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#135483
A triac won't reduce the voltage - they're used to turn the supply off entirely for complete cycles. That still reduces the mean power delivered into the load (typically a light bulb), but doesn't change the voltage.

A transformer is the way to go, though it's an odd step-down factor. A variac might be the easiest way to achieve the voltage you need.
#135489
AndyC_772 wrote:A triac won't reduce the voltage - they're used to turn the supply off entirely for complete cycles.
While I agree with Andy,
Funny, we must be living on different planets...
triac-control.jpg
#135491
A GTO-triac might be used to clip off the higher voltage portion of the sinewave but then it's no longer a sinewave. Some devices don't mind this (ie - a heater or incandescent lightbulb), others do. The OP needs to provide some more detail as to his needs and end usage.
#135696
Ah yes sorry forgot to mention the reason. Basically a friend has bought a underfloor heating matt, and cut off a portion of if thinking he can just do that, so now instead of having the original resistance in the wire mat, he has taken some of the resistance out. I worked out from the original power supply and length of the mat he will need to cut the power down to about 180 as he has taken roughly 20% of the overall length off.

Would triacs be good for 10A and this type of application?
#135701
Yes a triac will work, however, the fix sounds like a potentially serious fire hazard. For a resistor, the peak voltage doesn't really matter (up to a point). The power is the issue and if the maximum allowable power applied to the mat is exceeded for "long enough", insulation could melt and/or burn and short circuits could lower the resistance further, leading to catastrophic failure. If the triac circuit should fail and allow full AC supply voltage across the compromised heating element, all of the above could take place. Best to get a new mat.
#135703
jremington wrote:Yes a triac will work, however, the fix sounds like a potentially serious fire hazard. ... If the triac circuit should fail and allow full AC supply voltage across the compromised heating element, all of the above could take place. Best to get a new mat.
I agree - triac dimmers usually fail "always on" (this has happened to several in houses I've owned) so it's just too risky IMO. If you really need to do this, I'd suggest using a transformer in an autotransformer configuration, as it will be a lot smaller (you could use a 500VA transformer instead of an 1800VA unit). Partly re-winding a toroidal transformer might be the way to go, if you know what you're doing.
#135706
jremington wrote:...potentially serious fire hazard. ...insulation could melt and/or burn and short circuits could lower the resistance further, leading to catastrophic failure.
With or without triac, you definitely need an appropriatedly rated fuse (takes care of short circuits...remember? :roll: ) plus an overtemperature protection embedded in the flooring. A winding thermostat as used in electric motors, for example. Use a contactor to control the whole thing, and should the thermostat trip, it shuts the contactor off and you can then manually restart the heater. That way a triac is safe to use (and much more convenient than a bulky, humming transformer full of valuable copper 8) )
#135726
On the other hand, if the mat was originally designed for 220 VAC, it will work fine and should be perfectly safe on 110 VAC. It will just provide less heat. Since P = V^2/R, the power output will be about (110/220)^2/0.8 = 0.31 of the original.