What is the Best Way to Control a Heating Element?

All,

I have a heating element that I want to control so that I can eventually make a PI temperature controller for a tank.

I have a 1500W heating element but I step down the voltage from 120V to 24V. The two options I know and can do are:

  1. Using a triac on 24V ac. However, I don't know how to tell if it misfires

  2. Using a full bridge rectifier to convert 24VAC to DC, and then using PWM on a SSR (the SSR is hooked up to the heating element)

Are either of these good (and efficient) options? Are there any other recommended options or methods? Also, is one option safer than the other?

Thanks

After reading and pondering your posting, I can only respond by letting you know that I strongly suggest you abandon this plan.

Consider you wrote that you dropped the current from 120V to 24V. You did not. You dropped the VOLTAGE and I am sure you do not realize the relationship between voltage and current. Which has me thinking that this project is way beyond your skill level. This project has a HIGH potential for being lethal and anyone who assists you is equally libel.

Yeah my bad I meant voltage, so no suggestions then lol?

rclark43: All,

I have a heating element that I want to control so that I can eventually make a PI temperature controller for a tank.

I have a 1500W heating element but I step down the voltage from 120V to 24V. The two options I know and can do are:

  1. Using a triac on 24V ac. However, I don't know how to tell if it misfires

  2. Using a full bridge rectifier to convert 24VAC to DC, and then using PWM on a SSR (the SSR is hooked up to the heating element)

Are either of these good (and efficient) options? Are there any other recommended options or methods? Also, is one option safer than the other?

Thanks

, None of those options make any sense when SSR to directly control your 120 volts are so cheap and readily available and work directly off an Arduino output pin.

Running 24 volts DC, PWM to a 1500 watt heater may even let it get a little bit warm, but not much more. And your SSR WILL NOT WORK with PWM, only AC.

Paul

rclark43: I have a 1500W heating element but I step down the voltage from 120V to 24V.

So you only want the 9.6ohm element to draw 2.5Amp, and produce 60watt. Do you have a DC supply for that, or an AC transformer. If you have a 24volt DC supply, then a simple logic level mosfet can be used to PWM the element on the low voltage side. Leo..

The usual way to control a heating element is with a PID loop driving slow PWM. Slow PWM reduces
interference from switching of high current (and for AC allows zero-crossing switching for even less
interference).

The typical time constants of heating elements are in seconds or tens of seconds so there is no need
for high speed PWM.

Using linear PWM rather than bang-bang control allows a PID loop to work in the linear domain and help avoid
overshoot.

There should never be any misfiring of a triac - that indicates a gross fault situation.

MarkT:
The typical time constants of heating elements are in seconds or tens of seconds so there is no need
for high speed PWM.

OP is talking about a “tank”.
Then you typically control what the heating element is heating, not the temp of the heating element itself.
That time constant could be minutes, or even hours.
Leo…

Wawa: OP is talking about a "tank". Then you typically control what the heating element is heating, not the temp of the heating element itself. That time constant could be minutes, or even hours. Leo..

All I'm saying is milliseconds is complete overkill, in the absence of a known timeconstant, 10 seconds is a fairly safe default value - even driving a mechanical relay.

Using a longer timescale makes it very tedious to test too...

MarkT:
All I’m saying is milliseconds is complete overkill, in the absence of a known timeconstant, 10 seconds is
a fairly safe default value - even driving a mechanical relay.

Using a longer timescale makes it very tedious to test too…

A couple of years ago I converted a drying oven to Arduino control. The initial cold start to 85C takes 15 minutes. A tank with liquid wound likely take much longer.

Paul

The reason I want to use 24 Volts (AC or DC) instead of 120V AC is because the element gets too hot with 120V AC for the purpose that I'd need (heating no more than ~2L of water to a maximum of 140F). Depending on the tank size and the time constant of heating the water I can modify the PI (or PID) parameters.

Now that I know the PWM needs to be very slow because of the large time constant what would be better, more efficient, AND safer?

  1. 24V DC heating the element with a DC SSR
  2. 24V AC being controlled by a triac?

Thanks

DC SSR is MOSFET based so will be more efficient as the voltage loss will be very small. Triacs lose 2V or so.

DC is easier to control too.