multiple diodes, one driver?

[neskusen]

Here's a simple question (I think), can you drive multiple diodes from one driver?
I know diodes have to be current limited, I'm just asking if you for example set a 5A driver to output 4 Amps and the connect it to 4 red diodes, will each receive 1A (equally divided), or not?

[VDX]

... this is common for high power diodes - have salvaged some 9Watt/9Amp-diodes from broken fiber-lasers, where they're wired in series of up to 12 diodes in a chain.

In an older medical laser I've found a red 20watt-module, where 60(!) 400mW-diodes were too wired serial in two groups á 30 diodes ... the chains were driven with 0.3Amps current at 160 Volts!!

Viktor

[neskusen]

I see. But what about connecting not in series, but the + and - of the driver to the + and -'s of the diodes?

[VDX]

... laserdiodes are regulated/driven over current - so you'll have to give every diode its own current regulator ... series are much simpler and cheaper ...

Viktor

[kecked]

DIODES are not purely resistive and change with voltage and temperature.

You can't load balance the diodes which is what you are asking to do just by putting them in parallel. One diode will preferentially draw more than an other due to internal resistance differences, thermal behavior as they heat up, and a few other items. You can put them in series and keep the current the same in all diodes. What happens here is that the voltage fluctuates instead of the current when in series. That doesn't affect the diodes operation because they are current devices and the single driver adjusts for this to keep the current the same. In the other case the current fluctuates and the voltage fluctuates.

Here is something I really had not though about until now. We usually add the diodes optical output together but it is possible one diode will be different in terms of the rise and fall times and threshold current as a result you could see some jumping in the low current operation resulting in weird effects in color when drawing images that use faint lines of a color. So you should try to match the diodes as much as possible in terms of the threshold currents at least. Rise and fall times should be so fast in diodes to not matter(so why did I mention it. Cuz it can happen). On the other end in terms of brightness your eye will be so swamped that I doubt you can see fluctuations on the high side of brightness.

Here is something that might help you

For driver running at 12V you can put 3 Reds in series (3v drop on the driver plus about 3V per diode =12v)

For driver running at 12V you can put 1 green in (3v drop on the driver plus about 8V per diode =11v)

For driver running at 12V you can put 1 blues in series (3v drop on the driver plus about 6-7V per diode =12v)

Reds are about 2-3V
Blues are about 6-8V
Greens are about 6-8V

So figure 3v drop on the driver and then add up the diodes to make up the supply voltage.

Now here is a question. Can you put two drivers in parallel to increase current? I'll let you work that one out. Clue: it depends.

[-bart-]

You can drive them in parallel, just give each diode a balancing resistor in series.
The value of this resistor should be at least 20% of the impedance of the given diode. The driver should be capable to drive the increased voltage.


Downside of this method is that if one of the diodes goes openloop, the remainder of the diodes will be overdriven accordingly.
As where driving them in series the remainder of the diodes will survive in this situation.

[kecked]

There is no upside to driving in parallel and the load balancing resisters limit the current you can put into the diodes. I've tried this and the result is not worth the effort. Additionally it is self limiting. But technically you are correct. It's a good way to fry a lot of diodes quickly. In series one pops and the light goes out but you still have the rest of the diodes alive. Plus you can just short one diode at a time to find the problem diode. I have not tried it but I think temco resistors might be needed as well to compensate thermal drift in the diodes resistance.

[neskusen]

Thanks for the input everyone.

So figure 3v drop on the driver and then add up the diodes to make up the supply voltage.

What do you mean by 3v drop on the driver?

BTW this is the driver I'm considering: http://www.ebay.com/itm/5A-analog-li...oAAOSwiYFXHE-T

[catalanjo]

What he means is that you should adjust the power supply voltage so that when driving full blast the supply voltage is only 3volts higher/more than the output voltage from the driver to the diode string.
This avoids excessive heat being generated by the drive card.
(In fact this depends a bit on the driver architecture so he has included a small extra margin to make sure you still get full power from the laser diodes).

Cheers
PS. Careful with lasorb choice and placements if you do this!

[VDX]

... check, if 100kHz max. modulation bandwidth is enough for you - most cheaper drivers can only handle up to 50kHz.

I need more, so I developed my own high current drivers with capabilities of up to 4MHz modulation, but the effective output modulation depends on the drawn and output currents too - for currents of 1Amp its something around 3MHz, but for 10Amps it's more like 1MHz max. and for 40Amps this will go down to 100kHz with the same components to get the full current switched ... here I'll have to find components with this MHz switching specs/capabilities too, what's rising the price nearly ten-fold ...

Viktor