Could someone please expalin how this quad op amp is being utilised? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 11, 2015, 10:18:45 pm
content: I know it's a big ask, but would someone be kind enough to briefly explain the 4 stages of this LM324 quad op amp circuit please?Basically it's an aftermarket automotive device. The input is from a pressure transducer that runs off a 5 volt reference voltage from the vehicle's engine ECU. The transducer reads intake manifold pressure, more pressure seen, more volts out to the ecu input measuring manifold pressure. This voltage would normally go direct to an ECU input, but this device intercepts it. The ecu uses it to determine fuel required and ignition timing required. It also checks the voltage from the transducer, (and hence manifold pressure, it's for a turbo charged engine) does not rise above a preset level set in the ECU programming. If it does, it kills the fuel to the engine by shutting down the injectors.This schematic is of a commercial device used to clamp the voltage from the pressure transducer at a fixed level, allowing more turbo boost to be run without the ECU seeing an over boost condition. In such conditions it cuts fuel to the engine. What I therefore expect this device to be doing is allowing voltage at the input to pass through unchanged until a preset level, perhaps set by the variable pot. At which point the voltage is clamped and although input voltage rises, the output then remains fixed. So although boost pressure in the manifold is rising the ecu thinks it has stopped rising and is still at a safe level, so no fuel cut off. I am not sure why it needs a quad op amp and what each stage is doing. Nor am I sure about the two diodes on the pressure transducer input wire. Thanks.

author: c4757p
author_rank: Super Contributor
timestamp: February 11, 2015, 10:32:43 pm
content: Lower left buffers the signal, of course.Upper left appears to be a self-biased voltage reference. The Zener diode will determine the output voltage and then the output voltage will in turn determine the Zener current, making it more stable.Upper right is a precision clipper. If the buffered voltage from lower-left goes above the reference voltage from upper left, it'll switch negative and pull the voltage down through its diode.Lower right is a final buffer with a current limiting resistor.

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: February 11, 2015, 10:38:19 pm
content: The diodes are clamping the input to 12V/0V at the input buffer stage

author: dannyf
author_rank: Super Contributor
timestamp: February 11, 2015, 10:43:06 pm
content: The opamp with the zener is a constant current source: the opamp's output and the opamp's inverting (thus non-inverting) input has a 5.1v voltage drop. That means the 12K resistor has a 5.1v drop, regardless of supply.So the voltage over the 12k resistor, 5k8 resistor and the 5k pot is a constant. That voltage is divided by the 15k/16k divider (the output is roughly 5v = 5.1v/12k * (12k+5k8+5kpot) * 16k/(15k+16k)) and "rectified" to generate a voltage - that voltage is ***only*** generated if the divider's output is LOWER than the buffered input signal.So, if the input is greater than 5v, the output is 5v. If the input is lower than 5v, the output is the input -> a voltage limiter.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 11, 2015, 10:48:42 pm
content: Quote from: dannyf on February 11, 2015, 10:43:06 pmAre you saying that it appears to have a safety feature enabled, in that it will clamp the output voltage after X input volts until input volts rise to 5 or more volts, then it outputs 5V (which would trigger the engine ECU into shutting the fuel off). It thus allows some clamping, but retains a safety element if the boost level goes "berserk". Or have I misunderstood the last paragraph? Thanks for the replies!!!

author: dannyf
author_rank: Super Contributor
timestamp: February 11, 2015, 10:55:08 pm
content: Yeah. It limits the output to some present voltage levels - adjusted by the pot.

author: retrolefty
author_rank: Super Contributor
timestamp: February 11, 2015, 10:59:34 pm
content: Quote from: Chris Wilson on February 11, 2015, 10:48:42 pmYes, misunderstood. Device will allow sensor measurement to pass unmolested to output as long as it's less then 5V, any sensor value higher will be clamped to 5V to ECU. It's a voltage limiter. I'm sure a much simpler design could be created that would be just as capable.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 11, 2015, 11:07:07 pm
content: OK, sorry, I was hoping it may have had a safety regime. Thanks for clarifying. I was surprised it used a quad op amp. Here is a PCB layout for a similar device that is simpler. I have not had a chance to pen out a schematic though, I only have this right now to show:

