Spot The Mistakes - Page 1
Forum section: electronics / projects

author: Colin55
author_rank: Contributor
timestamp: February 16, 2013, 05:19:54 am
content: I want you to list and discuss the mistakes in this circuit:

author: smashedProton
author_rank: Frequent Contributor
timestamp: February 16, 2013, 06:16:50 am
content: Flip d2 and zd1

author: Colin55
author_rank: Contributor
timestamp: February 16, 2013, 06:20:19 am
content: Flip d2 and zd1 D2 and Zener-1 are correctly placed.

author: amspire
author_rank: Super Contributor
timestamp: February 16, 2013, 06:21:30 am
content: Quote from: smashedProton on February 16, 2013, 06:16:50 amNo - they are correct.Here is my list of errors:----------------Start of listEnd of List---------------That was easy!

author: Rerouter
author_rank: Super Contributor
timestamp: February 16, 2013, 06:55:49 am
content: many things,Firstly you have a 9V battery charging off of mains via 68 ohm, very bad, will over volt and explode,secondly, what is the purpose of d3?third off, what is this thing meant to do? it appears to slowly increase the current flowing through the buzzer as VR1 changes, however i dont see the need for the op amp in this case :?forth, that led is sucking down a good 30mA, its value should really be reduced,etc,

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2013, 06:59:16 am
content: Pretty sure that resistor network can be simplified down to 3 resistors, one 22M, one 10M and the 3M9 one.Battery will have a short life at the charge current it gets, unless it is made from 8 AA NiCd cells.

author: amspire
author_rank: Super Contributor
timestamp: February 16, 2013, 07:23:53 am
content: Quote from: Rerouter on February 16, 2013, 06:55:49 amA very common way of charging NiCd batteries - they can handle continuous overcharging at less then 0.1C. Not ideal, but not a mistake. It was the method used to charge NiCd on the cheap in the era before battery charger ICs. Doesn't work well with NiMH and not at all with Lithium batteries.Parts were much more expensive then, and designers were brutal in minimizing costs. You do not add a second opamp, a zener, a transistor and some resistors to make a battery charger when you can use a single resistor instead.QuoteProbably to reduce the rectified mains voltage by 0.6V. If 8 NiCd cells were used to make up the 9V battery, then would give a maximum voltage of just over 1.4V per cell with nominal mains. The 9V winding will probably give out a little more then 9V under a light load, so it may be 1.5V.QuoteAs the OA71 germanium diode heats, its reverse current increases increasing the voltage on the opamp output, until the LED goes on and the alarm is turned on. You need a low very input current opamp to detect the increase in diode leakage current. We are probably talking about leakage currents below 1uA.This circuit is probably from the 1970's and the RCA CA3140A was one of the first mosfet input opamps - it was revolutionary at the time and everybody was designing circuits with this magical new low input current device. The usual opamp of this era was the uA741, and its input current could get over 1uA when it was warm - way to much for this circuit.QuoteThe LED is to warn you there is a fire. It will go on before the Piezo buzzer goes on, so think of it as a preliminary warning. It is meant to be bright. The LEDs of the era were not very efficient, so it is no use running them at 1mA.

author: amspire
author_rank: Super Contributor
timestamp: February 16, 2013, 07:41:09 am
content: Quote from: SeanB on February 16, 2013, 06:59:16 am22M is not a standard value. The standard resistors go up to 10M and the costs greatly increases above 10M. The designer probably wanted to minimize the number of different resistor values, so he came up with this design using multiple 10M resistors. It obviously could be done differently with less resistors, so I have to put this down to a choice by the designer. Definitely not an error.QuoteIt absolutely will be a NiCd battery. NiMH batteries were past the era when an OA71 would be chosen in a design. Lithium batteries were decades away.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 16, 2013, 07:46:12 am
content: R11 seems like it might be too small. At just 100 ohms it looks like it will allow too much current to flow through T1 when VR1's wiper setting is too low. This would also exceed the op amp's maximum output current.

author: amspire
author_rank: Super Contributor
timestamp: February 16, 2013, 07:49:15 am
content: Quote from: TerminalJack505 on February 16, 2013, 07:46:12 amThe opamp will current limit when the fire really gets really hot. It can take it so it is no problem.

author: Colin55
author_rank: Contributor
timestamp: February 16, 2013, 08:08:36 am
content: It is a HEAT DETECTOR using the germanium diode to detect temperature."Pretty sure that resistor network can be simplified down to 3 resistors, one 22M, one 10M and the 3M9 one."That's the first correction.Battery will have a short life at the charge current it gets, unless it is made from 8 AA NiCd cells.The charging resistor is too low."forth, that led is sucking down a good 30mA, its value should really be reduced, " You are near to the point. Check the output current of the op-amp.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 16, 2013, 08:22:51 am
content: Quote from: amspire on February 16, 2013, 07:49:15 amI think I would still like to see a larger value for R11 and have it in series with the zener.

