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<font face="Helvetica, Arial, sans-serif">You cant use a normal
on-off thermostat with a modulating control. <br>
The simplest thermostat for use with an SCR or TRIAC would have
what is called proportional control.<br>
For example, if the temperature fell below 68F by 0.1 F, 100W
would be delivered to the heater, of the temperature dropped to
67.8F, 200W would be provided and if it dropped to 67F, 1000W
would be delivered.<br>
Because a smaller amount of heat is added with small temperature
drops, the temperature does not oscilate as much as an on-off
control with a 3F deadband as described in the example.<br>
More sophisticated thermostats use proportional-integral controls
or proportional-integral-derivate controls (PID) which I wont go
into. Suffice it to say, we can control the temperature at the
thermostat to better than +/-0.1F with a PID thermostat control
and an SCR in our lab setups.<br>
<br>
You are correct that a relay and a thermostat with a tight dead
band (dead band= temperature difference between when the
thermostat turns on and off) can result in pretty good control if
the heating device does not have a lot of thermal mass (like an
electric baseboard!).<br>
In an experiment in my own house, I used a "simple" on-off
temperature control with a 0.1F deadband to control air
temperature to +/-0.2F simply by having the heater turn on for as
little as 10 seconds and then off again when the sensed
temperature rose 0.1F.<br>
As you state, all you need is a durable relay. It is often easier
to use an SCR or Triac than it is to get a relay that will durably
(e.g. 10 million cycles before failure) control large electric
flows. For tight temperature control and larger heaters (eg e.g.
0.1 F and 1500W), a modulating control is the more common
solution. For smaller loads (100W?) and broad temperature ranges
(0.5F) solid state relays tend to be better. For very small
electric loads (10W or less), SSR (solid state relay) technology
is routinely used.<br>
<br>
<br>
</font>
<pre class="moz-signature" cols="0">Dr John Straube, P.Eng.
<a class="moz-txt-link-abbreviated" href="http://www.BuildingScience.com">www.BuildingScience.com</a></pre>
<br>
On 11-12-19 2:09 PM, Corwyn wrote:
<blockquote cite="mid:4EEF8C03.7080308@midcoast.com" type="cite">On
12/19/2011 9:28 AM, John Straube wrote:
<br>
<br>
<blockquote type="cite">So, for example, a baseboard of 1500W
output may turn on for 10 minutes,
<br>
then turn off and the room cools down for 20 minutes, and then
the cycle
<br>
repeats. The temperature falls to, say, 68F, the heater turns
on, the
<br>
temperatures rises over the ten minutes to, say, 71 F, and the
switches
<br>
off again.
<br>
With a a modulating control like a Triac, a very smart
thermostat would
<br>
attempt to adjust the output of the heater to 500W continuously
(because
<br>
500 Watts for 30 minutes delivers the same energy as 1500W for
10
<br>
minutes every 30 minutes). This means the average current is
lower, the
<br>
electric heater runs at a lower temperature, and the room
temperature
<br>
stays more constant.
<br>
</blockquote>
<br>
This sounds great on paper (phosphors), but I don't see how it
helps in real life. If your thermostat is only accurate to 1.5
degrees (hysteresis of 3 degrees as you baseboard example) then
the temperature is going to rise until it reaches 71 and then
adjust; same as the full bore baseboard. It is not like it KNOWS
it will need 500W in advance.
<br>
<br>
If the thermostat is more accurate, then you might as well use it
on the full bore (presumably cheaper) baseboard. With a really
accurate thermostat, you could probably get down to 1 minute on
out 3. Does anyone really think that isn't good enough? That
they need modulation by the second? That the thermal mass of a
baseboard heater even responds that quickly?
<br>
<br>
I guess there could be an advantage of using a triac over a relay
if the MTBF is significantly longer (more cycles). Is that the
case?
<br>
<br>
<br>
Thank You Kindly,
<br>
<br>
Corwyn
<br>
<br>
</blockquote>
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