Low On-Resistance - Solid State Relays - Hewlett-Packard / Agilent

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Low On-Resistance

Solid State Relays

Application Note 1046

Introduction

The on-resistance is an important

specification for a solid state relay

that uses MOSFETs at its output.

In general, a lower on-resistance

rating will allow a higher contact

current rating. The HSSR-8060

and HSSR-8400 are single-pole,

normally open, solid state relays

(SSR) with very low on-

resistances. Each SSR consists of

a high-voltage circuit, optically

coupled with a light-emitting

diode (LED). When a control

current flows through the input

terminals of the SSR, the LED

emits light onto a photodiode

array. The photodiode array,

illustrated in Figure 1, generates

sufficient voltage and current to

operate the FET driver circuit and

also to drive the gate-to-source

voltages above the thresholds of

the two output FETs. This

application note describes the

main characteristics of the HSSR-

8060 and HSSR-8400, suggests a

control drive circuit, and presents

various applications for the SSRs.

Additional information regarding

SSRs in general can be found in

Hewlett-Packard’s Application

Note 1036.

Summary of

Characteristics

The HSSR-8060/8400 is packaged

in a 6-pin DIP, but only ﬁve pins

are used. Pins one and two are the

anode and the cathode of the

input LED, respectively. Pins

four, five, and six, at the output

side of the SSR, can be configured

as either Connection A or Connec-

tion B as shown in Figure 2. With

Connection A, the signal at the

output of the SSR can have either

positive or negative polarity. This

means that the SSR can pass

either ac or dc signals. With Con-

nection B, the signal at the output

of the SSR must have its polarity

as indicated in Figure 2b. In this

conﬁguration, pins 4 and 6 are

tied together, and the SSR can

control dc signals only. The

advantage of using Connection B

is that it places the two output

FETs in parallel with each other,

rather than in series.

Figure 1. Circuit Diagram of HSSR-

8060/8400.

Figure 2. HSSR-8060/8400 Schematic.

5965-5978E

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This conﬁguration reduces the

output on-resistance of the SSR

signiﬁcantly and increases its

output current capability by a

factor of two. Figure 2 also deﬁnes

the polarity for the input side of

the SSR. The HSSR-8060/8400

turns on (its contact closes) with a

minimum input current, IF, of 5

mA at a typical forward voltage,

VF, of 1.6 V. Operation at higher

currents causes faster closure of

the contacts. The SSR turns off

(its contact opens) when VF is

equal to 0.8 V or less.

Both the HSSR-8060 and the

HSSR-8400 have guaranteed

input-to-output insulation voltage

ratings of 2500 Vac, 1 minute.

Additionally, the HSSR-8060 has

an output transient rejection of

1000 V/µs at 60 V, and the HSSR-

8400 has an output transient

rejection of 1000 V/µs at 100 V.

The input-to-output transient

rejection speciﬁcation of both

SSRs is 2500 V/µs at

1000 V.

The HSSR-8060 has an output

withstand voltage rating of 60 V at

room temperature. If the SSR is

used as shown in Connection A to

pass ac signals, then 60 V is the

maximum amount of peak positive

or negative voltage that should be

applied across the output contact.

The HSSR-8060 is distinguished

by its low on-resistance, R(on), and

large output current capability, IO.

At room temperature, with

Connection A, the maximum on-

resistance of the HSSR-8060 is 0.7

ohm, and the average output

current rating is 0.75 A. With

Connection B, the on-resistance is

reduced to 0.2 ohm and the

average output current rating is

increased to 1.5 A. As mentioned

in the data sheet, the on-resistance

speciﬁcation for both the HSSR-

8060 and HSSR-8400 refers to the

resistance measured across the

output contact when a pulsed

current signal is applied to the

output pins. The use of a pulsed

signal (≤ 30 ms) implies that each

junction temperature is equal to

the ambient and case

temperatures.

