Zener failed short. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 27, 2020, 02:32:04 pm
content: Recently I implemented a zener diode and a fuse as over voltage protection. Unfortunately it failed short, and took out a L7812 linear positive voltage regulator and two TIP147 darlington transistors with spectacular results of magic smoke. The circuit survived but the regulator and transistors needed replacing. The offending zener diode has been removed, and confirmed short testing with a multimeter. I hadn't realised zener diodes fail as a dead short most of the time.So this is a 12 Volt power supply, I will add a picture of this PSU, just have to dig it out of the cupboard in a bit. I have two potential crowbar circuits instead (which I should have used from the outset) What I'm not sure of is which crowbar circuit to go with. Any tips on which circuit to go for would be great, thoughts on this appreciated. The PSU is 12.80 volts at a maximum of 10 Amps power rating. Intended application is HF Radio use. Thanks for looking in.

author: Gyro
author_rank: Super Contributor
timestamp: August 27, 2020, 05:51:36 pm
content: That's one of the endearing features of Zeners, they almost always fail short on overload (unless high enough to blow them apart), allowing them to punch well above their weight in overvoltage protection applications.

author: bob91343
author_rank: Super Contributor
timestamp: August 27, 2020, 10:32:53 pm
content: Why didn't the fuse open? I don't see it in the circuit.

author: MasterT
author_rank: Frequent Contributor
timestamp: August 27, 2020, 11:49:07 pm
content: I don't like a circuits, zenner is passing short non-limited current till SCR fires up, and it's very slow device. I'd put 470 OHm resistor in series, and get rid of capacitor that slow everything down even more.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 28, 2020, 12:51:02 am
content: Thanks for the repys, the schematics above are ones I've found in searches. If they post, the images below are of a 12 Volt 10 Amp power supply. It's a L7812 linear regulator and a pair of TIP36C power transistors. the toroid is rated at 15 Amps. I'm not sure there is room in there for a crowbar circuit, the fuse is 10 Amp ceramic type, it didn't blow when the zener diode went short. I'm just looking for some short circuit and over voltage protection.

author: MasterT
author_rank: Frequent Contributor
timestamp: August 28, 2020, 01:44:54 am
content: I find this for OVP topology:https://www.homemade-circuits.com/over-voltage-protection-circuit-for/and one more, combined OVP & OCP:https://www.mdpi.com/1424-8220/16/6/867/htmThough, BJT has to be replaced by MOSFET, properly sized for current.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 28, 2020, 02:35:14 am
content: Thank you for the schematic, i'm not 100% i could implement that on to copper strip board, maybe if i loo at it and break it up into manageable sections i could. In my psu above, i've quickly realized i am not going to get 10 Amps out of it. unloaded i get 12.90 Volts, 50 watt load 12.50 Volts, 100 watt load 11.50 Volts. My heatsink is not big enough, and i should have mounted the regulator on its own heatsink. Darlington transistors might keep the voltage up a bit better. Its only took me 2 years to finish this. I just need a simple over voltage circuit that can be laid out on strpboard. I increase current with 1mm copper wire soldered to the tracks that need high current ability. I wish i had room for pcb development, but i just don't have the space.

author: ledtester
author_rank: Super Contributor
timestamp: August 28, 2020, 04:11:00 am
content: What does your power supply circuit look like? Can you post a schematic?Here is one that simply adds a stage to the output of an existing supply. Don't know how well it will work:http://electronics-diy.com/lm317-overvoltage-protection.php

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 28, 2020, 07:53:50 am
content: Somebody told me that crowbar circuits are most reliablebut it seems not.For a voltage sensitive power supply (battery charger) I used a low-resistance Mosfet switch controlled by TL431 with added hysteresis (one more transistor). Most of time switch is closed, and it is opened rapidly with voltage higher than 16 V.Of cause I use protective TVS diodes or varistors at power supply output too, to protect it from high-voltage strikes when connection-disconnection occure.

author: exe
author_rank: Unknown
timestamp: August 28, 2020, 11:27:21 am
content: Why do you need a crowbar?

