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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Wikipedia
The term 'pulse transformer' is commonly used in many quite different contexts, e.g. 'Trigger transformer' for scr etc. where the transformer passes a pulse - but the waveform is not critical .
To me the main characteristics are tight pri:sec coupling (low leakage flux/inductance) and basically all of the other properties mentioned in the wikipedia article.
As you referred to an 'electric fence' what do you mean by 'pulse transformer' ? - trigger transformer for an scr?, step-up transformer? .....
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
The transformer sees a pulsed input from the discharge of a capacitor. I charge a capacitor to 500v and use an SCR to discharge the capacitor through the transformer primary. The secondary voltage is close to 20x the primary voltage. The pulse is a few microseconds in duration.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Well in this application you would want pulses to give the animals a quick shock, rather than say a MOT which would just fry the animal as soon at it made contact to the fence.
If you shorted out the fences HT output it would not trip the MCB where as the MOT would, which is needed outside where a fallen tree or plant could cause the fence to touch earth.
So I guess you could say your pulse transformer setup is current limited.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
For this application things like waveform integrity, close pri:sec coupling etc. are not important.
The magnetic path can be closed (e.g. 'flyback' core without airgaps) or open (e.g. ferrite rod or bar) The only two important things are; 1 Insulation. 20 kV insulation is not too difficult for pulses, many small layers for the secondary helps. ........the main concern here will be moisture ingress as the unit will be outdoors. Potting in wax or epoxy or similar would be required. 2 Core saturation, the product of (core area) x (number of turns) should be enough to handle the integral of (V.dt) ... volt.seconds. ....... as few primary turns as possible is best as this requires proportionately less secondary turns.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Sulaiman wrote ...
........the main concern here will be moisture ingress as the unit will be outdoors. Potting in wax or epoxy or similar would be required.
I second this. When I made a homemade step-up pulse transformer using nothing but electrical tape for insulation it would work fine but testing it outside in the dark would show that every-time there was a pulse of around 20kV on the output corona was also visible around the transformer at the same time of the pulses. Especially if the output was unloaded, even though it did not arc-over it would probably eventually eat away at the insulation.
This is a homemade one I made running. It was a really crude job but it worked and got me interested in high voltage.
The core is a ferrite rod. Its all wound on a piece of PVC pipe from wickes, secondary coil is lots of turns of 40SWG enamelled magnet wire (one layer) and the primary is a few turns of single cored copper wire from some spare twin and earth mains wire.
Doubt it would last long out in the British weather as a fence energiser though
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pulse transformer vs other transformers
