ThirtyTwo

thx

im only making the boost go up about 3 pounds so im not too worried about detonation, plus if you run a higher octane it reduces the chances so im going to run premium which is usually 97.

Im not using it for an RX7 though, its for a chrysler 2.2 motor. But the map sensor has the same voltages so it should still work for me

I have mine setup so that my wastegate doesnt get any pressure until a certain point and then it starts regulating, so its kind of like pinchng the line but with more control. but yeah it makes a big difference, spools up quick and gives me tons of boost. 8)

ThirtyTwo

well i built it, which was a major pain in the butt using a prototype board, but everything seems to work well. im kind of worried about a couple solder joints that are pretty close to touching on the op-amp but hopefully it will last me for a while until i build another one and make it better.

i put another trimmer from 5v to the input and adjusted it up and when it got to the voltage i set at R3 it stopped right on the money

thank you guys all for your help, i couldnt have bult this thing without it. i think im going to get into electronics as a hobby, i had a fun time making the voltage clamp. and i learned alot so thats cool 8)

ThirtyTwo

Ok i put it on my car and the 12v keeps zapping the IC. So i ran a simulation with the 12v being 5v instead, and everything worked good in the simulation. But when i built it the output is always around .1v higher than the input I think its making the engine run too rich, so is their any way i could add a resistor or something to the output to make it the same as the input???

The clamping action is working great though

Roff

LM358 output won't go close enough to the positive rail to work with vcc=5V. If you are getting it to go to 4.7v, and you are getting +100mv offset, you probably have a bad op amp. LM358 works up to 32V. If your car battery is killing it, it's probably from huge spikes on the +12V wiring. Add a series 470 ohm, 1/8W (or higher wattage) resistor between the battery and the op amp vcc pin. from the op amp vcc pin to GND, add an 8.2V zener in parallel with a 100uF electrolytic cap and a 100nF ceramic cap. This should clamp and filter out the transients, and also prevent oscillations, which op amps are prone to if the power supply is not decoupled.

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Ron

ThirtyTwo

could the op-amp be zapped by too high an amperage? The 12v on my car is at 6AMPS. When i was testing it on a computer power supply everything worked fine, but the 12v amps were really low

Roff

No. The circuit only draws as much current as it needs. Your car battery, unless you have a fuse in the circuit (which you should add, if you don't already have one) can supply much more than 6 amps - as much as a hundred amps or more under heavy load, like starting.

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Ron

ThirtyTwo

ok I didnt really want to order those parts and have to wait another couple days so i started looking for another 12v source and i found a regulated 12v off the computer. I hooked up to it and the op-amp isnt burning up anymore.

i replaced the op-amp but im still getting about .02v higher on the output than the input, its not much but i think its making my motor run crappy

do you think i could put a big resistor between output and ground or is there a better way to do it?

thx

ThirtyTwo

Ok im going to make another one in the future that is better, the one i made works fine but it has about 8 resistors adding up to around 90k of resistance from output to ground to drop the extra .02 volts so im going to figure out the exact resistance i need and use that in the next one. And i want to add what you said to make it more durable to voltage spikes.

but when i put the stuff you said in my circuit maker simulator it didnt work???

i didnt know if the capacitors were supposed to be in series or parallel, but when i tried with them in series it wouldnt even simulate and in parallel i get -226.7mv(note: in the simulator both triangles must be supplied with power and ground)

i couldnt define the capacitors as being electrolytic or ceramic, maybe that has something to do with it???

Attached Images

voltclamp.jpg (25.3 KB, 375 views)

ThirtyTwo

woops sorry, i flipped the diode around and it works now

so that shematic with the diode flipped is what you meant right?


lol also maybe you guys could help me with this, id like to take that circuit and add another trimmer and have that trimmer set a safety point so that if my boost spikes too high it will cut my engine and save it from blowing up. so input = output until voltage at trimmer1 is reached and then it clamps it at trimmer1's voltage but if input reaches voltage at trimmer2 it either clamps it at trimmer2's voltage or just lets input = output again.
sry i keep going on and on with this thread but i dont know much about electronics

Roff

Yes, flip the zener. And change the connection on the input protection diode as shown below.
To protect your engine from excessive boost, just limit the voltage on the end of your boost set pot to the appropriate level. How are you deriving that voltage (+5V?) now?

