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A little transformer advice wanted ...

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john1

Active Member
Hi,

I would like to fit up a small filament lamp as an indicator
to a water heating element, to show when the element is on.

The heating element takes roughly 10 Amperes, the lamp could
be any of those little 2 watt bulbs.
I was thinking of a 24v 50mA type, i have a few of them.

I was thinking of trying to use a small audio transformer
salvaged from an old transistor radio, i was just going to
work by trial and error, till i got it right ...

Unless someone could give me some guidance on it ?

The primary would simply be the wire that feeds the element.
Its unlikely to need two turns, but i don't know yet.

Cheers, John :)
 

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Roff

Well-Known Member
Why don't you use an NE2 neon lamp with a series 100k, 1/4w resistor? No transformer required.
 

john1

Active Member
Hi Ron,

yes, that was of course my first option.
But i would like to be sure that the heating
element is actually working, and those neons
don't show too well in daylight.

So, any ideas on such a unit ?
I was wondering if a ferrite doughnut might
be a better bet, what do you think ?

John :)
 

Klaus

New Member
Here's another clever idea I saw used on some old equipment we have here which has 6 indicators with small 24V bulbs. They used just an appropiate sized small capacitor to pass sufficient current for the bulb staight off the mains (240V) voltage. If you experimented with a bulb/capacitor series circuit in parallel to your heater you might get your result without a transformer.
Keep in mind the capacitor has to be rated for mains voltage and also the bulb wiring & base fitting.
Klaus
 

motion

New Member
Yeah, the transformer is the simplest solution. The turns ratio is 1:200 to get 50mA at 10Amps primary current. Try with 1-2 turns on the primary and 200X turns on the secondary and increase the no. of turns until the bulb starts to light up. Use a minimum of turns because you could be generating high voltages if the bulb were to burn open.

If you need less current, you could replace the bulb with those ultra bright leds @10mA current and it would need far less volts to turn them ON. You will surely get by with passing a wire through the iron core. Add a low voltage zener diode (~5V) in parallel with the LED for safety.
 

Phasor

Member
Yeah, the transformer is the simplest solution.
I disagree. I think a series capacitor is simpler.

For 240V, 50Hz supply, and a 24V 50mA lamp, the value of capacitance required is 660nF (theoretically). Since 660nF isn't generally available, try 680nF.
 

motion

New Member
He wants to monitor if there is current through the heater. He might be worried that the heater may burnt out for intance. Your solution detects just the voltage across the heater.
 

Phasor

Member
He might be worried that the heater may burnt out for intance.
True, but I think you read into it too much (unless I missed something...). Why don't we let john tell us?

EDIT:
But i would like to be sure that the heating
element is actually working
Oops, I did miss something... :oops:
 

john1

Active Member
Hi Ron, Klaus, motion, Phasor,

Yes, a cap is a very easy way, thats what i'm using at
the moment. The value, i found optimal is 0.47 Mfd.
But this gear is old, and it keeps blowing bulbs.
I think its due to the odd poor connection causing too
much current through the cap due to spikes and or rough
bits on its supply.

Also cos this machine is old and cranky, i still go over
to it and listen carefully to hear the 'hiss' from the
element, just to be sure that it is working as it should!

So i thought, if i can run it with a ferrite hoop or a
small transformer, that would suit me fine. I would know
the element was running if the bulb is on, and roughness
on the supply shouldn't affect it.

As to the open circuit voltages on a 'current transformer'
yes, they do give very high voltages with no load, they
are made so that several instruments can be connected to
them in series with almost no effect on the intended
current to the instruments, that is a specific situation.
With a 'current transformer' one could short together the
output and still get the intended current. The output
current is determined (mostly) by the resistance of the
output winding.

In this case, it would act more like a normal transformer
with a fairly high differential. (well thats my opinion)

I feel that LEDs would look out of place on this old M/C
but the suggestion of limiting with zeners has occurred to
me in a slightly different way, i tried to limit the excess
voltage that the lamp may get by putting a small neon bulb
across it. That didn't work and after two days i lost
another lamp, upon inspection i saw that the neon had its
resistor in the black insulation tubing right close up to
the bulb - so it wouldn't have done much limiting!
If i try that again i will include a four or five hundred
ohms in line too, and remove the resistor from the neon.
I know the neon could only limit it to sixty-plus volts,
but i reckoned that the lamp could probably take that sort
of spike alright.

Thank you all for your interest and suggestions, i think
i will make a start on winding some turns!

Regards, John :)
 
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