Voltage clamp for IECEx compliance - Page 1
Forum section: electronics / projects

author: pj
author_rank: Contributor
timestamp: August 11, 2025, 02:02:20 am
content: Hi all,I have a battery powered device that I'm looking to get IECEx certified, one of the parts I need to design is a voltage clamp. It needs to clamp somewhere in the range from 5V to 7.5V.I'm really struggling with this and would appreciate any design tips.The parameters I'm working with are:- Device current is 500mA max - According to the standard this then becomes 0.5A x 1.7 x 1.5 = 1.275A when the safety factors are included- Power 1.275A is 16.3W @ 5V to 24.4W @ 7.5V- Temperature derating to 50cOn of the issues I'm having is the zener diodes tend to go from 5W directly to 50W (stud mount), there does not seem to be any diodes available in the 10-20W range. I can chain two zener diodes together however the lower voltage zeneres tend to have significant reverse leakage currents.Any thoughts?There is a fallback where I can build a semiconductor controlled voltage limit circuit but I would prefer a passive solution if it can be achieved.For reference these are the allowable solutions from the standard.ThanksQuote

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 11, 2025, 11:21:07 am
content: Quote from: pj on August 11, 2025, 02:02:20 amWhat's wrong with just using five 5.1V, 5W zeners in parallel?

author: tszaboo
author_rank: Super Contributor
timestamp: August 11, 2025, 11:28:37 am
content: I assume you want to build a safety barrier and use a fuse (1.7 rating)Which standard are you using? IEC 60079-11:2023?

author: pj
author_rank: Contributor
timestamp: August 11, 2025, 02:34:41 pm
content: Quote from: mtwieg on August 11, 2025, 11:21:07 amUnfortunately they won't accept items in parallel because with manufacturing differences etc it's not guaranteed they will all switch on at the same time.Quote from: tszaboo on August 11, 2025, 11:28:37 amYes that is correct, 500mA fuse with safety factors applied x1.5 and x1.7.I'm using the latest version, IEC 60079-11:2023The best option I have so far is two chains of 7 diodes in forward position, this will block until the voltage is over 6V. Power is shared over 7 diodes easily. The actual voltage swings a lot with temperature but I think that's fine as under normal conditions the circuit will be running at 3.3V.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 11, 2025, 02:47:10 pm
content: Quote from: pj on August 11, 2025, 02:34:41 pmI'm not familiar with the standard, but this on its face is silly. They don't have to "switch" at exactly the same time/voltage. They just have to share power enough for any one to avoid overstress.Though you would want to make sure the parallel zeners have positive temperature coefficients (might need to use a higher voltage like 5.6-6.2V).If parallel zeners are strictly not allowed and you need a sharper "knee" than series diodes provides, then you'll have to use a simple active circuit. A power BJT controlled by a TL431 is a common implementation.

author: tszaboo
author_rank: Super Contributor
timestamp: August 11, 2025, 02:51:11 pm
content: It's difficult to find zeners that fulfill these requirements. Because even if they fulfill the power rating, then you need to keep the temperature down to avoid thermal ignition.If you are using these as fault condition voltage limiters, so they don't conduct in normal operation, then a crowbar circuit is easier. Otherwise maybe limit the current and power with extra resistor in series with the fuse.Also, if you distribute power across multiple parts, usually this means that you need to verify the temperature of everything in worst case conditions, according to section 9.3. I've just noticed that compared to the last standard, they don't ask for 30 minute stabilization of the temperature in this standard, which is nice.Usually, if you can, the easiest solution is to limit the current as much as possible. Measure the current of your device and select a fuse based on that. Or rewrite firmware to use as little peak power as possible.Quote from: mtwieg on August 11, 2025, 02:47:10 pmThe standard doesn't forbid doing this. But you need to be able to prove beyond doubt, that with worst case tolerances you fulfill all the safety margins, that can be a whole ordeal itself.

