6

WARNING

This project as currently described is badly flawed in a number of ways that are liable to lead to complete failure of the equipment. As it is stated that it is intended to be used in a life support system, equipment failures could result in fatalities.

It must be understood that advice given here is by way of comment and, even if followed, is in no way certain to be adequate to ensure that lives are not endangered.

The poster should seek competent professional assistance before implementing any system for this purpose. - RM

Added: Dude8604 has indicated that this system is for a respirator for his own use. That makes the concerns just as valid BUT the risks are up to him to judge.

I'm building a 12V UPS system to run some critical medical equipment without power interruption. The power noeds to switch over immediately so the equipment won't shut off. I have it finished except for one problem - the relay takes about 5 seconds to switch over. I've traced the problem to the 13.8V switched mode power supply, which is connected to the relay coil and the normally open terminal. The output of the power supply stays at a voltage high enough to keep the relay closed, but too low to keep the equipment running for those 5 seconds.

I have 2 1N914 diodes in series with the relay coil to reduce the voltage to the coil to closer to 12V coil spec. Do I just need to put in a Zener diode with the right voltage to keep the relay barely closed? This doesn't seem like the best way to do it since the power supply's output capacitor won't discharge for a long time.

Or is there a way I can rapidly discharge the power supply's output capacitor low enough to switch the relay without wasting a lot of power through a resistor in parallel?

Here's a link to: the power supply

the relay

On a related note, I'm considering switching from relays to transistors. To get a "normally closed" position, I think I'd have to use a depletion mode P FET (with Vth=0? does that exist?), but they're nearly impossible to find, especially for a decent price. Or a P JFET, but the ones I've seen can only handle a few mA, unless I use it to drive the gate of an enhancement mode FET, but I'd prefer a single transistor solution.

Thanks!

edit: Here's the circuit diagram. My question is about the relay towards the bottom right. The rest is for charging and might not be the best way, but that's a separate issue for now.Schematic

edit 2: Simplified version: simplified version

Russell McMahon
  • 147,325
  • 18
  • 210
  • 386
dude8604
  • 73
  • 1
  • 7
  • can you show us a really simple block diagram of the system, and the connection to the relay that you want to switch? Is the equipment run off mains power normally, and when that fails you want your system to take over and switch in your own 12V supply? – KyranF Dec 13 '14 at 07:53
  • @dude8604 Out of curiosity, what kind of medical equipment is this for? – Nick Alexeev Dec 13 '14 at 08:48
  • Added diagram to main post. I'd like it to normally run off mains power, but there are DC adapters for it too, so I think it's simpler to always use DC than to add an inverter. – dude8604 Dec 13 '14 at 09:22
  • @NickAlexeev It's for respiratory equipment, so it's extremely important that it doesn't fail. – dude8604 Dec 13 '14 at 09:24
  • 1
    RadioShack doesn't explicitly disclaim fitness for life support purposes, so I guess you are good to go with the power supply.`` – jippie Dec 13 '14 at 10:15
  • Oh wait the [relay vendor](http://www.automationdirect.com/static/specs/adpolicy.pdf#page=3) does: `Our products are not fault-tolerant and are not designed, manufactured or intended for use or resale as on-line control equipment in hazardous environments requiring fail-sale perfor- mance, such as in the operation of nuclear facilities, aircraft navigation or communication systems, air traffic control, direct life support machines, or weapons systems, in which the failure of the product could lead directly to death, personal injury or severe physical or environmental damage.` – jippie Dec 13 '14 at 10:19
  • yeah only special and expensive shit is legally supposed to be for medical equipment - also for military. – KyranF Dec 13 '14 at 11:29
  • 4
    Dude8604 - This **IS** the right place to come for help. **BUT** you must listen carefully. I about never point it out but in this case it's a good idea. Please note my "rep". I'm oldish and highly experienced (50+ years experience :-( ) and 'know what I am doing' with electronics and have so far come closer to killing myself than anyone else. | Your enthusiasm is commendable but there are enough things wrong with what you are proposing that could cause the system to fail that it would be extremely wise to start again with assistance from competent people. .... – Russell McMahon Dec 13 '14 at 23:47
  • 1
    .... People here are willing and able to help but the starting point is NOT to patch up this circuit but instead to understand your requirement so as to be able to help suggest a suitable solution. What country are you in. What / who will the respirator be used for? If you are in the US this may be illegal and if legal would certainly expose you to the risk of severe legal penalties if this is for human life support. | It MAY be that there IS in fact good reason for you to be building such a system (war zone / developing country with no hope of a professional system .... . But ... – Russell McMahon Dec 13 '14 at 23:52
  • 1
    ... we need to know such things in order to provide best help. What WhatRoughBeast wrote is technically fairly good and, as he said, it's not complete. | Advice you have received on charging may not treat the batteries at all well (too low rather than too high a charge rate). | Much more. Help us help you by telling us more about your requirement and situation, as above. – Russell McMahon Dec 13 '14 at 23:55
  • @RussellMcMahon Thank you for your help and concern. To answer your questions, it's a ventilator which I use, so there are no legal concerns, but obviously doesn't make safety any less important. I live in the US. So I still want to make this but I'll take steps to reduce the consequences of it failing. So I'm now planning to use this for a secondary tier backup for the critical breathing equipment which doesn't have a built in battery, and primary backup for the critical equipment which has a built in battery and for the less important equipment where power failure wouldn't be dangerous. – dude8604 Dec 14 '14 at 06:32
  • For the critical devices, I'll connect them to a medical rated UPS. The power supply I'm building would power the UPS, so if there was a problem with the system I'm building there would be that layer of reliable power. But my system would keep the UPS running for much longer than its internal battery would alone. Does that sound safe? If so, I'll need to build something similar to what I posted about, with some differences, which I'll post in my next comment. Or should I just post a new question? – dude8604 Dec 14 '14 at 06:32
  • 1. Most UPS systems take 120 VAC but the battery system will only put out 12 VDC. Does anyone know of a 12VDC input medical UPS? Or will I need to add an inverter? 2. What's the best way to charge multiple batteries while providing constant output power with mains power on or off? I'd like it to be able to rapidly charge the batteries so I can minimize generator time if the power is out for a long time. Thanks. – dude8604 Dec 14 '14 at 06:46
  • I should have more time to discuss this a day or so from now. With due care taken a number of UPSs should allow extension of the battery bank. This would ideally be done in consultation with the manufacturer (maybe being jut a tad light [tm] on what the load is :-). ie IF it is done well and IF it is clearly your concern then it sounds potentially reasonable to me. They may want more reassurance - and morally some sort of legally binding disclaimer by you may be desirable. Give me a nudge a day or so from now if no more progress made via others and you need my input. – Russell McMahon Dec 15 '14 at 09:26

