Safety device interaction???

[Rocketman]

Here's a question for some of the engineers here.

What is the effect of a collapsible spring with a switching regulator in a mod?

As the spring contact resistance goes up from heat what is the effect on regulator operation?

Could there be a point in the operation that source impedance (cell + contact resistance) that drops regulator output, cooling the spring contact, and generating a damaging oscillation?

The collapse of the spring is gradual, right? The first effect is increased resistance at the contact point. Might be a good thing with a straight thru mod with a switch. But what about a regulator that depends on a low and stable source impedance?

 

[CraigHB]

I haven't used a spring like that so I wouldn't know how much resistance they have, but you could measure it yourself. Just measure the voltage across it under load then divide out the current. I'd be surprised if it's over a tenth of an ohm, at least it shouldn't be.

There shouldn't be anywhere near enough input impedance to cause the regulator to malfunction. If that were the case, there'd be a lot of wasted power and heat. You'd be dumping all that power across whatever is causing the high input impedance instead of delivering it to the load.

Protected cells will limit current regardless of what is connected to them so the spring wouldn't collapse due to something like an atomizer short in the first place.

For a buck regulator and standard regulator with unprotected cells, the regulator normally limits input current so the spring would not collapse due to an atomizer short.

For an unregulated mod or boost regulator with unprotected cells, the spring will cycle as it makes and breaks contact (assuming the short persists), but you would know something is wrong since the device would not be functional. It would still keep the cell from going up in flames.

 

[Str8V8ping]

You could use a PTC fuse

 

[Rocketman]

I was just asking because it seems springs have been burning little holes in the protection boards of cells in some high powered flashlights. Usually associated with a LED module failure.

 

[MadmanMacguyver]

I gave up on fancy or maybe it should be simple saft springs when one failed my testing phase on one of my mods...anything I make must be SAFE not just mostly safe...I use Protection boards and replaceable fuses now...occasionally when space is at a premium a pair of PTC fuses but my preference is for minimal resistance so I tend to keep to mini blades when space is available for the connectors...

 

[CraigHB]

I was just asking because it seems springs have been burning little holes in the protection boards of cells in some high powered flashlights. Usually associated with a LED module failure.

Yes, that would be a problem, but a protected cell should never allow enough current flow to heat up the spring. Selecting a spring with resistance that high is a design blunder. First, you don't particularly need a fusable spring with a protected cell (unless you just want a backup) and second, the spring should not collapse under currents the cell can tolerate. You're blowing off a lot of power in the spring if it's heating up under normal operating currents.

I agree about PTC fuses. They really are the best solution for primary or secondary protection. I back up my over-current protection with my LiPo powered boosters. I have two levels of over-current detection (hard and soft short) plus a PTC fuse as a secondary.