The HSSR-8400 has an output

withstand voltage of 400 V at

room temperature. If the SSR is

used as shown in Connection A to

pass ac signals, then 400 V is the

maximum amount of peak positive

or negative voltage that should be

applied across the output contact.

Similar to the HSSR-8060, the

HSSR-8400 has a low on-

resistance and large output

current rating. At room

temperature, the maximum on-

resistance value is 10 ohms, and

the average output current

capability is 0.15 A. With

Connection B, the maximum on-

resistance is 2.5 ohms and the

average output current rating

increases to 0.3 A.

The output current rating of an

electromechanical relay (EMR) is

usually limited by its ability to

interrupt that current when

opening. The output current rating

of the HSSR-8060/8400, on the

other hand, is limited by the

highest junction temperature

(125°C) its MOSFETs can

withstand. This junction

temperature is a function of the

on-resistance, the load current,

the thermal resistances, and the

ambient temperature. As the

junction temperature rises, the on-

resistance also rises. To limit

power dissipation at higher case

and ambient temperatures, the

output current rating must then be

derated. It is important for SSR

specifications to include this

derating effect. The data sheets

for both the HSSR-8060 and

HSSR-8400 include graphs that

show the effect of temperature on

the output current rating, IO. If

these SSRs are operated within the

“safe area of operation” indicated

on these current derating graphs,

the corresponding “power versus

temperature” graph illustrates the

maximum amount of power

dissipated by the SSR. Operation

within this area ensures that the

steady-state junction temperatures

remain below 125°C.

The output current derating

graphs for the HSSR-8060 are

shown in Figure 3. The output

power dissipation versus case

temperature graph is shown in

Figure 3. HSSR-8060 Output Current

Derating Graphs.

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Figure 4. The following example

uses these graphs to calculate the

MOSFET junction temperatures

and the maximum recommended

value of the case-to-

ambient thermal resistance,

θC-A.

Designer’s specifications:

HSSR-8060, Connection A

IO = 500mA

TA = 25°C

Data sheet specification:

Typical Output MOSFET

θJ-C = 55°C/W

For this example, assume that the

above conditions have been

speciﬁed by the designer.

According to Figure 3b, for

IO = 500 mA, the maximum case

temperature allowed is 86°C. At TC

= 86°C, the maximum output

power dissipation is 0.3 W,

according to Figure 4. Therefore,

the maximum power dissipated by

each MOSFET is 0.15 W. Hence,

the maximum junction

temperature of each MOSFET is as

follows:

TJ= θJ-C(PO) + TC

= 55°C/W (0.15 W) + 86°C

= 94.25°C

Now, to calculate the maximum

recommended θC-A, the following

formula must be used: θC-A = [(TC

- TA)/Ptotal]. The maximum power

dissipated by the input LED is

equal to [(IF)(VF)] = [(10

mA)(1.85 V)] = 0.019 W. There-

fore, the total power dissipated by

the SSR must be less than 0.319

W. Hence, the value of θC-A must

be no greater than

[(86 - 25)/0.319], or 191.22°C/W.

This will ensure that TC remains

below 86°C. One suggestion for

minimizing θC-A is to enlarge the

pc-board copper traces

surrounding the output pins of the

SSR. Another suggestion is to

force cool airﬂow across the

board.

Maximum Signal

Frequency

When using the HSSR-8060/ 8400

to control ac signals, the maxi-

mum frequency of the signal may

be limited by the off-capacitance,

C(off), of the relay. The off-

capacitance is voltage dependent

and is specified as 135 pF typical

for the HSSR 8060 and 60 pF

typical for the HSSR-8400 at VO =

25 V. The data sheets for both

SSRs include graphs for the

typical output capacitance versus

output voltage. Besides the off-

capacitance, the maximum signal

frequency depends on the load

impedance, the circuit configura-

tion, and the amount of

attenuation required by the

designer. The attenuation refers to

the amount of signal passed

through the contact in its OFF

state versus its ON state. For

example, -40 dB of attenuation

implies that the current that

passes through the contact in its

OFF state will be one hundred

times smaller than the current that

passes through the contact in its

ON state.

