how to wire multiple diodes to one driver?

[mk64n]

I have found cheap source of 4.5 Amp high current Arduino and DAC controllable laser drivers.
If I have many diodes I want to power at once (0.8 Amps each, 4 of them , http://www.mitsubishielectric.com/s.../laserdevice/ldindis/638nm/ml501p73_p1245.pdf ) can I and how can I wire these to the driver?


I think its possible in series right? But then the driver needs to be be able to output 4*3 volts right? ANy other options I have?

 

[ARG]

Putting laser diodes in series is the best option. If you run them in parallel you need resistors in series with each diode to limit the current, without the resistors you run the risk of one diode drawing more current than the others.

In series they need (num diodes)*(voltage drop per diode), at the current of a single diode and in parallel they need (num diodes)*(current per diode), at the voltage of a single diode.

 

[Benm]

I would depend on the driver, these diodes only take 2.5 volts or so at rated current, so putting 4 om them in series requires a driver that supplies 10 volts at 800 mA or so.

One thing you could do is run them as 2 pairs with a small balancing resistor (say 0.22 ohms or so) for each pair, from a driver that was designed with blue diodes in mind. That'd give you about 5 volts forward voltage to be deliverd at 1.6 amps - quite a typical value for such drivers.

Since these diodes are case-isolated you can do either without much of a problem even if mounted on the same heatsink.

 

[mk64n]

How can I calc what resistor I need, given the max operating current and voltage of the diodes?
I don't have problem with a series setup its just I dont think the drivers I have in mind can supply so much voltage.

One is a modified Simpledrive up to 4.5 Amp output and the other is a Flexmod.

 

[Benm]

When sharing current between two strands the resistors depend on how different the diodes actually are. This is very hard to say from a distance - they could be very similar (all from the same batch etc) or quite a bit different.

There is a way to test for this though: Connect each diode to a proper current source, set the current to what it will be in your final product, and just measure the resulting voltage.

If these voltages are very similar (like 0.01 volts from another) you can use a small value, but if they are larg (say 0.1 volts apart) you need larger resistors. You can also use this way to pair diodes a bit so you get strands with similar total voltage drop at a given current.

 

[Cyparagon]

1ohm 2W on each string would be plenty. This would add 0.8V of drop to each string (in accordance with ohm's law of course) which would be way above any drift in forward voltage. You could probably get away with a smaller resistance, but it is more risky and would only serve to slightly improve efficiency.

Series operation is still better, if your driver can supply 10-12V without a problem.

 

[Benm]

The differences should not be that large really, but if you want to err ot the side of caution your could go for 1 ohm in each strand.

I recommended 0.22 ohm since that could easily be done with standard 1/4 watt resistors.

You would need some resistance to prevent thermal runaway in any case: forward voltage drops when diodes heat up, so the hotter pair/strand would 'short out' the cooler pair if you did nothing about it. Even with pretty similar diodes that could escalate into a problem especially if they are -not- sharing a heatsink.

All in series would be preferable if you have a driver that supports the voltage drop and proper electrical isolation between diodes... the normal case-insulated scenario should be plenty to allow a handfull of volts between pins and case.

 

[mk64n]

So do I understand correctly that there's no harm in using resistors with higher resistance as balancing resistors, just to be on the safe side?

And what is the idea of running them as 2 pairs, instead of having a balancing resistor for each individual diode?

Thank you all so much.

 

[Benm]

The downside of using larger resistors is that they will dissipate more power for a given current, and also require more output voltage from the driver. It's not real harm, but the first means they have to be larger resistors and the latter may run into the limitations of your driver at some point.

Running 2 pairs instead of 4 diodes comes from the low forward voltage drop of these red diodes. It is about half that of blue/blueray diodes, so a driver for those could run 2 red diodes in series no problem. Since you wanted to use 4 diodes logic is to make 2 strands of 2 to match the bluray driver output voltage, as well as current in your case, as many drivers can supply 1.6 amps.