2 Answers2

6

Jesus wept. Moderators - please close this question. Poster is asking for help in building equipment which is likely to get people killed. Worse, folks are helping him.

That said, let me expand. I'm assuming that when you say "critical medical equipment" you mean exactly that. If you are exaggerating, then I'm overreacting. If you're not, then you really need to pay attention.

First, you don't know what you're doing. I'm sorry, this is not a form of disrespect, but you need to be aware of your limits. Under other circumstances this would be cause for a certain amount of hand-holding, but other circumstances don't include putting faulty medical equipment into practice. In the event of a failure, you are at best looking at the possibility of a major professional negligence lawsuit, and at worst criminal charges (negligent homicide). Grieving relatives can be remarkably vindictive.

The following list should not be considered a check list of what you need to address to finish your project. Please. I'm trying to indicate to you why you need to back off.

  1. Are you aware of the certification requirements for medical equipment? Are you going to get your system certified? If not, why not?

  2. Your delay is caused by the fact that, according to the data sheet, your relay dropout voltage is about 10% of nominal - that is, 1.2 volts, more or less. Once the relay operates (pulls in) it will remain in contact until the coil voltage drops below the dropout voltage. This is part of the data sheet, and is standard relay behavior.

  3. Putting diodes (zener or otherwise) into the coil circuit will reduce the coil voltage, alright, and cause the relay to drop out earlier. And when the coil voltage is reduced to 80% of nominal the relays won't operate, either. Why don't you realize this?

  4. A dropping diode (zener or otherwise) will dissipate exactly as much power as a resistor which drops the same voltage at the same current. Why don't you realize this?

  5. Your relay coils are rated for a maximum continuous coil voltage of 110% of nominal, that is 13.2 volts. Until you put in a dropping diode you were exceeding the coil rating. Why don't you realize this?

  6. As has been commented, after you charge one of your batteries you run the risk of burning the contacts on the charging relay. Why don't you realize this?

  7. You're using a non-medical-rated power supply. This alone will get you sued if there's a problem. Why don't you realize this?

  8. You're using the decay of your power supply to cause dropout of the relays. The fast way to do this is to directly monitor the power line input. Why don't you realize this?

  9. You don't show it, but you need to monitor the DC power supply and the batteries, and the relay operation in the event of power failure, and provide an annoying alarm that cannot be silenced. Have you done so? If not, why not?

  10. What provisions have you made for the effects of a lightning strike on the power lines?

  11. Your power supply uses banana jacks for output power. What is the long-term reliability of banana jack/plug connections? What will prevent careless contact from disconnecting it? Do you have any idea how hard it is to make things foolproof? Fools can be so very clever.

  12. If the power supply quietly fails, the batteries will take over - until they're discharged. See point 6. You cannot simply assume that someone will notice the failure condition before the equipment shuts down.

