Hi steve and DoubleO,
On the whole I agree with Steves last post with however one reservation. (sorry!).
Using half wave rectification using a single diode is acceptable where the load (RL) is a resistive one, or possibly inductive, a dc motor maybe.
An led is, by its very nature is a rectifier in its own right! Its a diode which emits light --- yes? And like an ordinary diode will only conduct in one direction.
So, my only concern is still the reverse breakdown voltage of the N24BY, a known Maplin spec of 5v. As we stated earlier with no current flowing in the circuit, irrespective of where the series limiting resistor is placed, this reverse voltage of approx 5v is too close to the breakdown voltage of the N24BY.
Therefore Steve I still think it prudent to:
a) place the additional diode AFTER the led where its increased Vr threshold ( assuming the diode you stated has a higher Vr than the led), will act as a barrier to protect the LEDor,
place additional diode in parallel (but reversed) with led, and thus bypassing it when the dc is in neg half cycle. Using this we should remember to take out the volt drop across this additional diode (0.7v) when calculating series limiting resistor.
Sorry Steve, maybe I'm wrong , but I think that using the additional diode the way you stated do'es not negate the reverse polarity issue , Its still there. With no current flowing the assumed 5v of the neg cycle would still be present at the diode when its reversed biased. No?
Anyway, this tranny of double O's , 10v peak to peak, mean, rms, center tapped.... WE DONT KNOW!!
So because of this maybe we should err on the side of caution. That breakdown voltage of the led is too close to the neg value of approx 5v and is still a concern.
So Steve, what have you been doing today? Like me I reckon, stuck in doors cos its P*****g down!
Wheres Double O? Out in the sunshine I expect!! Well if he reads these posts when he gets back, he will probably go and buy some batteries for his LED's!! Come on Double O be more specific, what do you want a super bright blue led for anyhow?!? Let us know what application you want for your led then we can be more specific in sorting out the probs. ( agree Steve?)
Steve, you are quite right about the use of 78 series regulators and they are my prefered choice when sourcing more than one led. However have you used a constant current device? These are ideal led drivers , up to a max of 20ma. Irrespective of voltage change, and hence speed of an engine using DC, these will only source a constant current depending on the value of one additional resistor. I believe that Hornby use these devices in their current diesel locos. Although I have not opened up my Hornby loco's, these devices, which are in the same package as a BC 108 transistor, can be seen on the engine pcb. Basically, once the led reaches its full working voltage, the light remains constant irrespective of controller voltage, under dc control. However, my Bachmann diesels do not have this led circuit. The brightness varies with dc controller voltage, and looking at the pcb on them , only have a series limiting resistor. Hope this, as an aside, is of interest.
WHERES DOUBLEO?? Given up on his LED circuit and gone down T' Pub!!