Would these transistors work?

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wahqui
 
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Would these transistors work?

Post by wahqui »

Hi
I want to use a single of these transistors. And drive 8 IR-LEDs.
On a single output of the Atmel µC.

Would this work?


PNP: ON SEMICONDUCTOR - BC327-25ZL1G
NPN: NXP - PBSS4021NZ

Data sheets are available at this site.

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philba
 
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Re: Would these transistors work?

Post by philba »

I'd stick with an NPN transistor for this. You plan on putting all those LEDs in parallel? What forward current are you shooting for?

uoip
 
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Re: Would these transistors work?

Post by uoip »

In addition to the questions philba asks, what voltage do you have available to drive the LEDs? If you have enough voltage available, you can put the LEDs in series, which reduces your total current requirements, at the minor cost of increasing the voltage which the transistor must be able to handle in the off state. Or maybe you can use a combination series/parallel arrangement where you have two parallel strings of four series LEDs, or four parallel strings of two series LEDs. Each string will require its own dropping resistor (or other current limit) in order to balance the current between strings.

wahqui
 
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Re: Would these transistors work?

Post by wahqui »

philba wrote:You plan on putting all those LEDs in parallel?
Yes. All in parallel.
philba wrote:What forward current are you shooting for?
This is the IR-LED: VISHAY - TSHF5210

The data sheet said:
Absolute maximum ratings (Test condition: 100µs):
Peak forward current: 200 mA
Surge forward current: 1,5 A

Basic Characteristics (Test condition: 100µs, 1A):
Radiant intensity: 1800 mW/sr

I've chosen these LEDs because this was the highest radiant intensity I've found.
I've orientated me on the highest indicated currents.

I'm not sure how much base-current would drawn on these transistors in this case. I assume 6 or 8 LEDs is a safe condition to operate.
But I would choose even more LEDs if possible.

As power supply I want to use 2x AA-batteries. And if this is not sufficient I would add a second battery-holder in parallel or choose a rechargeable Lithium-*-Battery.

edit: typo
Last edited by wahqui on Mon Feb 13, 2012 5:26 pm, edited 2 times in total.

wahqui
 
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Re: Would these transistors work?

Post by wahqui »

uoip wrote:Or maybe you can use a combination series/parallel arrangement where you have two parallel strings of four series LEDs, or four parallel strings of two series LEDs. Each string will require its own dropping resistor (or other current limit) in order to balance the current between strings.
What do you think about to use this kind of LED-(boost-converter)-drivers?
TAIWAN SEMICONDUCTOR - TS19371CX6 RF - LED DRIVER
It has an PWM input.
If the cap and inductor (mentioned in the sample circuit) on the output is removed the LEDs could be modulated at 36kHz, I think.

edit:
Switching frequency is 1,2MHz and current on the PWM input is only 65µA.

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philba
 
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Re: Would these transistors work?

Post by philba »

good point on serial LEDs. 2 AAs will give you 3.2V for 2000mAh. which will do 2 emitters in series. doubling your battery life - good idea.

The TS19371 is probably not a reasonable solution. You mention 36Khz so I assume you are using this for IR remote control on a 36Khz carrier. Nowhere in the datasheet does it say that you can go above 10Khz PWM control. It might work but the datasheet doesn't give a clue about it.

wahqui
 
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Re: Would these transistors work?

Post by wahqui »

philba wrote:good point on serial LEDs. 2 AAs will give you 3.2V for 2000mAh. which will do 2 emitters in series. doubling your battery life - good idea.
The forward voltage of the IR-LEDs is 2,3V (@100µs,1A) and 1,4V (@100mA). I'm not sure what's better for long range (2-LED-strings or all in parallel), because the batteries decrease the voltage in the course of the discharge. A rechargeable lithium-*-battery is maybe the better choice, because of the lower voltage drop.
But I understand your point. :o
I assume for 2 LEDs in series I need 3 AA-Batteries.
I've chosen a 4700µF cap for voltage stability. Maybe this is a little bit overdimensioned but won't be a problem.
philba wrote:The TS19371 is probably not a reasonable solution. You mention 36Khz so I assume you are using this for IR remote control on a 36Khz carrier.
Yes. Before this thread was moved it was in "TV-B-Gone-Kit". This was a stronger sign about my development plan. :wink:
philba wrote:Nowhere in the datasheet does it say that you can go above 10Khz PWM control. It might work but the datasheet doesn't give a clue about it.
I noticed it but could imagine it relates on power efficiency, losses in the inductor, EMI or something.
OK, I will drop this driver but give it maybe a chance in the future.
I have this driver already on my list for an LED flash light. :D

edit:
The maximum specified I/O current of the µC is 40mA. Would this work if I replace the transistors with these and choose only one NPN (for Q1-4, in this schematic)?

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philba
 
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Re: Would these transistors work?

Post by philba »

a couple of points:
- 100 uS pulses means 5Khz so don't be expecting to get 36Khz with 1.5A pulses.
- Vf is well behaved until you get to about 100 mA. I would go with 100 mA and put 2 emitters in series but you can do anything you want.
- The PBSS4021NZ looks to be good.
- you are going to have to create a series of 13.89 microsecond pulses. maybe shorter. keep that timing in mind.
- the emitter you've chosen is centered at 890 nm and falls off rapidly on either side (datasheet, fig 7). I thought most IR receivers were tuned for 940 pr 960. They may be sensitive enough to pick up 890, though. I know the vishay receivers are pretty tolerant.


I would bet the TS19371 speed issue is about stability of the switcher's control cycle, not efficiency. I think this is why they go on about dimming control rather than directly PWMing the chip.

wahqui
 
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Re: Would these transistors work?

