Oil furance ignition transformer. Proper resistance for primary

Quite possible! Lots of leakage inductance in something like this.

That's probably worst case, and with lots of phase lag (low power factor). No way is it delivering 250 W to the spark, that would burn up the points quickly.

If it is bad, most likely the secondary has broken down insulation.

Classic problem in small engine ignition coils, too. They run fine at first, then breakdown with internal sparking when hot. If the spark weakens and then fails when hot, inspect the secondary wires and spark points for breakdown, and if none is found, replace the transformer. It is a SIMPLE device. If power is applied, you shoudl get spark. if not, it is either sparking someplace else or within the transformer.

Jon

There are two types of transformer controls: interrupted and intermittent. If you have an interrupted control, the transformer is actually shut off s ome time after the flame is established. This prolongs the life of the ign ition transformer and points to almost a decade. Intermittent means the tr ansformer runs the entire time the burner does, so the trans and points are being used even after they are no longer required.

Someone already mentioned that coils can fail when hot, and burner transfor mers are no exception. And btw, when they're right, they will fire a gap w ell over half inch if the points are shaped properly.

If you live in the States, home stores such as Home Despot carry "ignitors" , which are drop in replacement for the old style transformer. They mount and are wired in the same manner, but use a high freq switch mode arrangeme nt. They are less than $40, so it's a good idea to have one at home as the se things are generally timed to fail nights, weekends, and holidays.

SOunds ok to me.. The wire is of small gage so you would expect some DCR readings of significant value, especially on the secondary side. Jamie

"micky"

** The primary impedance with no load is actually much higher than your naive calculation.

Ohms law has nothing to do with it.

If you check similar size 120VAC transformers, you will find their primary resistances are all around 3 ohms.

With 2 amps of current in the winding, the heat generated ( given by I squared R) = 12watts.

Expect a similar amount of heat in the secondary and you can see why the things get quite warm.

.... Phil

What you have sounds correct.

I have here a new Dongan 250VA, 8000 volt 23ma ignition tranformer. It's oil filled and has lovely steatite insulators.

The primary resistance is 2.7 ohms and seconday is 18.5k. For a plain shunted ignition transformer the numbers look safe, and it's close to what you have.

Thanks, and thanks everyone for your helpful answers.

It seems there was probably a problem with the transformer anyhow, only when it got hot.

Because my oil nozzle wasn't changed every year (not since fall of 2012, actually) it failed (clogged) this past December after a little more than 2 years.

When I replaced it, the furnace ran fine for 90 minutes and then stopped. I reset it but it happened again after a while. And maybe a 3rd time.

All of the houses here started with the same furnace, and when one of my neighbor's got a new furnace, I asked if could have the old burner. So I have spare parts for almost everything. I changed the transformer, not too long after the OP, before New Years, and it's worked fine ever since.

I'm thinking it may have had this problem for many months even, but normally the furance runs no more than 20 minutes at a time. The house is always about 68, and to keep it at 68 just takes no more than that. The nozzle is usually replaced when it's not cold out. But by letting the house get cold, 50^ and colder than that out, it had to run for 90 minutes, heating the transformer more than normally.

If I had turned off the furnace after twenty minutes and let it cool for

20, over and over, I might have gotten the house warm again with the old transformer. Of course this is a lot more convenient and it didn't cost me anything. And I wouldn't have wanted to try such an unlikely plan knowing no more than I did then.

This is all contradicted by one occasion in December I think the furnace stopped when it had not been running long, but I've rather successfullly ignored.that.

I should have measured the resistance of the replacement, but, typical for these days, I forgot. It's no newer than my old one.

FYI, there is a new design that isn't a simple transformer. They have them on the Home Depot webpage, under oil furnace transformer (but not ignition transformer)

. Apparently they work as replacements for the old style. (although in the pictures they were hinged at the front and mine is hinged at the left side, but a furnace parts place would probably have greater variety. )

Thanks, Leader. Cydrome salute W