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2013, 08:27:50 am
content: 3140 has current limiting that actually works, and it does not kill it if shorted at such a low supply voltage. Current sinking though can destroy it, as the lower bipolar has no protection and can be driven easily into over dissipation if shorted to the positive rail. Nice is the built in current sink so it always runs in class A at the output at low current, a nice feature.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 16, 2013, 08:42:16 am
content: Hmm. So the op amp basically becomes a constant current source in this case. I wonder if that will cause the voltage detector to fall out then.There also might not be enough current for the zener if that pot is turned to a higher value. A lot of low voltage zeners need a lot of current before they will conduct.

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2013, 08:58:55 am
content: Never had that problem with low voltage zeners, more you need a lot of current before it will actually have the rated voltage across it. Often they would conduct at less than half the rated current, and have a low voltage across them. To get a 3V3 device to 3V3 it is often needed to have at least 30mA through it, at around 1mA it will often drop less than 2V.Only good thing here is that as the electronics get warmer in a fire the alarm will be tending to turn on as the box gets hotter.

author: Colin55
author_rank: Contributor
timestamp: February 16, 2013, 09:08:14 am
content: The maximum output current for a CA3140 is 40mA. It will deliver 40mA to the LED and possibly damage it.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 16, 2013, 09:09:45 am
content: Quote from: SeanB on February 16, 2013, 08:58:55 amThat makes sense.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 16, 2013, 09:53:46 am
content: Quote from: Colin55 on February 16, 2013, 09:08:14 amI don't know if I agree with that or not. We don't know anything more about that component than that it is an LED. I could be some monster that can handle the current.It still seems like the circuit is pushing its luck, though, by relying on the op amp's current limiting feature. I'm guessing the circuit was built to a price rather than a quality specification.

author: CarlG
author_rank: Regular Contributor
timestamp: February 16, 2013, 02:29:44 pm
content: There's no decoupling capacitor. That's about the only error I see at first glance. Maybe a battery low detection feature is desirable, but no fault.BUT:the zener voltage of ZD1 must be wrong? I don't know of any nominal 3.1 zeners. I think it's a misprint and should be 2.1V. In any case, it's higher than I would have chosen.Ignoring current through sensor D5, IC1-3 is at about 1V (given V+=9V), thus setting the steady state output. The AC gain, i.e. when the leakage current through D5 rises fast enough (compared to tau = R8*C2), is 3.2, so max output is about 3.2 V, i.e just about the indicated VZof ZD1. Without checking any datasheets, even with VR1 in top position, the base current of T1 is reduced to say perhaps 10-100uA, depending on the temperature. I would have designed for at least an order of magnitude higher.Ok, let's say ZD1 ischanged to 2.1V: Then I still don't think the over current problem discussed above is an issue at all, since VR1 most certainly is trimmed in production. My guess is that the value of R11 is chosen from the values already existent in the design. It could have been omitted, but inserted as a basic current limit during trimming.The output is loaded by R10-LED1 (ignoring current through VR1). Assuming 3.2V output and 1.4V forward voltage, the LED current would be 18mA, which seems resonable. With 3.2V at TP2 and VR1 at top position, max current through R11 is about (3.2-2.1-0.7)/0.1 = 4 mA, provided the OPA can source 22mA.Anyhow, the CA3140 maximum output current isn't 40 mA for 9V supply. 40 mA is short circuit current (to V-) for +-15V operation. Max output current (typ) for 5V is specified to 10mA. For 9V slightly higher of course, but not 40mA.Regarding D3, it may have been inserted to keep the device alive in case of asingle failureshort in D1, D2 or C1.

