Electronics > Projects, Designs, and Technical Stuff
Restive load on UK supply
malc-c:
I'm looking for some guidance as I can't get my head round the possible ramifications of what I'm proposing to do.
In the US there are a couple of manufactures / suppliers of heat strip for use in reptile enclosures. It's rated at 12w per foot when run on US 120v supplies. Using basic Ohms law I calculated the resistance for a 12" length to be 1200 ohms, which would draw 100mA if connected directly across the 120v supply. Now punching the resistance into the formula for 240v and the power value is increased to 48w, with 200mA draw. Now I'm guessing that if this were plugged directly across 240v there would be a likelihood that the heater would burn out ??
The other issue in the mix is that the intention is to run the strip via a suitable thermostat, typically set to 32c, so whilst the strip would heat up quickly, the thermostat would then reduce the chance of it overheating ??
Could someone advise if running these strips on 240v (via a thermostat) would be a practical idea ?
Cheers
Malcolm
alanb:
my suggestion would be to use two strips in series. They would each have a voltage drop across them of 120V (assuming that they have identical resistance).
malc-c:
Never thought about that. Obviously it becomes twice as expensive, and fitting two strips would prove challenging as its doubling the area, but I guess its worth experimenting with
Thanks
Zero999:
Or use a small transformer to power the element.
This one has two 55V secondary windings, which need to be connected in series for 110V, which is near enough.
https://uk.rs-online.com/web/p/toroidal-transformers/6718971/
Gyro:
Running the thermostat at twice the voltage and twice the current could be an issue.
The main thing I'd be worried about though would be the time lag between the element and the thermostat operation. At four times the element output power, either the element will get really hot (red heat?) or burn out before the thermal mass of the thermostat catches up.
You could end up with intermittently grilled reptiles!
EDIT: Yes, I'd go for the step-down transformer too. It also removes worries about whether the insulation is up to 240V operation.
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