Thought i'd update this, just in case someone else wants this info too.
The chipamp LM3886 PSU board has an inbuilt circuit for an led power on light, the led being (usually) soldered to the board. It consists of a diode and smoothing capacitor in series with a 10k resistor to provide the correct current for the led. I heard back from Brian Bell who said the circuit would rectify 25vac to 34vdc. The transformer I have is 2x22vac so I recon my original estimate of around 30v should be close. Brian also said that the supplied 10k resistor should provide enough current for almost all led's. I think that could be optimistic in my case as the switch already has a resistor built in which I don't know the value of, likewise the led voltage drop or current so I can't use the maths to work out the correct resistor value. I think i'll start with the fitted 10k and work from there. That's if I get the amp fixed and working again after my episode of brain fade and magic smoke.
The chipamp LM3886 PSU board has an inbuilt circuit for an led power on light, the led being (usually) soldered to the board. It consists of a diode and smoothing capacitor in series with a 10k resistor to provide the correct current for the led. I heard back from Brian Bell who said the circuit would rectify 25vac to 34vdc. The transformer I have is 2x22vac so I recon my original estimate of around 30v should be close. Brian also said that the supplied 10k resistor should provide enough current for almost all led's. I think that could be optimistic in my case as the switch already has a resistor built in which I don't know the value of, likewise the led voltage drop or current so I can't use the maths to work out the correct resistor value. I think i'll start with the fitted 10k and work from there. That's if I get the amp fixed and working again after my episode of brain fade and magic smoke.
Unless you need a power on indication with the intensity of a small laser then I suggest a much lower current than 10mA for the LED. Modern LEDs are very efficient and will still give sufficient light output at currents of less than 1mA.
I recently built a capacitor ESR meter which was powered by a 9V battery and I tried to reduce the current consumption to be as low as possible in order to maximise battery life. I eventually managed to reduce the current for the whole circuit (1 IC and 5 transistors) to 4mA with less than 1mA for the green 'power on' LED. A 10K resistor was used in series with the LED from the 9V supply. It is still very bright at around 800uA.
I suggest you try different values of series resistors to get the intensity that you require from the LED. (A resistance substitution box is useful for this application and is a useful addition to the test equipment armoury.)
I recently built a capacitor ESR meter which was powered by a 9V battery and I tried to reduce the current consumption to be as low as possible in order to maximise battery life. I eventually managed to reduce the current for the whole circuit (1 IC and 5 transistors) to 4mA with less than 1mA for the green 'power on' LED. A 10K resistor was used in series with the LED from the 9V supply. It is still very bright at around 800uA.
I suggest you try different values of series resistors to get the intensity that you require from the LED. (A resistance substitution box is useful for this application and is a useful addition to the test equipment armoury.)
With the switch LED designed for 6V, the series resistance should be in the 100's of ohms. The 10k resistor will swamp that value, making little difference in the final current flow through the new LED. The 10k should provide a few milliamps for the LED at 30V. A recent project of mine used a junkbox LED with about 2mA for an ON indicator. Just about perfect - easily seen but without the glaring brightness common in commercial products. If I needed a nightlight I'd've built one.
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