author: pj
author_rank: Contributor
timestamp: August 11, 2025, 03:13:47 pm
content: Quote from: mtwieg on August 11, 2025, 02:47:10 pmThanks yes in the real world this would be perfectly acceptable but the standard is very strict and sometimes the interpretation does seem ridiculous. I do have an active circuit I can fall back on if I can't make this work.Quote from: tszaboo on August 11, 2025, 02:51:11 pmYes the zeners are very hard to find and I have discovered that the low voltage ones have significant leakage. Thanks I'll have a look at crowbars, trying to avoid the resistor in the main power path as it will drop the voltage and burn power. I have a cellular modem so I do need some power peaks of around 400ma but maybe a slo blo fuse could handle these pulses as they are only short.

author: cburgess
author_rank: Contributor
timestamp: August 11, 2025, 08:00:48 pm
content: A few quick notes:Will be battery be part of the equipment in the explosive atmosphere? - if so:Are you sure you need a triplicated zener clamp? ( I would expect this in "associated apparatus" )See the table in 60079:0 for maximum open-circuit cell voltages for different chemistries.If the zener diodes are on the output side of a DC-DC converter consider energy limiting the input side of the converter.The minimum battery internal resistance and fuse cold resistance can be used in your short-circuit current calculationsBattery internal resistance can also be your friend when it comes to dissipation calculations (matched power)Don't assume a 5W Zener will give you 5W of "usable dissipation" - surface temperature will most likely be the limiting factor.Don't forget to allow for component tolerances - that 5V 5% Zener needs to be 5.25V for your calculations - it all adds up...It's hard to avoid infallible current limiting resistors somewhere in your circuit - even though they are wasteful of power.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 12, 2025, 07:32:17 am
content: To protect the over voltage devices, be they Zeners, MOV, TVS or whotnot, a ptc fuse or ptc fuses in the line seems like a good idea.You can "amplify" a zener to get the power rating you need. A power transistor does the heavy lifting until the PTC fuse kicks in.Use Darlingtons and low power Zeners maybe.SCR or Triac crowbar for fast protection.A PTC or PTCs can be chosen to set a current low enough for the crowbar element to drop out pending fuse reset.

author: tszaboo
author_rank: Super Contributor
timestamp: August 12, 2025, 09:06:01 am
content: Quote from: pj on August 11, 2025, 03:13:47 pmHave you already done intrinsically safe designs? If not, I have to warn you it's not easy and straightforward to do them.You can skip over one part of the standard, and not realize it's importance. So you finish your design, submit to certification, and the review comes back with something critical. It can be absolutely design critical, something that means you need to practically restart your PCB design, or maybe your entire project. And once you do that, you are months behind schedule.My point is that it's a lot cheaper to hire an expert consultant for the project, someone who have done this before. Also, don't underestimate the documentation. About 70% of the effort in the project goes into that.Also, if you don't already have this: Have the specification of the hazardous location in writing. Signed by your supervisor. With date. Because it happens way to often that they realize it also needs to have dust protection as well. Or it's a Zone 0 application instead of Zone 1. When change like this happens, you might as well throw out all the work you did.

author: pj
author_rank: Contributor
timestamp: August 12, 2025, 11:56:12 am
content: Quote from: tszaboo on August 12, 2025, 09:06:01 amThanks this is great advice and yes I have been on the roller coaster for a few years now. I'm not a big company, just a one man band doing some hazardous area IOT on the side.I have 2 products certified now. The first one took me two years, I had to restart many times and at the very end I had to drop some features because there were last minute issues and I just needed to get it across the line.6 months after getting my product certified the satellite communication provider shut down and my device is now a partial paperweight.I'm having another go at it but this time I'm trying to protect myself against obsolescence. As you probably know there is only one company in the world that have a rechargeable lithium ion battery that can be recharged in zone 0. I used this battery in my first design so this time I'm replacing this with a LiFePO however this requires a boost to get up to 3.3V, which subsequently then requires a clamp/shunt.