  13. If the output is accidentally shorted, the output capacitors in your DC power supply may well burn or weld the contacts in your relay. What provision have you made against this possibility?

I'm sorry, but I'm out of energy here. But I could go on at length. Please, you are out of your depth in a pool where lives depend on your doing thing right.

Bence Kaulics
  • 6,353
  • 12
  • 33
  • 60
WhatRoughBeast
  • 59,978
  • 2
  • 37
  • 97
  • +1. Agree - except re closure. See following. He is better off staying here and being advised what he SHOULD do (whatever it is) than being kicked out and going elsewhere for "help", which may well fail him. | I have added a large text note to the top of the question. | I have flagged this for moderator attention. If you have not done so and have enough rep to do so please do so. I suggested that we do NOT close the question but instead try to convince him that he needs to do differently | NB: MAY NOT be in US or similar. MAY just perhaps be for eg animals. | Think about F6 & F7 :-(. – Russell McMahon Dec 13 '14 at 23:37
  • You're probably right, although there are probably less antognistic ways of saying that. But that's beside the point. This was originally going to be a secondary power source for long term power outages. This is for my own personal use so what I'm thinking now is to get a medical grade UPS which would last at least long enough for a good margin of safety. Then the supply I'm building would kick in to keep the UPS fully charged for at least several days. That way I would have the safety features and alarm of the UPS but would still be able to supplement the battery life. Does this seem safe? – dude8604 Dec 14 '14 at 00:01
  • So would the second circuit diagram I posted work for that? Even if I have the medical UPS I'd still like it to switch over quickly, and alarm to indicate that the power is out and also alarm when the battery is low enough that I need to think about making other arrangements and/or start the generator. – dude8604 Dec 14 '14 at 00:07
  • Oh. Well, if this is just for your personal use, you obviously can't sue yourself, so I guess I overreacted. Some. All the problems are still there, but if you're willing to take the chances I can't argue that hard. You still need to do a lot of research. Best of luck. – WhatRoughBeast Dec 14 '14 at 00:51
  • The simplified version won't work. You need a separate charger for each battery. Except for the switching issues your first version had the right idea. You might try this: Use the charging circuit in edit 1. Connect each battery output to the load via a high-current (>15 amps) diode. Set the DC supply to slightly higher than the peak battery charging voltage and connect it to load via a diode as well. Now no relay is required. If the DC supply voltage drops the batteries will take up the slack. You'll lose some energy to diode drop, but Schottky diodes will do (.66 volts @20A: VFT2045BP-M3/4W) – WhatRoughBeast Dec 14 '14 at 01:06
  • Why would he need a separate charger for each battery? And why does the size of the battery matter, or how many batteries there are in parallel? In solar power systems there may be even more batteries connected in parallel, working fine even when there is less charge current. The op didn't say how much current the respiratory equipment is using. And regarding the relays, I don't think he's going to use them. I say: keep it simple, no moving parts. And the reason that he's with an informed community here is exactly why I feel he should stay and we should answer. – PkP Dec 14 '14 at 07:05
  • @WhatRoughBeast Please excuse the following - I wanted to act asap in case others did not read the details and acted to close etc while not fully informed. I added a note at the top of your post and indented your original 1st paragraph. He seems to be going in the right direction. Feel free to remodify as you see fit. Thanks. RM – Russell McMahon Dec 14 '14 at 08:12
  • @clabacchio, WhatRoughBeast, are you saying that we shouldn't offer advice on making life support equipment? What about other potentially dangerous equipment? Where to draw the line? And if you need to make such equipment anyway, where to ask for help? – PkP Dec 14 '14 at 11:39
  • @PkP I just rolled back an edit from Russel that I considered more appropriate as a comment. I think we have to be really cautious in answering such questions, but I'm not against them. – clabacchio Dec 14 '14 at 12:21
  • @PkP - I agree that it's a hard judgement call. The problem with giving help is that it's not a comprehensive design review, and corrects only those problems the questioner thinks to ask about. As the saying goes, "It's not the things you don't know that get you, it's the things you know that aren't so." I acted somewhat in haste, but I was horrified by what I saw going on. More than anything I was trying to give dude8604 a wakeup call that he could not ignore. – WhatRoughBeast Dec 14 '14 at 21:24
  • Right, also I was not sure what to do, so I wrote a meta post about it at http://meta.electronics.stackexchange.com/questions/4023/how-to-handle-questions-about-life-critical-equipment?cb=1 asking the help of the community. And Nick very helpfully provided this previous discussion about a related matter. http://meta.electronics.stackexchange.com/questions/402/what-is-our-policy-on-dangerous-stuff?lq=1 There's just one answer to my meta currently. In the earlier case it seems a consensus was reached to try to help, but to do so carefully and notify clearly about the possible hazards. – PkP Dec 14 '14 at 21:28
  • I appreciate everyone's warnings and I agree, although I'd like to think I'd have realized it on my own eventually. Do you agree that, as I said in an earlier comment, if I use this as a secondary backup, with a medical UPS between this battery/power supply and the ventilator, it is no longer being used directly for life support? Does this address everyones concerns or is there something I'm missing? Also, now that I'm not using this for something as critical, and I'm not going to use relays anymore, should I just start a new question? I think this thread may be confusing to anyone new to it. – dude8604 Dec 14 '14 at 22:27
3

