using led's to bias a current source
Posted by: minime on 16 January 2004
noticed some leds lit up inside my cdp someone on pfm suggested they might be to bias a current source.
is their anyone a bit tec minded maybe could explain this.
the red leds come on at power up then go off on power down.
they are located on a circuit board towards the back of the cd player.
thanks in advance
is their anyone a bit tec minded maybe could explain this.
the red leds come on at power up then go off on power down.
they are located on a circuit board towards the back of the cd player.
thanks in advance
Posted on: 16 January 2004 by Paul Ranson
What a potentially huge question....
Go to http://www.dself.dsl.pipex.com/ampins/discrete/ef.htm and scroll down to Fig 4. Q2, D1, D2 and the 100R resistor form a current source.
It works because diodes, when a current flows through them, drop a constant voltage, about 0.6v, as does a transistor's base-emitter. In the diagram the transistor base is at 1.2v due to the diodes, its emitter is at 0.6v and therefore there is 0.6v across the 100R resistor. Ohm's Law then says that the current through the resistor must be 0.006A. As the current flowing into the base of the transistor is negligible this current must flow into the collector. Strictly perhaps a current sink rather than source, but the transistor does its damndest to maintain the voltages and hence current.
LEDs are electrically diodes, but rather than dropping 0.6v when current is flowing it's more like 1.2. So you can drop an LED in in place of the two diodes and everything carries on working except that there's a pretty glow. Different types and colours of LEDs have different 'on voltages'. Prettiness is therefore tweakable...
Did that help?
Paul
Go to http://www.dself.dsl.pipex.com/ampins/discrete/ef.htm and scroll down to Fig 4. Q2, D1, D2 and the 100R resistor form a current source.
It works because diodes, when a current flows through them, drop a constant voltage, about 0.6v, as does a transistor's base-emitter. In the diagram the transistor base is at 1.2v due to the diodes, its emitter is at 0.6v and therefore there is 0.6v across the 100R resistor. Ohm's Law then says that the current through the resistor must be 0.006A. As the current flowing into the base of the transistor is negligible this current must flow into the collector. Strictly perhaps a current sink rather than source, but the transistor does its damndest to maintain the voltages and hence current.
LEDs are electrically diodes, but rather than dropping 0.6v when current is flowing it's more like 1.2. So you can drop an LED in in place of the two diodes and everything carries on working except that there's a pretty glow. Different types and colours of LEDs have different 'on voltages'. Prettiness is therefore tweakable...
Did that help?
Paul
Posted on: 16 January 2004 by Manu
Leds can be used as "constant" voltage ref (1.5v) on current source circuit instead of regular/zener diodes. Some manufacturers also use LED inside they units as "indicator" in test mode. In the case of a CDP, the second option is more probable.
Emmanuel
All opinions are my own, and reflect those of the organisation i work for, even if not stipulated.
Emmanuel
All opinions are my own, and reflect those of the organisation i work for, even if not stipulated.
Posted on: 16 January 2004 by minime
glad of the help chaps and thank you
i think i got most of that but i am now a bit worried that my cdp is in test mode.it's playing fine but how would i know?
[This message was edited by minime on FRIDAY 16 January 2004 at 22:04.]
i think i got most of that but i am now a bit worried that my cdp is in test mode.it's playing fine but how would i know?
[This message was edited by minime on FRIDAY 16 January 2004 at 22:04.]
Posted on: 16 January 2004 by Manu
Don`t worry. It works.
When it lights on doesn`t mean it is in test mode. What i have seen is, when set in test mode, the led is flashing at different rates depending on the condition of operation.
Emmanuel
All opinions are my own, and reflect those of the organisation i work for, even if not stipulated.
When it lights on doesn`t mean it is in test mode. What i have seen is, when set in test mode, the led is flashing at different rates depending on the condition of operation.
Emmanuel
All opinions are my own, and reflect those of the organisation i work for, even if not stipulated.
Posted on: 16 January 2004 by minime
thanks again the glow is to indicate everything is fine, the manufaturer has just mailed me after a week. it's also very warm in that area smells the same as a new naim.
once again even though it's not a naim cdp
i always seem to get great advice from the forum members.
thanks.
once again even though it's not a naim cdp
i always seem to get great advice from the forum members.
thanks.
Posted on: 17 January 2004 by prowla
Wow! is this an opportunity to make glowing innards showing Hi-Fi like the current fashon in PCs?
Paul Rowlands
Paul Rowlands
Posted on: 17 January 2004 by minime
lots of lights i feel a right prat now.