Help buiding a 12v 30 amp resistor system or pwm

Discussion in 'The Projects Forum' started by GC89, Mar 3, 2010.

  1. GC89

    Thread Starter New Member

    Mar 3, 2010
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    Hi guys I am new to the forum, I have looked around a bit and it looks like a great resource for learning. So I am re-designing the fuel system in my car and have come across an issue. For the controller I need to limit the pump speed at idle and low demand conditions to do this I need a resistor capable of handling 12v (13-15 realistically as most batteries output exceeds 12) and 30 amps. The pumps max draw peaks at 21 amps in extreme conditions so I chose to shoot for 30. The stock resistor pack is .7 ohm, it is a little light for the pumps needs so I am shooting for .5 ohms. So far I am having trouble finding any resistors that will meet my needs. I have also been told a pwm would be another option to control the pump speed but cannot find one that will handle the loads either. Is there a way to build a resistor or pwm circuit to do what I need, parallel resistors perhaps and if so what would you recommend.

    Thanks I appreciate the help
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, you're going to run into a problem in that your fuel pump takes time to change speed, therefore change output pressure/flow rate.

    You will have a "lag" between the time you apply power and the delivery of increased flow/pressure. If you stomped on the "loud pedal", you will probably experience detonation until the pump can overcome it's inertia and catch up.

    It's more usual to have the pump running at near capacity, and have a regulator control the pressure.
     
  3. GC89

    Thread Starter New Member

    Mar 3, 2010
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    I have accounted for the lag in tuning the switch point so it shouldn't be a problem, and the pressure is regulated. Its based of both manifold kpa and rpm and should be conservatively set. The pumps output at the low level alone would likely be enough to supply the engine with enough fuel even at wot. I am mainly doing this because the car does see average driving quite frequently and I want to prolong the pumps life. I am also putting an override switch in for times at the track, anticipated hard driving, ect.
     
  4. GC89

    Thread Starter New Member

    Mar 3, 2010
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    Can anybody point me in the right direction?

    what about this PWM

    Or this one

    They are both a little bigger than I hoped I could get away with but I guess thats what it will take when dealing with the amount of heat this voltage and draw will create
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Well, the 1st one looks very similar to another couple of 30A PWM circuits that I've seen; the one with the fan on top of the heatsink. One of the designs was pretty decent; I'll attach it. There are no values to the components, but wouldn't be too hard to figure out. The other design was terrible, and would definitely need a fan-cooled heat sink. My guess is the 1st one is of the latter terrible design, particularly because they state that PWM is at 400Hz. If you tried increasing the frequency, you'd fry the MOSFETs, since the gate driver circuit was poor.

    I've attached MX066.pdf - which was the better design.

    The 2nd link you posted looks pretty interesting. Since you can control it with 0-5v, that would make it relatively easy to interface with another control unit. Many PWM circuits require their control voltage to be in the range of 1/3 Vcc to 2/3 Vcc; in your case Vcc would likely be from 12v to 14v.

    If you're going to build something, you're going to spend at least $20 for parts, and it'll be a learning process.

    Have a look at the schematic I attached, and see if you understand it.

    [eta]
    Something else you'll need is a diode connected with cathode towards +V across the motor, and a small capacitor in parallel with it would also be a good idea. By small, I'm talking in the range of 100pF to 1nF (1nF=1000pF). The diode will have to have the same current rating as the motor.

    If you don't have a diode across the motor, the current flowing through the motor will have no place to go (reverse EMF) when the PWM circuit turns off. This will cause very high peak voltages on the output of the PWM circuit, which will very likely destroy the output transistor/MOSFET of the PWM circuit.

    [eta]
    Sorry if you feel like your request for help is being ignored. It's just that we seem to have a lot of people with questions, and not a great number of people who have time to give realistic answers.
     
    Last edited: Mar 5, 2010
  6. GC89

    Thread Starter New Member

    Mar 3, 2010
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    Thanks for the diagram. I understand it for the most part. Resistors, diodes and polarized capacitors I recognize however I am unfamiliar with the symbols labeled TR, ICL, and MF. I am definitely willing to put some time in to learn. Also since there are no values listed how would I go about determining them? Is a PWM the only option or would it be possible to build a resistor pack which I assume would be much simpler. I was also intrigued by the 0-5v control, my engine management system has v-outs capable of 0-12v which could open some possibilities but currently there are only two and they are both used. I am going to look into writing some of the other unused ports as v-outs as well.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Those are transistors; also called bjt's for bipolar junction transistors. There are both NPN and PNP transistors used in the schematic.
    There is an explanation of these in the Semiconductor section of our E-books.
    Link: http://www.allaboutcircuits.com/vol_3/index.html
    See Chapter 4.
    OK that is an operational amplifier, commonly referred to as an opamp.
    See Chapter 8 in the above link.
    Those are N-channel power MOSFETs. They are somewhat similar to bjt's in that the current path through the device is controlled by one terminal of the device. However, they are quite different.
    That's always the problem. ;) I'm afraid that I don't have the time to go into all that at the moment. You can get a head start by reading up in our E-books, as it's important that you understand how the thing is going to work.

