Published: 2020-11-11 | Categories: [»] Engineeringand[»] Optics.

In my attempt to reducing the cost of the [∞] OpenRAMAN spectrometer, I recently tested the MTD415TE TEC driver in replacement of the expensive TED200C lab controller. At contrario to what I experienced with [»] Thorlabs precision current driver, this TEC driver revealed to be a very good alternative to expensive professional TEC drivers!

Thorlabs proposes different versions of the MTD415 chip that ranges between 55€ and 75€ (ex-VAT). The “L” versions operates with LMT84 temperature sensor while the “T” versions works with 10kΩ thermistors. You then have the choice of the standard SMT chip or the chip mounted on a daughterboard (“LE” and “TE” versions). Since the LDM21 block I’m using includes a 10kΩ thermistor, I went for the TE version.

To evaluate the board without having to make additional circuitry myself, I also purchased the MTDEVAL1 board with it extra USB cable for 145€ (still ex-VAT). This board is really handy to set up the MTD415 chip PID settings and I really recommend buying one so that you don’t have to implement the serial communication with the chip yourself. Note that you will also need a 12V power supply that can deliver at least 2A. You can use about any high-current power supply that you can find as long as it can operate at small loads as well.

You will also need a SUB-D9 connector to plug into your LDM21 block but this is relatively straightforward. I recommend twisting the two wires for the thermistor for extra stability even though it is not mandatory.

The MTD415TE on the MTDEVAL1 board is shown in Figure 1.

Figure 1 – MTD415TE chip on the MTDEVAL1 board

One interesting thing to note if you look carefully at Figure 1 is that I had to wire the TEC element with reverse polarity! I have no idea why the indications are messed up but it is like that. Failing to do so will heat your TEC instead of cooling and vice-versa, which may eventually damage your LD so be careful.

The MTDEVAL1 board comes with a software that you can install on your computer to interface with the MTD415TE board. Despite a few bugs in the GUI, the software is relatively well done and easy to work with.

The user manual will guide you through a Ziegler-Nichols procedure to tune your PID by first identifying the gain at which the system start oscillating. A screenshot of the program during that tuning phase is given in Figure 2.

Figure 2 – Ziegler-Nichols tuning of the PID

The GUI assists you in getting the PID values from the Critical Gain and Critical Period which makes the process relatively easy when you are not used to PID tuning. The standard Ziegler-Nichols rules tend to produce slight overshoots however as can be seen in Figure 3. I tried to use modified Ziegler-Nichols rules to produce damped response with no overshoot but I was not able to make sense to the “Cycle Time” parameter of the configuration.

Figure 3 – PID response at not load (laser off)

This was not a big deal however as the system produced very little overshoot when switching on the laser at a 300 mA load. Actually, the temperature did not exceed the set point by more than 60 mK which is already quite good. After less than one minute the temperature settled within ±5 mK variation. This is illustrated in Figure 4.

Figure 4 – Very slight overshoot when switching on the laser

I then left the system running with the laser on to check how it performed over time. Everything was working fine until I ran into a catastrophic failure after 1h of operations. The driver stopped powering the TEC element while the laser was still on. The temperature quickly went up and could have potentially damaged my LD. When I noticed the problem, the temperature was already at 38°C.

A quick investigation revealed that the problem originated from the LDM21 passive cooling system. The LDM21 has a heatsink that does not allow sufficient cooling by natural convection. Just for the quick info, when I contacted Thorlabs about using the DJ532-40 with the LDM21, they told me the module was not compatible.

Because the passive cooling of the LDM21 did not handle the heat generated by the hot side of the TEC element, the TEC became hotter and hotter, requiring more current to keep the cold side at the 22.50°C set point. More current was then generating even more heat which was at the origin of an instability in the system since at some point the maximum driving current would be reached (1A in the default settings). When the maximum current is reached, the MTD415 safety features automatically switches off the device after a given delay. This is good to prevent the driver from over-heating but will lead to a catastrophic failure of the LD that is still powered up.

I was able to fix this problem by adding an extra heat sink with forced convection cooling on the LDM21 as shown in Figure 5. Fans are usually avoided in optical systems due to vibrations but here it works perfectly fine.

Figure 5 – Extra heat-sink mounted on the LDM21

With the fix installed, the system was perfectly steady and no increase in current could be noticed anymore over time. With the ambience at 22.3°C I was even able to operate the laser at 16°C by increasing the maximum TEC current to 1.5A which is the maximum allowed by the MTD415TE board.

The thermal regulation in the MTD chip is good enough as can be seen in Figure 6 with a 0.5A TEC load. For extra safety an auxiliary heat-sink may be added to protect the chip in case of long-term operations at high load.

Figure 6 – Thermal imaging of the board under a 0.5A load

In conclusions, the circuit works extremely well considering its price (the MTD415TE is sold 65€ ex-VAT). Tuning of the PID requires serial communication which can be done using the MTDEVAL1 board but the settings can then be stored into the non-volatile memory of the MTD415.

The only potential problem observed with the module is that it switches off automatically if it reaches it maximum current for too long. Hopefully, the module also proposes a STATUS output pin which toggles from Vcc to gnd if any errors occur which would switch off the TEC. It would be advisable to connect this STATUS pin to the ENABLE pin of the laser controller to automatically switch off the laser in case of a TEC failure.

Finally, the MTD415 also defines a temperature window and a delay after which it considers itself as stabilized. According to the datasheet of the product, with the default safety mask set at factory, this temperature window should not influence the STATUS pin. However, in our tests I noticed that the STATUS pin was depending on the temperature window by default. This is a good thing because it allows turning the laser on only when the TEC has stabilized, but it is not clear if it is normal or not that this mode is enabled by default. Note that it was not possible to switch on/off this mode using the provided software and it requires a raw serial communication to the chip. When using the temperature window, and knowing that we noticed a slight overshoot of about 60 mK at 300 mA load for the LD, I would recommend setting a temperature window of 250 mK with a 10 sec delay. The TEC will stabilize better than that but it ensures that the LD will be switched on only when the TEC ensures that the LD case is held at the set point ±0.25°C.

Also, considering the tests performed, it is safe to assume that the TEC will be able to operate the laser at full power at 22.50°C in a room that is maintained between 20°C and 25°C. I would not recommend operating this driver in a room hotter than 25°C but operations can probably be extended slightly below 20°C since heating is more efficient with a TEC than cooling. Note that it is also possible to use any set point between 20°C and 25°C for the LD used, so if your lab operates in a warm country you may be able to work in a 27-28°C environment by putting the set point to 25°C. When planning to operate the laser at full power under stress thermal load for the TEC, I would recommend putting an extra heat-sink on the MTD415.

That’s all for today! I will now look into the LD3000R module from Thorlabs to power up the laser :) I should then be able to come up with a much cheaper alternative to the expensive LDC205C/TED200C drivers for the OpenRAMAN project!

I would like to give a big thanks to James, Daniel and Mikhail who have supported this post and helped me buying this driver by signing up on [∞] Patreon. I also take the occasion to invite you to donate through Patreon, even as little as $1. I cannot stress it more, you can really help me to post more content and make more experiments!

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