# Unequal heating elements in series - brain fog trying to work it out!

#### rogerds

Sep 25, 2016
10
I'm hoping to repair an old egg incubator (hope this is the right place).and apologies
It was heated using x2 film heating elements wired in series, each one rated 115v 75w (I'm in the UK - mains @220v) controlled with a simple thermostat system
One has burnt out and i can't source identical replacement here, but I do have a film element rated at 240v 40w
I've been trying to calculate voltage across each if i connected them in series but I guess what I'm really trying to work out is can I connect them in series with a 220v supply without burning the house down?
Advice would be greatly appreciated - cheers

#### kellys_eye

Jun 25, 2010
5,131
Is this a case of mechanical 'fit' rather than electrical expediency? You could remove all the old elements and use an element from an electric iron (they are 'flat') as a mechanical solution.

If there is loadsaroom the world is your mussel (can't afford oyster) as slapping in any heating element close to the required max of 150W (total) will fix it.

For reference, the 115V 75W element will be around 1.5ohms and the 240V 40W version will be 6ohms. If you wired them both in series you end up with 7.5 ohms and a total power dissipation LOWER (i.e. 32W).

#### rogerds

Sep 25, 2016
10
thanks for that - yes... its mechanical fit, the film (its similar to the heating film now used for underfloor heating - which is what i'm trying to use) or.... and sorry for another question, could i add a resistor to the original (115v 75w) element to bring that up to 220v and wire them in parallel?? thanks

#### Alec_t

Jul 7, 2015
3,246
For reference, the 115V 75W element will be around 1.5ohms and the 240V 40W version will be 6ohms.
? Not according to Mr Ohm and Mr Watt.

The 75W element has a resistance of 115^2 /75 = 176Ω.
The 40W one has a resistance of 240^2 /40 = 1440Ω.
In series their total resistance is 1616Ω, so at 240V they would have a total wattage of 240^2 /1616 = 33.6W.

Last edited:

#### kellys_eye

Jun 25, 2010
5,131
Dang. ... and I used the ^2 button too! Either it's not working or I missed it! Either way, of course, you are corrrect.

To the OP - a series resistance would work but it would be inordinately large and - effectively - a heat generator in its own right.

An old electric iron element would resolve it though - consider it. It's power is 'regulated' by the thermostat anyway so as long as the fitted element has sufficient output to get the incubator warm enough in the environment it's used in then it's pretty much irrelevant what that element is. Given 150W was the considered value for the original unit I wouldn't go any lower than that. Higher wouldn't be too much of an issue though - but don't go mad!

#### crutschow

May 7, 2021
520
You could use one of those 240V to 120V traveler adapters to power the two elements in parallel, which would then give a total of 75W + 40W = 115W.

#### rogerds

Sep 25, 2016
10
*now* i understand - thanks!.....maybe an explanation ( i should have taken a couple of photos) the heating elements are thin film (x2) (originals 950x45mm by approx 1mm thick) sandwiched between an insulated outer shell and an inner plastic lining, the case being cylindrical lying horizontal approx say 800mm diameter and chicken eggs are held in racks inside at 38 C for 3 weeks till they hatch.
One of the heating films had burnt through (literally!). my solution in the end was to buy a strip of underfloor heating film and cut two lengths of 750mm and glue them on top of the existing film(s) and as this film is quoted at 220v, 55w/metre, connect them in parallel.
Giving approx 40w each (and not use the remaining good heating film at all). Whilst the total of 80w is much lower than the original 150w it does seem to be able to raise the temp to the required 38C and hold it there - I'll let you know in 3 weeks time!
Thanks again for taking the time to respond - cheers - roger

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