Limits to high voltage varistors in series? - Page 1
Forum section: electronics / projects

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 06, 2016, 05:31:33 am
content: Im in need of protecting some semiconductors for short spikes as the line voltage nears 10-12kV, the semiconductors will handle up to 14kV.For various reasons Id like to use a bunch of 1kV varistors in series, and my question is, how do these behave when arranged in series?These semiconductors handle very low current (in the mA range), so there is a resistor in series to limit any overcurrents. The string of varistors will be placed after the resistor to take advantage of it

author: Marco
author_rank: Super Contributor
timestamp: July 06, 2016, 08:00:34 am
content: So you want to clamp spikes at 14 kV ... with a working voltage up to 12 kV? Varistors don't do that, their transition isn't sharp enough. The transition of TVS diodes isn't sharp enough either.

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 06, 2016, 08:21:31 am
content: Nope, I said I want to clamp at 10-12 with maximum allowable of 14kv for the semiconductors and a working of about 8kV peak.Heres the V-I curves for some Littlefuse varistors, when voltage doubles, current increases by a 10^4 to 10^5 factor. Given that I have an impedance at the input of the circuit I can create a larger voltage drop at the output.Anyway, what I was asking is how 10 varistors in series will behave. I'll guess I'll have to make some tests and try.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2016, 09:38:19 am
content: Should be fine.Arrestors for power company lines -- medium voltage -- are stacks of disks. Of course they're probably made for the purpose, maybe matched or selected in some way? But I have no idea.If you don't expect large energy surges, such that any single component will fail, then you should be set.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 06, 2016, 11:23:29 am
content: Quote from: MasterTech on July 06, 2016, 05:31:33 amYes, you can wire MOVs in series, but that might not be the best approach depending on what you are trying to do and where this device is connected to the mains. One alternative to a stack of MOVs is a single MOV rated for nominal line voltage in series with a GDT (gas discharge tube) arrestor with a much higher breakdown voltage (Littelfuse makes small, board mounted, axial-leaded types that breakdown around 8.5kV). The MOV provides more consistent/predictable breakdown and blocking voltages while the GDT provides much higher standoff and peak power ratings.All that said, you will need to use very generous spacings (creepage, clearance, air gap) and multiple current-limiting resistors in series to withstand 10kV+ surges, regardless of the type of arrestor employed.

author: Marco
author_rank: Super Contributor
timestamp: July 06, 2016, 11:36:27 am
content: Alternatively try ~17x P6KE600CA, sharper transition than MOVs.

author: David Hess
author_rank: Super Contributor
timestamp: July 06, 2016, 03:16:14 pm
content: If they were graded first for breakdown voltage, would several avalanche rated rectifiers in series work?

author: MosherIV
author_rank: Super Contributor
timestamp: July 06, 2016, 03:54:47 pm
content: HiI do not want to hijack this thread because I have a questions about MOVs but not to do with the OP.Can some one answer (some of) my questions in this thread please?https://www.eevblog.com/forum/testgear/varistormov-for-ut61e/Thanks

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 06, 2016, 04:52:05 pm
content: Quote from: David Hess on July 06, 2016, 03:16:14 pmThis is actually perfectly acceptable practice and you don't even need to grade the diodes for breakdown voltage (that's only necessary if connected in parallel).

author: Marco
author_rank: Super Contributor
timestamp: July 06, 2016, 07:08:51 pm
content: Quote from: David Hess on July 06, 2016, 03:16:14 pmHell you could use MOSFETs if you really wanted ... I don't see why you wouldn't use TVS though if you want semiconductors, they are avalanche diodes.

author: calexanian
author_rank: Super Contributor
timestamp: July 06, 2016, 10:39:02 pm
content: You are squarely in the range of Spark gap and discharge tubes. There are also large MOV's for lightning protection on high voltage lines, but they are very large and expensive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 07, 2016, 03:47:02 am
content: The difference is: TVSs are rated for lots of peak power.Most devices are rated for some amount of avalanche, but only as a last resort. MOSFETs get saturated with charge carriers and keep conducting for hundreds of nanoseconds (something like reverse recovery, but following avalanche).Diodes don't share evenly over the junction, and fail at much lower peak power levels. (In the olden days, impurities -- which cause very sharp points of breakdown -- were common, and even every-day rectifiers were ESD sensitive!)Schottkys are a particular pain: the few that are rated for it, are rated for about 1mJ. The schottky junction itself doesn't avalanche. The PN guard rings, that encircle the schottky region, do that. There's... just not much of 'em.Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 07, 2016, 07:44:34 am
content: I was thinking avalanche rated rectifiers because they are available with much higher breakdown voltages so fewer would be needed but since their breakdown voltages are not tightly controlled, they might need to be graded.Avalanche rated rectifiers differ from normal rectifiers in that their junction is processed to create uniformity so that the entire junction breaks down at once. This results in a sharp knee versus the soft knee of a normal rectifier where different areas of the junction break down at different voltages concentrating power dissipation in small areas. The situation reminds me of secondary breakdown in bipolar transistors.I have never seen avalanche rated rectifier used for surge protection but as far as I know, semiconductor TVSes are basically the same thing with tightly controlled breakdown voltages. They are good for the usual applications like inductive load switching and series connected diodes strings for high voltage rectification.They are also available with massive breakdown power ratings but I suspect junction capacitance would be a problem with large devices.I was just throwing this out there as an idea. I considered several other alternatives and this or a series string of TVSes is what I would try first.

author: Marco
author_rank: Super Contributor
timestamp: July 07, 2016, 08:29:25 am
content: Quote from: David Hess on July 07, 2016, 07:44:34 amThey are usually stacks of 1kV diodes in disguise though right? A 9 kV microwave oven diode seems almost exactly in the right area, both in working voltage and breakdown.

author: David Hess
author_rank: Super Contributor
timestamp: July 07, 2016, 09:33:27 am
content: Quote from: Marco on July 07, 2016, 08:29:25 amThey can be starting somewhere above 1000 to 1500 volts but TVSes, or at least the ones I can find, only go up to about 1/2 or 1/3rd of that.A 9 kilovolt microwave oven diode may be a good example however it is not the type of device I would look for.