MACS Monthly Newsletter - 2017 - JAN1

©

Total Vehicle Climate and Thermal ManagementTM

January 2017

By Dave Hobbs - MACS Field Correspondent

Relays and Solid State Controls for HVAC
In this month's MACS Service Report we're
going to take a dive into the world of electronic
control devices encountered in today's mobile
A/C systems.

changed in many decades. Thanks to computers, what has changed in the past three decades
are the different nuances of relay spike suppression and winding currents.
Take the way we used to turn on an electric
cooling fan as an example. (Figure 2) A cooling

Delphi

General Motors

Relay Controls 101
Relays are pretty simple - right? Well for the
most part yes. Using the principle of electromagnetism a simple coil of wire (Figure 1) can

Figure 1: Relays are electro-mechanical switches. Common failures are the
winding becoming open or shorted and the contacts becoming pitted / burnt
causing an open circuit to the load the relay controls or an excessive voltage drop.

Figure 2: The simplicity of yesterday's circuits! A simple relay controlled by a thermal fan switch, a fuse for the control side of the relay
and fusible link for the high current side of the cooling fan was all there
was to it. No C.A.N. bus! A higher current side (the contacts) being
controlled by a lower current side (the winding). Notice the numbering on the terminals and the diode in parallel across the winding to
suppress the voltage spike that occurs when the field in the winding
collapses as the cooling fan switch opens.

create a magnetic attraction on a pair of electrical contacts (held apart by a spring) in order to
pull those contacts together much like a switch
you might flip. While multitudes of relay varieties such as NC (Normally Closed), NO (Normally Open), SP (Single Pole), DP (Double Pole),
etc. exist in our auto electric / HVAC worlds,
the general operation of a 12-volt relay hasn't

system thermo switch would provide a low current ground to the relay winding that was tied
to an ignition 1 circuit via a 20 amp fuse. One
relay contact was connected to the cooling fan

MACS Service Reports is the official technical publication of the
Mobile Air Conditioning Society Worldwide, Inc., P.O. Box 88,
Lansdale, PA 19446. The material published in MACS Service
Reports expresses the views of the contributors and not necessarily that of MACS. Every attempt has been made to ensure

January 2017

the accuracy of the content of MACS Service Reports. MACS,
however, will not be responsible for the accuracy of the information published nor will MACS Worldwide be liable in any way for
injury, labor, parts or other expenses resulting from the use of
information appearing in MACS Service Reports.

1

MACS Service Reports



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

MACS Monthly Newsletter - 2017 - JAN1
MACS Monthly Newsletter - 2017 - JAN2
MACS Monthly Newsletter - 2017 - JAN3
MACS Monthly Newsletter - 2017 - JAN4
MACS Monthly Newsletter - 2017 - JAN5
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MACS Monthly Newsletter - 2017 - FEB1
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MACS Monthly Newsletter - 2017 - MAR1
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MACS Monthly Newsletter - 2017 - APR1
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MACS Monthly Newsletter - 2017 - MAY1
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MACS Monthly Newsletter - 2017 - MAY7
MACS Monthly Newsletter - 2017 - MAY8
MACS Monthly Newsletter - 2017 - JUN1
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MACS Monthly Newsletter - 2017 - JUN3
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MACS Monthly Newsletter - 2017 - JUN5
MACS Monthly Newsletter - 2017 - JUN6
MACS Monthly Newsletter - 2017 - JUN7
MACS Monthly Newsletter - 2017 - JUN8
MACS Monthly Newsletter - 2017 - JUL1
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MACS Monthly Newsletter - 2017 - JUL3
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MACS Monthly Newsletter - 2017 - JUL5
MACS Monthly Newsletter - 2017 - JUL6
MACS Monthly Newsletter - 2017 - JUL7
MACS Monthly Newsletter - 2017 - JUL8
MACS Monthly Newsletter - 2017 - AUG1
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MACS Monthly Newsletter - 2017 - AUG5
MACS Monthly Newsletter - 2017 - AUG6
MACS Monthly Newsletter - 2017 - AUG7
MACS Monthly Newsletter - 2017 - AUG8
MACS Monthly Newsletter - 2017 - SEP1
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MACS Monthly Newsletter - 2017 - OCT1
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MACS Monthly Newsletter - 2017 - OCT8
MACS Monthly Newsletter - 2017 - NOV1
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MACS Monthly Newsletter - 2017 - NOV7
MACS Monthly Newsletter - 2017 - NOV8
MACS Monthly Newsletter - 2017 - DEC1
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MACS Monthly Newsletter - 2017 - DEC3
MACS Monthly Newsletter - 2017 - DEC4
MACS Monthly Newsletter - 2017 - DEC5
MACS Monthly Newsletter - 2017 - DEC6
MACS Monthly Newsletter - 2017 - DEC7
MACS Monthly Newsletter - 2017 - DEC8
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