How fast does a zener react ? - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: February 19, 2014, 12:44:28 pm
content: I'm putting a zener pre-regulator circuit in front of my voltage reg's to ensure that any high voltages do not reach the regulator. But this is automotive with fast and high voltage spikes (I am using a transorb too), is there any risk that a zener does not react fast enough ? My plan is that first the transorb will react quickly to fast rising spikes but because it could clamp at up to 70+V (33V rated) I need something else to help protect the regulator so I have put a resistor followed by a zener.

author: grumpydoc
author_rank: Super Contributor
timestamp: February 19, 2014, 12:53:51 pm
content: You can get MOVs down as low as 20 or 30V, wouldn't they be better (being designed for the job etc)?

author: Simon
author_rank: Unknown
timestamp: February 19, 2014, 01:35:42 pm
content: well I'm using one of those too but like I said a 33V one will hold at up to 70V so i need something else.

author: free_electron
author_rank: Super Contributor
timestamp: February 19, 2014, 01:41:34 pm
content: Zeners are slow, unless you have them already in conduction.

author: oldway
author_rank: Super Contributor
timestamp: February 19, 2014, 01:42:25 pm
content: If you have a zener pre-regulator, you certainly have also a serie resistance in the dc bus to limit the current in the zener diode.If you add a capacitor in parallel with the zener diode (let's say for example a 0.47µF 100V, you will "smooth" the high voltage transients and it will be safe. (RC filter)Transorb is not necessary.

author: Simon
author_rank: Unknown
timestamp: February 19, 2014, 01:52:01 pm
content: I have that but a 100V fast rising voltage dump wants to go somewhere, there is a reason why we use transorbs.......

author: madires
author_rank: Super Contributor
timestamp: February 19, 2014, 02:03:27 pm
content: What about combining a TVS and a MOV?

author: luky315
author_rank: Regular Contributor
timestamp: February 19, 2014, 02:04:50 pm
content: Use the right Transient voltage suppression diodes (TVS, TRANSZORB, ...) They are intended for this purpose. Zeners are way to slow.Unfortunately, MOVs have problems with higher temperatures.

author: Simon
author_rank: Unknown
timestamp: February 19, 2014, 02:06:13 pm
content: Well actually I'm not sure on the difference, it's an area I need to look into, it doesn't help that others that probably have as little knowledge as me in this area mix all the names up to suit

author: madires
author_rank: Super Contributor
timestamp: February 19, 2014, 02:27:33 pm
content: The TVS is quite fast and the MOV is a little bit slower but able to take more pain.

author: free_electron
author_rank: Super Contributor
timestamp: February 19, 2014, 03:15:35 pm
content: put a choke after the transorb.in---tranzorb to ground---choke--get one of those drum core types. that will block the pulse

author: Marco
author_rank: Super Contributor
timestamp: February 19, 2014, 04:26:22 pm
content: Quote from: Simon on February 19, 2014, 12:44:28 pmHow do you get a 33V breakdown voltage TVS to 70 Volt???BTW, at these voltages a "zener" diode is simply an avalanche diode, just like the transorb/TVS.

author: luky315
author_rank: Regular Contributor
timestamp: February 19, 2014, 04:31:50 pm
content: The physical effects behind Zener and TVS are basically the same, but thery are optimized for different things:Zener can work regulary in the breakdown / avalance region and are more precise.TVS should "avalanche" only if there is a fault, but they are way faster then Zener.A HF cap with high voltage rating directly after the input can (!) help to short fast transients directly to GND.

author: Simon
author_rank: Unknown
timestamp: February 19, 2014, 04:41:58 pm
content: Quote from: Marco on February 19, 2014, 04:26:22 pmIt was not a TVS I was looking at but in any case look at a datasheet for a TVS even a 33V one will "clamp" at over 40V it is because of the sheer power.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2014, 05:47:56 pm
content: These are all the same things, on a fundamental level. Zener diodes avalanche (they're technically avalanche diodes above 6V), which is a process that occurs as soon as the voltage reaches the junction. MOVs exhibit breakdown between semiconductor grains; same thing.The main difference is, MOVs are cheap and massive (many joules of energy withstanding), diodes are more compact and have lower internal resistance (the clamping voltage is closer to the nominal voltage).In either case, the clamping voltage will be about double the nominal voltage, so your circuit must still withstand that, or be able to filter it out.If you need to be absolutely sure, multiple stages are certainly an option. Some series resistance, an LC filter, then an MOV, will take the edge off massive pulses; this followed by more series resistance and LC filtering, and a TVS diode, further carves down the pulse to very reasonable levels. Best of both worlds. Note the explicit use of series resistance, which might even be implied rather than explicit (e.g., wire and PCB trace resistance). A dumb series resistor can do a lot to take the edge off a high-current spike.As for "as soon as the voltage reaches the junction", response is limited by lead inductance. A through-hole device will clamp more poorly than an SMT device. MOVs have high capacitance, so they'll respond slower than TVSs, comparing with equal lead inductances. Poor layout (e.g., routing power to the circuit first, then to the TVS as an afterthought) will have even worse performance.Filtering before and after the clamping elements is of use particularly on fast transients. Use series inductance -- air-cored inductance that doesn't saturate, mind: ferrite beads are useless on power rails! Low inductance ceramic caps are good as well, but beware their voltage dependency: as the voltage spikes past nominal, the capacitance drops sharply. A typical 1uF 50V 0805 size X7R ceramic is nominal (say +/-20%) only up to 15V! At its 50V rating, it's not even 0.2uF, and your filtering will suffer accordingly.Always choose the protection device for the lowest current or voltage rating that meets your input spec. A circuit that draws 1A should be fused at no more than 2A. A circuit that's specified for 18V maximum should have an 18V TVS on it (which will clamp to maybe 30V, so the components should be chosen to withstand more than 40V).Tim

author: edmundopt
author_rank: Regular Contributor
timestamp: February 19, 2014, 07:18:44 pm
content: I would sugest some reading, such as Littelfuse Application Note AN9768!Now, specific for automotive, you have the ST TA0338 technical article.It will be hard for you to have any issues after reading those.

author: PChi
author_rank: Frequent Contributor
timestamp: February 19, 2014, 08:34:00 pm
content: I have used small surface mount zeners that were quite fast enough to absorb ESD spikes. I have also seen larger (1 Watt I think) devices clamp an inductive load very quickly. What they didn't like was the massive, many amps current, large peak dissipation which appeared to cause a large drop in clamping voltage, rather like some secondary breakdown effect. They didn't last long either despite not getting hot.I think that zeners act very quickly but standard parts don't behave well and don't survive so best to use the correctly rated devices.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2014, 08:48:05 pm
content: More reading:http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDFIncludes some characteristics of common zener diodes (1N47xx, etc.) in ESD/TVS applications.Tim

author: edmundopt
author_rank: Regular Contributor
timestamp: February 19, 2014, 09:41:55 pm
content: Quote from: T3sl4co1l on February 19, 2014, 08:48:05 pmthat is a lot of reading