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 12, 2015, 12:52:26 pm
content: Here's the schematic of the dual op amp circuitI mentioned above, is the this functionally any inferior to the quad op amp circuit? Thanks

author: Andy Watson
author_rank: Super Contributor
timestamp: February 12, 2015, 01:02:26 pm
content: Quote from: Chris Wilson on February 12, 2015, 12:52:26 pmDependsDoes it perform its function? It is probably less accurate than the quad op-amp version - but if it works, what's the problem?The four op-amp version provides stable conditions for the zener - so I would expect the reference voltage to be more stable against changes in supply voltage.The two op-amp version has an output resistance of 1680 ohms - this may, or may not be a problem - depends what follows, i.e. the input impedance of the ECU.

author: wiss
author_rank: Frequent Contributor
timestamp: February 12, 2015, 01:11:19 pm
content: This sounds like a very simple way to blow up your engine, and also be certain that no warranty or insurance will cover the damages...

author: retrolefty
author_rank: Super Contributor
timestamp: February 12, 2015, 01:20:18 pm
content: Quote from: wiss on February 12, 2015, 01:11:19 pmTo be clear, for any new to electronics members around here, the device, as being used, is NOT a safety device, rather it disables an exisiting ECU safety function.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 12, 2015, 01:36:55 pm
content: Indeed, with this an abnormal boost level will not receive any safety fuel cut off AT ALL. Not to be used unless you know what you are doing and appreciate the possible consequences of the lack of the over boost safety cut off.

author: wiss
author_rank: Frequent Contributor
timestamp: February 12, 2015, 03:29:39 pm
content: I'm more of the opinion that if you do this, you do prove that you do not know what you are doing!Or you work with destructive safety testing, to make sure that now debris will be emitted from the engine with deadly velocities in the case of ECU failure for example...

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 12, 2015, 03:41:10 pm
content: Thanks, but I can assure you I know exactly what I am doing, from an engine perspective, it's my day jobElectronically I am a relative beginner.

author: wiss
author_rank: Frequent Contributor
timestamp: February 12, 2015, 03:56:26 pm
content: Ok, could you explain what it is you are trying to do?

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 12, 2015, 04:09:50 pm
content: I am building a fuel cut eliminator for a customer. To allow more boost without the engine ecu cutting the fuel. He and I are both aware of the risks, the commercial ones starting trimming the MAP sensor signal before a threshold voltage is reached, altering the pre cut fuel map.

author: dannyf
author_rank: Super Contributor
timestamp: February 12, 2015, 05:32:06 pm
content: QuoteDepending on what you meant by "inferior".The simplest would be to put a 5v zener on the ouptut, with a serial resistor.The circuits you have basically achieve that via a precision rectifier. Tough to tell which is better unless you can layout your criteria.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 12, 2015, 05:39:14 pm
content: Hi Danny, I really need some adjustability to precisely set the cut off voltage as near to the ecu switching level, I'll build both and test them myself watching the V out with the sort of running voltage swings an alternator based charging system gives. Thanks! The quad op amp is a schematic from a commercial, but obsolete device that many many people have used without issue. the other is from an Australian magazine that others, too, seem to have found fine.

author: BradC
author_rank: Super Contributor
timestamp: February 13, 2015, 01:18:53 am
content: A zener is a pretty bad idea. You'll find the leakage as you approach the knee will badly affect the output linearity. The quad op-amp circuit looks like a reliable way of doing it.The question that pops into my head is if you are clipping the MAP output, where is the extra fuel coming from? You tell the computer there is less airflow than there actually is, so it's not going to inject the correct fuel qty, or are you relying on the O2 feedback in closed loop to get enough juice? If so what stops the lean peaks when you tromp on the loud pedal before the ECU catches up in closed loop?