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 28, 2020, 01:32:39 pm
content: I wanted a crowbar circuit to protect against over voltage and short circuit scenario to the output. The circuit is really simple with two series pass transistors. These provide most of the output current above the 1 Amp the regulator provides. The only difference is a single silicone diode in the - Volts rail, pushing the output voltage to approximately 0.7 volts above the 12 Volts the regulator provides. So my unloaded output is approximately 12.80 volts according to the panel meter.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 28, 2020, 10:06:54 pm
content: I was going to start another thread, but the zener failing short seems to have done more damage than first thought. Now I'm getting 21 Volts on the output, I've change the regulator and transistors. And disconnected the panel meter to re test with minimal as possible to find this fault, I now suspect the zener going bad damaged a capacitor, maybe going short. I will have to remove the capacitors another day and see if there shorted. No matter what I tried I'm just getting the smoothed input voltage on the output.

author: ledtester
author_rank: Super Contributor
timestamp: August 29, 2020, 02:18:13 am
content: Were the zener and all of the other component failures due to an overvoltage condition? If so, how much overvoltage are we talking about?

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2020, 04:17:45 am
content: Quote from: MasterT on August 27, 2020, 11:49:07 pmExactly, the SCR gate just looks like a diode to the cathode so the current through the zener diode is essentially unlimited until the SCR turns on. In the example shown below from the Tektronix PS503A power supply, the zener diodes in the crowbar circuits have series resistors to limit the current.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 29, 2020, 01:29:14 pm
content: Completely forgot about a YouTube video I had watched ages ago now. It's quite a long video, but from what I can make out this guy has a usable circuit from this. He explains at the start about over voltage problems, worth a watch.The over voltage through my circuit was 21 Volts. But previous to this shorted zener, the only other thing I tried was a pair of TIP147 in place of the TIP36C transistors I started with, I thought the extra gain from the TIP147's would limit voltage sag under load. I will browse over the circuit again, but I can't seem to find the fault with the circuit.Help if I remembered to post the link doh..https://youtu.be/4qSKcUxwP4w

author: eblc1388
author_rank: Frequent Contributor
timestamp: August 29, 2020, 05:12:32 pm
content: It is very easy to add an SCR crowbar circuit to the power supply output.However, most do not consider what follows after the crowbar had operated, shunting the output to 0V. Now abnormally large current will be flowing. There must be other forms of protection in place or else the circuit will probably ends up in smoke or fire.Are you 100% sure that form of protection exists on your design? Have you tested it to make sure it will safely isolate your circuit after the crowbar had operated?

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2020, 05:43:38 pm
content: The other form of protection is some way to limit the short circuit current with the crowbar active. In the example I show above from the Tektronix PS503A power supply, the built in constant current limiting feedback loop limits the current to a maximum of about 1 amp, or about 0.5 amps if the power supply is installed in a low power mainframe, and if the pass transistor shorts, then a fuse on the input blows. Tektronix intended the SCR crowbar to protect against both pass transistor shorts, which would be typical of overheating, and user error, where the user sets the output voltage too high for the application; they included instructions for modifying the trip point of the SCR crowbar.Another common implementation places the SCR crowbar before the regulator which is why the regulator may need to be protected against reversed input to output voltage. This obviously relies on a fuse to limit the current.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 29, 2020, 07:42:26 pm
content: The PSU is not in use at the moment, I haven't put an SCR crowbar circuit together yet. I'm still trying to work out the over voltage shoot, protection so far is a 1 Amp fuse for the regulator, and 10 Amp fuse for the main output. Once I discover what's causing the over voltage, then I hope to implement some over voltage protection. As per my first post, one of those schematics. Thank you for the replys.

author: ledtester
author_rank: Super Contributor
timestamp: August 30, 2020, 03:35:55 am
content: Back to analyzing the initial failure...QuoteThe LM7812 is a pretty hardy device with lots of internal protection circuitry. Even the datasheet says:QuoteThe max DC input voltage is 35 V. So I'm wondering how the 7812 got cooked.The transistors you are using are pretty robust too, with V_CBO and V_CEO greater than 60 V. However the V_EBO is pretty low -- around 5V. It would be exceeded, for instance, if the base of the transistors got shorted ground somehow.Do you have an idea of how the failure occurred?

