Need a simple circuit design for heater and power indication....

[quizzical1]

Hi All,

Can someone help me with this?

We need to design a 120VAC circuit to power a 50 deg C PTC disc heater and have two power indicating lamps - one that shows power is ON and the other to show if the heater has failed. I realize power on could just be a neon lamp in parallel across the heater, but how can I indicate when the heater has failed?

Thanks,


Glenn

 

[MiketheEngineer]

I(f you lose AC - you probably can't unless you have a battery backup or even as a primary source??

 

[quizzical1]

Sorry Mike,

I should have explained it better.

We don't need a lamp on with no power - just two lamps: one to show when 120VAC is present and one to show if the PTC heater has failed (and power is still on). When power is off both lamps will be unlit. If heater has failed, the power lamp will be lit and the heater lamp will be off.

Glenn

 

[btrueblood]

Do you expect the PTC to fail open, or fail as a dead short, or both?

 

[btrueblood]

Sorry, should also mention that there is a third failure mode, in which the PTC loses thermal contact with whatever it is supposed to be heating, this causes the PTC to rapidly rise in resistance until it reaches thermal equilibrium in its new, hotter, higher resistance state. Your circuit may need to distinguish any of the three failure modes, depending on the characteristics of the PTC element.

 

[VEBill]

It might be simpler to add a digital temperature readout - a simple sub-$100 procurement and a simple 30-minute installation.

As opposed to a custom design effort to engineer a PTC failure detection system.

 

[OperaHouse]

There are small current transformers w2ith built in led. A wire is passed through the donut transformer on the way to the heater. This assumes that the heater is powered with AC.

 

[quizzical1]

Thanks All,

Yes, it's a 120 VAC line.
A digital readout is not desired - client wants just a cheap visual indicator for power on and heater working / passing current. Do PTC heaters usually fail open or shorted? I assumed open.

OperaHouse,

The PTC has 40 ohms @ 25C (360W). Can you steer me to the current transformer with built-in LED's?

 

[OperaHouse]

CR Magnetics CR2550-R Low Cost Remote Current Indicator with Red LED, 0.75 AAC Turn-On Point

Lower currents can be handled by additional turns.

 

[VEBill]

Do PTC heaters go to very low currents once they reach their high point temperature? It might require some analysis and/or testing (and a bit of luck) to cover the entire current range. Or add some circuitry to shunt any excess signal.

If the OP needs help with this, post details of the PTC heater (e.g. PN, link to data sheet).

 

[quizzical1]

Sure, it's a Spectrum P5010D050S401H PTC self-reg heater.

 

[quizzical1]

Thanks OperaHouse

 

[VEBill]

That data sheet doesn't provide the current ratio from max (at cold start?) to minimum (presumably) at stasis. If the current ratio is large (greater than about 5 or 10 to one) then perhaps the LED (or light bulb?) attached to your current transformer driven (DC?) power supply may vary too widely in brightness. One could add a few additional components to address this if required (Zener diode bypass, or constant current regulator, or voltage regulator, or...).

It's still an open question what the Imax/Imin ratio is for your heater. I'd call the "application engineer" at Spectrum and ask for this info. He/she might even have some other good ideas.

 

[GuntherA]

what about using a single phase current relay to just switch a 120VAC light across the power rail as needed? Omron K8AB-AS2 comes to mind.
Hard to tell without any actual indication of failure mode or even typical current curve for normal operating conditions.

Undercurrent / Overcurrent should be all you need. Not the cheapest option by a long shot though.

 

[OperaHouse]

The current could go down next to nuthing if it was a really insulated box. These devices don't care about current on the high end, the cores just saturate. The device is rated for .75A minimum. In a steady dtate situation the heater current could go down to a half amp. As I said before just wind two turns on the donut or more so it works at normal conditions.

 

[VEBill]

Sounds good.