energy present is a matter of how much charge stored on it. Matching up the capacitor to a particular squib would be about meeting the specification I mentioned earlier. You could store a lot of energy on a super-cap, for example, with very low voltage. But it wouldn't deliver the required energy in the required time since the voltage present would be too low to induce enough current in a short enough time. Most of the energy would stay on the capacitor. So the selection of the capacitor depends upon the load. The pairing must be arranged to meet the all-fire specification.

An inductor's (transformer here) energy is \$\frac{1}{2} L I^2\$ and is stored as lines of magnetic force in vacuum space regardless of the core material used. (In your case, you may want a gapped core.) But again, it is easiest to imagine that once you've selected your inductor/transformer, the energy present is a matter of Webers (volt-seconds, and the equivalent of charge on capacitors.) Again, the selection of the inductor/transformer depends upon the load and the pairing must be arranged to meet the all-fire specification.

For inductors/transformers this is often more of a problem. You may have noticed that the volume of a capacitor (for a given manufacturing technology) is proportional to the energy it can store. The same is true for inductors/transformers.

If you had a specification (you don't, yet), it might be possible to compute the required core volume needed to store the energy you want held there. (You might read this post from me which is one place where I discussed core volumes.) But that can't be done just yet because you don't have a specification. And I suspect you will be disappointed at the sheer size required, once you are able to provide an accurate specification (considering that you are imagining a single pulse here.)

Answer 5

This is the revised version of the logic circuit from yeserday including filtering components. I have also modified the load so it's using squib (firecracker) rather than buzzer. With extra safety clamping circuit. It works like a charm on breadboard.