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 30, 2020, 10:15:44 pm
content: No unfortunately, only powered up, and both transistors let out the magic smoke. The regulator might not have failed, but I won't use it after an episode like that. Could have been a large transient, I've got no soft start, or other clamping components etc.

author: Ian.M
author_rank: Super Contributor
timestamp: August 30, 2020, 10:25:30 pm
content: What do you expect to happen at power-on with 470uF at the regulator input charging through the base-emitter junctions of the pass transistors? Obviously they'll fully saturate till the charging transient is over and kick up the output voltage to close to the input voltage. Even 100nF of capacitance there at the regulator input and pass transistor bases is pushing your luck.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 31, 2020, 02:26:48 am
content: the only capacitance i used was 20000uf for rectified filtering, and 4700uf at the final output. i didn't want to much voltage sag under sudden heavy loads.

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2020, 02:42:34 am
content: So why post a totally inaccurate schematic in your reply #11?

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 31, 2020, 12:44:25 pm
content: Because the circuit operation is exactly the same as the schematic I posted, I only changed some capacitance values. The zener is not in the schematic, no one complained about that addition. The circuit I have is essentially the shematic above in my first post. It's not very helpful trying to be justifiably correct. At worst I made an error by not mentioning the larger capacitance earlier. It's unlikely to have shorted the zener diode.

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2020, 05:26:26 pm
content: As we are trying to diagnose a transient failure during startup, and for transient behaviour capacitor values and locatuions are *IMPORTANT*, sayingQuote from: davelectronic on August 28, 2020, 01:32:39 pmwas really unhelpful and distracting. You swapped out the transistors for Darlingtons (which I admit you did disclose elsewhere), deleted three capacitors, added another and changed the value of the only remaining one without telling anyone about the capacitor changes till I called you out on your posted schematic's transient behaviour. No wonder you are getting your readers confused!Is this edit of your schematic, omitting transformer and bridge rectifier on the left, and Zener or intended crowbar circuit and load on the right 100% accurate? I doubt it as I bet you haven't told us exactly where the fuse(s) are!

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 31, 2020, 08:20:03 pm
content: Sorry for the confusion. Your schematic is accurate.

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2020, 10:37:10 pm
content: Darlington transistors can be finicky in that circuit so their gain needs to be carefully controlled. They really are not needed but if you insist ...At a minimum, the 100 ohm input resistor should be significantly reduced in value so that full output current is reached when the regulator also reaches its full output current. The base voltage of the Darlingtons, when combined with their emitter ballast resistance, is about 2 volts so there should be 2 volts across the input series resistance as the 7812 approaches full output to maximize control over the transistors, which yields about 1 ohm.A better configuration is shown below, although not for the Darlington configuration. Two diodes are placed in series at the input to compensate for the 2 x Vbe voltage drop of the transistors and then the ratio between the input resistors controls the current sharing.

