Amplify current from a 4-20 mA temperature controller to drive a Peltier module

Question

I have a temperature controller (Chino DB1000) which has a PID output of 4-20 mA. With this, I want to control the current going to a Peltier thermoelectric module (15V, 6A).

I am doing this via an ON-OFF circuit with a DC solid state relay, a 15 VDC power supply, and a temperature controller for SSR. But I don't want to keep doing this since the ON-OFF is not good for the thermal control or for the Peltier itself.

Is there any simple circuit to convert the 4-20 mA from controller to a regulated current to the Peltier? The 4-20 mA are not the sensor signal, but the processed PID control signal from the controller

After searching similar questions, I think that the answer might be towards PWM or MOSFETs.

Thanks a lot!

Answer 1

When you PWM a peltier you run an already relatively low-efficiency device at an even lower efficiency. The heat pumped across the peltier is \$\propto I_p\$, while the losses due to joule heating are \$ \propto I_p^2\$, namely (\$ I^2R\$).

If you don't need to run your peltier at its peak efficiency for your application PWM is a much simpler approach.

You will have to decide if you wish to have a bi-polar output, namely source/sink current or uni-polar where it just sources current in one direction proportional to your 4-20 mA control signal.

A 4-quadrant programmable power supply like some of the Kepcos is one approach.

An LED constant current driver with a dimming input would be a lower cost approach.

Of course you can design a full custom amplifier with a bipolar output stage, or run the peltier as a bridge-tied-load.

EDIT:

If you go the pwm route, a sample PWM modulator LTC6992 simulates directly in LTSpice, and has a component configuration to make life easy.

Answer 2

I think you are at the answer already. You need to convert your 4-20 mA analog signal to a PWM signal to run the Peltier. The Peltier is not affected by running PWM providing you keep the frame rate high, anything over 1 KHz should be just fine.

There are professional solutions for 20 mA to PWM or of course you could convert to voltage and go from voltage to PWM.

While the battle between those for/against PWM of the peltier modules rages, I tend to believe those professionals such as TE, who almost exclusively produce PWM controllers. This paper shows tests of the Peltier modules at differing PWM frequencies. Notice particularly that in these tests you CAN see changes over the control unit with time, but the effect is below 2% in 2500 hours of operation.
Also this rather interesting Masters paper shows one reason to use DC control for low noise (which is an exception to most requirements).

TE have a range of controllers (all PWM drive) with lots of technical info on the site

Answer 3

As to efficiency concerns, other posters are right. You want to drive the Peltier with a constant DC voltage. To accomplish that we need some kind of switch mode power supply to supply that DC voltage.

Switching power supplies and Class D audio amplifiers are available cheaply in the target voltage and current range. They have the power handling capability to drive the Peltier, the question is how to get them to provide an electronically controlled DC voltage.

While the audio amplifiers have the right topology do drive the peltier in both directions, they would be harder to modify for DC operation. Take a look at this one for instance. Somewhere in there are some capacitors which if short circuited would turn it into a controllable DC voltage source. I personally wouldn't want to have to find them. The power supplies, with some external control circuitry are the easiest to make work.

Take a look at this module. It uses the LT3800 DC-DC step down controller. It's well suited for what I have in mind. The blue multi-turn trim pot near the input side. The one with the little brass screw you turn to change the output voltage. This terminal will be the feedback input.

The output voltage of the converter is fed to a resistor divider. The resulting feedback voltage is fed to the switching controller. The controller tries to keep the feedback voltage at 1.23 volts in the case of the LTC3800. IT does this by varying the PWM duty cycle. This produces a stable DC voltage at the output, which is what we want to drive the peltier.

Flip the board over and you'll see the trimpot terminals exposed. It's easy enough to trace out the layout and tell which one connects to the Vfb pin. when the converter is operating it'll have the reference voltage listed on the datasheet. Solder a wire onto that terminal and connect that to an external circuit through a resistor. This lets you vary the power supply voltage electronically. Be careful not to apply more than the rated absolute maximum(5v) to that pin though.

For driving in one direction, (Only heating/Only cooling), Connect one terminal of the Peltier to the power supply and the other to ground. The power supply acts as it was designed and provides a varying DC voltage which varies the amount of heating or cooling supplied. An external control circuit would be connected as described before. You'd just need an op amp some resistors and a potentiometer.

For driving the Peltier bidirectionally, you'll need two of them, Essentially, the control circuit would set one power supply at Vx and the other at (Vsupply-Vx). The converters would have to be modified since they're set to be non-reversible. To fix this, tie the burst enable pin to Vcc rather than Vfb (The manufacturer just copied the datasheet example layout). You can see it in the image on the Ebay page. The third and fourth IC pins from the right are shorted together, Cut the trace going to the Vburst pin and solder in a wire connecting it to the Vcc pin on the other side of the chip.

The control circuit would just be a 5v regulator, an op amp and some resistors, a capacitor, and a trimpot for the set point. 5V is chosen so the voltage on the converter feedback pin stays below the absolute maximum voltage.

Components are chosen to give the integral component a 10 second time constant.

The disadvantage with this is that you need to change the component values to tune the circuit. Using a microcontroller (EG:Arduino). to read the 4-20ma value, read a potentiometer to get the set point and produce an analog voltage would make the controller easier to tune. Just change the constants in the program. The code would be very simple and this would allow a computer to control the set point. Here's what that would look like:

You write some code for the Arduino. It reads the analog voltage coming in, converts that to a temperature, feeds that into a control loop and writes some analog value out that sets the voltage going to the peltier. You can even get another library to tune the control loop for you.