Controlling 3 phase heater's triacs manually. - Page 1
Forum section: electronics / projects

author: avitsur
author_rank: Contributor
timestamp: September 15, 2024, 11:13:40 am
content: Hi, I have a domestic water heater (Chinese production) that operates on 380V 3 phase. it took only a short while until it stopped functioning. the problem is on it's control board, from what I've checked, it doesn't provide signal to the three triacs that operate the heater elements. the triacs are - as can be seen on the pics - T40C8J. what I would like to do is to convert the heater to operate manually by omitting the original control board and to install a circuit that will control the gates of the triacs by a potentiometer.looking forward for your help.

author: moffy
author_rank: Super Contributor
timestamp: September 15, 2024, 11:38:13 am
content: Quote from: avitsur on September 15, 2024, 11:13:40 amA domestic water heater is normally fully on until the set point temperature is reached, which would mean that the Triacs are either fully on or off and the potentiometer would be for setting the maximum temperature, no phase control required. You would then have a slightly lower temperature than maximum to turn them back on, after they have been turned off, to prevent oscillations around the trip point.If the circuit is not fail safe you could run into an exploding water tank, you would need a pressure switch interlock as well, sometimes safety relief valves fail.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 15, 2024, 12:12:57 pm
content: It would probably be easier to repair the control board. Otherwise, what you need is a PID controller but all the cheap ones I have seen only support single phase phase angle control.

author: avitsur
author_rank: Contributor
timestamp: September 15, 2024, 12:51:03 pm
content: Quote from: NiHaoMike on September 15, 2024, 12:12:57 pmI've turned to the manufacturer and asked for the circuit diagram. that was the point that they vanished.

author: themadhippy
author_rank: Super Contributor
timestamp: September 15, 2024, 01:02:59 pm
content: Quotewhy?A simple comparator circuit should work fine, hardest bit is ensuring the 3 outputs are isolated from each other but nothing an opto coupler or 3 cant do.

author: avitsur
author_rank: Contributor
timestamp: September 15, 2024, 03:26:44 pm
content: Quote from: themadhippy on September 15, 2024, 01:02:59 pmCould you direct me where I can find a drawing of that circuit?

author: Zero999
author_rank: Super Contributor
timestamp: September 15, 2024, 04:12:47 pm
content: Quote from: avitsur on September 15, 2024, 11:13:40 amNo, that's not how to control a TRIAC, which is a switch that can either be on or off.Quote from: avitsur on September 15, 2024, 03:26:44 pmWhat sort of temperature sensor does it have?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 15, 2024, 04:43:58 pm
content: In applications such as this, the TRIAC's are never used for phase control, but just as solid state on/off relays that don't wear out. The water temperature is also not very critical. There are also regulations for such high current applications, depending on the power they draw. When the heater turns on, it should probably stay on for at least a few minutes before it turns off again.Because of te "three quadrant" thing of (most) triac's, it's better to pull current out of the gate (from a negative supply), then to put current into it. I've built light dimmers in which the uC was fed by a LM7905, but you do need some extra filtering and protection for the uC pins.For the rest, your biggest concerns should be for what happens if a Tricac blows, does not go off, and you boiler starts overheating / cooking. It probably has an overpressure valve, but a secondary system that turns the whole thing off in case of a serious fault is sensible for a system like this.

author: avitsur
author_rank: Contributor
timestamp: September 15, 2024, 05:19:35 pm
content: What sort of temperature sensor does it have?AS far as I can tell, pt100.

author: avitsur
author_rank: Contributor
timestamp: September 15, 2024, 05:27:14 pm
content: Quote from: Doctorandus_P on September 15, 2024, 04:43:58 pmAs you can see in the picture - there are three thermostats that protect the three heater elements and there is a relief valve on the hor water output port.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 15, 2024, 10:32:46 pm
content: Quote from: themadhippy on September 15, 2024, 01:02:59 pmLooks like a tankless heater, in which case PID would be essential for regulating temperature.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2024, 01:08:03 pm
content: Yes, if it's a tankless heater, then PID is required.For educational purposes and for my records, here's an example of a thermostat.The LEDs represent the emitters in a zero-crossing opto-coupler IC, such as the MOC3043, connected to the TRIACs, as per the data sheet. If required, add another LED in series to act as an indicator.https://www.onsemi.com/pdf/datasheet/moc3043m-d.pdf

author: avitsur
author_rank: Contributor
timestamp: September 18, 2024, 05:17:21 pm
content: Quote from: Zero999 on September 18, 2024, 01:08:03 pm