author: Colin55
author_rank: Contributor
timestamp: February 16, 2013, 05:32:32 pm
content: The maximum output current for a CA3140 is 40mA for a 15v supply. The supply will possibly be about 15v due to the transformer being lightly loaded and the output will be about 10v due to the arrangement of the 10M resistors and the fact that the output is connected directly to the inverting input. It will deliver about 40mA to the LED and possibly damage it.The op-amp will have UNITY GAIN in this arrangement and that is one of the big mistakes of the circuit.

author: mrflibble
author_rank: Super Contributor
timestamp: February 16, 2013, 05:52:22 pm
content: Problem unsufficiently constrained. Set "the mistakes" has no viable solutions.Put another way: crap documentation of intended functionality. ;-)That "Fig 1: blah blah" subscript hardly counts, but it does count for some. After all, that's why it says "crap documentation" instead of "nonexistent documentation".

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: February 16, 2013, 09:59:00 pm
content: Collin is well known for his drive-by shootings on ElectroTech.

author: amspire
author_rank: Super Contributor
timestamp: February 16, 2013, 10:26:59 pm
content: Quote from: CarlG on February 16, 2013, 02:29:44 pmI know we are always told to put decoupling caps across opamps, but circuits are often stable without them, and if the circuit is stable, then it is not a mistake to omit it.QuoteThe designer seems to have chosen readily available Mullard/Philips parts, so my guess is he would have used a Philips 3.3V 400mW zener. Since the Zener is rated at 3.3V at 10mA, it is probably functionally a 3.1 zener at the operating point in this circuit. The Zener curve becomes worse as the voltage gets lower, and a 2.1V zener is not great. The 3.1V zener is fine.QuoteIncorrect. The opamp output can go above 4.5V with the OA71 conducting. The resistor/cap combination on the opamp inverting input just increases the sensitivity to fast rising temperature.QuoteIt is possible the designer had bad experiences with caps from the 60s and doesn't trust them. I still suspect it is to lower the voltage to the battery slightly.I do not think this is a production design. It looks like it is designed from parts available from an electronics hobbyist store of the 70s. It appeared in Feb 2013 "Electronics For You" whatever that is, but no way did someone design this in 2013. Philips had an excellent range of NTC thermistors, and any production design would choose a thermistor over an OA71 for the job - far more accuracy, much cheaper circuit and probably the adjustment put is eliminated. The CA3140 could probably be replaced with a cheap transistor.This kind of fire alarm is of limited use since it will only activate when there is a raging fire right next to the alarm. By the time it goes off, it might be too late for you to escape. Obviously the instant smoke detector alarms appeared, circuits like this vanished.Quote from: Colin55 on February 16, 2013, 05:32:32 pmThat is what the designer wanted with extra gain to detect a fast rising temperature. How is that a mistake? It works with unity gain.

author: Colin55
author_rank: Contributor
timestamp: February 17, 2013, 12:55:09 am
content: This HEAT DETECTING circuit appeared in the latest copy of Electronics For You and they invited readers to Spot an Error in any of their circuits and win 1,000 rupees (about $20USD).I sent an email with 10 faults and waited for a reply.I then sent another email asking for a reply and got an answer from the designer of the circuit:Mr Mitchell is a Critic of all circuits published in EFY and he is tarnishing the Indian publications and authors. So I am not interested to reply to his queries.When you have a circuit with multiple faults, and get asked for a critique, I think it is side-stepping the issue, not to reply.I have pages of circuits that have been designed by this person and they all have major faults.I asked why the circuits were not checked for technical accuracy before publication as these poor designs were giving the wrong information to young Indians who are trying to learn electronics.The technical editor emailed back with: mind your own bussiness, (he can't spell business) we have enough people to look into the mistakes and take corrective action.In Australia, we corrected any faults in a circuit, in the following issue, but Electronics For You is hiding behind their mistakes and failing to acknowledge their existence.Most of the mistakes were picked up in a day on this Forum but some were hoodwinked into thinking the design was produced by a technical person.Here is my assessment of the faults, which they failed to acknowledge:1. What is the purpose of D3? It is not needed. The battery cannot feed-back into the power supply as diodes D1 and D2 will block the voltage.2. D1, D2 and D4 should be identified as 1N4002 to indicate the reverse-voltage only needs to be 100v. A 1N4007 is a 1,000v diode.3. The output of a 9v - 0v -9v transformer will be a lot higher than 9v on NO LOAD due to the poor regulation of these types of transformer. The output voltage is designed to fall to 9v on full load. This means supply rail will be a lot higher than 12v and the 9v battery will be overcharged via the 68 ohm resistor.4. R3 R4 and R6 can be converted to a single resistor.5. You cannot drive a diode through a 10M resistor and expect to get a result. Any voltage-change will be partially or wholly absorbed by the 10M resistor.6. What does C2 and R8 do? There is nothing at the join of these two components and they perform no function when connected in series. This type of arrangement is only used when the circuit is oscillating at a fairly high frequency. When it oscillates, the 10k and 22k resistors produce a voltage-divider that is detected by the inverted input of the op-amp. This feature does not apply when the change is very slow.7. The value of 10p for C3 will have no effect on the circuit and this capacitor is not needed.8. The maximum output current for a CA3140 is 40mA for a 15v supply. The supply will possibly be about 15v due to the point mentioned above and the output will be about 10v due to the arrangement of the 10M resistors and the fact that the output is connected directly to the inverting input. It will deliver about 30-40mA to the LED and possibly damage it.9. The op-amp will have UNITY GAIN in this arrangement and that is one of the big mistakes of the circuit.10. The value of R11 is too low. When the pot is turned fully clockwise, up to 30mA will flow through the zener and base-emitter of the transistor. It's just a bad design. Many of the values are far from "design values." Design Values are those you would find in a properly designed circuit.That's 10 mistakes in a single circuit.

