Running multiple diodes from a single drive via parallel connections.

[gizmoguy]

Hey guys,

So yet again I need some help

I am not really sure about building with lasers but I want to try and power multiple diodes using a single driver via parallel connections. Here's my plan

for the sake of this issue lets say I have a driver outputs 20v 10A
and I have several diodes lets say they need 1A 2V to be biased on and work so theoretically I could run 5 of those in parallel with some resistors to modify the voltage across each branch of the circuit.

something like this (sorry for the bad quality made it in paint a second ago

the only issue I found with this was as member 94Z28 pointed out if one of the diodes were to die the influx of current would destroy the rest of the diodes.

so I went back to the drawing board and thought of making a schematic that would control the voltage and the current output something like a buck control circuit with closed loop control. So the pulse modulation would change if one of the other diodes went out immediately. and put one of these in between each diode.

If anyone's ever made anything like this id be a lot of help if you let me know how you did it or if this configuration would work. from what I've found it switches in milliseconds I am not sure if that's quick enough to burn out the other diodes or not. thanks again guys

Edit: This would be for portable solutions.

 

[FireMyLaser]

Ideally you should connect the diodes in series if you have just one driver, but it can be hard to find a driver that that can supply the voltage, or if the diodes don't have a floating case.
Connecting in parallel will be fine if you use balancing resistors, and be wise to not push the diodes to the max power. Leave little headroom. I wouldn't worry about a diode burning out, I've never had one go open circuit myself. They just turn into an LED.

 

[diachi]

You can run the diodes in parallel just fine, but you need a balance resistor in series with each diode. I've seen 2.2Ohms suggested for 445nm diodes, I've seen 1Ohm used with ~660nm diodes. Obviously need to take into account the power ratings of the resistors.

Thankfully laser diodes tend not to fail open so much, most failures I've come across are purely optical so the electrical properties of the diode stay pretty much the same, they just stop emitting any laser light. That means the failed LD will still be drawing the same current and the others then won't be cranked up by the driver.

Still, you need the balance resistors due to differences in forward voltage between diodes.

Edit: FireMyLaser bet me to it!

 

[Alaskan]

Just be aware when running everything off the same driver, if you have a problem with the driver output all of the diodes might fail instead of just one. I've considered running diodes in series myself, have wanted to do so to save the money otherwise required for several drivers, but finding a driver which can run off of the input voltage needed for each driver in series was the problem I ran into. i.e. 7 + 7 + 7 + 7 volts requires more than 28 volts input, probably 29-30 VDC, depending upon how much drop-out voltage the device consumes, maybe more. I could easily find constant current drivers which would work, but then I was concerned whether they would spike or not when turned on, and how hard they turned on without soft start built into them.

 

[FireMyLaser]

Spiking is a non issue if you are using a laser driver. That's the point of using one in the first place. But if you still have reason to believe that it will spike, then put a cap on the output, no big deal. Also, what diode drops 7 volts?

 

[gizmoguy]

Alright sweet thanks guys you've been a big help what driver would you recommend for the M140 M diode. I plan on running a couple of these. @Firemylaser I most likely will run them in series due to current being a constant. I wasn't aware that they never really failed open XD. So I guess to simplify this as best as possible could I run Battery powers --> driver powers --> output of diode used in step up voltage circuit output ran to chain of diodes in series? I know ide lose some amperage but as long as im not running the diodes at full power shouldn't be an issue should it?

 

[FireMyLaser]

I'm not aware of any laser driver that's rated for diodes in series, but that just because I never find datasheets for the drivers. So potentially any step-up driver can do it, but I don't know which. It depends on how many series diodes, and at what current. At some point you can exceed the power rating of any driver, so watch out for that.
But assuming that you find a driver that can supply the needed voltage and current at that voltage, then just set it for the current that you want and that will be all.

 

[gizmoguy]

I'm not aware of any laser driver that's rated for diodes in series, but that just because I never find datasheets for the drivers. So potentially any step-up driver can do it, but I don't know which. It depends on how many series diodes, and at what current. At some point you can exceed the power rating of any driver, so watch out for that.
But assuming that you find a driver that can supply the needed voltage and current at that voltage, then just set it for the current that you want and that will be all.

Alright thanks man ill have to find a compatible driver..............

 

[Alaskan]

Spiking is a non issue if you are using a laser driver. That's the point of using one in the first place. But if you still have reason to believe that it will spike, then put a cap on the output, no big deal. Also, what diode drops 7 volts?

