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Need a boost....

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WillieL:
Can anyone give me some hints on boosting the output on this circuit?
I am severely mathematically challenged.   :doh:

I've put one together, but it's only putting out 68 mA and I'm trying to get 250mA out of it @ 300mV.

HELP?   :scratch:

AussieJimG:
What is the load when you get the 68mA at 300mV?

Jim

WillieL:
Around 1.2 or 1.3 ohms I believe.  Basically whatever the resistance is running through my meter, measuring between the output leads.

I can use a soldering iron, but don't the first thing about circuit design I'm afraid.   :Doh:

kwackers:
R3 limits the current.
Ohms law says I = V/R, so ignoring Q3 that's 3/47 = 63mA.

Simply reducing R3 may work, although obviously Q3 needs to be able to handle the current.

The other point is that to get 250mA through Q3 it needs to have enough gain.
Q1 and Q2 form a fixed voltage dropping aprox 0.7v each, so R2 has 0.7 volts across it  (Range: 0.7 to 1.4). That leaves 3-1.4 = 1.6v across R1 so the max current that can flow through R1 = 1.6 / 1k = 1.6mA.

However if you pull too much current through it you'll unbalance the whole divider chain so it's important the gain of Q3 is has high as possible.

Finally pulling 250mA from a D cell is a fair amount so you'll get some voltage drop on the rails which may compound your problems (and reduce battery life).

WillieL:
kwackers,

Thank you for that. I've made some changes and gotten some improvement, but still not where I need to be.  :(
I've tried reducing the R values to increase the current. This is where I'm at now:

R1 750 ohm, 1.0 W
R2 500 ohm, .75 W
R6 6.8 ohm, .5W
R3 6.8 ohm, .5W ------>  3/13.6 = 220mA correct? (R3 + R6)

Voltage drop across R1 is 1.63V (1.76V output shorted)
Voltage drop across R2 is .625V (.616V output shorted) Edit: End to end. Or did you mean between an end and the wiper? (Q3 base)  :scratch:

Oh, also I'm not using D cells. Power supply is a 3V 700mA wall wart.....

Still, I'm only reading 87mA at the output now. It will go much higher if I adjust R2, but then the output voltage goes sky high as well and I can't allow it to exceed the .3V limit.   :(

So is Q3 the bottleneck then?  :scratch:
If so, is there a different transistor that would work?

Again, I don't know what I'm doing so it's all guess work on my part.   :zap:

 :hammer:

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