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Topic: Understanding of auto-illumination circuit (Read 1 time) previous topic - next topic

TomGeorge

Tom
You forgot current limiting resistor for LED.
I thought about that but I can only see one resistor on the PCB and in the OPs circuit.
The IR LED might have its own.
Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....

ted


ReverseEMF

#17
Jun 03, 2018, 05:32 pm Last Edit: Jun 03, 2018, 05:49 pm by ReverseEMF
Tom
you have forgotten current limiting resistor for  LED
Since, for IR data transmission, like in the case of a Remote Control, the IR LED is typically driven to it's MAX Pulse Current rating, perhaps the designer of this circuit is relying on the Regulator's internal current limiting to supply the proper current.  As far as I can tell, it's not certain what that regulator part number is, but if it is an LP2980, then it has a max current of 50mA, which is just about right for driving an IR LED for broadcast purposes.

In fact, come to think of it, that moves in the direction of solving the Q1 shorting Q2 to ground, problem.  Since the IR LED will be pulling the regulator output down to:

   IR LED Forward Voltage + Q2 VCE(sat) ~= 1.6V

BUT, Q1, it seems, will, through the Q2 BE junction, pull the supply line down to more like 800mV to 1000mV, which should be too low to fire that LED!  Unless, Q1's VCE(sat) is really high, like 0.8V, or something.  That could be the case, if Q1 was a Darlington, but the OP's Diode tests on that part, seem to eliminate that possibility -- so, still a mystery!
"It's a big galaxy, Mr. Scott"

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ted

 Infrared 3W LED
- Wavelength: 850nm
- Diameter: 2cm
- Voltage: 1.5V~1.7V
- Current: 1500mA
- Replacement emitter board for flashlight

ReverseEMF

#19
Jun 03, 2018, 06:49 pm Last Edit: Jun 03, 2018, 07:05 pm by ReverseEMF
Infrared 3W LED
- Wavelength: 850nm
- Diameter: 2cm
- Voltage: 1.5V~1.7V
- Current: 1500mA
- Replacement emitter board for flashlight
Well, if that's true [and I'm not sure how you can know it is], then there is a huge issue of power dissipation.  Either that HY4D is more like a p-channel MOSFET, with a very low channel resistance [in saturation] AND that 4XY6 is somehow managing to limit current to 1500mA, without frying, or the HY4D is some mysterious marvel that can cut 3.3V down to the 1.5V~1.7V needed to properly drive that 3W LED...makes no sense what-so-ever!

To drop 3.3V down to even 1.5V [the least worst case condition] would require:

(3.3 - 1.5) * 1.5A = 2.7W!!!!

How the heck are either of those tiny little parts going to dissipate that kind of power?  I mean, they gotta be linear, 'cuz there are no Switch Mode related parts--anywhere!

BUT, I'm guessing that the HY4D is a p-channel MOSFET [one with a very low Gate Threshold Voltage] -- that's the only way, so far, I can see this thing even beginning to function properly.  But, that 3k3 resistor is rather odd--really doesn't need to be that low, if all it's doing is pulling up the Gate on a MOSFET [unless, maybe, that MOSFET needs to switch really fast--which might be the case if that LED really is 3W]!  BUT, really, I doubt it's a 3W LED [or there's more to this circuit than has been revealed]!  Another possibility is that the 3k3 resistor was chosen to set the sensitivity of the LDVR [by imposing a greater current load -- but, again, the variability of transistor Beta would seem to make that a rather touchy design decision.




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Please DON'T PM me regarding what should be part of the Public Conversation -- Let it ALL hang out!!
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ted

This is another one 2.4V

3W-850NM

Emitting Colour:  IR
Color Wave Length: 850NM
DC Forward Voltage: Typical: 1.6-2.4V
DC Forward Current: 700mA
Viewing Angle: 120°-140°
LifeSpan Time : > 50,000 hours
PCB dimension: Diameter 20mm

Post # 12 - VR1 adjusted to open Q2 not completely ......

ReverseEMF

- Replacement emitter board for flashlight
A flashlight for snakes/people wearing night vision goggles/the occasional human with IR vision?
A flashlight that only works in the proper amount of ambient darkness?
"It's a big galaxy, Mr. Scott"

Please DON'T PM me regarding what should be part of the Public Conversation -- Let it ALL hang out!!
Unless, of course, it's to notify me of a mistake.

ElCaron

A flashlight for snakes/people wearing night vision goggles/the occasional human with IR vision?
Yes?

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