For comparison, typical SSRs were

conﬁgured as simple series

switches and tested at room

temperature for maximum signal

frequency. Each SSR was tested

with a load resistor, RL = 100 ohm

and an output sine wave, VO = 1

Vp-p. The maximum signal

frequency of each SSR to obtain a

signal attenuation of -40 dB was as

follows:

HSSR-8060: 40 kHz

HSSR-8400: 65 kHz

HSSR-8200: 2800 kHz

The HSSR-8200 is another SSR

made by Hewlett-Packard. It has a

very low C(off) speciﬁcation of 4.5

pF maximum.

Figures 5a and 5b show a circuit

model and its off-state equivalent

circuit for a simple series switch

using an SSR. The frequency

response of this circuit is shown in

Figure 6. The break frequency, fB,

is the frequency above which there

is no attenuation in the signal.

Figure 4. Output Power Dissipation vs.

Case Temperature.

Figure 5. Simple Series Configuration.

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This means that the same

amount of signal will pass

through the contact whether it

is opened or closed. As shown in

Figure 6, the signal amplitude

decreases by 20 dB for each

decade decrease in the signal

frequency. A designer who

requires -40 dB of attenuation

when the SSR is off must have a

maximum signal frequency that

is at least two decades below the

break frequency. As an example,

the break frequency is about 800

kHz for a load resistance, RL, of

1kΩ and a C(off) of 200 pF. If a

designer requires at least -40 dB

of attenuation when the relay is

off, the maximum signal

frequency is two decades below

800 kHz, or 8 kHz.

To control higher-frequency

signals, two SSRs can be used in

the series-shunt configuration

shown in Figure 7a. In the ON

state, the series SSR is closed, and

the shunt SSR is opened. In the

OFF state, the series SSR is

opened and the shunt SSR is

closed. Figure 7b shows the

equivalent circuit for this OFF

condition, and Figure 8 illustrates

its frequency response. This

series-shunt configuration

produces a higher break frequency

than the simple series configura-

tion. The reason for this improve-

ment is that the break frequency

equation now uses the low on-

resistance value of the SSR rather

than the load resistance value. The

series shunt conﬁguration allows

higher signal frequencies to be

used or gives increased attenua-

tion at lower signal frequencies.

As an example, using two relays

with C(off) = 200 pF, R(on) = 6 Ω,

and RL = 1 kΩ, the break

frequency for the series-shunt

conﬁguration is about 66 MHz.

Figure 6. Simple Series Frequency Response.

Figure 7. Series-Shunt Configuration.

Figure 8. Series-Shunt Frequency Response.

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The maximum signal frequency to

ensure at least -40 dB of

attenuation is approximately two

decades below 66 MHz, or 660

kHz. Notice that because

R(on) << RL, the value of the load

resistance has virtually no effect

on the calculation of the break

frequency.

Control Drive Circuit

Suggestions

Operation of the HSSR-8060/

8400 requires at least 5 mA of

input current. A larger amount of

input current results in faster turn-

on of the SSR and a slightly faster

turn-off. A simple circuit for

obtaining the desired ON current

and OFF voltage is shown in

Figure 9. The logic series used can

be either TTL or CMOS, as long as

the current sinking capability is

adequate. Resistor R1 sets the

level of steady-state input current,

IF. The purpose of R2 is to bypass

logic-high leakage current with

sufficiently small voltage drop to

ensure an OFF-voltage less than

0.8 V. R2 is not required if the

logic output has an internal pullup

circuit that is able to satisfy the

OFF-voltage requirement of the

SSR. With open collector TTL

outputs, R2 is always required to

ensure that VF(OFF) < 0.8 V.

