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2-PNP laser driver (easy!)






Benm

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wall said:
Sorry for reviving an old thread, but this is the only driver I could find to power a case-negative diode from a single li-ion.
Are there other transistors I could use instead to achieve a lower dropout?
Click to expand...

Not really, the output transistor is driven near saturation in this circuit and will have a minimum dropout of 0.3 volts or so. The 0.6 volts is he base-collector voltage of the small transistor, and there is little you can do to lower that either.

There may be intergrated circuits that can perform the same function with lower overall dropout voltage, but replacing the transistors in this design will not make a big difference.
 
Hiemal

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... Sure, somehow integrate a low gate threshold mosfet into that circuit.

MOSFET's have on state resistance, so voltage drop doesn't factor into that equation.
 
Benm

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It is possible to use a mosfet indeed, provided is has a low enough Vgs to achieve full conductance. This only diggs into the ~0.3 volts required by the power transistor when its in saturation, so there is not much to be gained here.

You will always have the 0.6 volt drop, its simply the base-emitter voltage of the bc557, and used as the reference voltage to set the constant current.

There may be further tweaks that can be done, but i haven't found anything simple that performs as well as this circuit does.
 
W

wall

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Thank you all for your help, but is there any high-side switching/sensing buck I could use for an lpc-826?
 
Benm

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There are several, but discussing those here would go way beyond the scope of this topic. You can search around the forum to find numerous examples of both high and lowside switching drivers.

I posted this circuit as a simple option that uses off the shelf components to make a driver suitable for a situation where you have at least some room for a voltage drop. It's obviously better in performance compared to a LM317 approach that requires 4.25 volts to operate within spec, but its not a buck/boost/sepic driver that allows you to operate a laser diode near the supply voltage.

There are numerous topics on those though, with some very good designs. Downside is you will not be able to build those with random parts around the workshop, and will probably have to order specific ic's and inductors to make them work well.
 
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wall

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Can this 2-pnp driver also be used to charge lithium batteries from usb?
 
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wall

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But could it be used to safely charge a lithium battery from usb, even if CC only?
What charging voltage would it reach this way?
I already bought a 5- and 10-pack of these transistors for a case negative build, and I wanted to integrate a small lithium charger inside the host.
 
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wall

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Can I use a BD139 instead of the BD140?
 
Last edited:
lasersbee

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wall said:
Can I use a BD139 instead of the BD140?
Click to expand...

I would suggest that you do a little reading on the basics
of electronics and electronic components before atempting
to build any electronic circuits to bring you up to speed.

A quick look at the data sheets for those 2 transistors let
me know that the 2 transistors are not interchangeable...
One is an NPN type and the other is a PNP type...


Jerry

You can contact us at any time on our Website: J.BAUER Electronics
 
W

wall

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Will two BC557 be enough for about 200mA? Input 3~4.2V, output 2.8~3V.
I only have BC557s and BD139s.
 
Benm

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Nice to hear - did you intent on making it look like a creepy spider though? ;)
 
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wall

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It was just for testing, I need to trim all that to fit in a 14500 host somehow.
BD140 gets hot very quickly, should I heatsink it? It's going to power an LPC-826 at 250mA from a single 14500.
I'm trying not to mess up on my first build.
 
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