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Relay Methods

Transistor Project Relays

All the items below need to be considered before ordering relays and transistors that will be used together.

When working with relays, the transistor's maximum collector-to-emitter voltage, collector current and power dissipation ratings must be considered.

Look-alike relays have become scarce.  Undoubtedly on-line auctions will offer some vintage relays that will sell at collectible prices.

Work-alike relays remain easily acquired.  Manufacturers include ET Connectivity (Potter and Brumfield division), Omron and Panasonic Electric Works.  Check Digikey, Mouser and other major distributors for stocked part numbers.  Pricing groups include US$1.50 through US$5.00,  US$5.00 through US$10.00 and US$10.00 through way up.  Almost every relay a hobbyist could want fall into the two lower groups.

Before searching for a relay, text edit the requirements.
Coil Voltage <= ____________ VDC.
Coil Current <= ____________ mA DC.
Switching Voltage >= ____________ VDC.
Switching Current >= ____________ Amps VDC.
Coil Resistance >= ____________ .
Mass <= ____________ grams.
Size <= _____ cm X _____ cm X _____ cm.
or whatever units you use.
Number of contact operations per second.

Ideally coil voltage should be equal to or less than the supply voltage and equal to or less than 50% of the transistor's maximum emitter-to-collector voltage rating, whichever is smaller.  In a pinch (a.k.a. proof of concept deadline and maybe short lived seasonal decorations) coil pull-in voltage can be equal to or less than the supply voltage and equal to or less than 80% of the transistor's maximum emitter-to-collector voltage rating, whichever is smaller may be used.  In the latter case, circuit life may be short.

The manufacturer may or may not specify the coil resistance.  If not, measure the coil resistance before the relay sees any use.

Ideally the coil current should be less or equal to 50% of the transistor's maximum collector current rating.

The required relay contact switching voltage and contact switching current must be suited to the load.

The mass and size mostly come into play in radio-controlled models and telemetry balloons and rockets.  When hobbyists run up against operations per second limits, they usually change from relay to semiconductor switching.  In some cases a reed relay may be fast enough.

Thermal Runaway

As relays and solenoids often provide too little resistance to prevent damage from thermal runaway, the discussion repeats here.

Any bipolar junction transistor can enter thermal runaway.  Rushing to meet publication deadlines, writers sometimes neglect thermal runaway checks.  Therefore, a builder using hard-to-obtain or expensive transistors does well to include a minimal check.  Good biasing prevents thermal runaway.  However, simpler biasing preventing damage may suffice for hobby projects.  It may even provide more learning.

According to the power-transfer theorem, a transistor in a common-emitter configuration draws maximum power when its collector voltage equals half the supply voltage.  This gives the messy-looking, but conceptually simple formula

Pc = Vc*Ic = (Vcc/2)*((Vcc-0.5*Vcc)/Rc))


Pc = (Vcc^2)/(4*Rc)

Which rearranges to

Rc >= (Vcc^2)/(4*Pc)

Note 28 – To protect a 50 mW transistor from damage, the series resistance must be equal to or larger than values given below.
Battery            Resistance
4.5V                150 ohms
6.0V                270 ohms
9.0V                560 ohms

A large collector or emitter resistor does not prevent thermal runaway.  The circuit may still lock up in a “soft” failure.  However, the transistor will remain within its dissipation limits.  Precautions listed above prevent exceeding the transistor's voltage and current limits.

Inductive Kickback

Early transistor demonstration relay drivers often operated from resistive sensors such as CdS cells and thermistors.  The slowly changing (by electronics standards) resistances led to slowly changing currents in the transistor's base and collector electrodes.  Consequently, inductive kickback protection could be omitted.  Many sensors in use today produce much more rapid switching.  Consequently, transistors require protection from inductive kickback produced in relay and solenoid electromagnets.

In most cases, any 0.2 to 1 amp silicon diode of rectifier placed in the non-conducting direction parallel with the coil provides protection.  In the 1950s such diodes were more expensive than resistors and capacitors.  Hence, a “snubber” was used.  A snubber consists of a 0.22-uF capacitor wired in series with a 22-ohm resistor.  The snubber was then wired in parallel with the electromagnet.