Post by wahqui »

philba wrote:a couple of points:
- 100 uS pulses means 5Khz so don't be expecting to get 36Khz with 1.5A pulses.
- you are going to have to create a series of 13.89 microsecond pulses. maybe shorter. keep that timing in mind.
I calculated: 1s/(36000Hz/2)=0,000055...s=55,55...µs (@36kHz) Is this not correct?
philba wrote:- Vf is well behaved until you get to about 100 mA. I would go with 100 mA and put 2 emitters in series but you can do anything you want.
I looked at (datasheet fig. 3: Pulse Forward Current vs. Pulse Duration). My interpretation was that the possible current is lower at long pulses and higher at short pulses.
Is 100 mA the constant current?
uoip wrote:Each string will require its own dropping resistor (or other current limit) in order to balance the current between strings.
The schematic of the TV-B-Gone has no dropping resistor between the IR-LED. So I need a resistor? How is the current without resistor like in the schematic?
philba wrote:- the emitter you've chosen is centered at 890 nm and falls off rapidly on either side (datasheet, fig 7). I thought most IR receivers were tuned for 940 pr 960. They may be sensitive enough to pick up 890, though. I know the vishay receivers are pretty tolerant
I read that the maximum sensitivity of silicon-IR-transistors is at ~850nm. Could it be that (even if this is the case) that the IR-filter of most IR receiver is the limiting factor?

edit:
I would choose this IR-LED then: VISHAY - TSAL6100
But the radiant intensity is 1000mW/sr (@100µs).

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philba
 
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Re: Would these transistors work?

Post by philba »

well, let's see 1/36000 = 27.78 microseconds. That's the period at 36kHz. You can use any math you feel comfortable with, though.
No dropping resistor - probably relying on the internal resistance of the battery to not fry the transistors and LEDs. Did you really think you were going to get 12A out of a AA?

wahqui
 
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Re: Would these transistors work?

Post by wahqui »

philba wrote:well, let's see 1/36000 = 27.78 microseconds. That's the period at 36kHz. You can use any math you feel comfortable with, though.
Ah, yes. This makes sense. :D
And the half time of the period is the pulse?
(edit: This was your result I see now. Thanks! :o)
philba wrote:No dropping resistor - probably relying on the internal resistance of the battery to not fry the transistors and LEDs.
Did you really think you were going to get 12A out of a AA?
Not with AA but with LiFePO4-batteries (some 3,2V-Cells can handle 90-160A continues discharge current). :wink:
Or with many AA-batteries in parallel and big caps. :wink:

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philba
 
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Re: Would these transistors work?

Post by philba »

OK, have fun making blue smoke.

wahqui
 
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Re: Would these transistors work?

Post by wahqui »

philba wrote:OK, have fun making blue smoke.
LOL. I didn't said that I want to do it with LiFePO4-batteries. :wink:
My knowledges about transistors is limited. And optionally I can figure out and compare some options.
I want still rest at a realistic level and don't want trial-and-error-strategy. :wink:
I want to avoid disappointments if I order wrong parts.

Is my assumption correct, that AA-batteries replaces the resistor (between the LED) by his internal resistance?

(Only for my understanding:)
If the internal resistance of the battery is low (like LiFePO4) I would need an resistor between the LED? And the calculation is the same as calculation of the series resistor for LEDs?

Many thanks for your patience. You was definitively a great help. Thanks. :o

edit: Someone build a TV-B-Gone with 20 LEDs.
In this Video a Photo is shown (@14m30s). Design Noir - The seedy underbelly of electronic engineering
I'm not sure how this was done.

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philba
 
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Re: Would these transistors work?

Post by philba »

well, to be honest, you can't expect to get it perfect the first time. If you haven't blown a few components, you just not trying hard enough.

I'd look at the length of your pulses (yes, 1/2 the period). Then study the data sheet. Pay attention to the absolute max section but also look at typical characteristics. Study the graphs as well. There is a wealth of information in there - you just have to BANNED it out. With LEDs they will tell you pulse maximums - current and length - for several pulse lengths. Try to stay below those unless you like ordering parts a lot. Sometimes they specify duty cycle - you will have a .5 duty cycle while transmitting a code. You can probably push it a bit because your codes don't last that long.

By using a a resistor to limit the current, you aren't betting on the battery gods favoring you. It won't hurt if you really didn't need. But if you omit it and you did need it those gods are unforgiving. Why take a chance?

wahqui
 
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Re: Would these transistors work?

Post by wahqui »

philba wrote:With LEDs they will tell you pulse maximums - current and length - for several pulse lengths. Try to stay below those unless you like ordering parts a lot. Sometimes they specify duty cycle - you will have a .5 duty cycle while transmitting a code.
Ah. Now the graph (datasheet fig 3) makes more sense for me. :D
In other words: The value 0,5 means correlation between on-time and off-time. Value 1 means 100% (always on).
In my case (0,5) I can go to ~200mA peak forward current but average current would be ~100mA.
More current would need longer cool-off-time. If I don't cut off the connecting leads it could act as heat sink (maybe better for reliability).
philba wrote:By using a a resistor to limit the current, you aren't betting on the battery gods favoring you. It won't hurt if you really didn't need. But if you omit it and you did need it those gods are unforgiving. Why take a chance?
"battery gods"; nice wording! :D Yes, the battery god is not predictable for me. I have to confide. :o
I think 3x AA-NIMH (1,2V) and 2-series-LEDs-strings (1,35-1,6V@100mA) would be a good choice. (3*1,2V=3,6 ... 3,6V/2=1,8V).
philba wrote:well, to be honest, you can't expect to get it perfect the first time.
If I only could. :o Maybe in version-2 some day. :wink:

Many thanks! :o

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