Electronics > Beginners
Transistor Switching
<< < (5/7) > >>
james_s:
Reminds me of the first time I played with LEDs when I was about 4 years old. I remember wondering why they get so hot and don't last very long when I connect one to a 9V battery.

Like most future engineers seem to have done, I also tried connecting a NE-2 lamp to 120V prior to learning what a ballast resistor was.
MK14:

--- Quote from: Hero999 on July 08, 2018, 04:27:43 pm ---
--- Quote from: tpowell1830 on July 08, 2018, 01:37:57 pm ---
--- Quote from: ruffy91 on July 08, 2018, 01:06:03 pm ---Regulate current, not voltage.

--- End quote ---
BINGO!

Where do you put the resistor on a transistor hookup for current regulation? Keep in mind that you will need to regulate from approximately 50 µA (not 0) to 100 mA. Still, not the method that I would recommend.

--- End quote ---
The resistor needs to go in the emitter side, to regulated the current, which can then be controlled by varying the base voltage with the potentiometer.

Also what's wrong with regulating down to 0mA?

And 100mA is too high for a small 5mm LED.


--- End quote ---

I hope you don't mind me criticising your (apparently), adjustable 0 to 20 mA (approx), constant current circuit.
But, the way you have drawn it, the 100K pot, will probably not do anything, for around half its travel.
If, you changed it, so that the bottom lug of the pot, which currently goes to ground, was instead, connected to the point where the two diodes meet. Which should be around 0.6 to 0.7 volts.
Then most of the transistors Emitter/Base voltage drop will effectively be cancelled out.
Hence the adjustable pot, should vary over nearly all its range (rather than only half of it).

On the other hand, it is possible, my suggested change may reduce or cause instability. I would have to simulate it, or build it, or calculate hard, in order to find that out.
tpowell1830:
Okay, so if you want to dim an LED, your best option is a PWM (Pulse Width Modulator) circuit. The circuit allows the user to adjust the width of pulses, without changing the frequency of the pulses. This means that the effective voltage varies while the power supply voltage remains constant. This effectively regulates the average current applied. The LEDs will dim from dark to fully lit based on the regulating resistor in series with the LEDs or single LED and the voltage applied (power supply voltage).

A PWM circuit can be produced with discrete ICs, such as a 555 timer circuit or a µC with the proper connections and software. There are many ways to produce a PWM, just do a search for PWM circuits or pulse width modulator circuits.

I have supplied a couple here:

We have to do Dave's version of course.






Hope this helps...
Zero999:

--- Quote from: tpowell1830 on July 09, 2018, 01:50:50 am ---Okay, so if you want to dim an LED, your best option is a PWM (Pulse Width Modulator) circuit. The circuit allows the user to adjust the width of pulses, without changing the frequency of the pulses. This means that the effective voltage varies while the power supply voltage remains constant. This effectively regulates the average current applied. The LEDs will dim from dark to fully lit based on the regulating resistor in series with the LEDs or single LED and the voltage applied (power supply voltage).
--- End quote ---
Why is PWM better?

I can see why it's better than varying  the value of a series resistor, but don't see why it's better, than regulating the current. Simple PWM, with a resistor and no regulated power supply, will cause the LED's brightness to change, with the power supply voltage. A current regulated driver will keep the brightness constant. The only thing I can think of is, some phosphor LEDs, such as white ones, may change their colour, at low currents, but it's not something I've noticed myself.


--- Quote from: MK14 on July 08, 2018, 07:41:39 pm ---
--- Quote from: Hero999 on July 08, 2018, 04:27:43 pm ---
--- Quote from: tpowell1830 on July 08, 2018, 01:37:57 pm ---
--- Quote from: ruffy91 on July 08, 2018, 01:06:03 pm ---Regulate current, not voltage.

--- End quote ---
BINGO!

Where do you put the resistor on a transistor hookup for current regulation? Keep in mind that you will need to regulate from approximately 50 µA (not 0) to 100 mA. Still, not the method that I would recommend.

--- End quote ---
The resistor needs to go in the emitter side, to regulated the current, which can then be controlled by varying the base voltage with the potentiometer.

Also what's wrong with regulating down to 0mA?

And 100mA is too high for a small 5mm LED.


--- End quote ---

I hope you don't mind me criticising your (apparently), adjustable 0 to 20 mA (approx), constant current circuit.
But, the way you have drawn it, the 100K pot, will probably not do anything, for around half its travel.
If, you changed it, so that the bottom lug of the pot, which currently goes to ground, was instead, connected to the point where the two diodes meet. Which should be around 0.6 to 0.7 volts.
Then most of the transistors Emitter/Base voltage drop will effectively be cancelled out.
Hence the adjustable pot, should vary over nearly all its range (rather than only half of it).

On the other hand, it is possible, my suggested change may reduce or cause instability. I would have to simulate it, or build it, or calculate hard, in order to find that out.

--- End quote ---
Thanks for your comment. I realised this later, when I was away from the computer.

Yes, connecting the resistor to the node where the diodes meet will help, but it still might not go all the way to zero. Another resistor, with a slightly lower value, than the pot, in series with the bottom side of the pot will fix it, although there will still be some dead-band.
MK14:

--- Quote from: Hero999 on July 09, 2018, 07:51:44 am ---Thanks for your comment. I realised this later, when I was away from the computer.

Yes, connecting the resistor to the node where the diodes meet will help, but it still might not go all the way to zero. Another resistor, with a slightly lower value, than the pot, in series with the bottom side of the pot will fix it, although there will still be some dead-band.


--- End quote ---

Yes, that is also a good or even better, way of improving it.

Afterwords, I also worried, because, although diode junctions typically are between 0.6V and 0.7V, at low (but not too low) currents, in the LED part of the circuit. Very little current, will flow into the base of the transistor (in that circuit), especially if its Hfe happens to be high.
That might cause the base/emitter voltage, to drop below 0.6V, depending on the characteristics of the actual transistor, that is being used. There is probably significant transistor, to transistor variation, on parameters like that, even for identical types of transistors. (e.g. at 50 microamps base current).
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod