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SECTION 3 - SERVICING COMPONENTS
IV. THERMOCOUPLES
All temperature controller and high limit functions are based
on input from thermocouples. When two wires composed
of dissimilar metals are joined together and one of the ends
is heated, a continuous current flow is generated. Middleby
ovens use a Type J (iron constant) thermocouple. The iron
wire increases the number of dissimilar metal junctions in
the circuit.
Middleby has used several types of thermocouples. All
can be replaced with the current version. Refer to Figure
24 for illustrations.
A two-lead thermocouple with a small retaining clip.
Used for EITHER temperature-sensing OR high-limit
functions. This type is no longer available.
A four-lead thermocouple with a small retaining clip.
This is a dual-function unit that sends its signal to the
temperature controller and to a separate high limit con-
trol module. This type is no longer available.
A three-lead thermocouple with a separate ground lead,
a shielded cable and a large mounting flange. This is
the only type of thermocouple available from Middleby
at this time. If it is used to replace a four-lead dual-
function thermocouple, two complete and separate ther-
mocouples will have to be installed.
flange
Figure 24 - Thermocouples
clip
Cable
(3 leads)
Cable
(2 or 4 leads)
Current 3-lead
thermocouple
Obsolete 2-lead
or 4-lead
thermocouple
A. Part Number Reference - Thermocouples
Part numbers are provided ONLY for the current, 3-lead ther-
mocouple kits. These thermocouples include instructions
for replacing all previous thermocouple designs, in all oven
models.
P/N Description
33984 Kit, thermocouple, PS360-series or PS555/570
series
33985 Kit, thermocouple, PS200-series or PS536
B. Thermocouple Functions
1. Temperature-sensing thermocouple(s)
All thermocouples monitor oven temperature. However,
Middleby uses the term temperature-sensing thermo-
couple in reference to thermocouples that send their
signal to the temperature controller instead of to a sepa-
rate high limit control module.
On ovens that do NOT include a separate high-limit
control module (this includes most PS200-series and
PS360-series gas ovens), this thermocouple signal is
used to monitor the oven temperature for regulating the
gas flow/heater activation, AND to activate the high limit
warning in case of an overtemp condition.
Middleby ovens use from one to three temperature-sens-
ing thermocouples, depending on the oven model.
Temperature-sensing thermocouples are used on all
oven models and are connected to Terminals 7 and 8
on the temperature controller. Based on the signal from
this thermocouple, the temperature controller:
Calls for heat from the burner/heating elements to
maintain the set point (output from Terminals 4 & 5
for on-off electric and gas ovens; output from Ter-
minals 15 & 16 for variable pulse electric and modu-
lating gas ovens).
Controls the cooldown feature if the HEAT switch
is turned off while the oven is above the cooldown
temperature (output from Terminals 9 & 10).
On ovens without a separate high limit control mod-
ule (most PS200-series and PS360-series gas and
electric ovens), the oven temperature sensed by
this thermocouple is also used to activate the high
limit warning (output from Terminals 11 & 12).
2. High limit thermocouple
Middleby uses the term high limit thermocouple in
reference to the thermocouple that sends its signal
solely to the separate high limit control module.
If the oven does not have a separate high limit control
module, there is no high limit thermocouple. On these
ovens, the temperature-sensing thermocouple(s) con-
trol high limit activation.
C. Troubleshooting
1. Open thermocouple
An open in a thermocouple will cause the temperature
controller to read the maximum temperature (usually
909°F/487°C). This causes the following:
On ovens without a separate high limit control mod-
ule, an open thermocouple triggers the high limit
alarm. This shuts down the burner. After the open
has been corrected, the high limit will need to be
reset to allow the oven to resume operation.
On ovens with a separate high limit control mod-
ule, an open temperature-sensing thermocouple
causes the temperature controller to stop calling
for heat. This shuts down the burner.
On ovens with a separate high limit control mod-
ule, an open high limit thermocouple will trigger the
high limit control module to shut down the burner.
Note that the high limit control will need to be reset
after the open is corrected before the oven can re-
turn to operation.