Jump to content
Electronics-Lab.com Community

On/Off heater controller using triac


Recommended Posts

In a previous post, I asked about designing a controller to regulate the temperature of a small tank of water.

Ante suggested using a triac to control power sent to the heating element. After some reading, I also agree that this is a good technique; however, I haven't been able to find any circuit examples on the web.

My problem seems fairly simple. I have an aluminum tank with a flexible heating element wrapped around the outside surface of the tank. I would like to set a desired temperature using a potentiometer. I have a temperature sensor IC's that measures degrees Fahrenheit (10 mV per deg F). I put the IC in some copper tubing and sealed the end, making a temperature probe. This probe will be submerged in the water tank. I figure that a couple of comparator circuits could be used to compare the temperature from the probe with the desired setpoint and these comparator circuits could then control voltage to the gate terminal of the triac.

Anyone know of an example that uses a simliar technique?

The triac I have is NTE5629 rated at 400V and 4A. (Here's the datasheet http://www.nteinc.com/specs/5600to5699/pdf/nte5629.pdf). The triac came with a mica washer, which I assume goes between the metal part of the triac and the heat sink.

Thanks in advance for any help.

Darrin

Link to comment
Share on other sites

Darrin,

For a start: Do you know the power consumption for the flexible heating element? A 4A triac on 230VAC (if this is your voltage) will limit the power to about 900W. For efficiency and ease of interfacing to temperature measuring circuit I suggest you look at the MOC3041 or MOC3061. The datasheet http://vitsch.net/datasheets/mov3041_optoisolator_triac_driver.pdf or http://www.fairchildsemi.com/ds/MO/MOC3163-M.pdf will give you info on snubbers for triac circuits. This Zero-Cross Phototriac Driver Optocoupler will galvanic separate the mains from the rest of your circuit and make it simple to connect the rest of the circuit.

Ante ::)

Link to comment
Share on other sites

Just from experimenting with the heating element with 110V AC, my water tank gets too hot, too rapidly.

The resistance of this heating element is VERY low, about 20 Ohm. From Ohm's law, this would be around 600 W (if my math is correct).

So, I need even less power than 600W. I only got the triac with such a high voltage rating, because I read somewhere that it was wise to buy components with higher volt/amp ratings than you actually need.

I will look into those triac drivers that you mentioned.

One thing I'm concerned about is that the pins on the triac are very close together (about 2

Link to comment
Share on other sites

Thanks Ante.

I ordered a couple of free sample MOC3041's from Fairchild. I found a schematic in the data sheet that has me a little confused.

On page 5 of http://www.fairchildsemi.com/ds/MO/MOC3041-M.pdf, there is a circuit that shows 115V AC connected to the MOC3041. At pin 2, there is a symbol that looks like a plug. I believe this is a symbol to a 2 input NAND gate, but I'm not sure. ???

I want to make sure I know what I'm doing before I go messing with the AC. :o

It seems like I could just put a relay betwen pins 1 and 2 to toggle this circuit. ???

Thanks again for you help.

Darrin

Link to comment
Share on other sites

I'm can't post an image of my circuit (I don't have a website), but I can post a web page of something very similar.



Instead of using a thermisor to measure temperature, I'm using an IC (LM34), which outputs 10 mV/deg F and is much more linear than the thermistor.

I don't know how this type of circuit would interface with the NAND gate on the triac driver IC. Any suggestions?

Thanks again.

Darrin


Link to comment
Share on other sites


Darrin,

You need to convert the output (10mV/degF) to pulses that can control the opto isolator triac. The lower the temperature the longer the pulses should be until it reaches the desired temperature.

Ante ::)






Ok. I'm starting to feel really stupid. I just want to use the triac so I can limit the power to my heater (via phase control). In other words, I probably won't be varying the phase angle while my heater is ON. I'm building a VERY crude bang-bang controller. When my voltage from my temperature sensor is too low (comparator will tell me this), I just want to turn on the heater until the voltage reaches the temperature set point. I'm not sure I understand how to use the NAND gate, since I'm dealing with an analog signal. Can I just run pin 2 of the MOC to ground?

Thanks again.

Darrin
Link to comment
Share on other sites

I guess what I'm really building is a solid state relay for my heater, like this one from Tony van Roon's tutorial on relays:

From: http://www.uoguelph.ca/~antoon/gadgets/relays/relays.html

relayssr.gif

In my application, the heating element would be the load, instead of the light bulb. Correct me if I'm wrong, but the components C1 and R5 are a snubber circuit to prevent false triggering of the triac, correct? It is also my understanding that the phase angle of the triac output is controlled by the resistors R3 and R4. Is that correct? (the R4 in the figure is labeled 2k2 -- 22K?)

Or is the phase controlled by the MOC? I'm confused? ???

I can certainly build this circuit as is and experiement with it. It's my guess that using a pot for R4 would allow me to vary the phase, but it's just a guess.

I certainly have appreciated your input. Hope I'm not flooding this board with my dumb questions.

Darrin

Link to comment
Share on other sites

I think I may have found my "poor man's" solution.

dimmer-switch-1.jpg

I'll just use my temperature sensor and comparator circuit to control ON/OFF of the dimmer switch and adjust the pot on the dimmer until I'm happy with the response characteristics.

I may try my hand at controlling it with PWM to the phototriac driver when I learn more. I'm sure it's somewhat similar to motor speed control using a 555 timer, but I'm still not clear on how the pulses get synchronized with the AC frequency.

Thanks again for all the tips.

Darrin

Link to comment
Share on other sites

I don't mean to beat this into the ground, but I'm very curious.

What would happen if the pulses were sent to the triac driver such that they never crossed the zero intercept of the AC signal?

In other words, what would be the output of the driver if the pulses were entirely contained within the AC waveform? Excuse the crude drawing (below).

. .
. .
. .
. .......... .
. | | .
. | | .
===================== zero volts =========

Thanks.

Darrin

Link to comment
Share on other sites

Hi Darrin,
If your input pulse occurs during the high-voltage "excluded-firing" time of the mains, then the triac won't turn-on. That is why zero-crossing triacs are used for bang-bang controlling, not phase control.
A triac needs only one pulse to turn it on for each half-cycle of the mains. Once it is on, it stays on, until at the end of each half-cycle when the current through it reaches zero, then the triac turns itself off.
Lamp dimming (or contol of the power to your heater) is controlled by delaying the firing (turning on with a short pulse) of the triac during each half-cycle of the mains. With a short delay, then the triac fires early during each half-cycle and the triac is on nearly all the time, so the load gets full power. With a long delay, then the triac fires late during each half-cycle and the triac is on only for the short time that remains in that half-cycle, so the load gets a small amount of power. See, this is PWM, but at only 60Hz.
The variable resistors in Ante's lamp dimming circuits control the amount of delay and have a fairly high AC voltage across them. It would be difficult to design a circuit to vary a resistance with temperature, and work with a fairly high AC voltage.
So maybe it is best for you to use one optically-isolated triac that is switched on and off with your temperature sensor and comparator. That triac can apply power to the "lamp dimmer" that controls power to your heater. Then adjust the dimmer's control for the speed that the heater heats-up.
Get a high-power dimmer. The little one that you showed is good for only about 500W max.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
  • Create New...