TIP127 current calculation....

[koolguy]

Hi,

I am using TIP 127 for controlling current of led as load, i have connected the emitter to +12V and collector to led series of 4 (2.2 x 4= 8.8V) using SMPS.
The base is connected with pre set 10K resistance to GND of 12V.
data sheet says 5A with 120mA...maximum..

 

[koolguy]

The base-emitter voltage is 5V, i have connected 12V with 10K pre set.....

 

[ericgibbs]

hi Ritesh,

A TIP127 is a PNP Darlington transistor.

The Collector should be connected to a Negative Supply with respect to the Emitter.

What is the driving source for the 10K Base resistor, do you have a diagram to post.?

 

[koolguy]

here it is...........

 

[ericgibbs]

hi,
The Base must be negative with respect to the Emitter on a PNP, connect the 10K to a -Vsupply to turn on the TIP

 

[MrAl]

Hi,

I agree that to turn this on you need a resistor from the base to ground or something similar. Depending on your LED current requirements however the resistor may have to be sized a bit lower than 10k. That's because the gain is roughly 1 amp per milliamp, so to see 1 amp collector current you need a min of 1ma into the base as a starting point, and for a full 5 amp collector current you'd need at least 5ma into the base but maybe a little more than that. I would think that a 1k resistor should be able to drive it regardless with a 12v supply (after the roughly 2v base emitter drops that leaves about 10ma base current).

There's another problem however, because since this is a Darlington the collector to emitter voltage drop will be higher than a single transistor. That means you might see a drop as high as 4 volts across the transistor. Again, this may or may not work with your LED's depending on their actual voltage and current requirements. At 1 amp collector current it might be ok and you could always test it statically first.

 

[large_ghostman]

I cant always understand Ritash too well, so I might have it wrong, I think he needs 8.8V for his leds, so the TIP127 is cutting it fine??? No offense Ritesh, I just find you hard to understand at times, your English is better than my Indian . If I have it right I think it wont work with the TIP. But the boys who know will put it right .

 

[koolguy]

hi,
The Base must be negative with respect to the Emitter on a PNP, connect the 10K to a -Vsupply to turn on the TIP

sorry base was with GND only...
but how to control current i think TIP is not suitable for controlling current in between 300mA to 1.5A

 

[large_ghostman]

I think your main problem will be as pointed out the voltage drop, you need 8.8V and the tip can lose around 4V (ish), so for a 12V supply
12V - 4V = 8V you are nearly 1V out.
How much current do you actually need? are these normal leds? or high power ones?
I am not sure why you need the TIP127. Maybe a 2n2222 would be better?

 

[large_ghostman]

IF I have read the TIP127 datasheet correctly, then 300mA -1.5A, is going to take you way outside the SOA with the voltage you need.

 

[koolguy]

I think your main problem will be as pointed out the voltage drop, you need 8.8V and the tip can lose around 4V (ish), so for a 12V supply
12V - 4V = 8V you are nearly 1V out.
How much current do you actually need? are these normal leds? or high power ones?
I am not sure why you need the TIP127. Maybe a 2n2222 would be better?

It was working but the current controlling was not fine so, 2n2222 or something else to use pls tell?

 

[large_ghostman]

What current do you need?
Forget 2n2222 if you need over 800mA, for some reason I had in my mind they can take upto 1.5A, but I have checked and my memory was wrong, the datasheet says 800mA.
Could you use a MOSFET? Sorry but it is all guess work with the information you have given.

 

[ericgibbs]

What current do you need?

ritesh said: controlling current in between 300mA to 1.5A

 

[large_ghostman]

Ah sorry Eric, I wasnt sure, in the first post he mentions the datasheet and 5A, that kind of stuck in my mind, then you posted as i did

 

[ericgibbs]

It was working but the current controlling was not fine so, 2n2222 or something else to use pls tell?

hi ritesh,
You will find that trying to control the current to stable value by using a single transistor will not work.

Look up voltage to current converter circuits

EDIT:
This is an example circuit ONLY.

 

[large_ghostman]

hi ritesh,
You will find that trying to control the current to stable value by using a single transistor will not work.

Look up voltage to current converter circuits

EDIT:
This is an exampel circuit ONLY.

Is he trying to control the led current with the transistor or just switch the led with it? I am a bit confused

 

[ericgibbs]

Is he trying to control the led current with the transistor or just switch the led with it? I am a bit confused

I 'suspect' he trying to control the current thru the LED's in order to control the brightness.
EDIT:
In a linear way

 

[koolguy]

Is he trying to control the led current with the transistor or just switch the led with it? I am a bit confused

actually i am not using series resistance and i want to switch led, as i work on led display..

 

[large_ghostman]

Sorry Ritesh I will leave this to Eric, I understand what Eric is saying but I get confused when you reply. I am curious tho, are you using the transistor to adjust the brightness?
I dont understand why you dont have a resistor to set maximum brightness on each led, then maybe adjust brightness down from that point with the transistor?? Then you only have to deal with a max of 80mA. But I am pretty sure I am still not understanding what you are actually doing, so I will watch

 

[audioguru]

He wrongly thinks that the darlington output current is "controlled" by how much base current it gets so he does not use a current-limiting resistor in series with the LEDs.
He does not understand that each TIP127 darlington can have a current gain much higher than the minimum amount of 1000. He also does not understand that the current gain changes when the temperature changes.

He also cannot understand the Maximum spec's on the datasheet:
1) The maximum allowed base current is 120mA which does not mean that for its maximum allowed collector current of 5A then the base current must also be maximum!
The minimum hFE is 1000 at a collector current of 3A (it is not shown for 5A because then it works poorly) then the maximum base current used is 3mA, not 120mA. But the actual current gain might be 3000 or 5000 when it is hot then the base current is only 0.6mA.
2) The maximum allowed reverse base-emitter voltage is 5V. But it is never reverse in his circuit. The datasheet shows that the forward base-emitter voltage is a maximum of 2.5V at a collector current of 3A and is less at lower current or could be much less. With the 12V supply and the 10k series base resistor then the base current is about 1mA.
3) He uses a "preset" variable resistor for the 10k base resistor then it can be adjusted low enough for both the preset resistor AND the base of the darlington to burn out. The LEDs will also burn out.

He said that he wants to "switch LED" in a display. But I think he wants to demonstrate how quickly his LEDs will burn out in his circuit that has no current-limiting.