author: amspire
author_rank: Super Contributor
timestamp: February 17, 2013, 02:01:50 am
content: Colin,You have not identified a single fault. Most of your statements are not correct. Using a 4007 instead of a 4001 is very common - the 1n4007is often the same price and sometimes cheaper. 1N4001's can be hard to find at times and really only exists for historical reasons that are now irrelevant. You do need a compensation cap on that opamp otherwise it is not stable at unity gain - it is 100% guaranteed to oscillate without the 10p cap. The 10Meg resistors do not swamp out the diode. At low temperatures, it is the diode that swamps out the 10M resistors. C2 and R8 increase the sensitivity to a fast rise in temperature. The maximum that can flow into the transistor base is probably 15mA with the pot at full (and it will not be at full after adjusting) and 15mA will not damage the transistor or zener so that is absolutely OK. There are no "design values" for a transistor that you should adhere to - you can do absolutely anything you like with a transistor as long as you do not exceed limits and blow it up. Even Maximum ratings can be exceeded if you know what you are doing - eg using a transistor in avalanche mode for a fast pulse generator. I can go through the whole list, but you have not identified a single fault. Sorry about that.

author: BravoV
author_rank: Super Contributor
timestamp: February 17, 2013, 04:32:02 am
content: As EE wannabe, learned a lot from this thread by reading the heated discussion alone, thanks guys.

author: mrflibble
author_rank: Super Contributor
timestamp: February 17, 2013, 04:39:03 am
content: Quote from: BravoV on February 17, 2013, 04:32:02 amI see what you did there... I wonder if that fire detector picked it up.

author: CarlG
author_rank: Regular Contributor
timestamp: February 18, 2013, 09:12:35 pm
content: Quote from: Colin55 on February 16, 2013, 05:32:32 pmWhere do you get that from? As I said before, the datasheet spec of 40mA is at 30V (+-15V) supply. And that's the short circuit current. What says it would be the same with the output loaded.QuoteI really doubt that it's capable of delivering 40mA through R10+LED1, until proved otherwise. Partly due to the facts above, partly due to the "light load condition" you refer to for a supply voltage of 15V won't exist when both the LED and buzzer comes on.

author: CarlG
author_rank: Regular Contributor
timestamp: February 18, 2013, 09:15:44 pm
content: Quote from: amspire on February 16, 2013, 10:26:59 pmThen we have different opinions about that. As I see it, there's no way to prove 100% stability under all conditions. To me it isn't good engineering practice to spend timetryingto prove that a circuit like this - i.e. an opamp without any HF decoupling at all - is stable under all conditions, instead of just put in the decap. I certainly wouldn't let that pass on a peer review. Then again, of course this argument isn't as relevant here as in professional design, but even for a hobby project or breadboard I would put the decap in.QuoteYou might be right, I won't argue about it.However, I see several arguments against labeling the device with some sort of intended operating point instead of the actual device type. Fortunately, I've never seen that in a schematic (until now?), and I don't think I ever will (again).QuoteI guess that you get this value (4.5V) from assuming ca 25-30mA through R10+LED1, but I'm not convinced that CA3140 can deliver that much (here). More realistic than 40mA, though.For some reason I overlooked the steady state condition when D5 is "saturated", which of course is inexcuseableQuoteI wouldn't argue against that, it could be inserted for more than one purpose.