Perhaps you could just connect the 13.8 volt supply in parallel with the battery? The system always "runs from the battery" but also recharges the battery always when mains is on. If the battery is a lead-acid battery then you should have no problems. 13.8 volts is a good voltage to charge the battery to, but if you're worried that the battery charger might actually drain the battery (normally this wouldn't happen), then you can also add the diode.

PkP
  • 7,567
  • 2
  • 23
  • 42
  • Is that really okay for the batteries? Wouldn't it charge too quickly and overload the power supply? They're 5 x 100Ah deep cycle batteries which are charged individually with relays on a timer (in hindsight maybe not the best design) and switch to being in parallel when the power is out. – dude8604 Dec 13 '14 at 09:00
  • 1
    It's a bit dangerous to *not* have the lead-acid batteries always connected in parallel. If you charge them separately, they will probably have different voltages when you *do* connect them together in parallel. And then, hundreds of amperes of current will flow from the strongest battery to anothers and sparks will fly in your relays. I would have the batteries always connected in parallel so that they always maintain the same charge level and use a power supply with current limit, or indeed a battery charger, as the power supply. – PkP Dec 13 '14 at 09:22
  • 1
    Dude, you say that "It's for respiratory equipment" - I'd say that it's of paramount importance that you run the respiratory equipment directly from the battery. You don't really need those relays for anything and if they should fail... I don't even want to think about it. Plus as I explained in my previous comment, I think those fuses F1...F5 will blow the instant you switch all the relays on together. – PkP Dec 13 '14 at 09:31
  • 1
    A "true" UPS has batteries permanently powering the output (respirator) with the main supply keeping the batteries "topped up". Think of an Ipad, running while plugged into a charger. When you lose power (unplug the Ipad) the batteries keep on going. You might use a relay to detect loss of mains, and give a warning beep (because hey, that backup UPS battery won't last forever). – Alan Campbell Dec 13 '14 at 09:31
  • Yes, a battery charger will be ok. To be sure, you can check the ratings on the battery charger if it says anything about a *maximum* size of the battery (it shouldn't) and there wouldn't be any harm in contacting the manufacturer of the battery charger about whether he recommends the use of the product in life critical equipment or not. – PkP Dec 13 '14 at 09:36
  • @PkP So the second schematic I just posted would work fine/better? – dude8604 Dec 13 '14 at 09:52
  • Yes, that one looks good. Just don't add a new fully charged battery to the system at such a time that the other batteries are drained. There's a possibility of a fuse being blown at that time. – PkP Dec 13 '14 at 10:08
  • @PkP - No, alas. Going in the right direction but still not adequate | 5 x 100 Ah = 500 Ah. Charged with a 10 AH charger = C/50 rate. He will probably never be able to boost / equalise at that rate. – Russell McMahon Dec 13 '14 at 23:58
  • Hi, @Russell! An interesting point, but I'm not sure I understand what the problem is (and would be very interested in any insights to the real world operation of lead acid batteries you might have). Surely the batteries charge with whatever surplus current there is available? For a reference, my parents previously had this solar power system with 3x60AH batteries and charge current usually less than about 2 amperes. The system worked fine, providing some TV watching happiness during the weekends and charged during the weekdays. Are these two systems not comparable? – PkP Dec 14 '14 at 07:16
  • Hmm, the recent additions to the question call for rapid charging, and that changes the situation somewhat. – PkP Dec 14 '14 at 11:33
  • Rushing - more anon. To obtain maximum lifetimes and capacity from large and expensive lead acid batteries (and others) they need to utilise a "topping charge". Also requirements for float mode and regular deeper discharge vary. A small solar panel may "bulk charge" the battery to most of its capacity but not have enough current at the extended voltage required to properly equalise and fully charge the battery. The charger quoted in this case does not have the capability at all as it is designed to run only in bulk or float mode. The charger is not bad - just not necessarily suited. .... – Russell McMahon Dec 14 '14 at 13:58
  • .... @PkP Your parent's batteries may or may not be being well treated. More data needed. Rushing ... 3am - got up to do a specific task. Need to resleep shortly for a long important day coming. More later ... – Russell McMahon Dec 14 '14 at 13:59