Project: Madness (a Fan/Electronics modification)
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Associate
Which LED's are you using. Whats is the stated max current and voltage?
The voltage drop measured across an LED will be deceptive if nothing is current limiting it.
The voltage drop measured across an LED will be deceptive if nothing is current limiting it.
They are Ultra bright Orange 3mm LEDs with forward voltage of around 2.1v (measured on a different project). The max current is supposed to be 30mA with typical around 20mA. I've no idea what the max voltage would be, the place I got them from states 1.8-2.2v.
Will have a look at it today and see what voltage drop I get when adding resistors.
Will have a look at it today and see what voltage drop I get when adding resistors.
Soldato
Then the max is 2.2v . . . . 
Associate
then the resistors should be (assuming the 12V supply, 2.2v LED @20mA)
(12 - 2.2 - 0.1)/0.02 = 485 ohms
(12 - 2.2 - 0.1)/0.02 = 485 ohms
Yes I always use 470 ohm to current limit a single Orange LED and will give them a shot. That low voltage/high current thing threw me as I wasn't aware of the behaviour of LEDs that's all.
From what I've read and experienced you'd expect if it were anything like 200mA and 12v it would make the LED go pop instantly, that's why I'm confused.
From what I've read and experienced you'd expect if it were anything like 200mA and 12v it would make the LED go pop instantly, that's why I'm confused.
Associate
Yeah it wont be at 200mA but it will be high, it wouldn't last very long if the LED was constantly on but as it is flashing you probably would get away with it for a while.
I like your project, it makes quite a nice effect
I like your project, it makes quite a nice effect
Tested with a 470 ohm resistor and they aren't particularly bright. Connected up a pot and under 200 ohm the orange LEDs seem to go a little red, suggesting they are getting a bit much. My power supply was on 7.5v for the actual circuit and it was extremely dim with that, better at 470.
Might just grab some 250 ish resistors and see how it goes.
Have left it with pot attached at 200 to burn for a while so I can check for colour change.
Fitting a 2k2 resistor to the base on the first transistor didn't make a visible difference.
Ammeter hasn't arrived yet.
I also tested 4 orange LEDs in series on the emitter with no
Resistor and all 4 LEDs lit full bright.
Thanks. I think the effect is nice too. Well worth the 36 transistors, 36 resistors and 18 capacitors.
I should do a cost breakdown to see what this set me back.
Might just grab some 250 ish resistors and see how it goes.
Have left it with pot attached at 200 to burn for a while so I can check for colour change.
Fitting a 2k2 resistor to the base on the first transistor didn't make a visible difference.
Ammeter hasn't arrived yet.
I also tested 4 orange LEDs in series on the emitter with no
Resistor and all 4 LEDs lit full bright.
Thanks. I think the effect is nice too. Well worth the 36 transistors, 36 resistors and 18 capacitors.
I should do a cost breakdown to see what this set me back.
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Associate
They will be bright as they will be well over the rated 30mA maximum, the only thing limiting current is the slight resistance over the transistor. You should calculate the values of the resistors to account for this.
I've no idea what resistance is inside the transistor but now that my el-cheapo multimeter has arrived I can determine what current is flowing.
With no resistor as schematic I'm getting around 170 mA.
With a 470 ohm resistor I'm getting 18.3 mA. The voltage measured from the transistor emitter is around 10.7v from a 12.4v regulated source. So I guess the resistance of the transistor is dropping the voltage a touch.
With no resistor as schematic I'm getting around 170 mA.
With a 470 ohm resistor I'm getting 18.3 mA. The voltage measured from the transistor emitter is around 10.7v from a 12.4v regulated source. So I guess the resistance of the transistor is dropping the voltage a touch.
Associate
that's a large amount to drop over the transistor, is that with the circuit switched on constantly? It may be the it isn't fully turned on. You would usually expected to only loose around 0.1V over the transistor, well unless the current was really high.
With some experimentation I've determined that to get slightly over 20mA I need a 390 ohm resistor attached. The voltage drop over the transistor decreases as the circuit draws more current. Is this a voltage divider effect? Not really sure how that stuff works yet.
The circuit is fully powered with no switch on my breadboard.
The circuit is fully powered with no switch on my breadboard.
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Will be modding another fan (probably the translucent one to see what it looks like) and making a junction out of a terminal block. I could use crimped terminal connectors but double terminations and triple terminations are dodgy.
Waiting on 390 ohm resistors.
Could do with some 19-20 core cable too but that's hard to find.
Waiting on 390 ohm resistors.
Could do with some 19-20 core cable too but that's hard to find.
Associate
I don't think its the voltage divider effect, that shouldn't vary I would say its to do with the resistance of the transistor changing.
The more the transistor turns on the less voltage it should drop over it as the collector to emitter resistance will reduce. The Vbe sat should be around 0.6v
I just checked the data sheet
http://www.fairchildsemi.com/ds/2N/2N3904.pdf
It states a max of 0.85V which is the voltage required at the base to switch it on fully.
The more the transistor turns on the less voltage it should drop over it as the collector to emitter resistance will reduce. The Vbe sat should be around 0.6v
I just checked the data sheet
http://www.fairchildsemi.com/ds/2N/2N3904.pdf
It states a max of 0.85V which is the voltage required at the base to switch it on fully.
As you mentioned earlier putting a resistor on the LED slows down current flow and now makes the LED delay much much longer. Too long in fact, although it might be better now at slower cycle speed.
Instead of 10uF I'd need to use 1 or 2uF. This makes perfect sense as with 170mA of flow is 8 times more than the LED gets now.
Bit of a bummer really as I now have to buy 2.2 uF capacitors as well. Oh well I suppose I will have plenty of 10uF caps fir future projects. Using 1uF caps makes them a little too short to dim and 2.2 uF seems about right, better than before in fact.
I've wired up two circuits on breadboard and hooked them up to a switch and can see the difference in orange colour.
Instead of 10uF I'd need to use 1 or 2uF. This makes perfect sense as with 170mA of flow is 8 times more than the LED gets now.
Bit of a bummer really as I now have to buy 2.2 uF capacitors as well. Oh well I suppose I will have plenty of 10uF caps fir future projects. Using 1uF caps makes them a little too short to dim and 2.2 uF seems about right, better than before in fact.
I've wired up two circuits on breadboard and hooked them up to a switch and can see the difference in orange colour.
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Associate
Yeah you are experiencing the fun of analogue circuits, its just one big balancing act 
Just to confirm it doesn't matter if I get 63v caps rather than 16v or similar as for some reason the lower voltage ones cost a lot more than the comparatively higher ones.?
Thanks.
I'm loving this tinkering. I didn't really expect it to be perfect first time out but I wasn't that far off and it does sort of work.
Thanks.
I'm loving this tinkering. I didn't really expect it to be perfect first time out but I wasn't that far off and it does sort of work.
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Associate
yeah 63v caps will be fine, just that they will be noticeably bigger
Associate
Just a side note when you do make the final thing you could use matrix board instead of strip board (I seem to find it easier to compress things) and maybe make it on 2 boards and stack them.