/diy/ - Do It Yourself

So I was pulling apart an old hotline electric fencer energiser, it bumps up from a 6v dc battery to pulses of a couple of thousand volts.

It had a low voltage circuit that then connected to a transformer.

In the low voltage circuit there was this big round thing about an inch high and 1.5 across bolted flat to the board that I don't really recognise any clue what it might be? sorry I don't have any images to share of the actual board

I found a replacement board on sale on ebay with the same component in the bottom left

Any idea what it is or does?