Electronics > Projects, Designs, and Technical Stuff

resistor wattage for hotplugging snubber?

<< < (4/5) > >>

k8943:
One way or another, bringing the conversation to the matter or current did help clarify what had to be learnt and have now verified that a 1ohm / 4.7uF combination indeed will work quite well. (Have also in the meantime load/heat tested electrolytics and a bunch of other things!)

Thanks a lot everyone.

k8943:
With hindsight found this quite interesting. Chipped a way a little and small update.

Tried 1ohm 0603 resistor in the snubber it became 4ohm! However two 2ohm resistors in parallel took the load. (Tested hotswapping with linear bench supply set to 24v, since it seemed to give more violent peaks than switched supply and 28AWG wiring used within project.)

Used 1206 1ohm resistors in testing, then looked more closely at the pulse curves in the post at beginning of thread and realised 2 * 0603 resistors might give better pulse resistance and work out cheaper.

Discovered a 0805 4.7uF ceramic had about the same snubbing effect (with 1ohm) as 3 * 2.2uF 0603 (6.6uF) caps, but 1206 4.7uF cap was way better than both. Size matters?

Preferred recipe therefore 1206 4.7uF ceramic and 2 * 2ohm 0603 resistors.

Capacitor 1276-2789-1-ND is 9.5 cents per thousand.

Then went off in another direction, what about a zener?

For example Digikey: SMBJ24D-M3/IGICT-ND

Voltage breakdown occurs at 27v and the cost per 1000 is 11 cents. Can handle "600W" pulses.

If, say, a pulse ran to 40v and the voltage drop across the diode is (not actually sure from datasheet?) 25v, that leaves 15v that needs to go somewhere. A 10ohm resistor in series => 1.5A current (not really sure how to size this). This is so massively with specs maybe a smaller zener would handle it? However didn’t see an obvious cost saving insodoing.

Do zeners get used for this sort of thing?

Siwastaja:
Bigger ceramic is better because it has closer to the rated capacitance even at high DC bias voltage. The nonlinearity of a small ceramic amplifies the overshoot effect. In a snubber, it's less effective.

Zeners do work and are used, but the curve isn't very steep, and there is unit variation so that you are limited to reducing the overshoot to about 30-40%, of course a great improvement if you have 100% overshoot in beginning. With an RC circuit, you can get to almost 0% overshoot, and this works over a wider voltage range. But if your aim is to protect the semiconductors, a zener does great job, and you can be allowed to ring more at lower input voltage, since only the worst case matters.

Combining both approaches can be very good.

k8943:
Thanks. See the advantages of the RC solution better now with context.

Thinking that the "curve" you refer to is the line between the "minimum breakdown voltage" (which is fairly well defined) and the "maximum clamping voltage" (which is indeed terrifyingly high). Sort of see that if the series resistor was chosen to limit the maximum (pulse) current through the zener in such a way that the maximum voltage across it was much lower (than the max), then the voltage across the resistor will be significant enough such that the total voltage drop is nonetheless high. RC it is!

T3sl4co1l:
...But, the idea of using an NTC here is still an option, no?

May not be practical, simply because the amount of power draw is so low that it won't heat up appreciably.  Well, if that's the case, it won't be wasting much, either, right? :-//

In that case of course, an ordinary resistor would do as well.  Take for example the 4.7 to 10 ohm power resistor commonly seen in fluorescent/LED lamps: this serves as inrush limiting as well as fusing, and has a small impact on efficiency.

- The relative merits of electrolytic and ceramic have been mentioned, so I won't go into further detail.

- TVS is good for this sort of thing, but also as mentioned, it's unlikely to be quite good enough, with so little margin available.  For a 24V supply, I would pick at least a 36V converter, preferably more.  The TVS would be "24V" rated if it's 24V nominal (say from a regulated supply), or 28 or 33V or more if a linear supply or battery is involved, and the peak surge voltage will be in the 40-50V range, hence the higher converter rating.  28V is too low though.

- The last option, that hasn't been mentioned yet: extend the operating voltage, and deal with surge in whatever way is necessary.  This is impractical for heavy loads, but the light load in question here is quite easily handled, say with a DN2625 or similar depletion FET, in a zener-reference-pass-regulator configuration.  (The DMOS with G-D pullup resistor has low voltage drop, until gate voltage is limited by the zener; the high drain voltage rating allows quite loose protection to be used, even a MOV.)

I did this for an automotive application last year; the device drew maybe 20mA at 28V, and withstood 60V continuous and 100V load dump without a flinch.  (The FET limiter was set to kick in at 40V.)

Tim

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod