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ArcticMyst Security by Avery

Small 3v driver (info request)

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Aug 21, 2008
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I'm looking for information on how to build a tiny 3v driver circuit which i can use to power a red or ir (green) diode. I'm trying to fix a few broken green pens by replacing the diode (808's from stonetek) and the driver.

The pens take 2 AAA batteries, but with the diodes needing 2.9v the dropout of a normal adjustable regulator (at over 1 volt) is probably too high and also i've not been able to get one to produce the (relatively) high current needed.

So what i'm looking for is something that can output around the 200mA at 2.9v with 3v input and is small enough to fit inside a pen/aixiz module. I'm assuming regulators are out so maybe something with op amps like the original driver only alot simpler.

I enjoy electronics tinkering so will prefer to make them myself rather then buying a pre-made driver, you can't learn anything if someone else does all the work for you  ;)
 





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I dunno if it's possible but I think in this case, a zener diode, cap and resistor might work.

If u can find a 2.9v zener then its perfect, if not maybe a 3v one, and use two resistors to form a potential divider, then tap off to get 2.9v?

I think that's probably the only type of driver that doesn't consume voltage. I heard that even the best LM317s take at least 0.5v away.

(I'm new to this, so I'm going theoretical here. I need more experience)
 
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idd, that might work :D

I'm currently reading up on op amps, seems you can feed the output back into the input to make an automatically adjusting stable output voltage which you can then combine with a tranistor and resistors to generate a stable high current control.
It seems to be the method used on the dx laser drivers although they are much more complex then i'm thinking of making.

Once i have something which i think can work i'll post the schematic here so hopefully someone with better knowledge can look it over ;)

The mountain is looking smaller...
 
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One could use this circuit. It's the p-channel version of the driver on this thread.-> http://www.laserpointerforums.com/forums/YaBB.pl?num=1220866048

You only need to choose R1 and R2. For your app Running 2.9V to the diode from a 3v supply, you want the max voltage at that resistor to be 0.1v. Ohm's law lets you calculate this- E=IR where I is your desired max current. and R is the resistance value. So for a 400mA driver you would do-
  E(0.1)=I(0.4) * R
so R1=0.25 Ohms. And since 0.4 *0.1 = 0.04 watts  we can use a small (1/4 or 1/8 watt ) resistor here. But make sure to use a 1% ;)

For R2, we need max voltage to be 0.1v to match the max voltage at R1. So with a 1k pot we use a value close to 29k for R2 (1/8W, 1%).

The above is just an example. You'll want to plug in your max current and supply values to get appropriate values for R1 and R2...

cheers,
kernelpanic

[edit] This circuit uses the op-amp/feedback method you posted previously. But we don't use a normal bipolar transistor as that'll increase dropout. Mosfets have an extremely wide range and the on resistance is usually measured in milli-Ohms. So they work well for low-dropout applications. Personally I like to design the supply current a bit high. So if I'm gonna drive a long-die open can diode at 420mA, I'll build a 500mA driver... [/edit]
 

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That looks to be pretty much what i want, big thanks there kernelpanic :D

I originally mentioned a transistor instead of a mosfet because, from what i've read, mosfets seemed more like switches so you would have less control over the output current. However, my knowledge is limited so i'm glad there was someone to point that out ;)

btw, i'm planning on making it with smd parts scavenged from old dvd roms and hdds. It's amazing the stuff you find on those! The dvd board alone has easily over £100 worth of parts if i bought them individually, but instead i got it for £0 as it was donated to me by a mate after the mechanism broke.
His loss is my gain :cool:
 

rkcstr

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Most green lasers use an IR diode that is case positive, meaning the diode's positive is tied to the metal case, versus the usual ground (or negative) case. It looks like Senkat's IR diodes are probably the same way.

In that case, if you're replacing the diode of a green laser, you'll probably need a negative-side regulator with common input/output positive (so it can be case positive).

Here's a circuit that's on Sam's Laser FAQ that looks like it would work well for your needs, EU38 driver:
http://www.repairfaq.org/sam/laserdps.htm#dpsclc
 

caleb

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iv been wandering if this is possible. :)
anyway, is the driver in post #3 a 'boost' driver. if so, can i input 4.5V.
does it have similar charachteristics to the lavadrive?

thankyou.
 
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First draft...

Theres inevitably mistakes within it so please be ruthless, also it currently has no adjustment ability as i'm not sure on the correct placement.
I added a zener as that seemed to be the prefered method for every other op amp voltage regulator circuit i've seen (including green laser drivers) although maybe kernelpanics voltage divider would be a better option in this case?

Bits...

Q1 - protection mosfet (brutally plagurised from the rkcstr micro-drive ;))
Q2 - mosfet
IC1 - op amp
C1 - 10uf input (smoothing?) cap
C2 - 1uf smoothing/delayed start?  (copied from other driver designed so i'm unsure of it's exact use)
C3 - 1uf LD cap
ZN1/R1 - input voltage divider (this bit i have the least amount of confidence in)
R2 - 1r0 current control resistor (i'd like this to be fixed at 1 ohm so people can do the mV = mA test across it)
R3 - 100k drain resistor (copied from other driver boards)
D1 - reverse voltage diode (probably not needed with the protection mosfet?)

The capacitor values are based on some smd caps i have, 10uf is max but they can be alot smaller (e.g. 0.1uf)
I don't know what values the parts of the ZN1/R1 divider should be, after a bit of research it turns out the red LDs prefer 2.5v and the IR LDs like 1.9v. Can't remember where i read that 2.9v value from now   ::)

The values of Q1, Q2 and IC1 are still at the 'i've no idea' stage.
 

