# [EVDL] PTC heater question



## EVDL List (Jul 27, 2007)

If (as I recall) ceramic heaters are also called PTC,
they produce more heat with more airflow. Ie, they
are self-regulating. I left my exact blower in on the
Civvy, and can work it at any one of the 4 positions. 
Not necessary to have zeners.



> --- Frank John <[email protected]> wrote:
> 
> > I understand that these heaters are typically wired
> > so they work when the heater fan control is on full
> ...


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## EVDL List (Jul 27, 2007)

> Frank John wrote:
> > I understand that these heaters are typically wired so they work when the heater fan control is on full blast. What would happen if it's used with less than full air flow? Will it heat up quicker than self limit? My fan speed control uses a single relay with variable resistor for intermediate control and would need something like a zener diode in the circuit if I want the heater contactor to only work at high speed. I'm just not sure if this is necessary or not.
> >
> 
> ...


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## EVDL List (Jul 27, 2007)

In my experience the heaters work great at any fan speed. Heat output 
tends to go up as the fan speed is increased although high fan may 
actually result in less heat. PTC heaters have a funny resistance 
curve. They tend to be rather high resistance when cold, drop as they 
warm, then rise again as they get hot.

I just checked the resistance (off, cold) of the heater in my Datsun 
and its 55 ohms (less than 3 amps start-up current for a 1500 watt, 
120 volt unit.) I tested another heater core by covering it on both 
side with newspaper (no air flow, not even convection.) The current 
quickly ramped up then tapered down to 2 amps, it might go even lower 
but I wasn't trying to start a fire 

I suggest tapping the fan resistor at the low speed input. That 
resistor is usually only a few ohms so the voltage drop to drive the 
heater relay with the fan on high is minimal. I suggest you use that 
power to operate a small relay with the heat on switch. The output of 
that relay should drive the heater relay. The reason for the double 
relay setup is so that if you turn the heater fan off with the heater 
on the heater relay will not see a slowly dropping voltage causing it 
to drop out slowly. Save that abuse for a small 2 to 5 amp relay that 
is only handling 12 volts on the contacts (at under 1/2 amp.) A 
snubber across the heater relay is also a good idea because a relay 
powerful enough to break pack voltage at 10 or more amps can 
generally put out quite a reverse spike when its powered off.

Paul Gooch




> Frank John wrote:
> 
> > I understand that these heaters are typically wired so they work
> > when the heater fan control is on full blast. What would happen if
> ...


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