author: pj
author_rank: Contributor
timestamp: August 12, 2025, 11:58:37 am
content: Quote from: cburgess on August 11, 2025, 08:00:48 pmThanks for the notes, i missed the 5% tolerance which will no doubt be required.In terms of battery I have a certified ex ia design already that has a solar panel + rechargeable lipo however I'm switching to a LiFePO battery and this is why I need the boost/clamp.

author: Whales
author_rank: Super Contributor
timestamp: August 12, 2025, 12:42:13 pm
content: QuoteDoes it specify a duration this protection needs to hold for? I hope not permanently, otherwise you'll need to strap a reasonable size heatsink to it.

author: pj
author_rank: Contributor
timestamp: August 12, 2025, 01:00:18 pm
content: Quote from: Whales on August 12, 2025, 12:42:13 pmYes it needs to handle steady state (permanent) of 9.5W (my original power calcs are wrong). This is what makes finding components so difficult. The 7 diodes works well because it gets split to 1.36W per diode which is achievable.

author: jkostb
author_rank: Regular Contributor
timestamp: August 12, 2025, 04:51:13 pm
content: Hi,According to the standard you DO NOT need to apply 1.5x on top of 1.7X the fuse current!So if you design the voltage safety shunt consisting of fuse and zener then this is the usual way:1) Select appropiate fuse to limit the current through zener diodes.2) Zener diodes needs to be able to handle 1.7*fuse current. So max power dissipation zener diode is Zener voltage (including tolerances)*1.7*fuse current3) Calculated power in step 2 must be 2/3 of maximum allowed power dissipation.4) Maximum allowed power dissipation is determined from max ambient temperature, thermal resistance and junction temperature zener diode.Hope this helps

author: tszaboo
author_rank: Super Contributor
timestamp: August 12, 2025, 08:50:49 pm
content: Quote from: jkostb on August 12, 2025, 04:51:13 pmCurrent x 1.7, then in step 3 you apply the 2/3 rating. 1/(2/3)=1.5So in a way, you do need to apply.Quote from: pj on August 12, 2025, 11:56:12 amWell congrats, it's quite an achievement. After having two products, you probably have enough experience to handle most of the procedures and what comes with it.

author: Whales
author_rank: Super Contributor
timestamp: August 12, 2025, 10:59:32 pm
content: Quote from: pj on August 12, 2025, 01:00:18 pmWoah.Apparently TO-220 TVS diodes are a thing, but they seem to have a useless at DC (according to their safe operating area or pulse width graphs).Your chain of diodes might be a good bet. You will want to choose and test them well to make sure their knee isn't too gradual. Depending on how much air exposure your device gets: don't be surprised if you need 5x5cm area of dedicated copper planes (or more) to keep their junctions cool. I don't know the form factor or housing of your device so I can't comment more, there are some PCB & junction temp approximation calculators on the web that might be useful.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 13, 2025, 01:54:31 pm
content: Quote from: pj on August 12, 2025, 11:56:12 amSounds like a very interesting project! I'm a bit confused about the numbers though... I thought LiFePO has a slightly lower cell voltage than Li-ion, so why would changing to LiFePO add a requirement to boost voltage? And how is a 5-7.5V clamp useful? Is it for the input of a step-down charger?

