# Pnp transistor

Discussion in 'General Electronics Chat' started by Peter basolo, Jul 14, 2015.

1. ### Peter basolo Thread Starter Member

Nov 24, 2014
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What is the name of a 6 volt pop transistor? I know a lm317t is 1.25. Also is there a name code for transistors? I'm using this for a blue laser diode I have that I am making a diver for. can you check the schematic for me? This is a slightly modified shcem. Form tingawinga7 I didnt make this. Will this give me a final load of 6 volts at 130 ma? I need this fast I'm about to go to radio shack to buy the parts thanks in advance

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2. ### ian field Distinguished Member

Oct 27, 2012
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You should be able to do the constant current you need for the laser diode with just the 317 and a current sensing resistor - unless its a very big laser diode.

Feed the 317 output to the laser via the current sensing resistor and connect the voltage sense pin to the diode as well.

As the laser draws current, it develops a voltage across the sense resistor - when that voltage reaches the 317 1.25V threshold; the 317 throttles back and limits the current.

So find out what current the laser needs and calculate the resistor to drop 1.25V at that current.

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3. ### Peter basolo Thread Starter Member

Nov 24, 2014
38
0
Interesting. Could I cut out the pot? Also does the 317 cut current? A 9 volt puts out 180 ma but I want 130 ma how can I achieve this

4. ### dl324 Distinguished Member

Mar 30, 2015
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R = V/I. 1.25V/0.13A = 9.6 ohms. If you use a 5% value, you can use 9.1 or 10 ohms.

P=IV, so you need 0.13A * 1.25V = 0.1625W; use a 1/4W resistor.

5. ### dl324 Distinguished Member

Mar 30, 2015
3,391
653
In the US, 2N3906 or 2N2907 are frequently used for general purpose circuits. Their complements are 2N3904 and 2N2222, respectively.

I assume the voltage you're referring to is collector-emitter or collector-base breakdown voltage. You won't find a discrete general purpose transistor with such a low breakdown voltage. The transistors mentioned above are 40V and 60V, respectively.

One other thing. You seem to be lax in your use of capitalization. In electronics, capitalization is often critical. Milliamps is abreviated mA, not ma.

6. ### ian field Distinguished Member

Oct 27, 2012
4,460
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If you don't know the correct current to calculate the sensing resistor for, you can temporarily fit a pot and turn it up till the laser starts lasing - but its all too easy to turn it up too far and destroy the laser diode.

If you mean a 9V transistor radio battery - it won't supply 130mA for very long.

Presumably the 180mA you stated is the mA/h capacity of the battery - that doesn't mean it will supply 180mA for an hour, the measure most manufacturers use is that the battery will supply 9mA for 20h. If you draw more current than that, the claimed mA/h capacity starts to shrink.

7. ### ian field Distinguished Member

Oct 27, 2012
4,460
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What I described turns the 317 into a current limiting regulator.

8. ### ian field Distinguished Member

Oct 27, 2012
4,460
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To drive a laser diode?!

9. ### dl324 Distinguished Member

Mar 30, 2015
3,391
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Why not? OP said he wanted 130mA. When I worked at HP Labs in the late 70's, laser diodes didn't start lasing until around 100mA. It's generally much lower now, but I just did the calculations the OP needed to get 130mA from an LM317 configured as a current source.

10. ### ian field Distinguished Member

Oct 27, 2012
4,460
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My misunderstanding - I thought you were referring to just a dropper resistor.

11. ### Peter basolo Thread Starter Member

Nov 24, 2014
38
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If a 317 puts out 1.25 what puts out 6 volts? And I need more than 20ma can I use 2 18650 batteries in series?

12. ### AnalogKid Distinguished Member

Aug 1, 2013
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1,301
READ page 14 of the LM317 datasheet. It has the schematic for using the 317 as a constant current regulator, which is what you want for a laser diode. For that schematic, Iout = 1.25 / R1, or R1 = 1.25 / 0.130 = 9.6 ohms. Buy two resistors, 10 and 220 ohms, and put them in parallel as R1.

Delete both 1N4001 diodes from your schematic.

ak

13. ### Peter basolo Thread Starter Member

Nov 24, 2014
38
0
Obviously I'll need bigger resistors/ diodes

14. ### ian field Distinguished Member

Oct 27, 2012
4,460
792
The 317 has a 1.25V difference between output and Vin pin - its normally used as an adjustable voltage regulator by connecting a potential divider from output to GND and feeding the tap back to Vin - when the tap is at 1.25V less than the output, the 317 stabilises at that level.

Used as a current regulator, the output current flows through the sense resistor and develops a voltage across it - the Vin pin is connected to the load end of the sense resistor, so when the voltage across that resistor reaches 1.25, the 317 stabilises.

15. ### Peter basolo Thread Starter Member

Nov 24, 2014
38
0
I bought the parts you guys were so much help thanks I'll let you know the outcome!

Aug 1, 2013
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17. ### Peter basolo Thread Starter Member

Nov 24, 2014
38
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Well it's finished. I haven't tested it out yet but it doesn't look bad for my first circuit board!

18. ### atferrari AAC Fanatic!

Jan 6, 2004
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Peter, I do not intend to disturb your way to success but please be aware that the LM317 is not a pnp transistor. It is a three terminal voltage regulator.

19. ### Peter basolo Thread Starter Member

Nov 24, 2014
38
0
That's how I wired it the title of the thread is misleading thanks for the input

20. ### atferrari AAC Fanatic!

Jan 6, 2004
2,673
785
Your schematic shows a "PNP" on what I believe is the LM317.