Hi,
I am trying to use my Trex Junior to power and control a Thermoelectric Cooler (TEC) using a PC.
I have connected 1 TEC to one of the 2 Trex motor outputs, I have connected the COM serial pins to the serial port of my PC ad I am successfully sending commands to the Trex via the Trex Configurator v1.0. My power source is a Gwinstek Lab power supply tuned to 5.0V DC
I am running the Trex at a “motor speed” between 1/127 and 10/127 corresponding to a max (measured) output current of 0.5A at a PWM frequency of about 600Hz.
I have encountered the following problem:
While the TEC generates the expected temperature difference for any set speed, the TEC heats up a lot, i.e. there is no cooling.
Also when the set speed is above 50/127 the Trex powers down the TEC, even if no current limit was set.
The TEC is a Marlow DT3-8 with a resistance of 0.4 Ohm, Max Current 7.4 A, Max Voltage 4V.
I called Marlow and they advised that the cooler requires a smooth DC current for optimum operation. A ripple factor of less than 10% and a PWM power source below 1KHz.
Here are is my question:
Is the over heating due to the fact that the power source is 5V while the TEC accepts 4V max?. Isn’t the Trex effectively reducing the output voltage through the duty cycle? I calculated that at a speed of 10/127 the TEC should be seeing a 0.4V voltage, so what’s going on?
I did not solder the 0.1 uF capacitors provided with the Trex Junior. Does this make any difference ?
I am not in “single motor” mode and I have left one of the two Trex outptuts open. Do I HAVE to connect all 4 wires to the load and go single motor mode?
I know that the Trex was not built for this application but I believe the Trex is a nice piece of electronics and it might do the job.
I think you’re asking for breaking your parts with your setup since you’re out of spec. in so many ways. When the TReX Jr (and all of our motor controllers) use PWM to adjust the speed of a motor, they are switching between full power and no power. When you apply 5 V to a 0.4-ohm load, you’re going to get over 10 amps flowing, which is out of spec. for the TReX Jr and your Peltier cooler.
You don’t need the 0.1 uF capacitors for your setup.
You do not HAVE to connect both motor drivers, but you need to pay attention to the specs, which in this case include a peak of 5 A for one, so it does not make sense to try to pull 12.5 A out of it.
However, I think your overall setup might still work and that your perception of things not working are a result of the physics of the Peltier device. In general, you’re putting power (volts times current) in, so the net effect is that you’re heating. One side will be cooler than the other, but to get one side to be below ambient temperature, you really have to dissipate the heat from the hot side quick. So, my recommendation is to try with a big heat sink and fan on the hot side, and to put in a power resistor (around 1 ohm; make sure it can handle the power!) in line with your cooler so that it doesn’t get as much current. If you have them, you could also use a few of the coolers in series.
Jan,
Thanks. I have connected in series a 2 Ohm resistor at the TRex Jr output. I see now less over heating.
The temperature differential generated with the TRex is quite precise and repeatable. A 1/127 speed incremental gives me about 0.3 Celsius of gradient between the hot and the cold side of the TEC. I am measuring the temperatures using platinum RTD sensors.
I have purchased a Low Voltage Pololu Controller too, and I want to try it with the TEC. The Low Voltage model should allow me to remove the step down resistor.
