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jander6442

jander6442

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I wanted to build a unit that housed 2 modules  2 PHR  diodes running off of one lava v4 is it possible to use 1 driver

for this ?. I'm under the impression from other threads that let's say don't say for sure but , come close to this subject...

If I bridged the wires from driver to diodes how can I achieve this or [highlight]is it possible at all[/highlight]. I've read on the array

set ups, but they are a little beyond me... ::) I'm not looking for really high mW #'s just two side by side beam's from the same host...

I would app. any and all input on this  :) :) :) If there is a section for this I appologize in advanced.
 





You can just hook the lasers up in parallel to one driver, as long as the two lasers together don't draw more current than the driver can handle. For two PHRs @ 100mA, you'd just need to set the flexdrive to 200mA.. Make sure your connections are at 100%, though. You don't want anything coming loose..
 
ElektroFreak said:
You can just hook the lasers up in parallel to one driver, as long as the two lasers together don't draw more current than the driver can handle. For two PHRs @ 100mA, you'd just need to set the flexdrive to 200mA.. Make sure your connections are at 100%, though. You don't want anything coming loose..
Click to expand...

In parallel you mean diode 1 gets a positive and diode 2 gets a positive and they share the neg or....

Not that this is crucial, just information gathering at this point... Thank you!
 
jander6442 said:
[quote author=ElektroFreak link=1240838761/0#1 date=1240841816]You can just hook the lasers up in parallel to one driver, as long as the two lasers together don't draw more current than the driver can handle. For two PHRs @ 100mA, you'd just need to set the flexdrive to 200mA.. Make sure your connections are at 100%, though. You don't want anything coming loose..
Click to expand...

In parallel you mean diode 1 gets a positive and diode 2 gets a positive and they share the neg or....

Not that this is crucial, just information gathering at this point... Thank you!
[/quote]

^That's exactly right.
 
If the two diodes have fairly identical electrical operating characteristics, putting them in parallel could work OK. The only problem is that PHR's can be notoriously dissimilar. This could cause one diode to try to hog all the current and burn out. Perhaps hook up each diode separately set at 100mA, and measure the voltage drop across the diode. If both diodes have a near identical voltage drop, OK to try in parallel. If one diode is say 5.2V and the other is say 5.6V it might not be good. LM317 drivers are fairly cheap to make, so why not use two drivers to be safe. (Personally I hate waiting for new LD's to arrive...) If driver size is a problem, I will use something like an AMS1117 or similar and surface mount resistors and caps.

Let us know how this works out.
 
billg519 said:
If the two diodes have fairly identical electrical operating characteristics, putting them in parallel could work OK. The only problem is that PHR's can be notoriously dissimilar. This could cause one diode to try to hog all the current and burn out. Perhaps hook up each diode separately set at 100mA, and measure the voltage drop across the diode. If both diodes have a near identical voltage drop, OK to try in parallel. If one diode is say 5.2V and the other is say 5.6V it might not be good. LM317 drivers are fairly cheap to make, so why not use two drivers to be safe. (Personally I hate waiting for new LD's to arrive...) If driver size is a problem, I will use something like an AMS1117 or similar and surface mount resistors and caps.

Let us know how this works out.
Click to expand...
This is what I was afraid of. The dissimilar properties of the two diodes and the "hog"
like charateristics of certian diodes. Diodes are a one way electrical vavle ,but in the neutral side I would also think that the shared ground would equilize the current draw. BTW please stick to topic of OP... ;) thanks
 
billg519 said:
If the two diodes have fairly identical electrical operating characteristics, putting them in parallel could work OK. The only problem is that PHR's can be notoriously dissimilar. This could cause one diode to try to hog all the current and burn out. Perhaps hook up each diode separately set at 100mA, and measure the voltage drop across the diode. If both diodes have a near identical voltage drop, OK to try in parallel.
Click to expand...
It gets worse.  

Most diodes (including the violet PHR-803's) have what amounts to a negative resistance characteristic: as they draw more current they heat up and their voltage goes down.

So if you start with two diodes in parallel that are exactly matched, what happens is that one of them gets a just bit warmer at random (a butterfly landed on it, for example).  Since it got warmer, its voltage goes down and it draws even more current.  The other diode now has a bit less current, so it cools and its voltage goes up, so it draws even less current.  This potentially creates a thermal runaway situation and the hotter diode may burn out.

There are two classic solutions to this problem:

1) Thermally couple the diodes together very tightly.  That is, mount them close together on the same heat sink with low thermal resistance from the diode to the sink.  This way, when one diode heats up, the other diode heats up.

2) Put a small valued resistor (dropping, say, 0.5 v to 1.0 v in the case of the 5.5 v PHRs) in series with each diode.  The idea is that resistors in series add together, and a large enough positive resistance added to a negative resistance creates a net positive resistance.  

The higher the resistance, the better the sharing, but the more wasted power, and you may run into a limit on the voltage output of your driver.

Also, as billg519 mentioned, you can measure the voltage of each diode.  Then you can adjust the series resistance for each one to get more equal sharing.

jander6442 said:
This is what I was afraid of. The dissimilar properties of the two diodes and the "hog" like charateristics of certian diodes... I would also think that the shared ground would equilize the current draw.
Click to expand...
Sorry, no... :)

Putting them in series works great, but you would need to electrically isolate them, and your driver will have to put out double the voltage.

Finally, this thread deals with a similar situation:  4 diodes better than 1. DIY Array on the cheap
 
^Matching the diodes up like billg said would work fine at first but in the long run (assuming you've taken temperature into account by using TECs or at the very least a decent heatsink), as the diodes age and their characteristics change, you would also end up with one hogging current from the other. I don't know of any way that you could counter this completely except to use optical feedback. You could also construct a custom driver capable of driving and regulating multiple diodes. This could be as simple as having two or more LM317 circuits on the same board..
 
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