# DC indicating lamp

Discussion in 'General Electronics Discussion' started by siddarth, Feb 12, 2013.

1. ### siddarth

2
0
Feb 12, 2013
How to make DC indicating lamp, so that it can give same brightness to all voltages(min 12VDC and max 220VDC)

2. ### Harald KappModeratorModerator

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Nov 17, 2011
This wide range is not easy.

If you don't have an auxiliary supply, you need a power supply that can take from 12V to 220V and convert it to the operating voltage of your indicator lamp.

If you can use an auxiliary supply (e.g. 12 V), you can build a simple circuit like this:

The resistors in conjunction with the zener diode (e.g. 5.6V) limit the voltage and current to the base of the transistor. The transistor turns on when the voltage at the input is high enough to drive the base current required.
Note that this is very simple basic scheme with lots of drawbacks. For example at high sense voltages the power lost in the series resistors will be comparatively high. Also the circuit has no well defined turn-on and turn-off thresholds. All these will require a more sophisticated circuit possibly using amplifiers and schmitt-triggers.

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3. ### siddarth

2
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Feb 12, 2013
Thanks for your help...I dont have auxillary power supply....

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The alternative is to determine the current required to operate the lamp and use a constant current supply to provide that current.

However, the power dissipated at 220 volts would be large.

For example a lamp running at 50mA (a low current for a lamp) would require the constant current circuit to dissipate about 10W at 220V

5. ### Harald KappModeratorModerator

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Nov 17, 2011
The supertex LR8 could be used to that end. It has an input voltage range from (VOUT +12V) to 450V. Therefore it will require a bit more than 12V for correct operation.
Page 2 of the application note shows how an LED can be driven. If you use a high brightness LED operated at e.g. 5mA, the power "burnt" in the regulator is approx. 1W.

Why do you need such a wide range? What is the application? Maybe there are better wayss to achieve the same goal?