LD driver configuring questions

[kecked]

No I mean how did you measure the Vf?

http://meettechniek.info/active/diode-dc.html

vf is the forward voltage across the diode when lasing. This article explains it better and shows the circuit. Most people graph current against Vf. I say when lasting because I'm not sure if the name Vf fits prior to threshold.

it really is that simple. Assume 2.6 for red. 6 for green or blue and you should be fine. If it doesn't lase due to insufficient voltage add more or remove a diode depending on the driver. You are over thinking this.

[Maltes]

it really is that simple. Assume 2.6 for red. 6 for green or blue and you should be fine. If it doesn't lase due to insufficient voltage add more or remove a diode depending on the driver. You are over thinking this.

I dont know if im over-thinking this or if Im just not getting it.
Im very fine with going with a rule of thumb of 2.6 and 6. These numbers come from adding the voltage and forward voltage right?

[Maltes]

Back to the "original" question. I'm having issue with Simpledrive (5A version).

When you suggest to turn the Bias up until I see a weak led-like glow, do you mean it will be visible on the wall at dark or on the glass lenses? I didn't notice any glow no matter how much I turned the Bias up. So I set it to minimum again.

I then started turning the Gain up. The modulation input was a constant 5V input (from my laptops USB port).
I did see the NDB7875 finally lasing. Fired up my Laserbee power meter and got the diode to 800 mW.
But something must have been very wrong, the driver mosfet was burning hot, I didn't measure the temp but from my experience if I can't stand something to the touch if its 80C or higher. So I immediately turned off the power. Checked the cables, they were not warm and handle 6Amps fine for my other projects. I checked the square housing the diode was in, it was barely warm.
I'm not sure what the issue was but I'm pretty sure the mosfet shouldn't get that hot, not to mention a large capacitor is right next to it.
This is the heatsink I used on the mosfet:


I can use a larger heatsink no problem, but I don't think its a heatsink issue, it is a 4.5A driver that was only used for <1W laser output. If at that power it was so hot I don't think getting the NDB7875 to its 3000mW power would be possible with any sized heatsink on the driver, so I must have set up something wrong. Maybe it has something do do with the Bias pot?

[catalanjo]

:

Back to the "original" question. I'm having issue with Simpledrive (5A version).

When you suggest to turn the Bias up until I see a weak led-like glow, do you mean it will be visible on the wall at dark or on the glass lenses? On lens or whereever you like I didn't notice any glow no matter how much I turned the Bias up. So I set it to minimum again.

I then started turning the Gain up. The modulation input was a constant 5V input (from my laptops USB port).
I did see the NDB7875 finally lasing. Fired up my Laserbee power meter and got the diode to 800 mW.
But something must have been very wrong, the driver mosfet was burning hot, I didn't measure the temp but from my experience if I can't stand something to the touch if its 80C or higher. So I immediately turned off the power. Checked the cables, they were not warm and handle 6Amps fine for my other projects. I checked the square housing the diode was in, it was barely warm.
Voltage drop over driver probably too high - reduce PSU voltage until only 1.8vdc drop @ max I
I'm not sure what the issue was but I'm pretty sure the mosfet shouldn't get that hot, not to mention a large capacitor is right next to it.
If the large capacitor is getting hot, it's a totally different can of worms !
This is the heatsink I used on the mosfet:


I can use a larger heatsink no problem, but I don't think its a heatsink issue, it is a 4.5A driver that was only used for <1W laser output. If at that power it was so hot I don't think getting the NDB7875 to its 3000mW power would be possible with any sized heatsink on the driver, so I must have set up something wrong. Maybe it has something do do with the Bias pot? I doubt it

Hope this helps
cheers
PS ...for clarity ....drop of driver = PSU volts less LaserDiode volts. or difference of input to output volts of driver card.

[Maltes]

Sorry i dont understand, do you mean the voltage coming out of the PSU is too high or too low? BTW it is 12.5 Volts. The diagram says the driver can be powered by a 5-24V PSU.

PS. Capacitor had gotten hot because it was physically very close to the hot mosfet I think.

[catalanjo]

PSU voltage much too high .......(better would be 7.5v, or even 9v if you want to run flat out) unless you are running two greens or two blues in series !

From memory (not terribly reliable) the forward voltage for this diode is = or less than 5.5V.

For every amp thru diode (with PSU above 12vdc) you will have approx 6.5W of heat in driver FET.....Ouch!
Cheers

[Maltes]

The manual says it should handle about 40W heat dissipation max. But I don't know much about mosfets as you might have guessed so I didn't know what that meant. I could only assume "5-24V PSU supported" meant I could use any and it would do the dc-dc conversion without glowing red. I guess 24V support is for when using the driver with LEDs?

Anyway, the V adjustment pot on the 12V PSU I dont think rotates lower than 11 Volts...
Do I need a non 12V power supply?

[catalanjo]

NEED is perhaps a bit of an exaggeration .....you could put a ruddy great heat-sink on the FET and just dissipate the heat, or use a Buck module, or add another diode in series with a PBS cube, or change to a lower voltage PSU. I would recommend the latter!

I think Phillip, (Live Lasers or Phritzler on PL) has high power drivers with on board DC-DC converters, & TEC and high temp cutoff etc. if you are really attached to the PSU .

Cheers

[Maltes]

I've only ever seen 5V, 12V and 24V switching power supplies. Could you please link to what kind of PSU you have in mind?
Thanks for all the help.

I could get a large PC heatsink on it but I doubt it would solve the issue because its so hot and I don't want to risk overheating the capacitors and everything else on this tiny driver while performing this experiment. Haven't burned anything yet but I wouldn't want to try my luck some more.

[catalanjo]

Yeah...finding "in between" voltage PSU's with a decent amount of current is a PTA.

Most people here on PL have a Lab supply so that they can decide which voltage to look for before dishing out the boodle.


You can always do a cheap bodge as "proof of principle" by getting a load of diodes and stringing them in series so as to spread the heat ...you will drop about 0.5v over each diode (they will need some cooling as well eg. small fan).

Link will be tricky since I don't know which side of pond you are.
As someone has already pointed out ...it makes it much easier to help out if you fill in your profile with your approx geo-location.

If you were in UK I would suggest
http://www.maplin.co.uk/p/worldwide-...r-supply-l11bq
or
http://www.ebay.co.uk/itm/OH-1048A09...cAAOSwBadTqej6
or
https://www.tlc-direct.co.uk/Products/SK661406.html
or
https://www.amazon.co.uk/dp/B00OVXYBFC?psc=1
or
https://www.amazon.co.uk/DROK-Consta...WY5KT0QG1J5AK6 = probably best for you !
in USA
http://www.ebay.co.uk/itm/PLEXTOR-AP...AAAOxy7odR0RB7

These are cheap and cheerful and just to get you on right track while you are looking for something a bit more powerful and more suitable for chassis mounting.

Ideally an adjustable PSU or buck to reduce voltage drop across driver to a minimum....try Bangood or Ali and see what Chinese suppliers have to offer.
https://www.amazon.co.uk/gp/product/...2F3L3 *nyancat = cheapest buck convertor for high current

Alternatively start a new PL thread "looking for adjustable or 8vdc PSU" and see what other members come up with.

Cheers
PS. I think you are confusing "temperature" with "heat", a much bigger heat sink will spread the heat (approx 5 to 7W @ 1amp), and thereby reduce the temperature along with the paranoia!