let's see if i'm getting this right.

[audioguru]

The little MPSA14 darlington transistor and most LEDs will blow up with a current as high as 750mA.

The 16 ohm resistor will have 750mA in it if both the transistor and the LED are shorted. Then it will have the entire 12V supply across it and it will heat with 9W.

 

[mybuickskill6979]

the box(from rs) states there npn type switching transistors the back states typical HFE of 200. VCE of 30v IC of 800mA
power disipation of 1.8 watts. nothing about darlington.
the reason i went with such high current is because in the end when i get my array going it will be the 750mA K2's

 

[mybuickskill6979]

would i have to change something minor about the circuit (Drawing) to make it possible to make it work? and work right. or am i WAYYYYY off? i don't know why this is so hard lol.

 

[audioguru]

You and RadioShack are WAYYYYY off!

Look at the datasheet for a little MPSA14:
1) NPN darlington transistor.
2) Minimum current gain at 100ma= 20,000.
3) Absolute maximum Ic= 300mA.
4) Max power dissipation if the ambient is not hot= 625mW.

Len showed you how to wire the circuit with an ordinary transistor, an ordinary LED and a 5V supply.
Why did you change everything around and use 12V?

You will need to use a small power transistor like a TIP31 (don't get it at RadioShack) and use Len's wiring.

 

[mybuickskill6979]

okay i'm messing with a TIP42 PNP right now i found the the emitter and collector BOTH need a P charge. and the base needs to go to negative, at least thats the way its working for me. i put a resistor to the LED to the emitter. P to a pot to the collector. and a pot from the neg to the base. at a minimum of 36 mA the (and the led being connected of course. and the base pot being at lowest adjustement. for the circuit to work. to the collector is 36mA. as long as i have those to measurements i can use the neg pot to adjust brightness. i'll draw it up because i may have confused somebody but does that sound about right for this transistor? the voltages are all 12v. it's got me confused but thats a good thing.

the reason i'm using this trans is cause its all i can get my hands on besides the darlingtons i have. and those things seems to never be truly closed!!

 

[Sceadwian]

Never trust Radio Shack packages. I got a box of transistors from there and they had the pin's labeled incorrectly, melted my bread board when I hooked one of them up acording to the pinouts on the package.

 

[mybuickskill6979]

okay. on the TIP42 datasheet(https://www.electro-tech-online.com/custompdfs/2007/02/TI2FTIP42C.pdf)

its says pretty much the same thing as the package. except alot more negatives. i've tried hooking things up in every sort of way i could imagine and nothing blown yet but the only way i can get anything is the base to neg and the other two to pos or the neg side of something from the pos.

 

[mybuickskill6979]

okay i have an observation to make. i tried putting a resistor to the LED the the led to the emitter on the tip42 PNP. it works i grounded the base and it works. so i put a jumper from the LED resistor to the emitter on the trans and put the LED from the collector to neg(made sure i had the polarity right) and connected the base again. it lights but just barely. this leads me to the conclusion, tell me if im wrong, but my conclusion is this. PNP's at least this one. use a BIGGER signal to make a SMALLER one, sort of opposite of what i've read about tranny's but its an observation. is this true?

 

[mybuickskill6979]

the more resistance i put on the base the dimmer the led gets the more the brighter. dude tomorrow i'm gonna go to RS to see if i can get some standard NPN's

 

[ljcox]

I can't be bothered trying to follow your written description of what you're doing.

A picture is worth a 1000 words.

See the attachment and Figure 2 from the TIP42 data sheet.

Edit, I just noticed from Audio's post that you're using a 12 Volt supply.

So it will be a good exercise for you to alter what I did below for a 12V supply.

 

[mybuickskill6979]

i will post a pic on the morning of my current "CIRCUIT" and will try to alter as shown. yup 12v. i'm stuck with it i shorted out the PSU i was using. and can't seem to get it to work again. so i'm stuck with a broken wall wart that i broke the sliding voltage selector on. one question. in the pic i see a -5V on the pos side. Wheres that comming from?

 

[fingaz]

To put it simply. . .

That's because PNP's are opposite to NPN's. I think that's what's confusing you. If you look at the tip42 datasheet, all of the voltages and currents are NEGATIVE.

It's probably best to get used to how NPN's work before you go onto PNP's, that will be less confusing.

If you really want to try this circuit, don't worry about -5v and 0v if you don't have a negative supply, just use +5v instead of 0v and 0v instead of -5v (ie. reverse the polarity)

 

[AllVol]

Note that in Ljcox diagram, he is using an N-P-N transistor for illustrative purposes. Make sure you are also, as using a PNP will cause all kinds of grief while you are learning.

AllVol

Too much, too fast, makes ones head swim.

AllVol

 

[Sceadwian]

It's very simple. On an NPN a positive voltage/current is applied to the base. On a PNP a negative voltage/current is applied to the base. It's just a matter of polarity. Some things are easier with PNP's but NPN's are more common

 

[Hero999]

Normally you can subsitute PNPs in a a circuit that uses only NPNs if you reverse the power supply.

 

[Nigel Goodwin]

Normally you can subsitute PNPs in a a circuit that uses only NPNs if you reverse the power supply.

AND any other polarity sensitive devices, diodes, electrolytics etc.

 

[mybuickskill6979]

yeah sceadwian i noticed that when messing with them yesterday, and by choking the neg to base with a pot the led will dim.
heres my circuit sorry for the horrible shot but you can see the stuff at least.
**broken link removed**
i'll list the stuff.
the upper most pot is set to 420 ohm and it stays there, its connected to the 12v pos.
the second pot is the adjustment for the negative to base.
on the right most side is the led which also goes to neg. i found yesterday as well that if i put an LED on the base side its just of flow and restriction to light both!!

 

[mybuickskill6979]

To put it simply. . .

That's because PNP's are opposite to NPN's. I think that's what's confusing you. If you look at the tip42 datasheet, all of the voltages and currents are NEGATIVE.

It's probably best to get used to how NPN's work before you go onto PNP's, that will be less confusing.

If you really want to try this circuit, don't worry about -5v and 0v if you don't have a negative supply, just use +5v instead of 0v and 0v instead of -5v (ie. reverse the polarity)

yeah fingaz. i have NPN darlingtons i don't think thats a good place to start lol! hopefully today i can go to RS(crappy place but its close) maybe they'll have a nice npn! Lately i've been losing alot of respect for radio shack!!

 

[Hero999]

AND any other polarity sensitive devices, diodes, electrolytics etc.

Obviously.

 

[ljcox]

i will post a pic on the morning of my current "CIRCUIT" and will try to alter as shown. yup 12v. i'm stuck with it i shorted out the PSU i was using. and can't seem to get it to work again. so i'm stuck with a broken wall wart that i broke the sliding voltage selector on. one question. in the pic i see a -5V on the pos side. Wheres that comming from?

It is not on the "positive" side.
It is a negative voltage since the collector and base of a PNP need to be negative with respect to the emitter.

The photo you posted does not help me to see what you have done.
A circuit diagram would be much better.

Do you have a scanner?
If not a photo of a hand drawn sketch would suffice - assuming that you don't have a drawing package such as PowerPoint.