Forums

Dealing with mains power transients

Started by Sylvia Else July 7, 2016
I'm looking at protecting a power sense circuit from mains power 
transients, using a resistor and a zener diode.

Spice says it works well, but when the zener conducts, essentially the 
entire transient voltage appears across the resistor. A high voltage 
transient spike is likely to be very narrow, but I can find nothing to 
say I can put, say, 5000V, across a 1K resistor, even for a microsecond. 
Indeed, it seems entirely possible that a current would flow across the 
outside of it, rather defeating the point of having the resistor there.

Any thoughts?

Sylvia.




On Thu, 07 Jul 2016 13:05:09 +1000, Sylvia Else wrote:

> I'm looking at protecting a power sense circuit from mains power > transients, using a resistor and a zener diode. > > Spice says it works well, but when the zener conducts, essentially the > entire transient voltage appears across the resistor. A high voltage > transient spike is likely to be very narrow, but I can find nothing to > say I can put, say, 5000V, across a 1K resistor, even for a microsecond. > Indeed, it seems entirely possible that a current would flow across the > outside of it, rather defeating the point of having the resistor there.
Some resistors come with pulse ratings. There's a power/time relationship that's not just as simple as dumping a constant energy pulse of any width into the thing, so you want to specify a resistor that comes with real live charts on the data sheet. I probably don't know the whole story, but I would assume that the voltage rating on the same series' high-resistance parts would apply for transients on the lower-resistance parts. -- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
How about a very small spark gap ? 

Also, what about the Zener diode, isn't it in peril here ? 
On 7/07/2016 1:25 PM, jurb6006@gmail.com wrote:
> How about a very small spark gap ? > > Also, what about the Zener diode, isn't it in peril here ? >
It just draws enough current through the resistor for the latter to drop the transient, so I don't think it's in danger. Sylvia.
On Thu, 7 Jul 2016 13:05:09 +1000, Sylvia Else
<sylvia@not.at.this.address> wrote:

>I'm looking at protecting a power sense circuit from mains power >transients, using a resistor and a zener diode. > >Spice says it works well, but when the zener conducts, essentially the >entire transient voltage appears across the resistor. A high voltage >transient spike is likely to be very narrow, but I can find nothing to >say I can put, say, 5000V, across a 1K resistor, even for a microsecond. >Indeed, it seems entirely possible that a current would flow across the >outside of it, rather defeating the point of having the resistor there. > >Any thoughts? > >Sylvia. > > >
I believe a residential service entrance will only see about 3kv if there is a fault. Maybe the NEC has the details. Cheers
On Thu, 7 Jul 2016 13:05:09 +1000, Sylvia Else
<sylvia@not.at.this.address> wrote:

>I'm looking at protecting a power sense circuit from mains power >transients, using a resistor and a zener diode. > >Spice says it works well, but when the zener conducts, essentially the >entire transient voltage appears across the resistor. A high voltage >transient spike is likely to be very narrow, but I can find nothing to >say I can put, say, 5000V, across a 1K resistor, even for a microsecond. >Indeed, it seems entirely possible that a current would flow across the >outside of it, rather defeating the point of having the resistor there. > >Any thoughts? > >Sylvia. > > >
You can get axial-lead high-voltage resistors into the 10s of kilovolts. And there are surface-mount resistors in the 3 KV range. Regular 1206 resistors are OK to a couple of kilovolts (I have some test data around here somewhere) and you could series a few of them, which would be cheap. Do you expect 5KV transients? I guess lightning could do that. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Sylvia Else wrote:
> > I'm looking at protecting a power sense circuit from mains power > transients, using a resistor and a zener diode. > > Spice says it works well, but when the zener conducts, essentially the > entire transient voltage appears across the resistor. A high voltage > transient spike is likely to be very narrow, but I can find nothing to > say I can put, say, 5000V, across a 1K resistor, even for a microsecond. > Indeed, it seems entirely possible that a current would flow across the > outside of it, rather defeating the point of having the resistor there. > > Any thoughts? >
** High voltage spikes certainly can damage resistors, both solid composition and ones that use spirals to increase the length of the conductors. High voltage types for special purposes exist but the average film resistor is rated for no more than 500V. However, common neon indicators are used directly across 240V mains with internal film resistors of about 200kohm with 0.25W rating and typically last for decades. The failures seen are all with the resistor going open. Connecting identical resistors in series reduces the spike and continuous voltage seen by each to a much safer level. The best idea is to add a suitable varistor across the AC supply input to your circuit - so protecting everything beyond it. .... Phil
Just put 5 1206 smd resistors in series. Look up the datasheet, one resistor can handle 1kV peak voltage

So 5 will provide good robustness against surges

Regards 

Klaus 
"Tim Wescott" <tim@seemywebsite.com> wrote in message 
news:Xp6dnfzBzolXVODKnZ2dnUU7-UmdnZ2d@giganews.com...
> Some resistors come with pulse ratings. There's a power/time > relationship that's not just as simple as dumping a constant energy pulse > of any width into the thing, so you want to specify a resistor that comes > with real live charts on the data sheet.
Right. If you see one that gives a single pulse rating, you can probably-kinda-sorta-hand-wavey-extrapolate around it. The slope is usually a goofy function of time, like t^(-1/2) (which implies diffusion, a classic thermal effect), or t^(-1/3) (..?!). Rarely, it is inversely proportional, even to fairly small time scales; this is mostly confined to the "high pulse" (bulk / composition) types. Naturally: if the datasheet doesn't say explicitly so, you're on your own. A guess is just a guess... until you start getting field failures back (or not, as the case might be!). Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
"Martin Riddle" <martin_ridd@verizon.net> wrote in message 
news:vnjrnbhd0um8mkq0hc4l70hgljfsrphv4n@4ax.com...
> I believe a residential service entrance will only see about 3kv if > there is a fault. Maybe the NEC has the details.
About that. IEC 60950-1 says 2.5kV is okay (though, to meet that classification, you also need single failure redundancy -- so, say if connecting resistors in series: use N+1 of them!). What you're actually getting is: statistics. Transients over 2.5kV are very rare (decadal, perhaps?), so it's useless building most equipment to handle such events. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com