In this application, the relay contacts will switch 12VDC, 5A. One single activation about every 5 minutes over a 3 hour operating session - so about 40 activations. Maybe only 10 sessions per year, so VERY low cycles on the contacts But I am also concerned about arcing effect on the control board (Teensy++2)
The switched load is a nichrome heater wire, 2.5 - 3ohms. Resistive, not inductive.
Do I need an RC snubber or a diode snubber? Sources vary on the answer.
If RC, the most common answer I find gives equations for R= 0.5xV (6 ohms here) and non-polarized C=1xA (5mfd here).
Many years ago, where I was working, we had two very large water valves controlled by 24 VDC solenoids in their pilot systems. Each valve had an OPEN and CLOSE solenoid hence 4 solenoid coils that had to be activated a few times per day. Each coil drew about 750 mA but the spike on the removal of power was several amps for about 1/2 second. These coils were being driven by relay contacts rated at 5 amps and after only a couple of days, the contacts were destroyed by excessive current. An RC snubber was tried but failed to protect the relay contacts. I placed a 1N4002 diode across the coils at the relay contacts with the anode going to the + 24 VDC supply and the cathode going to the coil wire. Note that the contacts were switching the + 24 VDC power to the coils. After 5 + years, one relay had to be replaced due to its' coil failure !!! The diode provided adequate protection for the contacts. Hope this helps !!
Tom
Relay contacts operating purely resistive loads do not need a diode nor snubber.
Both the diode and R-C snubber are used where an inductive load is being switched. Inductors do not like to have their current changed so on opening they try to keep the current flowing by generating "high" voltages. It is these high voltages that burn contact material resulting in premature failure.
The only caveat is if you have very long wires to your nichrome heater.
In your case a diode would do nothing and arguable the R-C could reduce contact life by increasing the initial on current.
The heater wire is less than 100mm (4") long, and very loosely wrapped - about 5 turns, 6-8mm dia.
I don't think it is very inductive (?).
I'll go with the diodes. The 2 longest wires are about 2ft long, the remaining 7 are all shorter.
I have some IN4004's that I can use.
Thanks!
MorganS:
If the heater is wound in a coil shape then it may have a meaningful inductance. I would add the diode as a standard practice.
Nah, unlikely. Compared to a tightly wound solenoid with a ferromagnetic core with µr in the thousands,
this is not going to be a significant inductance.
The only way to get significant inductance without ferro- or ferri- magnetic materials is use a coil
with a large loop area, so for instance a heating wire wrapped around a large cylinder. Normally
if such a heating arrangement is needed the manufacturer will use a non-inductive winding
technique anyway.