Temp sensor and safety switch

TacoHerder · Mar 26, 2012

Ive been thinking how nice it would be to make a laser that has a safety switch of some type and maybe a temp sensor in case your heatsink starts getting overheated. Anyone know where i could find something like that? Been looking, so far cant find anything small enough to place in a laser host.

rhd · Mar 26, 2012

I would take an approach similar to what they did here:
http://laserpointerforums.com/f67/how-pulse-laser-70292-2.html#post1020450
for pulsing lasers.

Unless you want to reinvent the wheel (or rather, the driver), why not just create a circuit that the power flows through prior to the driver? Then have it open the circuit when a thermistor registers above a particular temperature?

TacoHerder · Mar 26, 2012

rhd said:
Unless you want to reinvent the wheel (or rather, the driver), why not just create a circuit that the power flows through prior to the driver? Then have it open the circuit when a thermistor registers above a particular temperature?

I was thinking about that. If my switch had an LED light, would that throw off the power leading to the driver? Or should a low voltage LED switch work? I guess then i need to make sure my host has room for the extra internal circuit.

bad thing is im not skilled enough yet to make a circuit for the switch

I found a lot of temp sensors and stuff on Digikey.com. Now i just need to figure out what i can and cant use.

Sigurthr · Mar 26, 2012

The main problem is fitting it all in to the damn host, haha. I can whip up a circuit to shut off power above a certain temp pretty easily, but getting it made and made to fit inside a tiny host.... yeah total nightmare. The simplest circuit is still going to need a transistor (mosfet) that can handle the power being drawn by your driver, and usually this means the transistor needs to be heatsinked, compounding the issue.

Building a thermally protected labby? No problem. Building a thermally protected handheld? Ugh.

Just my $0.02

TacoHerder · Mar 26, 2012

Yup, i come back to that conclusion every other minute. It would be so nice though. How does WL have their safety swith set up? Unless they skip the heatsinking part.

Sigurthr · Mar 26, 2012

IIRC they have a thermistor on their driver which controls analog modulation (reduction of power at increased temperature). As RHD said, "reinvent the driver". It's a lot simpler than APC driver topology, and those have been around since the early nineties, if not earlier.

Any driver with analog modulation built in can be converted to automatic temperature control by simply forming a voltage divider with a thermistor.

Cyparagon · Mar 26, 2012

The thermistor is only present in the "krypton" models.

Ontologian · Mar 30, 2012

Have you ever seen Battle Los Angeles ? Make the lab section, fiber that < expletive > into a handheld.
This may help you though : Analog Power Management PDF

IT is more about transmission switches, but it may clear up some circuit information for you.
This tells you how those sensors work.

Temperature Sensor - Function

Very good information.

Cyparagon · Mar 30, 2012

The simplest option would be to simply use a thermal switch in series with the driver input. Something like this.

TacoHerder · Mar 31, 2012

Cyparagon said:
The simplest option would be to simply use a thermal switch in series with the driver input. Something like this.

MAN! Thanks for that. Looks like a very simple solution to helping protect the diode from excess heat and neglected use(not that i plan on doing any). When i build my next 445nm i want to give this sucker a try.

Benm · Mar 31, 2012

Sigurthr said:
Any driver with analog modulation built in can be converted to automatic temperature control by simply forming a voltage divider with a thermistor.

I suppose this would also work on a drive with an enable pin, as long as it has some hysteresis. Downside is that it will shut off abruptly, and may take some time to cool down enough before it turns on again.

I've never tried it myself, but it would be a good method of making a portable laser foolproof in terms of duty cycle.

TacoHerder · Apr 13, 2012

Cyparagon said:
The simplest option would be to simply use a thermal switch in series with the driver input. Something like this.

So i got a few of these to try out whenever i build my next strong laser. Do i have to put this before the driver, or can it be placed between the driver and diode?

Things · Apr 13, 2012

Put it before the driver.

Benm · Apr 13, 2012

^^ definitely! Otherwise the diode could fry when it re-engages and suddently connects the laser back to the driver which is supllying maximum voltage at that point (and charges its outputs caps to the max if it has any).

TacoHerder · Apr 14, 2012

Wow, ya im trying to avoid stuff like that. Thanks a lot guys. Ill most likely be using this before my dual drivers or a linear driver. Im not even sure how to go about putting this in front of a round driver.

flare09 · Apr 14, 2012

i too am working on something like this. i believe zener diodes have a point at which they let current flow, if that is correct, you could simply hook a thermistor to a low current zener and to power supply, then have the zener switch a lower power fet, just enough to light an led from the power. you wouldnt have to worry about heatsinking the fet that way. but you would have to switch off the power manually when the led comes on

Temp sensor and safety switch

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