You are expounding upon my very concern with that statement. What diode drops 7 volts? Single mode PL520 operates from 7.0 to 8.0 VDC and is the diode I was considering driving in series, decided against doing so.

http://imageevent.com/qdf_files/tec...dedatasheets?p=4&n=1&m=-1&c=8&l=0&w=4&s=0&z=2

 

[FireMyLaser]

Genuinely curious because the only diode I saw with that drop was the 488nm diode. It would be nuts if you had 4 of them.

 

[Alaskan]

The PL520 diode requires more voltage than any other diode I've used so far, the voltage is so close to two Li-ion batteries in series I've been able to power the diode without a driver board and it just wouldn't fail for me. You can see DTR driving one of his up to 7.6 VDC and comments that when pushed to high, the diode dims, but will recover if the voltage is reduced again. Probably the toughest little VIS diode we have, mine won't die. I might have a couple tenths of a volt drop in my wiring, but even using freshly charged batteries producing about 8.4 VDC, so far, my diode hasn't failed when directly hooked to the batteries and I've used it many times. If the batteries don't sag down to about 8.0 VDC pretty quick, I have seen my diode dim until they reach that, but that doesn't happen very often.

 

[gizmoguy]

The PL520 diode requires more voltage than any other diode I've used so far, the voltage is so close to two Li-ion batteries in series I've been able to power the diode without a driver board and it just wouldn't fail for me. You can see DTR driving one of his up to 7.6 VDC and comments that when pushed to high, the diode dims, but will recover if the voltage is reduced again. Probably the toughest little VIS diode we have, mine won't die. I might have a couple tenths of a volt drop in my wiring, but even using freshly charged batteries producing about 8.4 VDC, so far, my diode hasn't failed when directly hooked to the batteries and I've used it many times. If the batteries don't sag down to about 8.0 VDC pretty quick, I have seen my diode dim until they reach that, but that doesn't happen very often.

is there any specific way I could find the voltage drop for each of my diodes?
I plan on getting M140 M type diodes

 

[Alaskan]

If you had a driver and set it to the recommended current and then put a DVM meter across the drivers output leads feeding the diode, you could know what the voltage drop is for THAT diode at THAT current, but I don't know why you would do that instead of just looking for the data sheet, or ask here what the typical voltage drop is for your diode at its rated output power. My friend Google usually comes up with an answer for me, but I will admit, hard to find a data sheet for some of our laser pointer diodes, for some I can't get that.

 

[94Z28]

This should be the datasheet.
http://jezus.square7.ch/public/laser driver/M140 Datasheet.pdf


I could run a bench test for you using a LM-317 if you need me to, PM and i'll run all the numbers on a single diode through an LM-317 although these LM-317s have alot of drop to, I should be able to give you the rundown. /edit > Tomorrow I will, it's bed time.

I can tell you that a 12v input source with 1 diode driven by a LM-317 generates serious heat, and the amount of drop is considerable.

 

[gizmoguy]

This should be the datasheet.
http://jezus.square7.ch/public/laser driver/M140 Datasheet.pdf


I could run a bench test for you using a LM-317 if you need me to, PM and i'll run all the numbers on a single diode through an LM-317 although these LM-317s have alot of drop to, I should be able to give you the rundown. /edit > Tomorrow I will, it's bed time.

I can tell you that a 12v input source with 1 diode driven by a LM-317 generates serious heat, and the amount of drop is considerable.

alright thanks man sounds good

 

[Alaskan]

According to that data sheet, minimum 3.7, maximum 5.5 VDC. Diodes can require slightly different voltage from one to the other, even if the same model number, but a driver set to what ever current you have decided to run it at will automatically adjust the voltage so the diode draws the amount of current you set it for, so in that case, as long as you have enough voltage from your batteries and it isn't too much voltage for the driver, you don't need to concern yourself with it, with the following exception:

If the battery voltage is close to depletion, or even full charge for that matter, and that amount of voltage is too close to the amount required for the diode, your laser might have reduced output, or quit putting out light before the batteries are discharged through their full capacity, or only discharged half way. i.e., if your laser diode required 4.0 VDC for full output and the driver itself adds or drops an additional .2 volts, or two tenths of a volt, and your battery was only producing 4.0 VDC, then there wouldn't be enough voltage to drive the diode to full output power because it would only receive 3.8 VDC. I'm writing about a BUCK driver, not a boost, for boost drivers it's a whole different ball game.