Cheapest Power Relay at DigiKey

Relay Driver

Relays have been around for a long time and though often now replaced with solid state switches, they have unique properties that make them more robust than solid-state devices and are not going away. The unique properties are high current capacity, ability to withstand ESD and drive circuit isolation.

There are numerous ways to drive relays. In preparation for some of the more advanced relay drivers I will be posting in the future, I have listed a few basic relay drivers for your reference. Included are the following: High side toggle switch driver, low side toggle switch driver, bipolar NPN transistor driver, Darlington transistor driver, N-Channel MOSFET driver, and ULN2003 driver.

You may recall the TLC555 Relay Driver Circuit that I recently posted, it too is a low side driver.

Relay Driver Schematic

Relay Driver Circuits

Advantages of the low side driver

  • Easy to interface to low voltage logic circuitry
  • Far more interface options including the popular ULN2003 driver
  • Fewer components
  • Uses more commonly available and less expensive NPN drive transistors
  • Relay power may be sourced by a higher, unregulated voltage—reduces load on voltage regulator
  • Easier to interface relay economy feature—will be discussing this in the future
  • Industry standard technique

Relay driver, how it works?

Generally, we think on the high side because we usually place the power switch in the power lead as in Fig 1. The same may be accomplished by locating the switch in the low side or return lead as in Fig 2. When controlling relays via logic etc. it is far easier to interface to the low-side driver. For low power relays, a 2N4401 is a good choice (Fig 3). If you desire to drive a larger relay or want less base current, a Darlington driver (Fig 4) is recommended. If driving via CMOS logic, an enhancement mode MOSFET is a good choice (Fig 5). If you nave a number of relays or other loads to drive (like a 7 segment LED display), the ULN2003 is a great choice. (Fig 6).

Clamp diode

The clamp, free-wheeling or commutation diode provides a path for the inductive discharge current to flow when the driver switch is opened. If not provided, it will generate an arc in the switch—while the arc will not generally damage a switch contact, it will cause contact degradation over time—and yes, it will destroy transistors—been there, done that. The diode requirements are non-critical and a 1N4148 signal diode will generally work OK in low power applications.

The ULN2003 has internal clamp diodes. While these work OK in non-critical applications, I have had problems with them generating “glitches” in supposedly unrelated sections.

Avoid emitter follower drivers

I have not included an example of this because I do not wish to promote this technique. This is a high-side driver accomplished by an emitter follower transistor. They are frequently found on, but they are not the driver of choice for three reasons as follows:

  • It turns the relay itself into a voltage threshold detection device—while it may work, it was not designed for this function and the threshold may vary greatly from device to device.
  • The emitter follower transistor requires a base drive voltage that should go essentially to the positive rail or the relay may not pick up. Only relays that have a low pick-up voltage work OK.
  • The voltage hysteresis is extreme—once turned on, it is often difficult to turn off again.

For the future

  • Relay economy driver circuits
  • Voltage doubler relay driver

Glossary of undocumented words and idioms (for our ESL friends)

glitch –undesirable noise transient or spike (in electronics)

Preferred components for the serious experimenter

Cheapest Power Relay at DigiKey
The most inexpensive power relay at DigiKey.
Power Relay, 10A contacts, 12V, 30mA coil, TE Connectivity 1461069-5, Digikey PB1321-ND, $1.15 ea

Cheapest Signal Relay at DigiKey
The most inexpensive signal relay at DigiKey
Reed relay, 12V, 12mA coil, Coto Technology 9007-12-00, DigiKey 306-1064-ND, $1.08 each

2N7000 N-Channel MOSFET Transistor, 200mA, 60V, TO-92, DigiKey 2N7000TACT-ND, $0.39 each

MPSA29 NPN Darlington Transistor, 0.5A, 100V, hFE = 10K, TO-92, DigiKey MPSA29RLRPGOSCT-ND, $0.33 each,

2N4401 NPN Transistor, 600mA, 60V, hFE = 150 @ 100mA, TO-92, DigiKey 2N4401-ND, $0.20 each

ULN2003A Darlington Array, 7 Section, 500mA, 50V, DIP-16, DigiKey 497-2344-5-ND, $0.53 each



Join the conversation!

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  • Mario

    Hi there! Congrats on the website! I esaeicplly liked the iPhone Call Recorder (I actually found this website while searching for the app), and the whole fuss around it in Gizmodo .I’m a noob in electronics, just now starting using arduino (as I am more of a programmer..)So, if it’s not too much of a bother:I THINK I get the flasher circuit, though I’m not sure didn’t find a decent explanation on the web.. But why does the capacitator on the switch help with the flash timer?

  • Gopalakrishna Palem

    Thank you for sharing the thoughts.

    In the article it was mentioned “If you nave a number of relays … the ULN2003 is a great choice.”

    However, later it indicates “The ULN2003 has internal clamp diodes. While these work OK in non-critical applications, I have had problems with them generating “glitches” in supposedly unrelated sections”

    So wondering what is the right way to drive high-powered multiple relays using low powered supply (say drive 30Amp AC relays with 5v DC?)

    GK (Gopalakrishna)

    • Jim Keith

      Good observation! Actually, for standard applications (relay coils and display segments), sub-microsecond crosstalk is a non-issue due to slow armature operation time and visual eye persistence etc. In my case (multiple SCR gate drivers), external clamp diodes were necessary to prevent crosstalk.

      Also note that the ULN200x series includes the ULN2002 and ULN2004 that offer varying logic level input thresholds. Also, there is the ULN2804, an octal or eight section chip.

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