As mentioned earlier, turn-ON

time is influenced by the level of

input current. As input current is

increased, the turn-ON time

becomes shorter. However, it may

not be desirable to operate with a

high steady-state input current

because that would increase the

output offset voltage, VOS, due to

heat transferred from the LED

control to the contact side. Also, a

lower steady-state current

minimizes input power consump-

tion. In situations requiring fast

turn-ON but low offset, the

peaking circuit shown in Figure 9

can be used. When the logic

output is high, R2 assures that the

current through the LED is so

small that the capacitor is

completely discharged. Then when

the logic output goes low, a surge

of current flows through both R1

and R3 until the capacitor is

charged to the voltage across R1.

The steady-state current is set by

R1 alone. Thus peaking permits

fast turn-ON as well as low steady-

state current. Table 1 shows the

typical turn-on times obtained

with different values of resistor

R3.

Turn-on of the output MOSFETs

requires charging the gate

capacitances in the FET

DRIVER circuit. This charge is the

time-integrated photocurrent from

the photodiode array, and

translates into a certain amount of

current that must pass through the

LED. The corresponding amount

of LED charge is set by the value

of the peaking capacitor and the

voltage across R1. For this reason,

it is not necessary to change the

value of the capacitor when other

values of peak current are desired;

it is necessary only to change the

value of R3 and make sure that the

logic output is capable of sinking

the higher current.

Telecommunication

Applications

SSRs are commonly used by the

telecommunications industry.

Some examples of applications

Table 1. Typical Peaked Turn-on Times.

R3IF(PEAK) HSSR-8060 tON HSSR-8400 tON

(Ω) (mA) (ms) (ms)

– 10 (No Peak) 0.93 0.50

330 20 0.53 0.29

100 40 0.32 0.17

33 100 0.17 0.09

Figure 9. Recommended Input Circuit.

1-627

include on/off-hook switching, test

and maintenance equipment, PBX

and central-office switching, and

pulse dialing. Compared to EMRs,

SSRs are useful in these areas

because they are small and require

little board space. They have no

mechanical parts so they last

longer, thereby increasing the

number of operations that can be

performed. In addition, SSRs have

no contact bounce, arcing, or

acoustic noise.

In telephone loop applications, it

is often necessary to isolate the

telephone equipment from the

incoming telephone lines. Isolation

is important to protect the elec-

tronics from harmful voltages and

currents induced from lightning or

noise coupled onto the lines.

Modern line interface circuits,

such as those used for modems

and fax machines, consist of a ring

detector, an on-off hook control,

isolation, and surge protection. An

advantage of using an SSR as the

on-off hook switch is that it

provides both high-voltage

isolation and surge protection.

Figure 10 shows the SSR in a

telephone switchhook application.

In the ON-state, the SSR’s contact

on-resistance contributes to the

total impedance of the telephone

loop, which is between 500 and

2100 ohms. Therefore, the on-

resistance should be as small as

possible. At room temperature, the

HSSR-8060 has a maximum on-

resistance of 0.7 ohm, and the

HSSR-8400 has a maximum on-

resistance of 10 ohms.

The purpose of the on-off hook

switch is to connect or disconnect

the telephone equipment from the

PBX (private branch exchange) or

the PSTN (public switched tele-

phone network). When a person

wishes to place an outgoing call or

answer an incoming call, the relay

is turned on to allow current to

ﬂow between the tip and ring

conductors. In this relay applica-

tion, an overvoltage protection

device is often required to protect

the contact from possible

lightning damage. For example, a

metal oxide varistor (MOVTM) is a

bidirectional device that breaks

down and conducts heavily when

the voltage across it rises above a

threshold level. As shown in

Figure 10, an MOV is placed

across the output contact of the

SSR. The device protects the SSR

by limiting the tip-to-ring voltage

to a value below the maximum

load voltage of the SSR. The MOV

acts as a Zener diode but

dissipates more energy. When a

large current surge occurs, the

“Zener” voltage of the MOV can

cause significant overshoot. For

this reason, the SSR’s load voltage

must be higher than the highest

voltage of the protection device at

any given current surge. Most of

the SSRs used in telephone-line

interface applications are rated for

at least 350 to 400 volts. The

HSSR-8400 has a high contact

withstand voltage rating of 400 V.

