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Benefits between using a Relay or Transistor? Answered

I am unclear as to the potential benefits of using a relay versus a transistor and vice versa. Does some one have a good link or explanation? In the project that I am working on I need to be able to turn power on/off for a device that uses 500mA at 7.5V. Is there one that I should use? Everything is DC not AC if that makes a difference. Thanks in advance for your help.

7 Replies

icengBest Answer (author)2013-03-12
  • Relays can work with AC and DC,  Semiconductors are DC only.
  • Relays have zero closed resistance, Semiconductors have a forward voltage drop that wastes a little to lots of power.
  • Relays have infinite open resistance, Semiconductors have a
     leakage that can affect attached electronics.
  • Relays can operate at temperature extremes,  Semiconductors are limited to 95°C and a little below zero.
  • Semiconductors can operate at (  Megahertz ) speeds, Relays  are much slower at 200 hertz.
  • Semiconductors can be damaged and short due to voltage-peaking, secondary-breakdown over current, dv/dt and di/dt, Relays are primarily damaged by over-current.
  • Relays have a very high isolation from the control coil, Semiconductors as a rule are not isolated from the base, gate or trigger
  • Semiconductor switches almost never wear-out, Relays have a much shorter mechanical wear-out contact life.
  • Semiconductors can amplify analog signals, Relays can only open and close.
  • A semiconductor diode has almost no relay counterpart.

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iceng (author)iceng2013-03-12

More on relays;

A relay can carry more current then it can switch.
Switching capacity is determined by the contacts of the relay.
The more massive and conductive ( Silver not Copper ) the "buttons " are the more current a relay can switch.

When a contact Closes it bounces and draws miniature plasmic arcs
that pitt the metal button causing future hot and poor connections
which arc more and progressively worsen the overall switching action.

BTW when a switch opens a heavy DC current,
there is an Arc that also causes button pitting.
One day an abused contact will arc enough to weld closed and
then your relay switch is shot.....
This does not happen in a Semiconductor.

Switch contact manufacturers use red Vaseline ac a lubricant and
arc extinguishing agent to prolong contact switch life span...

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steveastrouk (author)iceng2013-03-13

+1 I can't think of anything to add !!!

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swamy201 (author)2014-08-08

use of relay in water level indicatior

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samaddon (author)2013-03-12

I in my opinion a transistor is better then relay cause they both can do the same thing "Switching of low or high voltages" but the difference is a transistor needs a very low voltage for switching (0.5volts or less)but relay need something around 6volts

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samaddon (author)steveastrouk2013-03-13

Yes he is right. But i was telling my opinion as i didn't use relays i commonly use transistor for switching. I use to mess up with DC circuit only . But the answer Iceng gave is better that me!

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