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2020, 11:03:44 pm
content: @davelectronic: Thank you.@David Hess: I definitely prefer your suggestion with a Sziklai pair pass transistor .@all,There's no easy way to tell how or why it initially lost regulation or whether the Zener cooked off just sitting there because its knee voltage was too close to the output voltage, but once the Zener failed short, the LM7812 regulator, with about 20V across it, would have rapidly gone into limiting, so the base drive to the pass transistors would have become fairly weak and they probably didn't have enough drive to pass enough current to blow the fuse. They then sat there cooking for long enough to blow them. At 20V input to output differential, a TIP147's SOA curve says it will die if it has to pass much over 6A for very long, even on a perfect heatsink. That's not enough for two of them to blow a 15A fuse before the transistors blow.How to fix it while still using Darlingtons? Well if you want the regulator's current limit to protect the pass transistors, the current through the transistors must be proportional to that through the pass transistors and must limit before the pass transistors pass excessive current. This wont be practical as the peak output current of a LM7812 can briefly reach 2.5A, but its only good for max. 1A continuous, so you'd need a *LOT* more transistors to let you derate them to only 40% of their SOA limit so they could handle the short-circuit surge. Therefore you've got to come up with an alternative protection circuit. One approach would be to monitor the drop across the emitter resistor of one of the pass transistors, and use it to turn on another transistor to rob them of base current when it exceeds about 0.6VAlso its advisable to let the LM7812 current increase in proportion to the pass transistor current so its under similar thermal stress. Then if all of them are on the same heatsink the LM7812's thermal limiting will also protect the pass transistors. To do that you need to compensate for the Vbe drop of the Darlington pass transistors, by feeding the regulator input through a couple of diodes, and make the currents proportional by feeding it through a resistor of N times the emitter resistors, where N sets the current ratio. The proportionality breaks down at low currents so to prevent the pass transistors turning on due to the regulator's quiescent current + minimum load current, you need another resistor across the diodes chosen to pass the required minimum current with approx 1V across it.Finally to crow-bar this safely, you need to sense the output but crowbar the input. To get rid of the excess output voltage without blowing the regulator when you crowbar the input, you need a really beefy Schottky diode across the whole regulator + pass transistor assembly, reverse biased in normal operation. Put the fuse between the reservoir caps and the rest of the circuit and use a big enough SCR or TRIAC to handle the I2t to blow the fuse, and everything except the fuse should survive a crowbar event. I prefer a precision crowbar circuit - see a TL431 datasheet for details.I've drawn up a possible arrangement in LTspice. Sim attached.Edit:design FUBAR- I under-estimated the leakage current leading to uncontrolled output voltage at max temperature with no load. ChangeR3to15 ohmsto fix. That makes the LM7812 do more of the work at low load currents, but that's no bad thing.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 01, 2020, 02:33:44 am
content: Thank you for all the help, i'm probably going to have to go over your theory a couple of times to see what is going on, as i said my maths is quite poor, ohms law not to much trouble, but multiple equations i find really tuff. I did try another transistor i have the MJH11021G PNP Darlington but it also showed exactly the same as the TIP147 output of almost twice the voltage.

author: David Hess
author_rank: Super Contributor
timestamp: September 01, 2020, 04:40:10 am
content: Quote from: Ian.M on August 31, 2020, 11:03:44 pmThat is from a very old National application note. (1) National recommended the Sziklai configuration because NPN power transistors were more common then but that hardly applies now. They did *not* discuss using PNP Darlingtons but I described how it can be done by using two diodes in series instead of the single diode which is shown in the example. The low value parallel resistance across the base-emitter junctions prevents the transistor from turning on at very low output current. The advantage of including the diodes is better control of the current sharing.Personally I have only used big PNP transistors but with good results.(1) National Semiconductor Voltage Regulator Handbook.

