Topic: Protect your diode from over voltage on the modulation input

[James]

Check this out.

satje and I were talking on Skype. He was going on and on about zener diodes used to eliminate spikes and overvoltages on 5 volt TTL or some such nonsense... I'm not really sure. I wasn't even listening. 

Anyway... I suddenly had this great idea! What about using a 5.1 volt zener diode to eliminate spikes and overvoltages on <= +5 volt color modulation signals! 

So I messed around quite a bit with a bread board and a bunch of different resister values. I found that putting a 5.1V zener in series with about a 24 ohm resister and putting that between the color modulation signal and ground, with the anode toward the ground, made it impossible to get above +5 VDC! 

I got to about 24 ohms by using what I have and putting two 47 ohm resisters in parellel.

I put my milliameter in series with the diode, resister combination and got less than 8 milliamps. I think that's pretty acceptable. The top measured voltage was right about 5.00V with this thing in place. The output of my DAC was anything from 5.00 to 7.8V. Below 5.00V it has no effect.

I even loaded the output of the DAC (op-amp) with 100K, 47K, 24K, 10K. All that was fine and had no effect on the output voltage. When I tried a 1K load, it dropped a bit.

First I thought about putting these on all four color outputs of my LaserBoy DACs, but then I thought it would make a lot more sense for EVERYONE who uses analog modulated diode laser drivers to put this on the modulation input of each laser.

If you setup a LaserBoy DAC properly, then all the color gains are set to output +5.00 volts when the sound card wave level is set to 100% and the master volume is reduced to just below clipping the circle. That means that no matter what you do with the Windows volume control, the card is already to the rails and can not produce any more voltage.

EDIT: (not correct! read to the end of the thread.) 

I still have some more testing to do. I need to measure the peak current draw of the whole box, sound card and correction amp, all gains to max, from the USB port.

In all seriousness, this was one of those super easy ideas that satje came up with. It might save tens of thousands of diode lasers all over the world!

If it doesn't work, you can blame him. 

James. 

[satje]

Nooooo nooooo, don't blame me  

Stupid enough, i was talking with James about driving the RGB lines.
As i do have analogue PCAOM wich is capable to 10 Volts, not really a issue.

But i begun telling that i want to switch to diodes....
Ooops, seems you have to be carefull with steering those.

So i thougt why not take a zener-diode to protect those  

Hope to rescued some with this stupid idea  

[James]

At this point, I would say TRY it before you totally depend on it!

Meter, meter, meter, meter, meter!

I need to see what happens if you put a negative voltage into it. I think I might put a signal diode on the output of the colors on the LB DAC and bias it with a bit of DC offset to make it impossible to output a negative color signal.

I'd like to hear from Dean, drlava or anyone else about these ideas (signal and zener)!

I showed it to my brother-in-law and he said it was a perfectly good use of a zener.

James. 

[meandean]

Anyway... I suddenly had this great idea! What about using a 5.1 volt zener diode to eliminate spikes and overvoltages on <= +5 volt color modulation signals! 

  Gee, Jimmy you're starting to learn some electronics!! Zeners do make good overvoltage protection, and they also protect against
undervoltage (negative values) since they work like a normal diode in the forward direction. I doubt that you need the series resistor though, because the op-amps in the correction amp are only good for about 18mA or so, and are short-circuit protected.
I would like to think that a decent projector design has some protection in it already, but you never know...

[James]

Gee, Jimmy you're starting to learn some electronics!!

Yes! In spite of YOU!!!

Now go and learn some C++ on your own, bitch! 

I like the 24 ohm resister. It was the only value that made the final output voltage almost exactly +5.00VDC. I tried zero ohms. I didn't like it! 

So what should happen if a negative voltage comes from the DAC?

James. 

[meandean]

Yes! In spite of YOU!!!

  I seem to remember using a zener in my correction amp design until somebody dumped it... Now, it's like some new thing
that nobody has ever heard of...

So what should happen if a negative voltage comes from the DAC?

 It should clamp at about -0.7v.

[James]

Connected like this:



Now, my question is.... Are we to assume that laser mod signals are ALWAYS with respect to GND?

That is The Question!

Where are those ILDA Spec Reps when you need them?

James. 

[James]

Yes! In spite of YOU!!!

  I seem to remember using a zener in my correction amp design until somebody dumped it... Now, it's like some new thing
that nobody has ever heard of...

OMG! I'm laughing so hard my sides hurt!

WTF R U talking aBOUT?

James. 

[meandean]

Now, my question is.... Are we to assume that laser mod signals are ALWAYS with respect to GND?

That is The Question!

 They are with respect to the low side of your individual color inputs, since ILDA has an individual low side connection for each channel.

OMG! I'm laughing so hard my sides hurt!

WTF R U talking aBOUT?

 Yeah, keep laughing as you've just now figured out what zeners can do...

Now go and learn some C++ on your own, bitch!

 What! So that I can write some crude console app that looks like something from the 1980's that nobobody wants to touch with a 10ft (3.048 Meters for all you metric fans) pole?

[James]

Huh? What? Oh Shit! I can hardly see to answer you! My computer is on fire from running this God Damn LaserBoy!!!

You just don't understand.... There is a fine art to writing such an enticing fucking pain in the ass application interface. It is truly annoying on so many levels.

James. 

[drlava]

FlexMod N2's have had this sort of protection built-in for a while. 

[satje]

Well i am glad to helped out with this.
Proven again that those minor things can be sooooo usefull.

Ferry

[James]

With a little bit of thought, this zener idea could also be applied to protect the laser diode itself; right across it's power pins.

Hmmmmmmm. Maybe I should put 5¢ worth of parts in a tiny white plastic package and sell it for $9.

What do you think? 

James. 

[meandean]

With a little bit of thought, this zener idea could also be applied to protect the laser diode itself; right across it's power pins.

 Zeners are more for clamping voltage in low current applications, so I'm not sure they would save a laser diode on the
drive side (a MOV would have a better chance)...
 
  BTW, you can connect 2 zeners in series back-to-back to clamp bi-polar voltages. You could take two 5 or 10v zeners to protect a
scanner input, or you could just use a MOV, as they come in many voltages.

[James]

I must have had my head up my ass again.

I don't know why I said you can't get more than +5 volts out of a color channel, if the correction amp is set up right.

That is NOT true!

So, I'm back to plan A.

I would HIGHLY RECOMMEND that anyone who uses analog modulated solid state lasers check into their diode driver designs and find out if they already have over voltage protection. If not, employ this simple thing!

I will make retrofit devices and send them out to all of the people I've made DACs for.....

As soon as I find my bag of 100 f&*king 5.1V zener diodes! I just had them a couple days ago. I've hidden them very well. 

I found them! 

James.