author: cburgess
author_rank: Contributor
timestamp: August 13, 2025, 09:44:45 pm
content: Have you considered putting the fuse on the input (battery) side of the the DC converter?Table 14, 60079:0 gives a maximum open circuit voltage of 3.6V for a LiFePO4 cell (carbon -ve electrode)This would give you (3.6V x 0.5A x 1.7 x 1.5 ) = 4.6W component rating to handle which is a bit more manageable.Even if you need to increase the fuse size to compensate for the additional current on the input it should be worth while - e.g 3.6 x 0.75A x 1.7 x 1.5 = 6.9WIt is also worth running the numbers with an infallible current limiting resistor on the input to the converter if you can tolerate a bit of inefficiency and volt-drop - it may be possible to lose the fuse altogether and that pesky 1.7 factor. For example a 1 ohm resistor gives you a matched power of 3.3W with the 3.6V cell.Note also for thermal assessment you don't need to apply the 1.5 safety factor - clause 5.2

author: pj
author_rank: Contributor
timestamp: August 14, 2025, 04:29:22 am
content: Quote from: mtwieg on August 13, 2025, 01:54:31 pmThe circuit requires 3.3V, the current lipo is 3.7V (charging 4.2V) so I'm using a step down. When I switch to LiFePO it will only provide 3.2V (charging 4.0V). I'll need a boost to get a steady 3.3V however under fault conditions, the boost converter is considered to produce an infinite voltage which needs to be clamped/shunted.Technically I could design for a lower voltage clamp however there are two factors I'm considering.I have a solar panel that can produce 7.5V, I don't want to design a clamp for 5V and then later on discover it needs to be 7.5V, covering worst case here.Because the clamp is designed for worst case scenario (1.2V vf) in the real world it will actually start clamping way before then, especially on warm days, once it starts clamping it will heat up reducing the clamp voltage even further.Quote from: cburgess on August 13, 2025, 09:44:45 pmYes I have a fuse there already for power limitation.The resistor is a good idea thanks, I just hate the concept of it, and I'm not sure how it will impact the function of the battery charging. I should run the numbers on it and not totally discount it. I imagine the boost will handle any drop in voltage.Quote from: Whales on August 12, 2025, 10:59:32 pmI found some that are SMD and can handle the power with standard footprints. The device is fully encapsulated so from memory the temperature rise is considered with all power sources combined, on the surface of the encapsulant/enclosure. Last time I had to make a sample device with embedded resistors, the lab applied the max power level until the surface temp stabilised.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 15, 2025, 12:38:37 pm
content: Quote from: pj on August 14, 2025, 04:29:22 amIf the clamp were solely due to concerns with the boost converter producing excess voltage, then I'd suggest adding additional OVP detection to the boost circuit (depends on the chip controlling the boost, of course). Alternatively, you could use a transformer-based buck topology like a push-pull or forward converter instead of a boost. These can output Vout>Vin (due to the transformer turns ratio), but their Vout is has a well-defined maximum, like a buck converter. Definitely more complex than the boost, but perhaps preferable to the clamp.QuoteOkay sounds like there are other power source involved... not sure I can give any more useful suggestions without knowing how they actually are used in the system, though.

author: tszaboo
author_rank: Super Contributor
timestamp: August 15, 2025, 01:03:55 pm
content: Quote from: pj on August 14, 2025, 04:29:22 amI know what you mean. There are have 95% efficient converters, and then we place a resistor before and or after them, absolutely destroying this efficiency. But you know, in the end the most important is to make something that is safe in these dangerous environments.Quote from: mtwieg on August 15, 2025, 12:38:37 pmThe standard doesn't specifically call for voltage limitations. But then without them, you can store energy in a capacitor for example, enough to set things on fire. So you limit the voltage, make the limiter redundant, depending on how safe it needs to be double redundant.

author: negativ3
author_rank: Regular Contributor
timestamp: August 15, 2025, 02:13:19 pm
content: Quote from: pj on August 12, 2025, 11:56:12 amA wellbore or wellhead monitor? Zone 0 that's a challenge.

author: pj
author_rank: Contributor
timestamp: August 17, 2025, 04:40:31 pm
content: Quote from: tszaboo on August 15, 2025, 01:03:55 pmYes this is true.Quote from: negativ3 on August 15, 2025, 02:13:19 pmSo it's a remote monitor for mobile helicopter fuel tanks (Jet-A1). These tanks get left in remote locations, offshore platforms, ships etc...I have a sensor that attaches to the face of the level gaugeThis attaches to the the solar powered telemetry box that sends back the level via satelliteInside the box is half encapsulated. All the pcb traces coming out of the encapsulant are protected with resistors.