author: Ian.M
author_rank: Super Contributor
timestamp: September 01, 2020, 06:18:38 am
content: Quote from: davelectronic on September 01, 2020, 02:33:44 amIts well worth wrestling with learning theLTspicesimulator. Its Free (unless you are in the semiconductor manufacturing business), and its professional grade as Linear Technology and their successor Analog Devices use it extensively in-house. The UI can be a little clunky and it doesn't have dumbed down animations and on-screen 'multimeters' etc. - if you want that with a pro-grade simulation engine, expect your wallet to rapidly go on a crash diet!Once you've got the hang of LTspice, it does nearly all the circuit maths for you. and spits out the results as voltages, currents, power dissipations etc. available at a mouse click, + pretty graphs - waveforms with respect to time or curves to represent sweeping a parameter value as I did above for load current. You rarely have to do anything more complex than back-of-an-envelope Ohms Law calculations to choose a resistor to start with, or maybe calculate a RC time constant to help you choose a capacitor.Now back to the excess output voltage question - any regulator can deliver too much voltage if you don't meet its minimum load current requirement. Lets look at the FairchildLM78xx seriesdatasheet. It says in the LM7812 table on page 8 parameter 'regload' (Load Regulation) is specified from 5mA to 1,5A as typ. 11mV, max. 240mV. The mV numbers aren't important but that 5mA lower limit of the specified range *IS*. Below that load current all bets are off. That's why I included R6 the 2K2 resistor in my design. Its got nom. 12V across it so it draws just under 5.5mA, to satisfy the minimum load current and have a bit left over to mop up leakage current through the pass transistors. Decreasing it to 1K to draw 12mA would help if they are notably leaky when hot. Its also a good place to add a LED power on indicator, as long as you reduce the resistor to allow for the LED voltage drop.On the same page we can see the max quiescent current is 6mA. That + the R6 min. load current must come through the regulator's input pin and R3 has the job of keeping the pass transistors Vbe low enough so they don't turn on significantly with no load on the circuit's output.Setting up a quick test jig in LTspice, to bias a transistor using the OnSemi TIP147 model[TIP147 datasheet]I provided with a voltage swept from 0 to 1V and measuring its collector current, with the effective temperature set to 150 deg C (Tj_max) leads to the conclusion we *ABSOLUTELY* *CANNOT* tolerate 1V Vbe at min.load current when hot as the pass transistors would leak 14mA each and that 2K2 resistor has only got 0.5mA to spare after meeting the LM7812 minimum load requirement.You could do a similar experiment on your bench with a 1.5V battery (use an alkaline AA, not a lower capacity battery, so the voltage holds up for the duration of the test) and a 1K pot across it to provide the base bias. Hint: include a suitable resistor in series with the transistor collector so you don't pop your multimeter's low current range fuse if you turn up the base voltage too high! Also its going to be more of a PITA in real life heating the transistor close to Tj_max - you'll have to put it on a heatsink with a thermocouple temperature probe with a smear of thermal paste in a close-fitting drilled hole right next to it and heat the finned side of the assembly with a hot air gun as close as you dare to Tj_max.Back to the LTspice results. Double clicking the curve title (labelled 'Ix(Q1:C)') and sliding the resulting cursor around tells me the leakage current becomes excessive (over the 0.25mA per transistor we can tolerate) with 420mV Vbe. The voltage across R3 will exceed that with only 4.2mA through it which means I've FUBARed the design in my previous post - if you switch off the load when the transistors are hot the output voltage will climb uncontrollably, and trip your crowbar. The fix is to decrease R3. It needs to pass 11.5mA without exceeding 420mV across it. Ohms Law says it cant be over 36.5 Ohms. That would initially lead you to a 33 ohm resistor, but so far this is just a sim-w@nk and I'd be happier with 15 ohms for a much greater safety margin in real life, as the pass transistors hFEmay be significantly greater than the 10K typical value the model uses, or its internal base pulldown resistors may be higher in value.TIP147 test jig sim attached.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 01, 2020, 11:53:48 am
content: Thank you for the explanation, this is really interesting. As a novice it's quite heavy reading, but I do get it, although I will go over it more than once just to be 100% that I've got it. From first reading, it looks like the configuration I used doesn't meet the requirements for the darlington transistors, like the minimum requirements are not being met to bios them in the on state . I might not have that quite right though. Thank you for the software simulation information, I'm primarily a Linux user, but can knock up a Windows machine for this software use. I'm going to do some more reading as well, but thanks again, your post is greatly informative.

author: Ian.M
author_rank: Super Contributor
timestamp: September 01, 2020, 03:15:25 pm
content: LTspice is tested to work under WINE, so no need for a windows PC or VM, unless you are really unlucky with your distro's WINE support.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 01, 2020, 09:25:41 pm
content: Usually use Mint, but have used others. Wine can be hit or miss, I've had mixed results using it. But thanks, I will certainly give it a go.