Quick electronics question

[cropwell]

otherwise the base is left floating which should never be allowed to happen

I should have remembered the lessons I learnt in the '70's when I was doing circuits with 4066's and leaving inputs floating, with unfathomable results (until the penny dropped). My emulation software is crap and out of date (IC library is woefully lacking), but it is OK for quick checks.
Yes, I put the voltmeter on to show what was happening across Q1 and did wonder why, but with the SW there should not be enough Ib to turn Q1 on. (???)

Even with strapping the base of Q1 to Gnd there is an e-v voltage of 10v with the switch open - curious... Putting a diode in series with the LED puts the emulation right, so I am going to presume that the circuit emulator has a large reverse leakage on the LED.

[Kitwn]

Even with strapping the base of Q1 to Gnd there is an e-v voltage of 10v with the switch open - curious... Putting a diode in series with the LED puts the emulation right, so I am going to presume that the circuit emulator has a large reverse leakage on the LED.

That agrees with LEDs having such a low specs for reverse voltage, usually only 5v.

So we can finally give m_c a definitive answer that might actually work! Strap the transistor base to ground via a large-ish resistor to avoid letting it float with the contacts open and add an ordinary diode in series with the LED.

[Doddy]

Nope, look at the current flow from base to 5V supply - it's modelled the reverse leakage current of the LED. This is the point where modelling with idealised circuit models will give you a somewhat distorted view of reality, the Vce of 6-7mV (on state) will be... unusually low. Similarly the reverse bias current flow through the LED will be highly variable during manufacture and you're likely to see Vc vary quite wildly with component selection and temperature. But the principle of the simulation is correct and shows that the OP pretty much got a workable solution - with the addition of a diode to limit the reverse-bias current flow through the LED. Of course, with this making the 21st post on the subject it would be quicker to breadboard the sucker and just test it :)

EDIT: Dammit - that was looking at new posts and believing post#20 was the latest.

[m_c]

Thanks for the simulation. I've been a bit quiet since posting the initial query as my head's been a bit cloudy due to a leg infection courtesy of a mishap at work last week :-/


The 'load' will be either 24v at 4K or 10K, or 12V at 10K.

I need to decide on components, as I probably have enough surplus parts. I think I've probably got suitable SMD resistors, LEDs, and either transistors or mosfets, but I need to dig through the collection and see exactly what I have and if it'll work.

[john swift]

only just found this thread

I would add a diode to block reverse leakage current through the LED when the switch is open

a 10K resistor between the base and emitter will prevent the base from floating when the switch is open

original circuit


revised circuit



john