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 13, 2015, 04:36:59 am
content: A simulator is a great tool to figure out what a circuit like this does.Attached is the output for a parameter sweep on 6 different settings of the potentiometer. (0%, 20%, 40%, 60%, 80%, 100%) You can see how it affects the output for input voltages from -2V to 14V.Basically the circuit clips the input to approximately 3.9V to 5.2V based on the potentiometer's setting.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 13, 2015, 11:23:26 am
content: Quote from: BradC on February 13, 2015, 01:18:53 amHi Brad. Most Japanese performance turbo cars run mad rich under heavy boost, mainly to keep the catalytic converter cores within acceptable temperatures. the makers have to do this, much to their chagrin. As such, and with the cats removed, there's a pretty big rich area high up the boost curve. One can utilize this by fooling the stock ecu to see less than the actual MAP sensor voltage. So long as you are careful not to allow runaway boost and stay within the bounds of reason you can make use of the excess fuel by ramming in more air. On a typical twin turbo 3 litre one might find it safe to go from a stock 0.8 bar boost to maybe 1.2 bar boost. The turbos are still just about within their efficiency island there. Any more and they are thrashing and heating the intake air to no overall benefit as air density takes a big dive. Obviously a change to a fully mappable ecu is the best course to take, but it's costly in terms of hardware, fitting and mapping time, many young lads are happy with the slight risk for improved performance.Utilising this excess fuel is commonplace. I also sometimes fit a mappable ecu, bin the cat(s) and run stock boost, but trim the high boost fuelling. Better HP and more MPG. Dirty exhaust emissions.

author: dannyf
author_rank: Super Contributor
timestamp: February 13, 2015, 11:49:15 am
content: QuoteIn that case, apply the input signal (bufferred if needed) to a precision but adjustable zener like 431 and you get a limiter to whatever the value determined by the 431. Much simpler than what you have.

author: BradC
author_rank: Super Contributor
timestamp: February 13, 2015, 01:09:47 pm
content: Quote from: Chris Wilson on February 13, 2015, 11:23:26 amQue? Manufacturers spend millions keeping the fuel curve as stoich as possible because excess fuel (hydrocarbons) in the exhaust gets "burned" in the cat and causes it to cook, ruining it.The only ones I've seen belching black smoke have been cars modified with gross hacks to the injection (like altering the CTS to make the unit think the engine is cold) to make it tip in more fuel. Unburned hydrocarbons will destroy a cat, and I'd be floored if _any_ Japanese (or otherwise) cars out of the box broke every emission rule in the book. I've certainly seen a few cars, and I've never seen a stock one that does that. 14.7:1. Not just a good idea.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 13, 2015, 01:22:27 pm
content: Just stating facts, most performance turbo petrol engines use rich mixture at the top of the boost / torque curves to keep cat and valve temps in check. Take a turbo petrol engined car on track, where you can keep your foot in it for long periods and the MPG will be truly shocking. The "normal" MPG figures for the same car will be good, because the makers are not forced to run at wide open throttle, full boost for extended periods when those figures are taken. Just because an engine runs closed loop it doesn't mean A: It has a wide band O2 sensor(s) and B: It's running closed loop in all parts of the maps. Anyway, we are swaying well off topic

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 13, 2015, 01:38:23 pm
content: Quote from: Chris Wilson on February 12, 2015, 12:52:26 pmThis circuit doesn't allow for fine-tuning like the other one.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 13, 2015, 02:32:45 pm
content: Quote from: TerminalJack505 on February 13, 2015, 01:38:23 pmIt would with a pot in place of the 1k and 680 Ohm resistors, wouldn't it? Thanks.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 14, 2015, 01:55:48 am
content: Quote from: Chris Wilson on February 13, 2015, 02:32:45 pmYep. The circuit would be functionally identical in that case.