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A boost circuit uses and oscillator and inductor to generate higher voltages than the input voltage. For those, you'd probably want a dedicated chip. I believe this design is a current sink. It's like having a reservoir of water and regulating the flow of the drain. Some folk have mentioned that these change their current regulation characteristics depending on the voltage of the battery though. Here's another design I remember seeing.
 
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I can clearly see the driver is made from two parts cut and paste! :D

Ah well, that's the best way to learn anyways!

for the R1/ZN1 voltage divider, I believe it's only providing a reference voltage for the op-amp, hence R1 should be as large as it possibly can be to minimise wastage. Then in that case, C2's value can be calculated using the RC time constant so you can decide how slow a start do you want the driver to have.

The other parts.... I dunno. I'm not familiar with transistors. Still reading up! :)
 
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Hmm, can't post links yet.. (using a workaround)
There's a really simple current regulator circuit here (add a www infront): allaboutcircuits.com/vol_6/chpt_5/15.html

And that website is basically a textbook, so it teaches alot of useful stuff. Go have a look!
 

rkcstr

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Viracocha said:
Q1 - protection mosfet (brutally plagurised from the rkcstr micro-drive ;))

No worries, I "brutally plagiarized" it from another member as well, cdanjo ;)

seen here
 
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The capacitor values are based on some smd caps i have, 10uf is max but they can be alot smaller (e.g. 0.1uf)
I don't know what values the parts of the ZN1/R1 divider should be, after a bit of research it turns out the red LDs prefer 2.5v and the IR LDs like 1.9v. Can't remember where i read that 2.9v value from now    

The values of Q1, Q2 and IC1 are still at the 'i've no idea' stage.

IC1 needs to be a single supply rail to rail op-amp. The lm358 is a good choice.

The mosfets aren't critical. But Q1 may be redundant. Q2 will protect the diode. Q1 only protects the op-amp, which probably doesn't need it.

But the zener diode/op-amp portion of your circuit probably wont work as you intend. Here's why-

The op-amp will try to make the inputs across the inverting and non-inverting inputs equal. So it will turn on the mosfet until the voltage across your sense resistor matches the voltage across the zener. The smallest zeners you'll generally find are ~3v. 3v across a 1 ohm resistor is 3 amps! If you wish to use a zener for stability, you'll need a voltage divider after it to bring the voltage down more.

Say you use a 3v zener and you're driving a long open-can at 400mA. With a 1 ohm sense resistor, you'd have 0.4v across it. So you need 0.4v at the input of the op-amp. (As an aside, 0.4v is a lot of waste at the sense resistor. I know you're trying to keep the adjustment easy, but a smaller value would be more efficient.) So you'd have to add resistors to the zener before the op-amp like this- ( Hope I didn't munge up your schematic too much.)

cheers,
kernelpanic
 

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Although Q2 would prevent current, would the addition of Q1 isolate the diode from the voltage complety? Might add extra protection...

hmm, pumping a 200mW diode with 3 amps is bad right? ;D

Using the voltage divider seems to be the way to go, in this thread http://www.laserpointerforums.com/forums/YaBB.pl?num=1188440672 with the reversed dx green schematic, the zener is at 4.3 volts (431) but they have a voltage divider which i think reduces the voltage to 0.13 of the original. So at 3v the opamp would get a max of 0.39v (very close to the 0.4v you suggested) with the zener acting like a maxium limit on voltage input, this will limit the opamp to 0.559v

Please, munge away! ;)

---

Checked out allaboutcircuits.com and it looks a pretty good site, i'll be giving it a proper read later.
 
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Latest layout below, i dropped the diode by the LD and added the voltage divider suggested by kernelpanic (big thanks for the help btw) with an extra resistor to set the minimum voltage.

Also worked out the components for it.
Q1 = FDN302P (p channel mosfet, 20v 2.4a max)
Q2 = FDN335N (n channel mosfet, 20v, 1.7a max)
ZN1 = BZX84C2V7 (2.7v zener diode)
IC1 = LM358D

C1 = 10uf
C2 = 0.1uf (118k resistance (approx) fills 0.1uf to 99% in 54ms.  http://www.cvs1.uklinux.net/cgi-bin/calculators/time_const.cgi)
C3 = 1uf

R1 = 100k
R2 = 1R0
R3 = 100k (drains C3 completly in 460ms)
R4 = voltage divider max level (see below)
R5 = 1k pot
R6 = voltage divider min level (see below)

The combination of R4 and R6 set the max and min voltage levels which can be set by R5, i've worked out 2 pairs of values so the driver can have a low range and a high range.
Low range - R1 = 18k, R2 = 100
High range - R1 = 7.5k, R2 = 220

This table lists the output voltage with 2.7v input, the 1 ohm resister means voltage = amps.
It's intended for the two low range or the two high range values to be used together, mixing them doesn't produce useful alternatives.

R1        R2        R5 = 0        R5 = 1k
-------------------------------------------
low      low       0.015v        0.165v      15mA -> 165mA
high     high      0.079v        0.439v      80mA -> 440mA
low      high      0.033v        0.183v
high     low       0.036v        0.396v

At least that is the plan ;)
 

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o.k. That looks like it'll work. Just make sure to use logic-level mosfets and you're golden. I still think Q1 might be overkill for a handheld(nice addon for a labby though). But having it would make multiple modes possible with a little control circuitry...

cheers,
kernelpanic

[edit] Oops. No it won't work. Q1 needs to be an N-channel...[/edit]

[edit] I lied. Actually an n-channel won't work (see post below). But reversing source and drain on Q1 should...[/edit]
 




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