MACS Monthly Newsletter - 2017 - JUN2

PTCs Electric Heaters - Not Just for hybrid Vehicles!

Drawing Courtesy Ford Motor Company

Photo Dave Hobbs

Positive Temperature Coefficient (PTC) heater elements have been around for well over a decade. You
may have noticed them as an option on the GEN
II Toyota Prius starting around 2004 if you work
on hybrids. If you're a diesel tech, you may have ran
into them there as well. Both types of vehicles seem
to have challenges with getting the passengers warm
quick and they are gaining popularity with gas engine
equipped applications as well. (Figs. 3 & 4) PTC heat-

Figures 5 and 6: To illustrate the principles of how resistance changes
with heat in PTC heaters, look no further than that halogen headlight on
your tool cart that you may be using as a substitute load in your voltage
drop testing. The filament in the bulb (Fig. 5, top) shows a resistance of
0.5 ohms. If we use Ohm's law to calculate how much current the bulb
would draw (assuming the filament's resistance didn't change with heat)
we should see 24 amps when the bulb is powered up. (I = E/R or more
simply 12 volts divided by 0.5 ohms = 24 amps) (Fig. 6, bottom) shows
that we see a bulb lit up but only drawing 3220 mA (3.22 amps). This
proves the resistance in the bulb's filament did indeed increase - to about
3.73 ohms when calculated with Ohm's Law.

Figures 3 and 4: Starting in 2004 the Gen II Toyota Prius had an optional PTC heater (Fig. 3, top) located at the front of the HEBA (Heater
Evaporator Blower Assembly) to take the edge off of the cold air blowing on cold starts. Since hybrids spend a lot of time with the ICE
(Internal Combustion Engine) off some kind of 12-volt auxiliary heater
made sense. Select Ford vehicles in both gas and diesel configurations
also use PTC heater located in approximately the same location.
(Fig. 4, bottom)

are a prime example. (Figs. 5 & 6) Compare to glow
plugs, some of which change very little in their operation internal resistance. (Figs. 7 & 8) There are certain ceramics that exhibit a temperature coefficient that increases
very rapidly as soon as a particular threshold temperature is
reached. Now, if you use these materials to make an electrical heating element, then when the resistance is low the element will take a large current and therefore warm up very
quickly. As the element warms up and reaches this threshold
temperature then its resistance becomes very large and with
a very large resistance the current drawn becomes very low
and so no longer capable of producing much heat. A PTC
element becomes somewhat self-regulating. Any additional
control circuitry is provided so a combination of typically 3
heating elements can be pulse width modulated to obtain
the amount of heat deemed necessary to supplement the hot
coolant heater system. The nature of the PTC heater gives

ers are sometimes referred to by OEMs as supplemental or booster heaters. PTCs work backwards
from most temperature sensors (a.k.a. thermistor)
on vehicles which are usually Negative Coefficient
Temperature technology. Instead of reacting to
temperature changes with a change of resistance
(as in the case of the thermistor) PTCs are fed voltage to create heat. Metals typically react to temperature
increases with increased electrical resistance. In some
metals the increase is not significant, but in others, the
resistance change is considerable. Light bulbs filaments

June 2017

MACS Service Reports

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2017