Temperature Shutdown - Auto duty-cycler (schematics)

Hey all. As many here may already know, I'm studying electronics at one of the biggest colleges of Brazil. So we already finished op-amps and I figured something useful that I didn't see interest. A temperature shutdown circuit.

How it works?

A 3mm thermistor is epoxy glued behind a laser diode or heatsink, for probing temperature. Using a 2.4 zener as source, the thermistor acts like a voltage divider. The signal from the thermistor is fed into an comparator.

When the temperature is high, a led lit up. Any higher and the driver is shutdown

I simulated that using a LM393 (very low power, uWatt comparator) and it worked fine.

The thermistor:


The schematic works for everything, the shutdown temperature (Ts) can be set anywhere in a wide range. The warning led is fixed at Ts - 5.



R1 = Zener diode resistor.
D1 = 2.4V zener
R2 = Voltage divider resistor
RT1 = 10k, NTC Thermistor. 3mm
R3-R4-RV1 = Temperature trip point settings
U1-U2 = Comparators (VCC and GND hidden)

More resistors should be added if hysteresis is needed, and output resistors if the comparators used are "open collector" type (as LM393).

How much it would cost?

I have no interest selling this. And I'm in Brazil, and I can't ship anything int'l too. Here, I bought everything necessary for two comparators for around $3.

My question is. Anyone have interest on having this functionality (temp shutdown)? Otherwise I'll not continue working on it.

posthere if you have interest. I just want you guys opinion!!

This is a neat idea. It certainly could help some folks.

I wish I had time enough to start on such a project.

Or put a $1 thermal switch in there. These are much cheaper, much smaller, much simpler, and higher current than the solid state system you're proposing. The only (minor) downside is that the trip-point is not adjustable.

Those cost $15 ea. in Brazil. I've never seen any cheaper thats why I started this project.

Anyways, the op-amp inputs can be reversed and turn a TEC or fan on...

Think I should change the thread name... :thinking:

I'm not seeing any schematics?

Are you just talking about a comparator with a temperature dependent voltage reference?

Without hysteresis included, the thing will just cycle the laser on and off too quickly. The diode temp will get just below the threshold allowing it to turn back on again for only a few moments.

Did you consider using a 555 based PWM circuit with a low carrier frequency?

I'm not at home yet. I don't have the schematics here.

But how you said, it is 'just' a comparator with a temperature dependent voltage reference.

It don't need hysteresis. The laser diode can't heat all of a sudden when turned on and off, to maintain the trip temperature... When it goes off, ie 60ºc, it won't goes down at the same moment. It will need a few more seconds to dissipate the heat and come back down to 59ºc, turns back on, a few more seconds to heat again, and the cycle repeats.

It will still need hysteresis, as the temperature will gradually change, it won't instantly drop from 60C to 59C. During the time it's transitioning between those temps, it'll oscillate. It's very easy to add hysteresis to opamps anyway.

Right, if you have a perfect set point of 60C, it will oscillate between 59.99 and 60.01C.

Where would you propose the thermistor be placed?

Question is, are you worried about LD temp? Or heat sink temp?

The thermistor proposed is a 3mm one. It can fit behind the laser diode, between his pins.

I thought the thermal resistance between the laser die and the diode body would act as a "natural hysteresis" if you people could understand me. Any way, 2 resistors are more than enough as hysteresis.

The physical hysteresis just means that it will not continuously oscillate at 59.9/60.01 but instead oscillate for a period of time while transitioning through the range. Electrical hysteresis solves the issue entirely. One of the reasons I love working with Schmitt triggers.

Leodahsan said:

The thermistor proposed is a 3mm one. It can fit behind the laser diode, between his pins.

Click to expand...

That's where mine has always been. Right in between my pins. :shhh:

Isn't a schmitt trigger essentially a hysteretic comparator?

Meatball said:

Isn't a schmitt trigger essentially a hysteretic comparator?

Click to expand...

Pretty much.

Great project Leo, thank you for sharing Great input Vets, awesome dialogue :beer:

~ LB

I made a 3x PT54 buck driver which used the onboard thermistors to reduce power when it heats up. When there is no feedback from the op amp, the output simply oscillated from full power to zero about once every second due to the thermal lag from the die to the thermistor. I added a feedback resistor across the op amp inverting input and output to reduce overall gain so it worked and power regulated smoothly.

So with an open loop comparator, I get what Leo was saying but due to the much shorter thermal path, the blinking would be much faster. I also vote for adding hysteresis. It shouldn't be too hard, just an additional resistor from output to non inverting input.

Sure, I just don't have time to modify the post now
I'm going to the college NOW...

Thanks for the headsup people! Will add hysteresis for sure and post how to insert a fan to cool, mosfet for shutdown, TTL for more advanced drivers, etc.

LPF has turned into an addiction....... it is just so much knowledge...

gotta go

Hmm, one thing that should be considered is how are you going to set the temperature tipping point? A thermistor (unless it's a temperature sensor, i.e. like TMP35 or LM35) would not have a calibration, and hence must be calibrated by measuring the temperature of the diode using a temperature probe or something. Sounds a little on the inconvenient side.

A possible idea is to use the LM35 or TMP35/36/37 sensors which output 1mV per degree, making it very easy to calibrate without even firing up the laser. You just need to use a multimeter. Unfortunately, the size is a little bit larger, around 5mm, although I think it can still fit near or just behind the diode in most setups.

What do you think?