Toaster like controller for 2 50 watts heating elements

Hi there,

I'm working on a personal project that might turn into a product. It has two heating elements that I would like to control with a potentiometer the same way a toaster does. I bought a toaster and took it apart to see if I can benchmark from it but I didn't see some sort of step down from 110v to a lower voltage nor a way to control the voltage or current.

The use case looks something like this: user puts food inside a chamber, moves the knob to determine how cooked it wants it, and then it presses a button. The knob determines how long the thingy runs for.

I was wondering if you guys could point me in the right direction - products I could base my circuit from or something I could breadboard at home and test it.
Is this possible while keeping a small footprint and relatively cheap PCB board?

Thanks a lot for any guidance

Is that two 50W heaters or one 250W heater?

Mike.

Pommie said:

Is that two 50W heaters or one 250W heater?

Mike.

Click to expand...

Two 50W heaters. Although I tested them and 25v and 1.3 A its plenty hot

Look up the Maillard effect. It explains why toast goes from no color to burnt so quickly. Basically, a slice of white bread is very reflective - much of the radiant heat is reflected away. As the bread gets a little color, less of the radiant heat is reflected, accelerating the toasting.

Matienzo said:

Hi there,

I'm working on a personal project that might turn into a product. It has two heating elements that I would like to control with a potentiometer the same way a toaster does. I bought a toaster and took it apart to see if I can benchmark from it but I didn't see some sort of step down from 110v to a lower voltage nor a way to control the voltage or current.

Click to expand...

Your entire premise is incorrect - there's no 'control' in a toaster.

All they use is a crude timer, mostly mechanical (bi-metallic strip), although some use electronic timers - but the bi-metallic one has the advatage that it automatically 'corrects' for the toaster already been warm after the first use.

You could easily do the same with a 555 timer, although it's rather limited other than for short periods - best option would be a microcontroller, and you could add extra features as well.

As Nigel says, toasters use a simple timer with on-off control to mains operated elements.

The so-called "microchip" toasters typically use something like a PIC 12 series MCU as a timer, plus a pot connected to its ADC for the user time setting (plus a relay or triac for controlling power to the elements).

With a bit larger PIC, a serial data digital display and a digipot or buttons, you can make a fully programmable timer to run for whatever time range(s) you need.

If you need temperature control, you can add a thermistor or PT100 type sensor and use an ADC in the PIC to read that and control power on/off appropriately.

If you wanted to make an improved toaster, a color sensor would be a good technology - pop the toast when it's just the right shade of golden brown.

Beside the confusions pointed out by Nigel, I think the OP's idea of the power required may be an order of magnitude off. Of course, he didn't say he was building a toaster - just something "like a toaster" - but remember, toast and warm bread are totally different things.

Unless the OP tells us what he/she is trying to achieve then it's all guess work. What are you trying to do?

Mike.

Alright let me start here so I can clarify

Pommie said:

Unless the OP tells us what he/she is trying to achieve then it's all guess work. What are you trying to do?

Click to expand...

I hope you are okay with me not telling you exactly what I'm trying to cook, but I'll explain further on the use case. I mention the toaster example because it is very simple from an user standpoint. Unfortunately for what I read it is too simple from an electronics point of view too. Again, I'm not toasting but cooking. I'm not cooking meat but we could use meat as an example.

Nigel Goodwin said:

Your entire premise is incorrect - there's no 'control' in a toaster.

Click to expand...

Point taken.

Nigel Goodwin said:

You could easily do the same with a 555 timer, although it's rather limited other than for short periods - best option would be a micro-controller, and you could add extra features as well.

Click to expand...

So, I have two heating elements that I would like to run at 25v and 1 A each for a period of time determined by the position of the knob. I used an Arduino nano with 2 mosfets to turn the heaters on/off. I would choose the cutoff temperature and then pick up the temperature with a 10k thermistor. I would like to avoid using thermistors because now that I know what temperature is the ideal temperature I don't need to measure it (I think) and I just want to set the time. The heating elements reach the ideal temp when they are run at 25v and 1A give and take. I might use a thermistor but as a safety feature in case something goes wrong and runs too hot.

I feel like the Atmega328 is an overkill for this project and I might be able to do it with a much cheaper micrcocontroller, but again, I'm not versed on micro controllers beyond my arduino projects.

I hope I covered all the questions.

If your heating elements take 1A at 25V then they are 25W elements - not 50W.

A few years ago I built a dryer for making biltong. It used**broken link removed** on a 12V supply as heating elements. It had three switches which had 1, 2 and 3 heating elements per switch. With the right switches on you could get anywhere from 50 to 300 watts in 50W increments.

Mike.

Pommie said:

If your heating elements take 1A at 25V then they are 25W elements - not 50W.

Click to expand...

They are rated for 50W but with 25W they get as hot as they need to be.

If you're happy programming an Arduino then you can use PWM to control two elements. That will give you from 0 to 100W.

Mike.

Pommie said:

If you're happy programming an Arduino then you can use PWM to control two elements. That will give you from 0 to 100W.

Click to expand...

Sounds good.

How about going from 110v to 25. For my crude prototype I was using this (25V instead of 12V) but it is too big and I would like the electronics to be all inside the gizmo.

That package seems pretty small for what it does. How much space do you have to work with?

With the two elements in series, you can run them direct from 110V with a suitable triac to control duty cycle.
(Or run the system from an isolation transformer for testing).

Use a zero-crossing opto triac trigger to isolate the live circuit from the electronics, eg. an MOC3062
https://www.farnell.com/datasheets/...56.1389781915.1549097982-164463952.1513000262

As long as the elements have a reasonably long time constant you can use "burst fire" control, eg. so many full cycles off then so many on. One on and three off would give 25% power.

That minimises interference problems as power is never switched under load.

Something like a BTA16-600 is about 1-00 and can switch far higher loads than you are using, it may not even need a heatsink in your application, or only a tiny one..