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We have a heater cartridge rated at 12 V DC and 50 W.
Can we feed it 18 V DC safely? In other words, push it to 100 W? Has anyone actual experience?

My reasoning is as follows. The manufacturers are likely to have a large safety margin with regard to their specifications because they must assume users will occasionally do crazy stuff like run it in air without a heat sink.

We will be putting it into a large heatsink and attempting to push the temperature to 500 oC (932 oF).

I assume these fail open circuit, but even if they failed as a short, the PSU should fold back.

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    \$\begingroup\$ I struggle to see what you mean. If you put it on a heatsink you can probably push more power, yes - but you wouldn't push the temperature of the heater as well - thats what the heatsink does, it helps getting rid of heat quicker so you dont get as hot. \$\endgroup\$
    – MrGerber
    Commented Jul 11 at 10:18
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    \$\begingroup\$ "The manufacturers are likely to have a large safety margin with regard to their specifications because they must assume users will occasionally do crazy stuff like run it in air without a heat sink." Kind of depends on whether this a serious brand or some cheap crap one, yeah? Cheap products tend to have zero margins. \$\endgroup\$
    – Lundin
    Commented Jul 11 at 10:40
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    \$\begingroup\$ When I was running with the MOAR POWA crowd, the mantra was 'turn it up until it explodes, then back off a bit'. You could try that. \$\endgroup\$
    – Neil_UK
    Commented Jul 11 at 10:52
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    \$\begingroup\$ Nobody but a fool will tell you that operating the heater outside of its published specs is safe. The best you should hope for is, if you tell what exact make and model it is, then maybe somebody will say, "Hey, yeah! I did that, and my lab didn't catch fire." Neil_UK's advice sounds pretty close to what I would do in your situation. I would try it, and find out. But, I would do it with the understanding that I would bear the entire blame for any bad outcome (burning the building down, sending somebody to the hospital, etc.) \$\endgroup\$ Commented Jul 11 at 12:49
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    \$\begingroup\$ The interesting side-effect of using something outside its absolute max ratings is that you have no guarantee that each device will behave the same. Suddenly the manufacturer change something about their process that doesn't impact the regular use case, but might change the failure mode significantly when used outside the limits. \$\endgroup\$
    – MrGerber
    Commented Jul 11 at 12:56

2 Answers 2

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Assuming (and this is very important) a swaged construction (which has the winding embedded in a crushed ceramic inner insulator- minimizing the temperature differential between the resistance wire and the sheath), I suspect you can get away with this if the sheath temperature can be kept well below the rated value.

Consider the below graph from a heater supplier:

enter image description here

For a given heater construction and mounting, reducing the sheath temperature from 1000°F (538°C) to 400°F (204°C) can somewhat more than double the allowable watt density for the same life etc. If the heater you have in mind has a wattage and sheath temperature rating then you may able to apply something like the above for extrapolation as well as derating the watt density.

Watt density is the key factor for heater rating- the watts per unit of heater area that is in contact with the object being heated.

As always when you stray from manufacturer's ratings, any bad things are on you and all testing must be performed by you (and there may be regulatory issues if the heaters are directly mains-connected or if they could cause a conflagration). So, at the very least apply a healthy safety margin and do sufficient testing.

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This all comes down to your use scenario and how well you can tolerate a device failure. It doesn't make sense to ask about it without providing such detail.

If your heating block is in the middle of a nuclear reactor and can only be serviced once a year, and people serviced by the reactor won't have any power until annual service if the device fails, then no, you should not gamble with using the device out of spec. If there's a real reason why you need to use the particular device in that use scenario, one option would be to do your own testing to assure the failure rate will be acceptable.

If you're building a coffee heater, and the failure mode is somebody has to wait to get hot coffee, this is different.

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