Restive load on UK supply - Page 1
Forum section: electronics / projects

author: malc-c
author_rank: Newbie
timestamp: July 02, 2019, 12:04:57 pm
content: 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 ?CheersMalcolm

author: alanb
author_rank: Frequent Contributor
timestamp: July 02, 2019, 12:09:01 pm
content: 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).

author: malc-c
author_rank: Newbie
timestamp: July 02, 2019, 12:16:16 pm
content: 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 withThanks

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2019, 12:48:38 pm
content: 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/

author: Gyro
author_rank: Super Contributor
timestamp: July 02, 2019, 12:51:37 pm
content: 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.

author: malc-c
author_rank: Newbie
timestamp: July 02, 2019, 01:26:01 pm
content: Thanks again for the suggestions. The thermostat is quite responsive using DS18B20 digital sensors. My concern was really the impact of running these 12w / foot heaters at four times their rated power.I have since found a manufacture of similar heat strip but made to run on 230 / 240v and meet EU requirements. Just waiting on a reply to my enquiry to have 30' shipped to the UK for use in my new hatchling rack. Failing that the use of a 240v to 120v transformer seems a logical approach.Thanks again for the suggestions...

author: jeroen79
author_rank: Frequent Contributor
timestamp: July 02, 2019, 01:47:57 pm
content: Quote from: malc-c on July 02, 2019, 12:16:16 pmIt would not be twice as expensive.You would need x Watt of heat and thus x/12 feet of heat strip.At 120V you connect the strip as it is.At 240V you cut the strip in half and wire the halves in series.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 02, 2019, 02:05:43 pm
content: What is actually stopping you to get heater cables those were already designed for 230-240V operation? We had such for turtles...

author: malc-c
author_rank: Newbie
timestamp: July 02, 2019, 02:42:05 pm
content: Quote from: jeroen79 on July 02, 2019, 01:47:57 pmI would have thought that I would need to use 2 x 12w 120v strips in series on 240v. If i had a single 12w strip and cut it in half, effectively making 2 x 6W strips and wired them in series would it not equate to the same as a single 12" 12w strip as the resistance would still be the same ?Quote from: dzseki on July 02, 2019, 02:05:43 pmIn the UK these rolls of heat strip are not available. You can get ready made 6" wide strips up to 47" in length, but are very expensive. The heat strips on a roll are around $2.50 per foot. As posted above, I've found a manufacture in the US that supplied the same product but at 230/40 volt rating.

author: jeroen79
author_rank: Frequent Contributor
timestamp: July 02, 2019, 06:41:03 pm
content: Quote from: malc-c on July 02, 2019, 02:42:05 pmNo.Think of the strip as two resistance in parallel.At 120V each draws 6W (50mA @ 120V).If you cut them apart and connect them in series each will have half the voltage across it.At 240V, each will still have 120V and each will still draw 6W.http://lushprojects.com/circuitjs/circuitjs.html?cct=$+1+0.000005+10.20027730826997+50+5+50%0Ar+192+112+192+176+0+2400%0Ar+256+112+256+176+0+2400%0Ar+192+240+192+304+0+2400%0Ar+192+336+192+400+0+2400%0Aw+192+48+256+48+0%0Aw+192+176+256+176+0%0Aw+192+208+128+208+0%0Aw+192+400+128+400+0%0Aw+192+176+128+176+0%0Aw+192+48+128+48+0%0Av+128+176+128+112+0+1+60+160+0+0+0.5%0Aw+192+80+192+112+0%0Aw+128+240+128+208+0%0Aw+128+80+128+48+0%0Aw+128+112+128+80+0%0Aw+256+80+256+48+0%0A370+192+48+192+80+1+1%0A370+256+80+256+112+1+1%0A370+192+208+192+240+1+1%0Ap+0+176+0+112+1+1%0Ap+0+304+0+240+1+1%0Ap+0+400+0+336+1+1%0Av+128+400+128+336+0+1+60+160+0+0+0.5%0Av+128+304+128+240+0+1+60+160+0+0+0.5%0Aw+128+304+128+336+0%0Aw+128+304+64+304+0%0Aw+128+336+64+336+0%0Aw+128+400+64+400+0%0Aw+128+240+64+240+0%0Aw+128+112+64+112+0%0Aw+128+176+64+176+0%0A370+192+304+192+336+1+1%0Aw+64+240+0+240+0%0Aw+64+304+0+304+0%0Aw+64+336+0+336+0%0Aw+64+400+0+400+0%0Aw+64+112+0+112+0%0Aw+64+176+0+176+0%0A