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. http://www.electroschematics.com/7103/555-tlc555-relay-driver-circuit/
Advantages of the low side driver
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).
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 electroschematics.com, but they are not the driver of choice for three reasons as follows:
For the future
Glossary of undocumented words and idioms (for our ESL friends)
glitch –undesirable noise transient or spike (in electronics)
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