Did I kill my driver?

A diode to protect against reverse polarity comes at signficant performance cost. The part itself is only a few cents, but it drops half a volt if it is a skottky diode, probably a full volt if it's an ordinary diode.

Half a volt may not sound that bad, but if your batteries run at 4 volts nominal it still wastes 15% of energy just for that protection.

There are considerably better ways to do this, like using mosfets to protect against reverse polarity. These are a bit more expensive and take up some more board space (at least 1 mosfet and 1 resistor), and still waste a few percent of power.

Another method is to use a fuse and 'crowbar' diode, which shorts reverse polarity so the fuse blows. This is only practical if your circuit already has a fuse for some other reason though, otherwise it's way too large for use in pointers.

So yes, there are good reasons to design drivers such that they get destroyed with reverse polarity, which have nothing to do with component cost.

Thank you for the information, and good ideas Benm! While I have a few power diodes laying around that can certainly do the trick, they get hot regardless. I took your advice and picked up a nice little p-channel mosfet. Its forward resistance is almost zero, it's voltage drop is close enough to 0 that it won't be noticeable, and the allowable gate voltage is much higher than the batteries that I am using, so I was able to solder the ground directly to the gate. It works great, and handles 20 amps without even getting warm. The cost was $.89, so more than a diode, but it's not contributing to the heat load, so well worth it. (Of course, I bought another $20 in parts from Digikey while I was at it...) Thanks again!

Glad to hear you were able to make use if this approach!

It's not that common to find it on battery operated products, as there usually is some physical way that avoids batteries inserted in reverse making elecrical contact. This mostly applies to button-top batteries though, like your typical AA/AAA consumer alkaline.

Using the p-channel mosfet is ideal if you have the space for it - losses are very low compared to series diodes and protection is good against randomly mis-inserted batteries (they will not do that good a job at high reverse voltages, but that doesn't randomly happen often).

Anything that can use flat-top lithium cells and such should really have reverse protection as it's not that hard to accidentily insert the batteries the wrong way around.

afaik this was your first post/thread...you would have been better off instead by making a polite intro thread in our WELCOME section--not to late to do that.. BTW.

I would have suggested an easier first-time build NOT considering even the high power which is asking for an accident-- those very familiar (think YEARS not days) have instinctively learned how to avoid an eye hit.. that takes TIME== not enthusiasm --
IMO we are safer w/o glasses/goggles than many new members with goggles.

I have started a Texas Social LPF group==always a smart move...want an invite?

Poisonednumber said:

So finally got my parts to build my first laser have been wanting to for a long time.

Parts list:

Diode:NUBM06 450nm Laser Diode In Copper Module W/SXD Driver & 405-G-2 Lens

Driver:4A Super X-Drive(SXD-V3)

Batteries: 2 18650 lg


So I wanted to do a quick test ....

.I already see problems coming . We have read this MANY times and the outcome is mostly the same ---death-of something

being QUICK is NOT being correct.



to see if it works so I put the two batters in series and wired the driver to the batteries and it turned on but only left it on for 2-3 secs at the most since I don't have my heat sinks installed..

Once again-- bad move--I NEVER do that. Connections should be soldered==and use of alligator clips avoided.


on driver and diode yet I tested it s few more times but I accidentally put one battery backwards

WHAT? Careless mistake.
since not all go the same way (BUT NONE go with each a different way) I suggest marking inside the tail cap w/ a fine tip sharpie + is pos to the rear and - if neg to the rear..btw reverse polarity protection usually works like this==very quick on-off *may* do no harm-- one second try or longer may only kill the driver a longer or repeated try will toast both

and it kinda sparked

haha there is no KINDA SPARKED--it did or it did not.

when I touched the wire to the battery

NO DO, NO DO..no 'touching'



then I realized I had a battery backwards I corrected it but it never came back on :/

Click to expand...

We suggest a cheaper & less powerful build for first timers for both safety and $$ reasons==
you paid WAY too much ..
for the 'lesson'.. could learned all that for $10..btw it is extremely rare that ANY seller replaces ANY killed/ dead parts- ever.:tsk:

but you may get a less $$ fix by only having what is bad replaced (by the seller)

PM anytime--next big TX laser meet is coming.
get lasers metered free and a WHOLE LOT MORE--search 'SELEM Lasershows' see ytube vids also TXLEM:beer::beer:

hakzaw1 said:

afaik this was your first post/thread...you would have been better off instead by making a polite intro thread in our WELCOME section--not to late to do that.. BTW.

I would have suggested an easier first-time build NOT considering even the high power which is asking for an accident-- those very familiar (think YEARS not days) have instinctively learned how to avoid an eye hit.. that takes TIME== not enthusiasm --
IMO we are safer w/o glasses/goggles than many new members with goggles.

I have started a Texas Social LPF group==always a smart move...want an invite?




We suggest a cheaper & less powerful build for first timers for both safety and $$ reasons==
you paid WAY too much ..
for the 'lesson'.. could learned all that for $10..btw it is extremely rare that ANY seller replaces ANY killed/ dead parts- ever.:tsk:

but you may get a less $$ fix by only having what is bad replaced (by the seller)

PM anytime--next big TX laser meet is coming.
get lasers metered free and a WHOLE LOT MORE--search 'SELEM Lasershows' see ytube vids also TXLEM:beer::beer:

Click to expand...

:wtf::undecided: :yabbem: :thinking: :drunk:

Jerry