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Boost controller design (using op-amp/transistor/solenoid)

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xr4tic

New Member
My electronics is a bit rusty, let me know what you guys think about the circuit below.

Basically, there is a 0-5V input that increases as boost goes up, and decreases as boost goes down.

The point is to open the solenoid when a certain boost setpoint is reached. The setpoint is adjusted via the 5k pot.

Let's say my target boost is 18psi (3.6V), so the pot would be adjust to 3.6V.

Below 3.6V, the solenoid is closed.
Above 3.6V, the solenoid is opened.

The solenoid will actually open/close once it reaches the setpoint to maintain the required boost (feedback)

Think it will work? Any comments/suggestions/upgrades?
 

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stevez

Active Member
One thing that I'd worry about is what happens when the output of the MAP sensor is at or near 3.6 volts. The circuit could just sit there and toggle between on and off. Differential is a term that some use to describe the difference between the "on" point and the "off" point. There is always some differential, so to speak, that is inherent in the system but if it's too small it will toggle or chatter - if too large you don't get the control you need.

In some cases a timer is added to maintain the "on" or "off" state and the time delay is long enough to prevent the instability but short enough to leave you with some control.

I'm the newbie when it comes to PICs (maybe BASIC STAMP as well) but wonder if this isn't a perfect application. You might include a timing function that you could program or have a timing function that is externally adjustable based on a pot setting.

The problem isn't new or unique so others may have better solutions.
 

Chippie

Member
Its proper term is Hysteresis.........use a comparator with a fixed reference voltage........the combo R1/R2 values set the trigger point and R3 sets the amount of hysteresis........
HTH Chip
 

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xr4tic

New Member
The voltage will never be constant at 3.6

When the solenoid is closed, boost will rise, when it opens, boost will drop. So in reality, it's always, on, off, on, off, etc once it gets to that point, so the voltage will always be above 3.6, below 3.6, etc. The duty cycle will vary depending on RPM and boost.

If I incorporate hysteresis, then there might be too much time between on/off cycles, and the boost may fluctuate constantly by 1 psi or so (as opposed to looking constant)
 

stevez

Active Member
There will always be some hysterisis (or whatever you want to call it) in the system - from the controller, mechanical intertia, distance a pressure wave has to travel, time to fill or empty pneumatic lines, etc. As long as you don't care how frequently the solenoid toggles I guess it doesn't matter.
 

xr4tic

New Member
The solenoid was designed for this function. Before, the ECU in the car controlled it, but now that I have a different turbo, and more boost, I have to control it myself.

So solenoid life shouldn't be a problem
 

xr4tic

New Member
stevez said:
I'm the newbie when it comes to PICs (maybe BASIC STAMP as well) but wonder if this isn't a perfect application. You might include a timing function that you could program or have a timing function that is externally adjustable based on a pot setting.
Ideally, in the future, I'd use a PIC so that I can have a fully programmable RPM vs. boost map, and also compensate for atmospheric pressure and temperature. One thing at a time, though :)
 

stevez

Active Member
I wasn't so worried about the solenoid as the equipment it was controlling. My knowledge of what you are doing is limited. I've seen chattering relays or other 'on-off' control tear mechanical equipment apart. As long as you've thought that thru.

I agree on the "one thing at a time" approach too. My personal goal would be to implement things like this with a PIC that has a simple program - since my programming skills are limited to the basics and probably always will be. The neat thing about the PIC is that I'd be able to make improvements via programming rather than reworking hardware.

The forum let's the rest of us think along with you and learn from that. I find it to be a nice mental break while I am at work.
 
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