# PTC heaters in series question

Thread Starter

#### danl

Joined Sep 24, 2013
60
Can two 12vdc, 220degC, PTC heaters be connected in series and powered by 24vdc? Both PTC's will have the same symmetrical contact area with a heat sink in an insulated enclosure.
PTC's are each in a 25x150x5mm aluminum enclosure. Heat sink is a 25x170x6mm rectangular copper tube (22x3mm internal c.s.), sandwiched between the PTC's, with internal fluid flow.
Dead cold, at 12V, current was 5.2A for a single PTC heater, so cold R=approx 2.4ohms.
Basic question is whether they will adequately balance in temp vs current response.
Thank you.

#### KeithWalker

Joined Jul 10, 2017
2,724
Can two 12vdc, 220degC, PTC heaters be connected in series and powered by 24vdc? Both PTC's will have the same symmetrical contact area with a heat sink in an insulated enclosure.
PTC's are each in a 25x150x5mm aluminum enclosure. Heat sink is a 25x170x6mm rectangular copper tube (22x3mm internal c.s.), sandwiched between the PTC's, with internal fluid flow.
Dead cold, at 12V, current was 5.2A for a single PTC heater, so cold R=approx 2.4ohms.
Basic question is whether they will adequately balance in temp vs current response.
Thank you.
They will not balance themselves when connected in series. If the is a temperature difference between them, the warmest one will limit the current to the other and have the largest voltage drop across it so it will dissipate the most heat.

#### MisterBill2

Joined Jan 23, 2018
14,280
They can be connected in series and they will operate. Probably they will be fairly close IF the mounting can keep them both the same temperature. But probably they will not be identical and so the one that limits current the most will affect how much heat they actually deliver. So it will be an interesting experiment.

#### KeithWalker

Joined Jul 10, 2017
2,724
They can be connected in series and they will operate. Probably they will be fairly close IF the mounting can keep them both the same temperature. But probably they will not be identical and so the one that limits current the most will affect how much heat they actually deliver. So it will be an interesting experiment.
They are Positive temperature coefficient devices so the warmest one will probably be damaged from the excessive voltage.

#### MisterBill2

Joined Jan 23, 2018
14,280
They are Positive temperature coefficient devices so the warmest one will probably be damaged from the excessive voltage.
They should self limit. I did say that they needed to be at the same temperature, closely coupled. And certainly the TS should watch what happens at the first power up.

#### KeithWalker

Joined Jul 10, 2017
2,724
They should self limit. I did say that they needed to be at the same temperature, closely coupled. And certainly the TS should watch what happens at the first power up.
How would you ensure that they are always at the same temperature? What is the specified maximum voltage for the devices?

#### MisterBill2

Joined Jan 23, 2018
14,280
The comment was to have them well coupled to the same heatsink, which would need to be a good heat conductor. The one the TS described, " Heat sink is a 25x170x6mm rectangular copper tube (22x3mm internal c.s.), sandwiched between the PTC's, with internal fluid flow. " could be adequate if they were coupled very well and equally.

#### KeithWalker

Joined Jul 10, 2017
2,724
The fact still remains that if one starts off a little warmer than the other, it will limit the current to the cooler one, causing an imbalance in voltage. The cooler one will become even cooler and the warmest one even warmer. That would cause a temperature gradient across the heat sink. If the positive temperature coefficient of resistance is high enough, it could cause the warmer one to have a voltage drop across it that may exceed the specified maximum voltage and cause damage.

#### MisterBill2

Joined Jan 23, 2018
14,280
You can also add a second 12 volt heater in series with each PTC device. That will totally solve the problem and improve the effectiveness as well.

#### KeithWalker

Joined Jul 10, 2017
2,724
You can also add a second 12 volt heater in series with each PTC device. That will totally solve the problem and improve the effectiveness as well.
No, it would just double the problem.
If you know the PTC value for the heaters, their maximum safe voltage and the thermal efficiency of the cooling system, you could do the math. The other alternative is to try it to see what happens, if you can afford to damage one of them.
The real solution would be to use a 12V supply or exchange the heaters for 24V versions.

Thread Starter

#### danl

Joined Sep 24, 2013
60
They will not balance themselves when connected in series. If the is a temperature difference between them, the warmest one will limit the current to the other and have the largest voltage drop across it so it will dissipate the most heat.
Thank you. Appreciate the rapid feedback.

Thread Starter

#### danl

Joined Sep 24, 2013
60
Thank you for the discussion.
I've read up a bit on ptcs. I haveno max V spec on these, but literature suggests this type usually has a a 2xV range for max volts. Saw one YouTuber take one from 12vdc to 120vac! And fully immersed in a beaker of water. Insane.
Simple ohms law assumptions...
If one heated fully and the other stayed dead cold (at about 2.5ohms) the max V drop across the hot one would be just below 24V.... supposedly within its max V design range.The cold one should catch up eventually, but not a reliable approach.
I bought a 12V supply that should easily handle the two or even three in parallel.
Thanks again.
I have another question about limiting the cold PTC surge current....going to new thread for that.

#### MisterBill2

Joined Jan 23, 2018
14,280
They are resistance devices, no inductance or capacitance. I=E/R and so the magnitude of such a surge is easy to figure.
And if a similar wattage heater is in series then the voltage may vary but it is not like these devices expire at 12.1 volts.

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