author: KerryW
author_rank: Regular Contributor
timestamp: February 14, 2015, 01:32:20 pm
content: It seems to me that the circuit could be vastly improved by the addition of 1 resistor between the diode (upper right on first svhematic) and the output op amp (lower right)Currently, the gain is0 - threshold -> Gain of 1> threshold -> Gain of 0With 5K resistor0 - threshold -> Gain of 1> threshold -> Gain of 1/3By setting the threshold and gain resistor you could increase the allowable boost without losing the safety cutout.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 14, 2015, 01:47:37 pm
content: Quote from: KerryW on February 14, 2015, 01:32:20 pmEDIT: I was assuming you were referring to the later schematic, and that's what I currently have breadboarded and discuss below, I see now you are talking about the Greddy circuit, sorrry.Just trying to work out what happens with this. The input volts need to match the output volts until 4.27 Volts is reached on the input. Then the output would stay clamped at 4.27VIt's vital input and output follow 1 to 1 right up to the clamp voltage level. Some commercial ones just skew the voltage from the get go, which also skews all of the fuelling. Not good at all.I have achieved that by changing the series 1K and 680 Ohm resistors in the divider for a single 1.2K The original circuit clamped at about 3.9V as it was designed to match the pressure sensor characteristic curve of a Subaru sensor.In an ideal circuit input and output would follow 1 to 1 until say 4.27V on the input. The output would clamp at 4.27.Then, should the input rise above 4.99V the output would jump to that figure, and fuel cut would still be initialised. The max output from the pressure sensor is its operating voltage, which is 5V from the engine ecu reference output feeding several different sensors.I'll try what you suggest it's sat on a breadboprd as I type, powered up. Thanks!

author: max_torque
author_rank: Super Contributor
timestamp: February 14, 2015, 02:29:52 pm
content: Always makes me smile when i see people "saving money" like this!Sure, you get to save £500 on the price of a proper remap, but spend £2000 on a new engine in a while instead.......(go on any car forum and type in the search box "blown my engine up" especially on the jap tuning forums and see what i mean)

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 14, 2015, 02:38:11 pm
content: I can only respond to commercial demand, if people want these things then I will supply themThe stock ecu's are not mappable, and a decent aftermarket ECU, like a Motec M800 with added options to run wide band Lambda, etcetera is over £2000 Sterling. Then there's making a loom adaptor, mapping time, blah blah. Gets into big money very quickly, often more than the value of the car. A blown engine in the sort of car these are likely to be used on is about 8K Sterling for a basic rebuild. But I do as I am askedMy own fun road car has a Motec M800, dual wide band Lambda and proper boost control and safety features, but not everyone can afford such mods, or perhaps wants to spend so much. Historically tens of thousands of this sort of gizmo will have been sold, if used responsibly they are OK.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 23, 2015, 11:59:02 am
content: Quote from: dannyf on February 13, 2015, 11:49:15 amHi Danny, I have gone for the dual (TL072) op amp circuit and it works well, but I have another question, sorry. For curiosity I built the adjustable Zener circuit and fed 0 to 5V from my bench power supply it drew no more than about 1mA and worked fine. I then did a sanity test with the real world MAP sensor feeding the circuit form its signal output pin, and the MAP sensor fed from the bench supply. The circuit then failed to work properly. I am assuming this is the reason for your "buffered if necessary" comment. Why would the MAP sensor not drive the adjustable Zener circuit direct, without buffering? What would be the cheapest and simplest way to add an OPAMP buffer, and would it need a 12V feed, or could it work off the 5V reference voltage the MAP sensor (and other engine sensors) runs off, derived from the engine's main ECU? Since building things I am learning a lot more than when I just read the theory about things, and it throws up the diverse issues even the simplest circuit may throw up for the unitiated like myself