Switching a 2.2Kw 240V kettle element using a Teensy

Experimentalist

Well-known member
Hi All,

In these days where power is at a premium I have a plan to pimp my kettle (I drink way too much coffee so it is very busy day to day). My plan is to put a thermistor in direct contact with the water and measure exactly when the water reaches boiling point. Thus, instead of waiting for the steam to bend the bimetallic strip, I want to switch the element off immediately somehow without waiting several seconds for the bimetallic strip to flip the switch. I figure that this could save a significant amount of money over a year. I am currently thinking of using a 240V 16A latching relay to switch the power on and off.

If I was switching an inductive load such as this using a DC supply I would put a fly back diode across the relay contacts. Since the power is UK mains power, 240V AC, how can I avoid the flyback causing premature failure of the relay contacts? My knowledge in this subject is limited but since it is AC I don't think I can use a flyback diode so what are the options? The relay I intend to use is the OMRON G5RL-U1-E (https://www.mouser.co.uk/datasheet/2/307/en_g5rl_uk-1570020.pdf). The datasheet states a few characteristics of the relay as shown below:

• Creepage distance 8 mm between coil and contacts.
• 10 kV Impulse withstand voltage.
• Ambient Operating Temperature 85°C
• Suitable for TV-8 rating. (SPST-NO (1 a))
• In-rush resistant and supports illumination load
• Compatible with capacitor load (IEC60669-1) (*)
• Compatible with the International Safety Standard for Electrical/ Electronic Household Appliances (IEC60335-1) (*) * G5RL-K1 A-EL-I-IA type

I am unsure but my guess would be that the "10 kV Impulse withstand voltage" may negate the need for any further protection.

Does anyone have any experience, better understanding or advice they can offer me on how I can ensure the relay is well protected to extend its useful life? Or is there a better way of doing the switching such as using IGBTs (Insulated-gate bipolar transistor) or power MOSFETs (metal-oxide-semiconductor field-effect transistor)? I found this document that shows how these devices can be configured to switch AC (https://www.irf.com/technical-info/designtp/dt94-5.pdf) but again I worry the inductive load may kill them.

If anyone has any advice I would be very grateful.

Yours
Ex
 
You don't need a flyback diode switching AC to a resistive element. You would need a buffer of some type like a transistor to drive the relay coil to boost the drive current and perhaps the drive voltage. You can't drive it directly off a Teensy.

Solid State relays are nice like AlanK mentioned and that is what I would go with. Very reliable and easy to use. Ensure you buy from a reputable dealer, the one in the link is probably fine.

I have dissected some of the Chinese counterfeit parts labeled as 'Fotek' and found that internally they used parts spec'd for 1/2 the current than is shown on the product label.
 
An advantage of most solidstate switches is that they have zero crossing switching to reduce electrical noise and spikes.
 
Thanks for the responses. Having done a bit of research I decided to go with this:

https://www.aliexpress.com/item/32842277618.html

I have ordered the SSR-80DA. This in theory is massive overkill since 2.2Kw at 240V should be drawing 10A by my calculations. The product page seems to talk a good job with regards to slating other manufacturers for using cheap components so we will see. It only cost £7 with the postage and taxes so I don't stand to lose much. The price differences for the different ratings made it a simple choice and it should be fine if it is really only rated at a quarter of what it says on the box. @KenHahn I was surprised when you talked about a resistive load because I assumed the kettle element was formed as a coil. Having read that I did a bit more research and the consensus seems to be that a kettle element can be considered resistive and has negligible inductance, which surprised me. It obviously had to be resistive to create the heat but I assumed it would also have quite a large inductive element (excuse the pun) to it. We live and learn. Has anyone used one of these with a microcontroller? It says 3 to 30V input I guess I just need to measure the current draw at 3.3V or 5V depending on which microcontroller I use and see if it is within spec of the micro digital pin? If not use a transistor to switch it I guess. Thanks again.
 
An advantage of most solidstate switches is that they have zero crossing switching to reduce electrical noise and spikes.

For switching 10-40A at 230V AC that seems to me a very nice advantage.

I suppose that's something to look up or is it so regular that almost all of them have it?
 
Thanks for the responses. Having done a bit of research I decided to go with this:

https://www.aliexpress.com/item/32842277618.html
You realise that that item will take up to TWO months to arrive.
Some AliExpress shipping is slated for 15 days but that is not an option with your order.
I am waiting for some RF Controlled relays to arrive that were ordered on 23rd May. They have reached the EU but still not arrived in the UK.
 
The windings of the heater would be relatively few in number and the resistive nature of the wire would help dampen any inductance effect. Being a large gauge wire has little effect. You can play around with one of the on-line inductance calculator to see the basic effect of playing with the variables: https://www.eeweb.com/tools/coil-inductance/

Most of the SSRs I have seen require a drive current somewhere in the 10-25mA range. A 5V Arduino is usually good for 20-25mA and a 3.3V Teensy for about 8-10mA drive current before a buffer of some type is needed. A transistor would work or a simple 74HCT type buffer IC would also work OK.

Your plan to overspec parts from Aliexpress will probably work OK from a tech standpoint. Whether you want to wait up to 2 months is another story. Here in the US, it usually takes about 2-3 weeks which isn't too bad.

Most of my experience has been with Crydom which are rock solid and available through normal distribution channels like Mouser and Digikey, but you are going to pay $40+ for them. If you knew you were going to be within 10A, you could probably get away with a 10A name brand device as the price starts to head north as the current rating goes up. 20A is usually up in the $55+ range.
 
Worst case with a jug heating element is to do with it's positive coefficient of resistance so will suck more current when cold.

For the sake of the exercise you might cover the element with cold water, perhaps toss in some ice blocks and measure it's resistance.
 
Worst case with a jug heating element is to do with it's positive coefficient of resistance so will suck more current when cold.

For the sake of the exercise you might cover the element with cold water, perhaps toss in some ice blocks and measure it's resistance.

Heating elements use nichrome which has very little change of resistance with temperature, a few percent in normal use.
 
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