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Regulating temperature of mains connected heating element: Skipping mains half cycles

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Not necessarily, it may work the same way as some 'thermostatic mixer taps' with a constant heat source but motorized mixer valve.

Brian.
 
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sorry Betwixt, i should have made clear that i am only speaking of "electric showers" here.....you are kindly referring to whats known in shower company marketing departments as a "digital shower".
(i only know this because i spent a few months in a shower co working on their "digital shower"...unfortunatley, i was not given access to their electric showers)
 

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Thanks, I also read the manual.
There are three flow settings. Then there’s the temperature dial for you to increase/decrease temperature with whatever flow setting. The temperature scale has fine gradataions, showing a lot of different temperature settings, and this can only be economically achieved with burst fire mains control. –Unless of course, the multiple graticule markings are just there for show….to fool the customer that it has fine resolution temperature control
 

Hi,

Let's assume you have
* 50Hz full wave regulation (smallest time step = 20ms)
* a 1kW heating element
* 1 liter of water

Then from the physics the limit is to regulate the water temperature with a resolution of about 0.005°C.

Klaus
 
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Having re-looked at the shower of post #21, I now agree with your words. It’s the flow rate that is being adjusted to give a wide range of temperature outputs.
However, there is a very big problem with this. It means that people may end up using a very low flow rate, which is very bad because it results in scaling up of the shower system….also, if your shower is on high power and high flow rate, then you quickly adjust the flow rate down…it can result in significant scalding…as well as temporary thermal shut down of the shower.
So this is why mains burst fire control, using a fixed flow rate, (or perhaps just two very similar flow rates) would actually be the best way to control an electric shower.
Please see point 13 (page 3 ) of the “Mira Décor” shower……

Mira décor shower manual (£400 electric shower)
https://resources.kohler.com/plumbi...-a-mira-decor-installation-and-user-guide.pdf

…this is diabolical “DO NOT switch the shower off and back on while standing in the water flow.
….when soaping up in an electric shower..it is essential to be able to switch the shower off….then back on again when you wish to rinse off the soap….
Burst fire mains control is the way forward.
 

Instant heaters with electronic temperature control are state-of-the-art (other products than that presented in post #21). According to power quality standards, they need to use full wave switching with not too fast modulation period.

- - - Updated - - -

I reviewed the specifications of some electronically controlled instant water heaters. Interestingly they are controlling the water flow by a motor valve, not the heater power.
 
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I reviewed the specifications of some electronically controlled instant water heaters. Interestingly they are controlling the water flow by a motor valve, not the heater power.
Thanks, yes you are right, controlling temperature by controlling the flow rate appears to be the main method....but it is a poor method....just cheaper....

What I would add is, that I have lived in some 35 different rental accomodations, and in virtually all of them, the electric shower had been pranged by the landlord to always run on maximum flow.
The reason for this is because adjusting the flow rate of an electric shower can lead to problems…..because if one restricts the flow rate too much, too suddenly, then the heater element overheats…and even though there is a thermal shutdown, it never really operates fast enough, and so cumulative damage is done to the heater element and other parts.
Another point about slow flow rates is that it increases limescale build….more so than faster flow rates.
This is why shower temperature adjustment by flow rate adjustment is a poor, el-cheapo solution to the problem.
 

I tried to understand this but did not get a single word. can anyone help me to understand this
 
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There are 29 posts in the thread, you should tell a bit more clearly what you mean with "this". Alternatively, what's your question related to the topic?
 
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Most 1st world countries require that the net average DC over a 24 hour period be 5mA or less - this to prevent earth stake corrosion

If you draw too much pos cycle only power ( or neg ) you can saturate the local supply transformer and blow the pole fuses

you will also upset any transformers on the same phase ( making them hum loudly ) and they can blow their fuses too ... this because you are unbalancing the volt seconds on each half cycle on the local mains ....
 
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So you want full cycles of control ( 20mS for 50Hz ) or multiples thereof, zero crossing detect opto coupler and a ckt that keep the firing on for 15mS - the SCR / triac will then turn off at 20mS all by itself ...
 

Thanks, we will end up using back-to-back power FETs with isolated drive due to the total 32A of current. We will parallel them.

What worries us is that I can see that if our zero-cross isnt pin point accurate, then we could end up with a net DC even if we do it in 20ms mutiples...though we can mitigate by having 10ms "gaps" (or multiples therof) between the (20ms x n) bursts.
 

The offset would only occur on the start of a conduction cycle, if you use SCR/Triac switching it will turn itself off accurately anyway.

Why use back-to-back power FETs when a triac is so much cheaper and easier to control?

Brian.
 

Why use back-to-back power FETs when a triac is so much cheaper and easier to control?
Apart from component costs, to reduce power dissipation at 32 A below triac level, you would need less than 18 mOhms Rdson for each of the two MOSFETs. Did you identify suitable transistors?
 

Apart from component costs, to reduce power dissipation at 32 A below triac level, you would need less than 18 mOhms Rdson for each of the two MOSFETs. Did you identify suitable transistors?
Thanks, no we didnt, but we will parallel them.
 

This is getting rather complicated and expensive. My initial thoughts if burst control is to be used is a triac fired with a ZCD opto-coupler, in turn fed from a comparator with the setting control on one input and a thermistor from the outgoing water flow on the other. Not a FET in sight and very little costs.

Brian.
 

Imagine a bad winter in your country. Imagine millions of multi kW heaters all running at full power at the same time. They are all doing “skip cycle” mains, where for example, mains current is only drawn for say 5 whole (20ms/16.7ms) mains cycles out of 6 whole mains cycles, repeatedly.... (The current is always commutated out/in at the current zero crossing).

This is like in the waveform of post #17 above.

Isn’t this going to in some way damage and over-stress the country’s mains electricity supply system?
 

Hi,

Millions of heaters ... are not synchronized. Every heater skips another cycle.
The result is an average .... you wont be able to recognize the cycle skipping at all.

Klaus
 
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