too much volts/ not enough current

[Dr_Doggy]

With my pic (50ma) output I am trying to switch my laser on and off(40khz), I am powering this with a dead 9v, so about 7V which works ok with the pic.

I am planning to use charged 9V but when I do that unregulated things dont power up, but nothing burns up, so I think I am planning to put a 5V regulator on source. (GOOD IDEA?)

anyway, so running my laser naturally uses the LM317 circuit, all over the web, but i need to integrate it;

CASEA) so i was thinking that i could hook up the input of the 317circuit to the pic pin out, which is laser's half powers at 50ma, (would this work maybe i should stick with this, half power means it will last longer!)

CASEB)but to run at full power (100ma) I should wire the LM317 to supply and switch it with a transistor wired to pic pin out?

CASEC)I am thinking that putting a buck converter on the pic pin out would be best(reduces pin volts to 2.5 and current up to 100ma, solves all my problems right!?

but would thses methods work at 40khz, and what component and config is ideal? (ie, transistor/buck converter, part no or specs)

 

[Reloadron]

Doggy, what is the correct voltage for the LASER? Also, what is the correct current for the LASER? I saw the 50 mA but no voltage. Full power would be what? Maybe 100 mA?

Your pic is likely a 5 volt device correct?

Given a choice and battery operation I would find a battery voltage that meets the demand of the LASER and then regulate down for the PIC. A LM7805 would work assuming your battery exceeds 7 volts. I would let the PIC drive a transistor like a simple 2N2222 to in turn drive the LASER. That would be simple as I see it.

Ron

 

[Dr_Doggy]

PIC is 16F688, but its running on 7v, (dead 9v)

um, its hard to get specs on laser, I read ~2V(100mA)on the web but its working on the 6v(50mA) pic output directly( now, just not well (not getting hot), i think it can get brighter).

is 2n2222 better than a 2n4401, I don't need the LM317? I am afraid of over current, so its oK to put base pin to pic, emitter to ground, and collector to laser ground, and laser vcc to 7505 outpin, will transistor drop 2.5v? will resistor somewhere in there help?

 

[Reloadron]

Well if we look at the data sheet for your PIC it would seem the maximum voltage it should be run at is about 5.5 volts so it is basically a 5 volt PIC even though it may work at about 7 volts. I am no PIC guru but I believe 5 volts would be a good and safe working voltage to assure long life and stability. Now as to the LASER? If we assume it is just a LASER diode it will likely run fine on about 3 volts. Therefore I would try the following. Use a 5 volt regulator like a 7805 to power the PIC and LASER. The output pin of the PIC can drive a simple switching transistor like a 2N2222. Depending on transistor try about a 2.2 K base resistor. Transistor emitter to ground and the LASER diode in series with the collector. Let's try about a 40 to 50 Ohm resistor in series with the LASER to keep the current at 50 mA or less through the LASER diode.

Short of more details I would start from there.

Ron

 

[Dr_Doggy]

no thats good, but ima cut out that last resistor(50ohm) since i want to get it to at least 90mA, and i can control flow by adjusting the 2.2k ohm , right?

 

[Reloadron]

The attached is what I would use. If you want for example 100 mA through the LASER diode and we assume the typical Vfwd of the LASER diode to be 3 volts we get:

Vsupply 5 volts - Vdiode 3 volts / 100 mA = 20 Ohms

Resistor R1 limits the current. Resistor R2 is the transistor base resistor. You want to use the transistor as a switch to handle the diode current. Actually a good value for R2 would likely be about 500 ohms. You want to drive Q1 into saturation.

All of this assumes Vfwd of the LASER to be about 3 volts which is typical for many small LASER pointing devices.

Ron

 

[Dr_Doggy]

ok thnx, i will do that! since were on the question though how do I calc R2 for future reference, usually i assume my collector current and divide it by the gain and use that current for the base. is that methodology correct?

 

[Reloadron]

Generally speaking you choose the base resistor based on the hFE of the transistor and the load. this is a pretty good read on the subject. Remember, we want to really drive the transistor into saturation as in this case we are using it as a switch. There also needs to be consideration on what your base driver current source can deliver however the PIC will source more than enough current. Give the link a read and remember choosing a base resistor value is not a 100% dead science when doing what we want to do, turn that sucker hard on!

The link should give you a good read and information to stash in the brain.

Ron

 

[Dr_Doggy]

okie thnx again!