431 Crowbar Circuit - A couple configuration questions. - Page 1
Forum section: electronics / projects

author: Spere
author_rank: Newbie
timestamp: April 15, 2015, 09:16:32 am
content: Hi,Been some time sense I had a chance to work on an electronics project and I've gathered some rust.I'm putting together a micro-controller based project to examine the voltages in two battery banks and open or close an isolation relay based on charging status, voltages, & a few other parameters. Part of the overall project is a crowbar to separate the battery banks - by opening an isolator relay - if the voltage goes over 15 volts.The controller uses a BasicStamp 2SX that I have from an earlier project. The battery banks are 12v nominal (14.4-14.6 typical) and a couple hundred amp hours each. The isolator relay coil pulls about 0.75A. The fuse in the crowbar (that powers the relay coil) will be between 1.2 and 1.5 A.Attached is both the schematic and an LTSpice simulation of the Crowbar sub-circuit. The simulation goes pretty well and breadboarding gave reasonable results.Sanity checks on the resistors and overall design would be greatly appreciated -- especially thoughts on survival over time or any glaring errors.Thanks Much!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 15, 2015, 11:53:39 am
content: General questions:- Is either battery naturally expected to suddenly produce a voltage substantially out of spec? Under what conditions would it do so?- How much current and voltage is expected of the supply (battery, charger, etc.?) and load? How fast should it switch, and how much fault current should it be capable of during that switchover, if any?- I assume this carries load current? The "relay coil" part seems to suggest otherwise, but I don't know why you'd be using a crowbar function for that.- As a crowbar circuit, this doesn't look very good. I think you are missing four things:1. The current is supplied by a transistor. First of all, why not switch this off, instead? It's much, much easier, and more than three orders of magnitude faster! (Under 10us versus over 10ms.)2. Attempting to clear a fuse, through a connection supplied by a transistor, is probably going to end poorly for one of them. I'll give you a hint: it won't be the fuse... As the saying goes, the transistor is the $5, three-legged fuse that's there to protect the $0.10 "fuse".3. Which suggests a misunderstanding in what a fuse is. It's a common thought -- it's nice to think, aha, I'll throw in a fuse because it opens when current is high! But, there are a lot of problems with that, and if you think about it, it becomes obvious that fuses *can't* work that way (example: they have to handle ordinary inrush current), and if you give it some analysis and read a few spec sheets, you'll find they are actually rather crude and difficult components to use that way.On the list of functions a fuse has, "protect the wiring", and "prevent fires or electrical hazards", have always been in the top three. Protecting semiconductors is somewhere at the bottom of that list. Yeah, it kinda sucks, but that's how it is.4. The circuit operation itself leaves a few things to be desired: although the voltage threshold will be pretty solid (the LM431 is properly biased), the operation will be very slow (1uF across the transistor?), it won't turn on the SCR very hard (~50mA), and it certainly won't do it for very long (presumably, as soon as the SCR starts turning on, supply voltage gets yanked down, which means the LM431 gives up -- job well done, go home everyone, nothing to see here!), which means the SCR will only be driven for some microseconds. If it has to discharge a big fat capacitor (or worse), it's not going to be very happy (SCRs have dI/dt limits -- too much current in too little time and they do their job exactly once...). Hard and continued gate drive helps alleviate this problem (it has to do with how much of the SCR is turned on at once -- strong drive (100mA+?), for a long time (10us?), is better than just tapping it on the shoulder).Now, I mentioned the transistor earlier -- the crowbar will actually do its job fine, because it doesn't have to switch much current (at least, from this side of the fuse, assuming the load doesn't backfeed current as this node is yanked on). Assuming the IC has 5V logic outputs, the H11AV2 opto will have about 18mA through its LED. Which will give 9-36mA through the phototransistor (CTR 50-200% limits, give or take). Which sucks for the opto, because that's up to 430mW dissipation (it's only rated for 150mW) and it's not being used as a saturated switch (which would be more consistent). In any case, 9-36mA base current into the D45H8, which has a minimum hFE around 60 (no rated max sadly, but maybe ~200 tops for an average worst case?), so the collector current won't be much beyond a few amperes. So, that transistor will current limit probably in the 1-5A range, everything as shown. Which might still be enough to blow the fuse, but it will take a long time, perhaps seconds. In which time, the transistor has probably failed already. (5A at 12V is 60W, which it could actually withstand, given a large heatsink, but if it's not heatsunk at all, it will only take a few seconds.)Tim

author: Spere
author_rank: Newbie
timestamp: April 18, 2015, 12:23:33 pm
content: Thanks very much for the feedback - Updated circuit is attached.Changed out the Opto & D45H8 to a 2N2222 powering a pcb relay. The relay contacts should be much better at taking the crowbar current.The only load current on this circuit is about 750 mA for the isolator relay coil. The heavy lifting is all done by the isolator relay - which has 200 amp contacts and is not shown in the schematic.The crowbar is a 'last line of defense' should several other safeties fail -- it will de-energize the isolator relay coil and isolate the battery bank from the charging system.Taking each of your points:1. I hadn't thought about rise vs. fall time. I believe the limiting time factor will be the Isolator relay.2. With the removal of the BJT, hopefully I have addressed this one. The fuse is max 1.5 amp and all the I2T calculations look good. (The pcb relay contacts are rated at 16A continuous and 30A for 4 seconds).3. You nailed the SCR IG, 50mA almost exactly. Resistors are now changed out and SCR Gate will see about 92 mA.4. The SCR should latch, after which the 431 becomes inconsequential. With the updates, the SCR gate should be hit pretty hard - subsequently hitting the fuse pretty hard - certainly a lot harder than previous.Happy for more thoughts and certainly willing to learn more!-- Scott