Way OT question about 240V transformer.

[Bearbonesnorm]

I've just received a 240V - 50V AC tranformer and in the space of 30 to 40 seconds it's now knackered It's for a feather burner I'm making for someone, anyway wired up the 2 primary connectors to a 5 amp fused plug and plugged it into the socket. Checked to see what the output was and both pairs of secondry connectors showed 26V which was as expected and all good.

Then connected 1 pair of secondary connectors to the burner (think hot wire cutter) and the wire heated nicely within 10 seconds. However, after a further 20 seconds I noticed a little smoke coming from the transformer ... so I unplugged it as you would. I now have that lovely burnt transformer smell and no output.

Any ideas before I conclude it's simply a crap transformer? Could only using 1 secondary output have burnt the secondary windings out?

Ta.

[MuddyPete]

Are you using a 240v/50v transformer or a 240v/50v/50v transformer? The description differs from the first sentence.
How is 26v output from 50v secondary terminals "good"?
What current does the load draw?
What current is the transformer designed to supply?
Do the instructions specify both outputs must be used simultaneously?

[Bearbonesnorm]

Are you using a 240v/50v transformer or a 240v/50v/50v transformer?

Sorry Pete, it's 240v primary with 2 x 24v secondary.

Do the instructions specify both outputs must be used simultaneously?

Instructions? It does state that the secondary outputs are independent but can be wired in series or parallel.

What current is the transformer designed to supply?

Rated at 50VA max.

What current does the load draw?

Don't know without working it out.

[MuddyPete]

What current does the load draw?

Don't know without working it out.

It's the actual load it draws that's critical, more so than the theoretical load (but theoretical load will indicate whether the transformer is at risk of being overworked or not).
Has the load been produced by a manufacturer that states the current drawn; or is it home-made?

[Bearbonesnorm]

Has the load been produced by a manufacturer that states the current drawn; or is it home-made?

Oh, very much a home brewed affair Pete. Basically, it's a 1mm Ni-chrome wire element.

There's an outfit in the US that sells transformers for hot wire cutters which is where I gleaned the transformer spec' from.

[NewRetroTom]

Have you got a multi-meter you could use to measure the resistance of this wire element? If you know that you should be able to work out how much current will flow at a given voltage.

[MuddyPete]

Is it possible to measure the resistance of the load (using an Ohm meter), then calculate the current it will draw?

This may help (see example 1) http://www.electronics-tutorials.ws/acc ... tance.html


(You don't have to draw the phasor diagram if you don't want to )

[Bearbonesnorm]

Resistance is 3 ohms which at 24v should give a draw of 8 amps. Can't see why that should have blown the transformer?

[Mike]

Was it a robot in disguise?

[Bearbonesnorm]

Took me a few seconds Mike.

[MuddyPete]

Morning!
Where were we...? Ah, yes...

Resistance is 3 ohms which at 24v should give a draw of 8 amps.

Correct so far. However: this article by a transformer manufacturer...http://www.csanyigroup.com/transformer-ratings...states

"KVA is simply the load voltage times the load current"

So: based on the spec of your transformer:
50VA = 24v x "maximum current it can cope with"

...and manipulating the equation gives...
50 divided by 24 = "maximum current it can cope with"

So, 50/24 = 2.0833 Amps = maximum current the transformer can cope with.

Thus: 50VA + 24v + 8 Amps = Smoke!

So why didn't the fuse blow? Well; this article https://electronics.stackexchange.com/q ... -blow?rq=1 states fuses blow when excessive current is drawn for a period of time, the key point being the relationship between "rate of current increase" and "time" in this case.

This article https://www.physics.ohio-state.edu/~p61 ... rrent.html (and many others like it) states "...currents between 100 and 200 mA (0.1 to 0.2 amp) are lethal."

The chart is worth a look, too:
https://www.physics.ohio-state.edu/~p61 ... urrent.gif


Worth remembering when tinkering about in the shed. Fun though it may be; personally I'd consign the home-made thing to the bin, then buy a system designed and made by a competent manufacturer that's within the capability of the transformer.

Please be careful.

Pete

[Bearbonesnorm]

Thanks for that Pete, I'll take a look at those links. It's the same spec transformer as used in hot wire cutters, so somethng's a miss somewhere.

Fun though it may be; personally I'd consign the home-made thing to the bin, then buy a system designed and made by a competent manufacturer that's within the capability of the transformer.

and

Please be careful.

Neither of the above compute

[Misterg]

Could only using 1 secondary output have burnt the secondary windings out?

Yes - 50VA is the total rating for the transformer. it's almost certain that each of your secondaries is (was) only rated for half that (~1 amp @24V).

Resistance is 3 ohms which at 24v should give a draw of 8 amps

Your calculation is correct. The resistance of the nichrome will increases a bit with temperature, but it still looks like too much load for your transformer.

There's a table here (near the end) that gives the current needed to heat various wire diameters to various temperatures - a 1mm diameter wire need ~10 amps to get to ~500°C.

Is it possible that the US device uses a thinner gauge of wire? (Thinner wire has more resistance for a given length, although you'd need to go down to ~0.25mm wire to match that transformer spec.)

(A 5 amp mains fuse will supply 1200VA before it breaks into a sweat, btw.)

[Bearbonesnorm]

Lovely, thanks for that lot Andy, that chart is most useful. I shall order some thinner gauge wire ........ and a new transformer

[BigdummySteve]

I take it that the device only needs to operate for a short time? If so you could use a ‘mod’ ( a device for powering e-cigs) one which uses twin 18650 batteries should be able to heat a short length of wire easily, failing that another battery device might work.

[Bearbonesnorm]

failing that another battery device might work.

I had thought about an smps Steve.

[Bearbonesnorm]

Thanks for the input folks ... now all sorted

[MuddyPete]

Is it earthed?

[Bearbonesnorm]

Is it earthed?

Yes Pete.

[MuddyPete]

Yay!

[Zippy]

Watt is all this chatter

[psling]

Watt is all this chatter

Stu sparked off a thread, there was some resistance at first but we finally got to the current situation. OMG.

[danielgroves]

Not bad for a Ohm project.

[benp1]

This all seems like a lot of faff when a pair of scissors could have cut those feathers instead

[Bearbonesnorm]

This all seems like a lot of faff when a pair of scissors could have cut those feathers instead

This ^ is true. However, in my defence I will say:

Scissors tend to leave a jagged edge on the feather which is both unsightly and more importantly, effects flight.
It's hard to cut intricate shapes with scissors, especially on small feathers.
When someone asks me to make them something, I try and do it to the best of my ability and produce something as nice as I can.

... but yeah, you're right