Oil furance ignition transformer. Proper resistance for primary.

Is it possible that a winding with only 4 ohms DC resistance would
have 52 ohms impedance at 60 Hertz**?

I have an ignition transformer for an oil furnace burner.

It's designed to make a continuous spark to ignite the atomized fuel
oil sprayed into the firebox. It runs on 120 VAC. And the
secondary is meant to provide a voltage that will jump across a
specified 1/8" gap, but probably will jump a 1/4" gap. I see that the
transformer secondary is rated at 10,000 volts.

Disconnected and measured with an ohmmeter it shows 20,000 ohms in the
secondary, and 4 ohms in the primary!!

Is that possible? Esp. the primary.

I don't know how to measure impedance at 60 Hertz.

The transformer primary uses 2.2A at 120 volts, according to its label

(The current in the secondary is 23mA. The transformer is a lot like
this one:
https://www.grainger.com/product/ALL...sformer-23M552
)

E=IR, R = E/I = 120/2.2 = a little over 52 ohms, right?

That would make the impedance at 60** Hertz about 52 ohms, right?

Is it possible that a winding with only 4 ohms DC resistance would
have 52 ohms impedance at 60cps**?

Or is the transformer bad?

Or what am I doing wrong?

The transformer seems to fail when it's hot, but it wasn't hot when I
measured the resistance of the windings.


**Or maybe 15 to 30KHz. Some replacement transformers specify that,
but I think it's only for quite modern furnaces. I don't think my old
furnace would have that. It has a circuit board with about 30 parts,
but I don't know what parts to look for that would be an oscillator.

Thanks.

micky wrote:


Oil furance ignition transformer. Proper resistance for primary.

Is it possible that a winding with only 4 ohms DC resistance would
have 52 ohms impedance at 60 Hertz**?

Quite possible! Lots of leakage inductance in something like
this.
I have an ignition transformer for an oil furnace burner.


The transformer primary uses 2.2A at 120 volts, according to its label

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.

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


Or what am I doing wrong?

The transformer seems to fail when it's hot, but it wasn't hot when I
measured the resistance of the windings.

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

On Saturday, December 21, 2013 1:39:21 PM UTC-5, micky wrote:

The transformer seems to fail when it's hot, but it wasn't hot when I

measured the resistance of the windings.


There are two types of transformer controls: interrupted and intermittent. If you have an interrupted control, the transformer is actually shut off some time after the flame is established. This prolongs the life of the ignition transformer and points to almost a decade. Intermittent means the transformer 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 transformers are no exception. And btw, when they're right, they will fire a gap well 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 arrangement. They are less than $40, so it's a good idea to have one at home as these things are generally timed to fail nights, weekends, and holidays.

In article , NONONOmisc07
@bigfoot.com says...

Oil furance ignition transformer. Proper resistance for primary.

Is it possible that a winding with only 4 ohms DC resistance would
have 52 ohms impedance at 60 Hertz**?

I have an ignition transformer for an oil furnace burner.

It's designed to make a continuous spark to ignite the atomized fuel
oil sprayed into the firebox. It runs on 120 VAC. And the
secondary is meant to provide a voltage that will jump across a
specified 1/8" gap, but probably will jump a 1/4" gap. I see that the
transformer secondary is rated at 10,000 volts.

Disconnected and measured with an ohmmeter it shows 20,000 ohms in the
secondary, and 4 ohms in the primary!!

Is that possible? Esp. the primary.

I don't know how to measure impedance at 60 Hertz.

The transformer primary uses 2.2A at 120 volts, according to its label

(The current in the secondary is 23mA. The transformer is a lot like
this one:
https://www.grainger.com/product/ALL...sformer-23M552
)

E=IR, R = E/I = 120/2.2 = a little over 52 ohms, right?

That would make the impedance at 60** Hertz about 52 ohms, right?

Is it possible that a winding with only 4 ohms DC resistance would
have 52 ohms impedance at 60cps**?

Or is the transformer bad?

Or what am I doing wrong?

The transformer seems to fail when it's hot, but it wasn't hot when I
measured the resistance of the windings.


**Or maybe 15 to 30KHz. Some replacement transformers specify that,
but I think it's only for quite modern furnaces. I don't think my old
furnace would have that. It has a circuit board with about 30 parts,
but I don't know what parts to look for that would be an oscillator.

Thanks.

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"

Oil furance ignition transformer. Proper resistance for primary.

Is it possible that a winding with only 4 ohms DC resistance would
have 52 ohms impedance at 60 Hertz**?


** 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

micky wrote:

Oil furance ignition transformer. Proper resistance for primary.

Is it possible that a winding with only 4 ohms DC resistance would
have 52 ohms impedance at 60 Hertz**?

I have an ignition transformer for an oil furnace burner.

It's designed to make a continuous spark to ignite the atomized fuel
oil sprayed into the firebox. It runs on 120 VAC. And the
secondary is meant to provide a voltage that will jump across a
specified 1/8" gap, but probably will jump a 1/4" gap. I see that the
transformer secondary is rated at 10,000 volts.

Disconnected and measured with an ohmmeter it shows 20,000 ohms in the
secondary, and 4 ohms in the primary!!

Is that possible? Esp. the primary.

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.

On Tue, 24 Dec 2013 02:32:34 +0000 (UTC), Cydrome Leader
wrote:

micky wrote:

Oil furance ignition transformer. Proper resistance for primary.

Is it possible that a winding with only 4 ohms DC resistance would
have 52 ohms impedance at 60 Hertz**?

I have an ignition transformer for an oil furnace burner.

It's designed to make a continuous spark to ignite the atomized fuel
oil sprayed into the firebox. It runs on 120 VAC. And the
secondary is meant to provide a voltage that will jump across a
specified 1/8" gap, but probably will jump a 1/4" gap. I see that the
transformer secondary is rated at 10,000 volts.

Disconnected and measured with an ohmmeter it shows 20,000 ohms in the
secondary, and 4 ohms in the primary!!

Is that possible? Esp. the primary.

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.

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.

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.

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)

http://www.homedepot.com/p/Unbranded...ce+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