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Yet another way to say it Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 2 Jul 2001 15:18:00
In Reply to: Relays and switches - flow of current ?, JeffB, Mon, 2 Jul 2001 12:57:41 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The way most relays work:
A relay is an electromagent and a switch. When the electromagnet is energized, it closes a set of electrical contacts (a switch). When the electromagnet isn't energized, the contacts are open.
All the systems in a car run on 12 volts. The difference is how much current they take, and that's why relays are used.
OK, why do this? Two reasons. First, is electrical current. You can energize the relay coil with very little current - maybe only 5 or 10 milliamps (5 or 10 thousandths of an amp). But you can make the switch contacts big - say contacts that can carry 10 or 20 amps. So you can use a small signal (low current) to switch a large current. In order to carry a large current, you need bigger electrical contacts.
The other reason - sometimes you want to open one set of contacts, and close a different set. For example, switch between low and high beams. In one relay position, current is connected to the low beam filaments. Flip the relay, and current is sent to the high beam filaments. You can also run a bunch of contacts with one electromagnet - for example, close the AC relay, and one set of contacts sends current to the AC clutch, while a different set of contacts send current to the fan on the radiator. Together, the fan and AC clutch would take more current than one set of contacts can handle, but with two sets of contacts, everthing works.
As a note, nowdays many relays are becoming 'solid state' relays. Instead of an electromagnet and a set of contacts, a solid state relay consists of transistor switches. No moving parts, so they theoretically last longer. But the same basic idea - a small current switches a large current.
, Tue, 3 Jul 2001 14:18:01, Tue, 3 Jul 2001 16:29:04, Mon, 2 Jul 2001 15:18:00 <-- Viewing This Message
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