    A resistor pack is how they did things in the "good old days". Just for fun, look up Gar Wood and his golf cart speed control mechanism. Gar was quite a famous person for his boat racing exploits over a half-century ago, and was very creative.
    [eta]
    Link: http://books.google.com/books?id=x9...resnum=5&ved=0CBoQ6AEwBA#v=onepage&q=&f=false

    But, if you think that the PWM box is large, wait until you see how large and expensive the resistors you'll need will be! :eek:

    That would be a good idea. ;)
     
    Last edited: Mar 6, 2010
  8. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
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    Just a thought:

    You only need a high power fuel pump for a fuel injected engine.
    (Carburettor systems generally use very low pressures, just to supply the float chambers in the carbs).

    If you change the pressure to the injector common rail, you will also have to change the injection timing as less fuel will flow through an injector in any given time.

    Have you compensated for this somehow?

    Another thought - an injection system must have a pressure stabiliser of some sort to guarantee a fixed pressure at the injectors so fuel quantities can be accurately predicted.

    If you restrict the power to the pump, you could get varying pressures as the throttle changes (so the engine runs rough) or the pump could simply stall due to backpressure.
     
    Last edited: Mar 6, 2010
  9. GC89

    Thread Starter New Member

    Mar 3, 2010
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    It is efi and yes there is a regulator. Pressure is resistance to flow, at low load there is little flow from the injectors and thus takes little flow from the pump to maintain pressure, and it is also impossible for the pump to stall. At high load the pump speed will be increased to maintain that pressure. So the pressure at the rail will be constant, aside from a 1:1 rising rate. I will also likely have to pull from the fuel map with the upgraded fuel system but thats no problem.
     
    Last edited: May 26, 2010
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    I don't follow your reasoning there...

    If it's an impellor type pump, pressure is proportional to speed; lower speed = lower pressure.

    If it's a positive displacement pump, pressure is proportional to torque (= current) and restricting the current will again drop the pressure.

    I'm just trying to get my head around how it can work?
     
  11. trader007

    Active Member

    Feb 27, 2010
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    or why again? even if he does accomplish this, will the pump life really be extended that much? most pumps run 120K miles at least....
     
  12. GC89

    Thread Starter New Member

    Mar 3, 2010
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    A regulator will bleed off any flow above what is needed to maintain the set pressure. So if the pump is capable of maintaining the needed pressure at 60% of its max speed (which it is) then the additional flow provided by running at 100% of its capability is bled off thus making the pump work much harder for nothing.

    Most factory fuel systems facilitate a high low speed system controlled by the ecu or even a full progressive map to control the pump. This is why they last so long and when you spend so much on an aftermarket pump it seems pointless to do anything to shorten its lifespan, especially when it is unnecessary. Also if the pump were to give it up at the wrong time there goes your motor as well which at this point will be worth more than the vehicle originally was.
     
  13. GC89

    Thread Starter New Member

    Mar 3, 2010
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    Ok I have read throught the e-books and think I have a grasp on what each components function is. Not enough to be able to calculate the values of the components but to understand what they do and how to put them together.

    I also have another concern with the design though. A member of another board who has done something similar said he does not like using 2 mosfets. He said relying on them like that can lead to very bad things if one should fail, ie the other will fail castostrophically which is not something I want going on in my engine bay.

    I also thought of a simpler way to control it. A map sensor puts out 0-5v in a linear scale in relation to its kpa range. If I could put together a pwm that would ramp from 20-30% at 0v to 100% at say 2v it could work perfectly and function as a progressive map. 20% flow at idle and full flow by 40 or so kpa.

    The other aspect I failed to consider is the pumps start up draw. Its max is 21a but I would expect to see much higher than that at the initial start up of the pump so I would think that would need to be factored in
     
  14. GC89

    Thread Starter New Member

    Mar 3, 2010
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    Heres the complete circuit diagram of how this will be implemented if it helps at all
     
  15. Bychon

    Member

    Mar 12, 2010
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    Mouser part number 71-HL225-07Z-1.0 = 1 ohm, 225 watts $11.71 each.

    Two of these in parallel for $23.42 plus shipping.
     
  16. GC89

    Thread Starter New Member

    Mar 3, 2010
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    Bringing this thread back from the dead! This project was put on hold but I now have the fuel pump installed and all new wiring run and have the time to get to work on this.

    So I understand the schematic and all the components but have no clue how to determine values for them, can anyone help me out?
     
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