Add a diode to my solenoid valve?

[Industrial Toys]

I would install Diodes on all coils, relays or otherwise. However, it does add one more thing that can go wrong and they always short closed. I can't think how many protection diodes I have replaced in my lifetime. A lot! Then it often destroys the driving transistor or chip as well.

If the coil is driven from an electronic switch, you probably do need a diode to protect the semiconductor junction. But if it is switched through a mechanical switch or smaller relay, where can the back EMF go when the switch opens?

If you want to experience the back EMF. Get a small relay and attach, then remove the leads from a battery while still holding on to the leads. It is rather unpleasant.

 

[bigdeano]

A capacitor should also work to prevent switch arcing, like is used in an old points ignition system.

 

[Industrial Toys]

Not sure, but I guess the condenser on an old ignition was probably designed to charge and then discharge creating the collapsing field of the primary coil thus creating the HV in the secondary.

 

[bigdeano]

Not sure, but I guess the condenser on an old ignition was probably designed to charge and then discharge creating the collapsing field of the primary coil thus creating the HV in the secondary.

It's to absorb the high voltage spike from the collapsing magnetic field in the coil and prevent point arcing until the points are open far enough to interrupt the current flow. Without the condenser there's a big arc and the coil energy is dissipated in the arc instead of in the spark plug.

 

[Industrial Toys]

So what happens when the cap fails? Does the motor get any spark at all?

 

[BukitCase]

Yeah, for about the nextt 3-4 MINUTES, poorly, til the points finish frying - you could always tell if your "condenser" was off value a bit - one way and the points would pit to one side, off value the OTHER way and they would pit to the OTHER side - just plain off (bad cap) and they'd crap out in just a few minutes... Steve

 

[k0ua]

If you want to experience the back EMF. Get a small relay and attach, then remove the leads from a battery while still holding on to the leads. It is rather unpleasant.

I learned that at a rather young age. But I did not understand what had happened until much later study. It did leave a "lasting impression"

 

[Texasmark]

I learned that at a rather young age. But I did not understand what had happened until much later study. It did leave a "lasting impression"

When conducting, the energy stored in the inductor is one half L (inductance of the inductor) x [I (current flowing at the time) squared]. The nature of the inductor (magnetic device) is opposition to a change in current flow. When you open the circuit, as you did, the inductor does all it can to keep the current flowing by increasing the terminal voltage to the limit of it's capabilities. So in theory the voltage could rise to infinity and the whack you got was determined by how much energy was stored in the solenoid and your bodies resistance. On a large inductance part running at a high current, the transient could kill you......knock your ticker off it's clock, or deliver a fatal burn.

 

[k0ua]

When conducting, the energy stored in the inductor is one half L (inductance of the inductor) x [I (current flowing at the time) squared]. The nature of the inductor (magnetic device) is opposition to a change in current flow. When you open the circuit, as you did, the inductor does all it can to keep the current flowing by increasing the terminal voltage to the limit of it's capabilities. So in theory the voltage could rise to infinity and the whack you got was determined by how much energy was stored in the solenoid and your bodies resistance. On a large inductance part running at a high current, the transient could kill you......knock your ticker off it's clock, or deliver a fatal burn.

It is an eye opener for sure..

 

[Industrial Toys]

Good to know about the old ignition.

So (silly me) has to wonder, if you don't have a diode and not a set of fingers to go through, what happens to that energy?

I too, learned that unpleasant shock from my 150 in one electronics kit.