Powering AC LED - old stereo

Thread Starter

frusciante89

Joined Nov 13, 2012
10
Hi everyone, I have an old Marantz amplifier with a broken lamp. The lamp is rated 8V AC 200mA. However, I used my multimeter to measure voltage and current: Voltage is 7.33 V AC and current is 5.68 mA (first silly question: do I have to use the AC amp or DC amp setting? cause the AC amp setting is reading a zero)
I want to replace the buld with an led, whose specifications are these:
http://www.sparkfun.com/datasheets/C...547W2C-A13.pdf
I suppose I have to use a diode, a resistor and a capacitor, but how can I choose the values?

Thanks for you help
 

#12

Joined Nov 30, 2010
18,224
This is a no-no on this site. It's a beginners site and the owners don't want them playing with wall power...unless I have misinterpreted your idea. Please clarify.
 

BillB3857

Joined Feb 28, 2009
2,570
It says in the OP that he wants to replace a lamp rated at 8V with an LED. With an 8VAC source, he will need an LED, a diode and a limiting resistor. I usually use a 1K for most indicator usage. The diode will be placed in reverse bias parallel with the LED.
 

#12

Joined Nov 30, 2010
18,224
There is still the measurement of 5.68 ma DC in a circuit that is supposed to run a lamp at 200 ma. That is enough to run an LED as an indicator but there might be something not working right in the driver circuit. If the driver circuit can not produce more than 5. 68 ma, that would fail to light the original lamp and maybe fool the repairman into thinking the lamp is bad.
 

BillB3857

Joined Feb 28, 2009
2,570
Measure the voltage source with the LED/Diode/resistor combo connected. You can then calculate a resistor value that will give you 20ma through the LED.
 

BillB3857

Joined Feb 28, 2009
2,570
What is the source of the 2.2VAC? For illumination of a meter, you will probably need at least 10 to 20 ma to an LED to get enough light out of it. It may be easier to use one of the lamps from a series string incandescent Christmas tree lights.
 

BillB3857

Joined Feb 28, 2009
2,570
Wouldn't the 1.4V drop due to the bridge use up most of the available voltage? Not much left when tying bridge to 2.2VAC. Am I missing something?
 
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