Telecommunication companies

may also use SSRs for test and

maintenance equipment. When a

subscriber reports a problem with

his or her telephone service, the

telephone company can use relays

to switch test equipment onto the

line to verify the problem.

Telephone companies might also

use relays to switch test

equipment onto a line to examine

the quality of the line. This is done

to locate potential problems.

Another application for SSRs in

the telecom industry is in PBXs

and Central Office Switching

Stations. SSRs may be used to

multiplex incoming signals, such

as concentrating several

subscriber loops onto a single

interface circuit. Or, SSRs may be

used in the cross-point matrixes of

these switching stations.

In a telephone line interface, SSRs

can also be used for the pulse

dialing function. With pulse

dialing, a relay is used to interrupt

the line current. The number of

line interrupts corresponds to the

specific digit that was dialed. A

“1”, for example, is identified by

one break while a “9” is identified

by nine breaks in the line.

Figure 10. Telephone Switchhook.

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Multiplexing

Sometimes, signals that need to be

measured require amplification or

conversion. As shown in Figure

11, multiplexing allows a single

device to process a number of

signals. This technique reduces

cost by using one device for a

number of channels rather than

one device per channel. Also,

multiplexing ensures that the same

amount of gain is applied to each

signal so that the ratios of the

ampliﬁed levels V1...Vn will relate

to ratios of their unampliﬁed

counterparts, E1...En. Compared

to EMRs, SSRs are especially

useful in multiplexing applications

because of their increased life and

reliability. Also, their fast

switching speeds allow more

efficient multiplexing of signals.

The HSSR-8060/ 8400 have turn-

on times under 1.8 ms and turn-off

times under 0.1 ms. Both relays

will turn on faster using the peak

circuit mentioned earlier.

In any application, it is important

for the amount of current passed

through the contact in its OFF

state to be negligible with respect

to the actual amount of current

being controlled. With power

switching applications, the leakage

current of the SSR is small relative

to the amount of current being

switched. In signal switching

applications, however, the amount

of current controlled by the SSR is

relatively low. Therefore, the SSR

should have negligible leakage

current. When multiplexing or

switching low-level signals such as

thermocouple outputs, an SSR

with a low leakage specification is

preferred. The HSSR-8060 has a

typical output leakage current of

0.1 nA, and the HSSR-8400 has a

typical output leakage current of

0.6 nA. Figure 12 shows an

example of a multiplexing system.

In this diagram, SSRs are used to

multiplex or scan low-level differ-

ential signals. The configuration

uses three switches per channel to

connect the signal HI, signal LO,

and guard to the measurement

system. In Figure 13, SSRs are

used in a flying capacitor circuit.

When relays 1 and 2 are closed,

the voltage is acquired from the

sensor and stored across the

capacitor. After relays 1 and 2

open, relays 3 and 4 close, and the

signal is read by the multiplexer. A

number of sensors can be

connected to the multiplexer in

this fashion.

Figure 11. Multiplexing and Demultiplexing.

Figure 12. Multiplex System.

1-629

Industrial Control

In programmable controllers,

input and output modules allow

microprocessors to sense and

control various loads. An ac input

module generates a logic level

voltage corresponding to the

presence or absence of an ac load

voltage. Likewise, a dc input

module generates a logic level

voltage corresponding to the

presence or absence of a dc load

voltage. Input modules receive

signals from a variety of instru-

ments on the factory floor,

including robotics assembly

equipment, chemical process

units, injection molding systems,

and so forth. An ac output module

allows logic-level voltages to

control a switch that turns ac

loads on and off. For example, the

output module of a process

controller might be used to

control the motor starters of

adjustable frequency drives,

position valves, or dampers. A dc

output module allows logic level

voltages to control a switch that

turns dc loads on and off.

