Re: Temp controlled DIY heating cables

I haven't been able to get my mail reader to copy the text from a large 
message that I am replying to, so to get the whole story on this, look back 
to issue #39.  Sorry for the inconvenience.

I don't think I was clear enough in my original posting for this idea, so I 
will try and elaborate.

The plan was not to make a 300 watt heater for a 55 gallon tank, I just got 
the 300 Watt because all the heaters up through that size are the same 
price, and I like to go with the biggest safety margin.  The actual heater 
was only going to be about 120 watts.  I think that this is a reasonable 
level for a 55 gallon, at least in my house.

I wouldn't think that the inductance of the transformer would be a problem 
if you are using 120 watts of transformer on a controller designed for 300 
watts.  The reflected impedance of the resistive cables on the secondary 
side will limit the current from the inductive kick.  In any case, I'll know 
for sure in a few days.

Finally, someone stated that cables that stay on all the time at a low level 
are better than something that switches them on and off as needed.  The 
rationale was that the low level ones would be on year around whereas the 
higher level ones would not come on in the summer (or come on very rarely).  
This is my first experience with heating cables in my own tanks so my 
rationale may be messed up (and I'll gladly accept any confirmation to 
this!), but it seems to me that the opposite would be true.  Let me explain.

Ideally the system would be designed so that the cables could be on at all 
times and just provide the required heat with no supplemental heater.  For 
various reasons this obviously won't work.  The two options then are to 1) 
Make a system designed to provide less heat than required and have an 
additional, thermostatically controlled heater to make up the difference 
(the typical DIY design) and 2) Place a controller on cables that will 
provide more heat than necessary (the typical commercial design).

It seems that the second option is better because it is closer to our ideal 
scanario, i.e. all the heat that is needed is generated by the cables.  If 
you don't need heat then you don't want the cables powered, no matter how 
weak they are.  If you do need heat then why generate much of it with an 
extra heater, thereby wasting a perfectly good opportunity to gain the 
advantages of the undergravel cables?  Assuming a controller can be made for 
DIY cables, I see no reason why it would not be superior.

I agree that too large a heater would result in much more time off than on, 
and possibly result in noticable temperature transients, but if a person was 
concerned about their high wattage cables operating for only a small 
fraction of the time in the summer then perhaps two circuits could be 
created.  One would be a winter mode with 120 watts and the other would be a 
summer mode of only, say, 30 or 60 watts.  Set up correctly this would only 
require flipping a switch or swapping some connectors a couple of times a year.

Any additional comments would be welcome.

jsprag at srv_net