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Need a boost....

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WillieL:
John,

Wow. Thank you for that! The diagram really helps.
So... I'll need to change R1 back to a 1K resistor then, is that correct?

I have plenty of the 2N4401 transistors so I guess I will use them instead of the darlington.
That means that Q5 is NOT required - correct? Also I don't see a diode in your drawing?   :scratch:

And thank you again for the idea about the adjustable over current LED. I would really like having that feature!   :clap:
I will also have to order a 100 ohm trimpot since I only have a 200 ohm on hand, but it will be well worth it to get this working the way I want it to.

Off to make another parts order.....   :palm:

Willie

kwackers:
Looking at Johns diagram the only thing that bothers me is R1 & R2 are the same value.

Roughly speaking Q1, Q2 & Q5 each drop 0.7v for a total of 2.1v leaving 0.9v across R1 and 0.7v across R2.
Whilst not the same they're pretty close and make the voltage drops across the transistors and the supply voltage overly critical.
I'd be inclined to reduce R1 or increase R2.

You still need Q5. Because of the pair of transistors used as a darlington the BE voltage will be 1.4v instead of 0.7, Q5 ensures the voltage at the base is high enough to switch on the darlington.
The main use of the darlington is the gain of it is equal to the gain of both transistors multiplied, this means the current you need into the base can be tiny in order to fully switch it on.

John Swift:
yes ,
 you will need to add Q5 and change R1  to  680 ohms to pass about 1.8mA through
Q1, Q2 and Q5   
 
assuming the transisors Q3 & Q6 each have a gain of 100
the darlington circuit they make will have a gain of 10000
and will only need 25 uA base current to output a 250 mA current

in the circuit diagram Q1 , Q2 and Q5 are used as diodes
and providing they are at the same temperature as Q3 and Q6 that form
the darlington transistor
the volt drop across the base emitter junctions of Q2 & Q5 (about 1.3V)
will be the same as the base emitter junctions of Q3 & Q6

the volt drop across R2 is fixed by Q2 to 0.65V (0.6V to 7V)

the output voltage across R4 will be the same as the voltage between the negative end of R2 (the connection to Q2) and the base of Q6
for a 0.3V output  the preset should be set about mid way



John

PS - WillieL
 you can use a 200 Ohm preset for the over current indicator

PPS  -Kwackers ,
I've had another look and  with the exra voltdrop across Q5 , it will be better with R1 being reduced to maintain the current through Q1 , Q2 and Q5

WillieL:
OK then, so.......

R1 680 ohms
R2 1K ohms
Add Q5
R9 100 or 200 ohm trimpot

I already ordered R9 (100 ohm) and a couple of darlingtons earlier today, but I'll have to get some 680 ohm resistors now.
I'm going to have quite a pile of extra parts laying around here....   :lol:

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