I've got a 300VA toroidal transformer wired to 120V mains and planned to add a 120V indicator lamp in parallel with the transformer to indicate power on. I hooked this up and tested it out with the transformer secondaries disconnected and it twice blew out the indicator lamp following power on after lighting briefly.
I confirmed that there is 120V at the input, fuse, and lamp holder. The output from the transformer seems roughly in line, given no load. There is no soft start circuit on the transformer and it is not blowing the 5A slow blow fuse on the mains. The bulbs were listed as 120V 0.003 amp. They were delivered in a plastic bag and are labeled B1A, which appears to be a 120v bulb.
Do I need a resistor in series with the indicator lamp to limit the current through it? Or am I trying to do something wrong/miswired something?
Thank you for the help.
John
I confirmed that there is 120V at the input, fuse, and lamp holder. The output from the transformer seems roughly in line, given no load. There is no soft start circuit on the transformer and it is not blowing the 5A slow blow fuse on the mains. The bulbs were listed as 120V 0.003 amp. They were delivered in a plastic bag and are labeled B1A, which appears to be a 120v bulb.
Do I need a resistor in series with the indicator lamp to limit the current through it? Or am I trying to do something wrong/miswired something?
Thank you for the help.
John
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Thank you for the quick answer. I made a mistake, it's actually 0.0003A (0.3 mA) so it should be a 470k resistor, right?
These bulbs are actually neon, so is that the reason that the resistor is required? With an incandescent, it wouldn't be necessary?
Yes, I think a current limiting resistor is necessary. Try the 470k, but the lamp may not ignite
with that high a resistor. Around 100k should work. A neon bulb drops around 90V when conducting.
If the operating current is 0.3mA and the neon bulb drops ~90V, then when 120Vdc is applied through a 100k, that resistor will allow 0.3mA to pass.
But it is operated from an AC source.
That probably changes the numbers. Maybe by a lot.
So you need to read the datasheet to find out what that 0.3mA represents. Is it peak, or rms.
Does the neon drop a known voltage?
But it is operated from an AC source.
That probably changes the numbers. Maybe by a lot.
So you need to read the datasheet to find out what that 0.3mA represents. Is it peak, or rms.
Does the neon drop a known voltage?
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