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Help configuring NPN Amp switch for 12V Relay

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audioguru

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Maybe the Vsrc signal is not 2.0VDC and 4.15VDC. Maybe it is pulses of maybe 5.0V with Pulse-Width-Modulation (look it up in Google) where the duty-cycle of the pulses determines the average voltage.

You can increase the value of C4 to average the voltage of the pulses but since we don't know what is the frequency then we must guess on the value of the capacitor. If the value of the capacitor is too high then the switching from 2.0V to 4.15V and back will be slow.
 
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slotcar330p4

New Member
Hi stuhagen,

I looked at your diagram and I have a few questions:

What do R4, D1, R5 do and why is R4 connected to 13.2 volt and not to the (stable) 10 volt ?

kind regards,

Peter.
 

stuhagen

Member
Hi stuhagen,

I looked at your diagram and I have a few questions:

What do R4, D1, R5 do and why is R4 connected to 13.2 volt and not to the (stable) 10 volt ?

kind regards,

Peter.
If you read back a bit, these parts are explained. The R4 is a "pull-up" resistor. The diode is protection for the LM311. R5 turns off the TIP31. As far as the Vcc to R4...I am not sure whether it is that big of a deal whether it comes for the regulator or not. It is a matter of the "path" in which the current takes in the firing of the TIP31.

I still have my "training wheels" on, so my explanations might not be the best.

Stu
 

stuhagen

Member
Ya, a scope would be nice. If you were to "guess" based on your knowledge on PWM, give me a few values that I can test with. I can drop a few different values and see what I get. All of my first designs did not even have C4. I put that in there for hysteresis after reading the LM311 sheet. I was having relay chatter thinking this addition would cure. But I also added R5. So I am not sure which of these 2 made the difference, but I do not have chatter, rather it is now just in reverse.

If you think that C4 is a possible cause/fix I may even try removing it all together and see what happens. If that doesnt cure this, give a few other values and may try that this weekend.

Stu


Maybe the Vsrc signal is not 2.0VDC and 4.15VDC. Maybe it is pulses of maybe 5.0V with Pulse-Width-Modulation (look it up in Google) where the duty-cycle of the pulses determines the average voltage.

You can increase the value of C4 to average the voltage of the pulses but since we don't know what is the frequency then we must guess on the value of the capacitor. If the value of the capacitor is too high then the switching from 2.0V to 4.15V and back will be slow.
 

audioguru

Well-Known Member
Most Helpful Member
All of my first designs did not even have C4. I put that in there for hysteresis after reading the LM311 sheet. I was having relay chatter thinking this addition would cure. But I also added R5. So I am not sure which of these 2 made the difference, but I do not have chatter, rather it is now just in reverse.
C4 is a simple filter, it does not provide hysteresis. A resistor from the output to the (+) input provides a little positive feedback for the snap action of hysteresis.

D1 and R5 are not needed.
R4 is needed because the output of a comparator is the open collector of an NPN transistor that does not source current like an opamp does.

If the action is reverse then swap the inputs of the comparator.
 

pfofit

Active Member
In your first post a long time ago you said.
The purpose to this is to switch over to a voltage enhancer to increase the speed of the fuel pump motor increasing its output.

1. Why do you want to do this?

Some of your earlier diagrams show an enhancer of 18 Volts.
2. Why are you hitting the fuel pump with such a high voltage?

If you want to see if the signal is PWM put your meter on AC voltage.
It's not a scope but if it reads anything more than a few millivolts, then it may be PWM .

3. Also, what year, make and model is this car?
 

audioguru

Well-Known Member
Most Helpful Member
Boosting the fuel pump pressure is used by guys who boost the turbo too much so that the engine blows its head off.

A turbo engine does not like a mixture that is too lean.
 

stuhagen

Member
Well, baffling as it may be, the circuit works perfectly after switching pins 2 and 3. Don't ask me why this would make it work. My only thought is the pull up resistor was causing some additional feedback to the LM and it was causing oscillation in the other configuration. Before I switched the pins, I could remove the Vsrc feed off the circuit and it would not do anything, like being dead. If I removed this on the bench test, the relay would trip. So my assumption was the actual signal coming into the circuit from the car was the culprit. I actually tried both circuit boards I had built. One with the diode and 1K removed, and both work the same. So all along it was the pins being reversed. Another possibility is the difference of reading either “0”v or 13.3v at Collector. Or triggered/non-triggered. Maybe having this voltage there in one way caused issues, but not the other.

As far as why I am applying 16v +/- to the fuel pump. It is “old” school, but by increasing voltage to the fuel pump, you increase it’s pressure. At a certain point when running full out, the car’s ability to supply adequate pressure drops off. This is dangerous and can cause serious engine damage. Most people just spend a few thousand and upgrade there fuel system with 2 fuel pumps and larger injectors. But in my situation, I am on the “line” of actually needing an upgraded fuel system. So this is a added safety net for the few times I hit this ceiling. The Kenne Bell BAP has been around for years and has being tired and tested to perform this way. Many data sheets showing how it increases the safety net. So for under $300, I have increased my safety net. I should also note that the whole other reason for this design is because this increased voltage (BAP) doesn’t even kick in until the car hits at lease 4000rpms. (the 4v signal) So the fuel pump only runs high for short durations.

Besides, being a EE from the 80's, what else am I going to do at night. I really enjoy tinkering with all this. Keeps me busy. Even if it all fails in the end, I enjoyed the effort, and getting back to something I had not done for a long time.
 

audioguru

Well-Known Member
Most Helpful Member
The fuel pump is not designed for 16V.
You will be passing a car on a two lane road and a truck is coming at you. Then your fuel pump burns out. SPLAT!
 

stuhagen

Member
The fuel pump is not designed for 16V.
You will be passing a car on a two lane road and a truck is coming at you. Then your fuel pump burns out. SPLAT!
Well, they are rated qt 14.4V....People have been running these for years with no real issues. Although there was one guy who ran his at 16V 100% of the time and it bit the dust after about a year. Mine would only be about 20% of the time at the most.

Stu
 

jawshoeaw

New Member
Hi, new to the forum but followed this thread start to finish.

One question.why are you using NPN transistor with a comparator? It shouldn't even work, unless your transistor is really sensitive. I would think you want a PNP transistor which works with the open collector of the comparator IC. The comparator's output isn't "output" it's more like an input or really more like a switch which opens a path to ground. So you can use a 12V or higher load such as whatever voltage the car provides and sink that voltage into the comparators "output" pin. Note that you are putting current into the LM339, not pulling it out. The PNP transistor is perfect for this - it senses the open ground in the comparator and allows current to flow to it's emitter with much high current than the 50ma the LM339 allows.

I'm new to all of this so maybe I'm wrong but I don't see how the NPN would even work. There is no current flowing to it's base, at least not from the output pin of the comparator. I wired up a comparator on a breadboard with a potentiometer and checked voltage off output pin. It does change as comparator output goes open/closed but it was like 0.2 volts or something. I could not trigger an NPN transistor to drive even an LED never mind a relay. The same exact transistor however has no trouble firing a 12v relay when triggered by a different IC, a 555 with actual current coming OUT of it's output.

Hope this helps and that someone can correct me if I'm wrong.
 
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