author: CarlG
author_rank: Regular Contributor
timestamp: February 18, 2013, 09:37:53 pm
content: Quote from: amspire on February 17, 2013, 02:01:50 amIn general, I agree with all you say here. Except for the necessity of C3; the test circuit for unity gain in the datasheet does not indicate any additional compensation.That is not to say it's a fault. The designer obviously thought it was a good idea to put it in, and without knowing the reason for that, it must be assumed that it was inserted for a good reason. It can't be classified as a fault.[EDIT: Moved the following to a separate post for clarity]It's probably not necessary to point out that anyone making a peer review and came up with a list like the one above saying "that is wrong, and that is wrong, etc etc" when every point could be argued against, wouldn't get much respect from hers/his colleagues.

author: Colin55
author_rank: Contributor
timestamp: February 18, 2013, 09:42:37 pm
content: amspire: I'm glad you don't design any circuits.You are entirely wrong in all your assertions.I have answered this on my website: Talking Electronics.comunder Spot The Mistake Page 15.

author: amspire
author_rank: Super Contributor
timestamp: February 18, 2013, 11:15:23 pm
content: Quote from: Colin55 on February 18, 2013, 09:42:37 pmA really stupid, ignorant and insulting comment. No need for personal attacks. You didn't even have the decency to address any of my comments. If you want to get personal, then I will leave you to it. I won't post again in this thread.QuoteI don't like the circuit much, but that does not mean there are any faults. They are design choices, but if the circuit was built, it would work - as long as the opamp is stable without a decoupling capacitor. As long as the circuit works, design choices are not mistakes. Of course different designers would make different choices, and many of those choices will improve the circuit.If you want to argue that a choice you wouldn't make yourself is a mistake, then your definition is different from mine, and there is no use arguing about it. How can there be 10 mistakes and the yet the circuit probably works perfectly? I haven't built the circuit, but I believe it will work.I wouldn't build it myself - a OA71 is a really poor choice for a temperature sensor and I would have hysteresis. I wouldn't design an alarm that needs a full blown fire first before it triggers. I would go for some kind of smoke detector, or radiant IR detector. They may as well put the alarm LED behind a label that says "YOUR TOO LATE! BYE"

author: amspire
author_rank: Super Contributor
timestamp: February 18, 2013, 11:35:04 pm
content: Quote from: CarlG on February 18, 2013, 09:15:44 pmi would put a decoupling cap, but if you test opamps without one, they often do not oscillate. I have seen many cases in production where manufacturing leaves out a whole lot of components that the designer put on the board simply because the product still works fine without them. I know the designer could give a very sound reason why he put every part in the circuit, but there is reality too. If the circuit is stable without decoupling caps (as it probably is), then the reality is it works without decoupling caps, and everything past that point becomes an argument about design esthetics.You would always put a decoupling capacitor in? Excellent - you are very wise and I would agree totally. But this is not our design.QuoteI think I am finished with it too.QuoteWhen I said 4.5V, I left out some qualifications. The pot has to be in a position that allows the CA3140 to get that high (I assume it will never be at maximum when adjusted), and I do not know the LED voltage since no LED type is specified. The designer may be using a LED array with LEDS in series but just used a conventional LED symbol in the schematic. Maybe even be using a flashing LED that were all the craze in electronics hobby shops in the 70's and that may need over 3V to operate and may even have its own current limiting. I never used one so I do not know.As long as the load is less then the current limit of the opamp, it could get to more then 4.5V. I would need the parts list to be more accurate.

author: CarlG
author_rank: Regular Contributor
timestamp: February 19, 2013, 12:31:01 pm
content: Quote from: Colin55 on February 17, 2013, 12:55:09 am[moved/edited the following from a previous post, since it's directed at this quote]It's probably not necessary to point out that anyone making a peer review and came up with a list like the one above, saying "that is wrong, and that is wrong, etc etc" when every detail on the list just is a design consideration, wouldn't get much respect from hers/his colleagues. If you don't understand or see the purpose of some specific design solution, be humble about it and ask for the reason of it.