Attached Images

voltclamp.gif (15.0 KB, 357 views)

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Ron

ThirtyTwo

oh i forgot, in the simulation i just put a set voltage to pin5 to make it easier to change; i have a trimmer between a +5v and pin5 on my actual board to adjust clamping voltage.

but for the boost protection, this voltage clamp actually makes my motor quite vaulnerable to overboost. the computer gets a voltage from the boost sensor and if it gets to like 4.3v(11 psi) it shuts my engine down, so im putting this clamp in to stop the voltage at 4.25 or so, that way the computer doesnt see past 10.5psi of boost and it wont shut my motor down. so even if im getting 20psi(4.95v) of boost, the computer only sees 10.5psi(4.25v)

the problem is that 20psi will destroy my motor so id like to make the clamp have another trimmer to set to a safety point where it will stop clamping and let "input = output" or "input = safety trimmer's voltage" which will let the computer see the real boost and shutdown the motor.

Roff

I think the schematic below will do what you want. The transfer function is posted below the schematic. The LM324 is identical to LM358, except it has 4 amplifiers instead of 2.

Attached Images

	boost_clamp.gif (5.7 KB, 345 views)
	boost_clamp_wave.gif (6.8 KB, 333 views)

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Ron

ThirtyTwo

wow thats awesome, thank you alot. now on to building it lol

ill post back on how things go with it, but it works awesome in my sim

thx again, it does exactly what i need it too

RedCarzRFaster

I wired it just like the attached image (post #4). I know this is from about a year ago or more but I was wondering if someone can chime in. My supply is only 5v but everything else is hooked up right. My input is 5v as well. The problem is if i clamp it at 0v i get an output of 1.6v and if I clamp it at 5v it only goes to like 3.something, not sure what i am doing wrong. I see in later posts to maybe flip a diode.

Blueteeth

Hi, sorry for my large posts in the other topic, I didn't realise it was specifically for 'boost', the pure analogue (opamps) approch seems to be to best route.

As for your circuit, it is designed to be used with a +12V supply. So powering it by 5v (same as the Vref, your input form the sensor) will mean it won't funciton properly. Also, opamps have a 'maximum output voltage' rating. This means that with a 5v supply, the output cannot swing too close to the power voltage. For the LF358, the datasheet, it says with a supply of +/- 15V (thats dual power supply) the output can swing 13v above and below 0v. So, it can't output the same range of voltage as its power supply. Basically, you're powering it by 5v, so, no matter how the circuit is configured, it won't be able to output higher than about 4V, or lower than 1V. When its powered by 12V it is capable of putting out +11v and 1V (with reference the 'GND'/0V)
It uses the 5v supply for the sensor as its voltage reference, and the 12V supply as its power.

I'm not sure if you know about 'dual rail' supplies (don't want to be patronising here).

The link provided in this topic, http://www.teamfc3s.org/info/articles/fcd/diy_fcd.htm, discusses it, but it doesn't actually require it. Its a bit hard to explain :/

Ultimately, if you wish to use that circuit, it must be powered by +12V from your battery. The Vref should be connected to the sensors power supply (+5V) and Gnd, is, well, GND, your chasis (preferably where the sensor connects). 4 connections in total.

If you must have it powered by 5V, then things could get tricky, as few opamps can output that close to the power supply (0.3v - 4.7). Although you did mention you wanted it clamped at 4.3, leaving 0.7 between output and power....*might* be possible. I'll wait for your repsonse before I post anything else, sorry I could be of much help this time,

Blueteeth.

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