Figure 14 shows an example of a

six-channel ac output module. The

HSSR-8060/8400 may be used to

sense and control signals in any

one of these input and output

modules.

Another application for SSRs in

industrial control equipment is on

scanner cards or matrix cards.

These cards may be used in larger

instruments such as program-

mable thermometers, temperature

scanners, and multimeters.

Scanners are very similar to

Figure 13. Flying Capacitor Multiplexer.

1-630

multiplexers, however scanners

measure sequentially while

multiplexers allow any order.

Figure 15 shows an example of

relays used on a matrix card. In

this configuration, a number of

sources can be connected, through

the device under test (DUT), to a

number of measurement devices.

In an automated test system, such

as a data acquisition unit, efficient

switching is important. In addition

to their fast switching speeds,

SSRs provide high voltage

isolation, which is often required

in industrial environments.

Another benefit of SSRs in

industrial control applications is

that they do not have mechanical

contacts, which could eventually

deteriorate from arcing or dust

particles.

Various Loads

Depending on the type of load, an

SSR may be required to withstand

a substantial amount of surge

current. A purely resistive load is

the easiest type for an SSR to

switch since it has no surge

current requirement. Other typical

loads of the HSSR-8060/8400 and

their related inrush versus steady

state currents are shown in

Table 2.

The surge requirement of the load

should be within the peak surge

current rating of the relay.

Therefore, an SSR that switches

one of these types of loads should

have current specifications that

Figure 14. Six-Channel AC Output Module.

1-631

meet both the steady-state and

surge requirements. Compared to

EMRs, SSRs are more tolerant of

surge currents because they do not

have contact bounce, which results

in arcing with EMR contacts. The

high-temperature arc could cause

melting and eventual degradation

of the EMR contact. With

Connection A, the HSSR-8060 has

a single shot, peak output current

rating of 3.75 A for a 100 ms

pulse width. The HSSR-8400 has a

rating of 1.0 A for a 100 ms pulse

width. For longer pulse widths, the

single-shot, peak output current

rating would decrease. Figures 16

and 17 show the results of an

experiment performed on seventy

units of the HSSR-8060 and HSSR-

8400. Each graph shows the peak

surge current values that we were

able to apply to the output of

seventy typical SSRs without any

one of them failing.

Another experiment was

conducted to determine the

maximum repetitive surge current

that twenty typical SSRs could

withstand. Ten units each of the

HSSR-8060 and HSSR-8400 were

tested for ﬁfteen minutes each

with surge current pulses applied

for 100 ms at 100 ms intervals

(fifty percent duty cycle). Under

these conditions, the maximum

repetitive surge current to failure

was 1.2 A for the HSSR-8060 and

0.25 A for the HSSR-8400.

Table 2. HSSR-8060/8400 Load Types.

Figure 15. Matrix Card Example.

Typical Inrush vs. Inrush

Load Type Steady State Current Duration

Small Solenoid 10-20X 70-100 ms

Fractional

Horsepower Motor 5-10X 200-500 ms

Miniature

Incandescent Lamp 20-15X 30-100 ms

Capacitive Load 20-40X 10-40 ms

1-632

Figure 18 illustrates the use of

SSRs in a lamp sequence control.

Some areas that use SSRs to

control lamp loads include process

equipment, navigational devices,

illuminated signs, and games. In

aircraft applications, SSRs may

control lamps for cabin lighting,

instrumentation lighting, and

status indicators. Compared to

EMRs, SSRs are especially useful

in aircraft environments because

they are immune to shock and

vibration and are unaffected by

electro-magnetic interference.

