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Thread: The big TEC driver thread!

  1. #11
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    I think drlava's design is nice, for a general purpose PID controller. We're all going to use these TEC drivers for the same thing though; cooling a laser. The conditions will be fairly similar across all users for us however, which opens up the possibility of a more specialised solution.

    As far as I can tell you need one PIDC120/150 for each laser as well, unless you mount them in the same block (which typically isn't possible with red diodes if you use for example a die4drive).

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    Yep I'm working on an efficient, switching, microcontroller PID TEC design but there are many simple low-cost analog solutions out there. Watch out many of the pre-packaged solutions have very slow response because they are not full PID.

    One of the benefits of the design based on the microcontroller is that when the blue/green lasers are manufactured, a computer can automatically scan the multivariate temperature space and choose the optimal setpoints, all only needing an RS232 port.

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    Not sure if that's newer, but Robin also has the Die4Thing which can do power metering and TEC drive and probably other things.

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    Cool

    See: http://www.edn.com/article/CA6615604.html

    I like this because one big pain in the posterior adjunct is placing the sensor in the right spot, this is a nice way to sense the heat flow.

    Steve

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    That's a brilliant idea! Makes it possible to control the TEC with just a microcontroller and H-bridge; no need for a thermistor.

    I see one problem with it though. It measures the temperature difference between the laser block and heat sink, meaning that it will be heavily dependent on the ambient temperature. Assuming we have a steady 20-25C that's not a problem, but in a hot venue or outside on a cold winter day it might just be.

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    Quote Originally Posted by drlava View Post
    One of the benefits of the design based on the microcontroller is that when the blue/green lasers are manufactured, a computer can automatically scan the multivariate temperature space and choose the optimal setpoints, all only needing an RS232 port.
    One of my other projects is closed-loop control of laser power (ie., driving DPSS a bit like "light mode" on an ion laser). This could work well with that!

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    Quote Originally Posted by heroic View Post
    One of my other projects is closed-loop control of laser power (ie., driving DPSS a bit like "light mode" on an ion laser). This could work well with that!
    Excellent, I think mixedgas was working on that too. The electronics would be relatively straightforward but someone would have to make a nice little feedback cell. You or Mecheng? Can't wait to test it out!

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    Quote Originally Posted by drlava View Post
    Excellent, I think mixedgas was working on that too. The electronics would be relatively straightforward but someone would have to make a nice little feedback cell. You or Mecheng? Can't wait to test it out!
    I've been doing experiments using a photodiode mounted off-axis (just below the beam) on a plastic peg. The spill light from a DPSS seems to vary in proportion to the beam output power, so unless you have an extraordinarily clean laser I think this will work without sapping power from the output beam. Plus it's cheap cheap cheap

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    Quote Originally Posted by tocket View Post
    That's a brilliant idea! Makes it possible to control the TEC with just a microcontroller and H-bridge; no need for a thermistor.

    I see one problem with it though. It measures the temperature difference between the laser block and heat sink, meaning that it will be heavily dependent on the ambient temperature. Assuming we have a steady 20-25C that's not a problem, but in a hot venue or outside on a cold winter day it might just be.
    It's an awesome idea, I agree. Similar to what's done in Li-ion chargers to watch their voltage, but I never thought of doing it with a TEC.

    The difference to ambient is an advantage because it can tell the TEC drive how hard to push to compensate in proportion to the push caused by the ambient difference. Doing it directly means no waiting for the thermistor's rate of change to be detected, although the differentiator time constant can be small. There is a thermistor in the circuit Steve linked to, and they're using full PID control too, this other trick just adds stability. I'm not sure that Seebeck measuring alone will be enough, for the reasons you mentioned. So the PITA of placing a thermistor still exists, but is probably minimised by that extra stabiliser.

  10. #20
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    Quote Originally Posted by heroic View Post
    I've been doing experiments using a photodiode mounted off-axis (just below the beam) on a plastic peg. The spill light from a DPSS seems to vary in proportion to the beam output power, so unless you have an extraordinarily clean laser I think this will work without sapping power from the output beam. Plus it's cheap cheap cheap
    That's an awesome idea too. Given that closed loop control can allow enough energy to accelerate a mass better than with open loop, it can help with some kind of widget to provide analog mod to a fixed output laser. Probably not fast enough to be useful though, but if it is, it's surely been done in the past. I might try this with a red diode though if it can make a better fade than current-sense control.

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