I have similar problems in Taiwan, unless I special order a rather expensive tranformer, I have to buy what is in retail stock and these don't always have the voltage I really want.
You also have to realize that the voltage is RMS so the peak voltage is actually 1.414 times that 25.2 volts. The peak voltage is important for avoiding failure of insulation, rectifiers, and capacitors.
25.2 Volts AC may actually be a good thing if your target is 24VDC. The bridge rectifier is likely to drop 2.0 Volts, so just the other side of it you have less than you want. And ripple filtering can also drop the voltage a bit. I'd look for something closer to 30VAC, do all the maths in detail. Test real bridge rectifiers rather than use 1.4 V 'rule of thumb' for the voltage drop.
You could always configure two 12VAC rated transformers in series if you require exactly 24 volts.
The voltage is a relic of the vacuum tube days. The typical filament voltage was 6.3V. Transformers were produced in multiples of that so we get 12.6V and 25.2V transformers. For power supplies it makes no real difference.
Try a regular hardware store for a 12/24VAC doorbell transformer. In the early days, the doorbells were run from 12 and 24 volt dry cells, so they stayed with those voltages. Many thermostats use these voltages as well and in very low power transformers.
I guess the bell is driven by a coil and most of them can withstand quite a bit of over-voltage. For one, the bell is not on for long periods of time - it is momentary use. And the coil wire is enamel coated copper that is usually rated for over 100V for insulation.
If there is no heat build up, it will be fine. If you use the bell extremely frequently, there could be a problem.
25.2/24 volts is 1.05 So 5% more voltage and maybe 5% more current and ~10% more heat. That's unlikely to cause a problem. The smaller transformer will also have a larger internal resistance. It's likely that the new smaller 25.2 volt transformer will actually run the bell at a lower voltage (due to internal losses) than the old 24 volt transformer.
what is quite funny is someone I work with was looking up center tap transformer and found transformers dancing..... .......
Then I commented "Thank god Michael bay did not add a transformer to his movies called something like 'VAC' ".
the nightmares that would of caused with Google ;p
If you think that's bad, try looking up examples on how to use a program called "dialog."
Zetsu
Posts: 186
Comments
You also have to realize that the voltage is RMS so the peak voltage is actually 1.414 times that 25.2 volts. The peak voltage is important for avoiding failure of insulation, rectifiers, and capacitors.
25.2 Volts AC may actually be a good thing if your target is 24VDC. The bridge rectifier is likely to drop 2.0 Volts, so just the other side of it you have less than you want. And ripple filtering can also drop the voltage a bit. I'd look for something closer to 30VAC, do all the maths in detail. Test real bridge rectifiers rather than use 1.4 V 'rule of thumb' for the voltage drop.
You could always configure two 12VAC rated transformers in series if you require exactly 24 volts.
I guess the bell is driven by a coil and most of them can withstand quite a bit of over-voltage. For one, the bell is not on for long periods of time - it is momentary use. And the coil wire is enamel coated copper that is usually rated for over 100V for insulation.
If there is no heat build up, it will be fine. If you use the bell extremely frequently, there could be a problem.
Lawson
"Possibly" not, but if you're concerned why not just get a 24 volt bell transformer from your local hardware (or Radio Shack)?
@
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