Upon turn-on, the current through

a lamp is very high initially

because of the Tungsten filament’s

low resistance at room tempera-

ture. The current decreases as the

ﬁlament heats up. Hence, the

inrush current can be reduced by

using a “keep alive” voltage across

the ﬁlament to keep it warm but

below the level of incandescence.

Similar to a lamp load, a capacitive

load will cause a surge current to

flow through the output MOSFETs

of the SSR, upon initial turn-on.

This surge current will depend on

the load capacitor value and the

rate of rise of the load voltage. In

addition, the frequency at which

the SSR is switched will affect the

output power dissipation. Ten

units of the HSSR-8060 were

tested at room temperature under

the following conditions:

Input current, IF = 10 mA (1 Hz)

Load, C = 100 µF capacitor

Load voltage, V = 60 V

Figure 16. HSSR-8060 Peak Surge Current Experiment Results.

Figure 17. HSSR-8400 Peak Surge Current Experiment Results.

1-633

Figure 18. Lamp Control.

Figure 19. Motor Reversing Control.

1-634

The load capacitor was charged by

the load voltage through an 80-

ohm series resistor. The output of

the SSR-under-test was placed in

parallel with the capacitor to

discharge it. After the testing,

each SSR was tested for 1.2

million cycles and passed. There

were no catastrophic failures or

parameter drifts.

The HSSR-8060/8400 can be used

to drive fractional horsepower

motors. A reversing control for a

synchronous ac motor is shown in

Figure 19. For motors that cycle

on and off frequently, an SSR is

often preferred over an EMR

because it can handle surges

better and does not produce EMI.

An SSR might also be used to

control small dc motor loads such

as those used in computer disk

drives, audio and video

equipment, household electronics,

or automotive electronics.

An SSR may be used to control the

input coil of an EMR, which is a

highly inductive load. Other

inductive loads include small

transformers, contactors, solenoid

valves, magnetic couplings, etc.

When SSRs drive inductive loads,

very high peak voltages can occur

across the output when switching

off the loads. The MOSFETs in the

output of the SSR are able to

withstand a reasonable amount of

inductive overload. For example,

ten units of the HSSR-8060 were

tested at room temperature under

the following conditions:

Input current, IF = 10 mA (1 Hz)

Load, L = 1-H inductor

Load voltage, V = 60 V

Load current = 670 mA

Each unit was tested for one

million cycles and passed. There

were no catastrophic failures or

parameter drifts. No overvoltage

protection for the SSR was

used in this experiment. However,

overvoltage protection is recom-

mended whenever the chance

exists for an event where both the

withstand voltage rating and

output power dissipation or surge

rating are exceeded, or where the

energy content of the transient is

very large as in lightning-induced

events.

Overvoltage Protection

Metal oxide varistors (MOVs) or

TransZorbsTM can be used for

overvoltage protection of the

contacts of an SSR. They both

break down and conduct heavily

when the voltage across them rises

above a specified level. For ac

voltages, either an MOV or a

bidirectional TransZorb can be

used. Both devices fail “short” so

that protection is always in place,

even though operation may cease.

As shown in Figure 20, the

protection device is placed across

the output contact

Figure 20. Overvoltage Protection.

1-635

pins of the relay and is used when

the contact is susceptible to

voltages greater than the rated

output withstand voltage, VO. For

adequate protection of the

contact, the protection device

should be in a fully conductive

state at a voltage just below the

maximum output voltage.

However, it must be in a high-

impedance state for any voltage

below the maximum line voltage.

When the SSR is used to control

small dc voltages, a single Zener

diode, illustrated in Figure 20a,

provides adequate protection.

Again, the clamp voltage of the

Zener should be greater than the

controlled voltage but less than

the maximum rating of the SSR

contact.

TM MOV is a registered trade-

mark of GE/RCA Solid

State.

TransZorb is a registered

trademark of General

Semiconductors.