author: floobydust
author_rank: Super Contributor
timestamp: August 17, 2025, 06:39:10 pm
content: I have done Zone 0 wellhead designs under 60079-11 that are similar - solar panel, battery, MCU monitor and cell modem.The company's original design when I came on board was to encapsulate the cell modem, which was a disaster. Any potting compound getting into the cellphone module would detune it. Manufacturing mistakenly connected the antenna MMCX to the GPS antenna port by accident. 100's of bricks were made. If you need to update the modem's F/W, you couldn't without access to all the modem's pins which were potted.The ambient temp requirement was -40°C to +60°C and the potting thermal expansion coefficient did shear parts off the PCB. It was overall hell.I managed to find a way to get rid of the potting compound entirely. It was not easy and I'd say this forum is incapable of discussions on it due to S/N ratio.The solar panel used was high voltage (21V) and a big resistor used to lower current for IS. Being up on a mast it was outside Zone 0 away from the wellhead but still piped in power there. The resistor pretty much killed off and wasted so much energy. Battery charging was pretty lean.I did not get to the next phase to use a buck-converter to drop it down to something easy for IS.A voltage-limiter was required at the battery and back of a napkin sketch reveals zeners are too much trouble, too big overall to dissipate that heat and meet the T-code.A colleague came up with a solid-state current-limiter and I had a good laugh when he ran it and the transistors got so hot, they desoldered and fell off the PCBIn the end I just used an SCR limiter and that works fine. Rarely it can false-trip and blow the fuse though, I think from lightning EMP on the solar lead-in cable, or maybe high dV/dt on power.A requirement in manufacturing was to test the SCR limiter, so I wrote test fixture S/W to do that without blowing the IS fuse.Other application - engineering for off-shore platforms was a lot of me signing off on things. Tremendous liability. Salt water corrodes carbon steel and other metals very fast so you have be extra careful about the hardware. Shutting down an offshore platform to do some work is pretty much impossible, they make $1,000's per minute some of those platform's wells. Going on a boat or helicopter trip to one is another gross experience because you are on a rotation with all the other crews.OP I can't make enough sense of your site's classification and overall requirements, to help much with design.Just saying, me using an SCR limiter saved a lot of cash$ in certification, it went quickly.

author: pj
author_rank: Contributor
timestamp: August 17, 2025, 08:09:00 pm
content: Quote from: floobydust on August 17, 2025, 06:39:10 pmI can totally relate to these experiences, I have so many bricks here with thousands of dollars of components in them (mainly modems). I can't bring myself to throw them out but it's not cost effective to try and dig them out of the potting. I had the same problem with shearing, the compound was heating up too quickly when curing and then shrinking back, pulling everything with it. The worst part was that I didn't know that was the problem, so some devices would just not work after potting, or worse they would fail randomly. It was only after I cut a box in half one day that I noticed it was happening.All my current boxes are partial paper weights now anyway because I went through certification based on using a Swarm modem however shortly after Elon bought out the company and shut it down.Thanks for the tip I will check out the SCR current limiter.

author: jonpaul
author_rank: Super Contributor
timestamp: August 17, 2025, 11:35:04 pm
content: We used a Motorola alternator load dump protector MR2525, MR2535 in 1980s for DO-160 Avaionocs bus protection.Check the TAZ Transient Absorbing Zener mfg for the voltage and application.http://www.microsemi.comAs dangerous vapors and fluids are involved with unlimited liability if your work can resut in an incident, I would strongly suggest you consult one of the many IECEx specialists rather than rely on a forum response.kind regards.JonLEGAL: THIS IS NOT TO BE CONSIDERED ADVISECONSULT A PROFESSIONAL