Coil Relay Overvoltage

[akaf24]

Hello, I'm a bit new to electronics, so this is probably a simple question. I am looking to to control the heated bed of my 3D printer with a relay. I'll buy the correctly rated relay (I have a 12 V power supply for the coil, and I'll have the contacts connected to a 24 V power supply for the bed). However, I do happen to have a relay laying around my house. The coil is rated for 12VDC (great!). But the contact terminals are rated for 80A/14VDC... My question is: does this voltage matter, and if so, why? The most current my heated bed will ever use is at most 10 A so does it matter if I have 24VDC across the contacts if only 8 A is passing through the contacts when it can handle 80A? Thanks. Here's the data sheet:

https://www.citrelay.com/Catalog Pages/RelayCatalog/A3.pdf

 

[Compositepro]

One key fact to understanding this is that the coil's magnetic field is at all times proportional to the current. So to collapse the field quickly requires that di/dt be large, which means a high kick-back voltage. This is what you get with a simple switch. The high voltage, however, will cause a damaging spark at switch contacts or transistor. A flyback diode will allow the current to continue flowing after the switch is open so it decays slower and delays relay operation. A resistor in series with the diode will increase the voltage, and thus power dissipation (rate at which the finite coil energy changes). An infinite resistor is the same as no resistor or diode.

A capacitor can be used in place of a diode. It acts as, in effect, a variable resistor. It allows current to continue to flow after the switch opens so the contacts can separate enough to withstand high voltage, before the current decreases as the capacitor charges. Capacitors (condensers) were used on the breaker points of automotive ignition switches. Without the capacitor the ignition coil energy would be lost in a spark at the breaker points rather than a spark at the spark plugs.

 

[waross]

Another valid explanation.
Our friend may "get it" yet.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Agreed Comp. Flyback, slower release than no flyback. But, nothing blasted with a HV arc.

Keith Cress
kcress -

http://www.flaminsystems.com