That explained a lot. I didn't realise the trace was based on the LM1949. The higher the clamping voltage, the faster the shutting time. Can you control the clamping voltage on the LM 1949? there should be an input to that effect.

Most ECU clamp it at ~50V. But >250V is almost comparable to the ignition coil (~300-400V at the primary).

The main reason why I was interested in the FJO unit- it has destroyed one of our IDC monitoring circuit, now I know why.

 

I have found the output transistior specs. with your help, thank you.


The transistor can drive up to 3A continuous and 7 A at pulsed mode. Voltage drop across the device is 1.5V. So, if you connect a 4.5 ohm injector directly to this driver, it will draws ~2.77A, not 3.11A. Based on 14V.

It appears that the ECU can drive this 4.5 ohm injector all day long at 100% DC. 4W of heat dissipation at the transistor. 2W at 50% DC. However, There is a case temperature rise consideration. 12W is quite a chunk of heat to dissipate.

Question, how are the transistors mounted, on a reasonable size metal base? Can youi send a picture of the ECU internal so that I can estimate the temperature rise at full duty.

 

I dont have a camera, but they are mounted to the outer case of the ecu

 

I think I will try to make a very rough guess, based on the dimensions of the ECU.
The thermal resistance (Rth) should be in the region of 0.5C/watt in free air.

20W of heat will rise the temperature of the ECU case by 10 C at 100% DC. I think it will be quite safe to run 4x 4.5 ohm injector continuously without resistors.

 

I ran 1680 Injectors directly no resistors for three years straight on my old Apexi never had a problem with the injector drivers

 

Good to know that. Rap the car up for the winter yet?

Do you know what is the winding resistance is?

 

My electronics knowledge is limited, but I did pick up the P&H add-on board has a single active flyback circuit for the 4 LM1949 drivers on the board. So it sounds like it's designed into the board and not part of the LM1949 IC. Looking at the LM1949 data sheet " typ. application circuit", they do show a 33V "Zener" diode just afer the injector symbol which I'm guessing is for clamping the flyback voltage.

 

That's a good question. I believe the answer lies in the common usage of a PFC, and not in any factual evidence to support that it is an intentional decision.

I believe they run whichever resistor is necessary to keep the Power FC from shorting/running hot, being damaged, etc.


Considering that I run a PFC, I do believe I will be running an FJO box to aim for optimal injector performance (which I will probably try to find some 1600/1680's in a 2.5 to 3 ohm range. Also, I do not want to run low impedance directly on the PFC.

 

It appear the only way to set the flyback voltage clamp is the Zener. Are you using a high voltage drive transistor?

You can improve the vlave closing speed by having a higer clamp voltage.

 

I believe the FJO will take all the guessing work out of the equation. Until the user knows the maximum drive capability of their piggyback device, the FJO is the way to go.

You are absolutely correct to follow this route. Sorry for the ignorance, who makes the PFC?

 

No idea, other then the circuit is the same as the one on page 10 here.
http://www.national.com/ds/LM/LM1949.pdf

 

The PFC is the Power FC made by Apex'i. It is a very popular choice among FD owners and even a few FC owners (when using the wiring harness converter that a company developed). I don't know if it is the simplicity of a plug and play install (with a few wires pulled/cut) or if it is easier to tune, or if it is price point. It could easily be a combination of all 3.

 

So, Here I am, kicking a dead horse.

BUT!!! I found an example of what I was saying:
FJO supplies 4a/1a P&H signal.

Now look here:
http://www.rceng.com/Peak-Hold-Injec...600-P42C5.aspx

RC P&H injector only NEEDS 2.5a/1a to operate at maximum performance

So this driver box really is a better option over using resistors.

There are examples of some injectors where this driver would not be optimal, and many examples where it would be optimal. (when compared to using resistors)

 

Some of these calculations are a bit over my head... I'm running the Rochester 1000cc injectors that are about 2.0 ohms with the FJO driver... Was it a better option for me??



.

 

Like you said , give it a rest and stop beating the same dead horse.

 

Chances are that you are running the injectors optimally. However, to truly know, you need to know what signal the injector itself wants to see at a minimum. The manufacture should be able to supply that information.

I know it is a dead horse... but now it is time to get into the nitty gritty details. Which P&H injectors will be best ran by the FJO... and which ones are best ran by a different driver. etc.

 

This is interesting. If 2.5A is good enough to give maximum performance, then putting an inline resistor of 5.6ohm is as soon as a P&H driven system.

This is probably due to the inductanc of their injector winding. It limits the switch on time.

 

So what's up with Crispeed's post, stating that you can use the FJO P&H box also on the OEM 550/850 injectors??

 

Not exactly. It means that the FJO driver will be able to close the field sooner allowing the injector to close at a more precise timing. A constant 2.5a holds the field open longer after current is cut off.

 

It will depend on how fast you allow the field to collaspe during switching off. The difference between field collasping at 2.5A and 1A the only difference.

If the clamping voltage of the P&H is lower than the OE+resistor, it can be worse.

 

How would FJO "supply" a 4 amp peak signal? That injector has an impedance of 5 ohms, so given a 14V power source, the max this circuit could flow is 2.8A. Since current draw in a circuit is determined by resistance (of the injector) and power source, it's not possible for the FJO box to run this circuit with a 4amp peak.

Someone else earlier stated in this thread that it's possible FJO limits their peak signal to 4 amps, and won't allow over 4 amps when an injector's impedance is below 3.5 ohms (given a 14V power source).

 

I have been meaning to give FJO a call and see if it is an adaptive 4a signal. How many ohms will it supply 4a to, etc.

I will post up what they say once I call them.

 

Please do. While you're at it, these questions would be good to ask. I will also give them a call and post my findings.

When you have injectors with impedance low enough to draw more than 4 amps: ex. 14V divided by 3 ohm (injectors) = 4.66 amp draw in the circuit. Would the driver add resistance to the circuit to keep peak amp draw at 4 amps and hold amperage at 1?

When using injectors with low impedance that will not draw 4 amps: ex. 14V divided by 5 ohm (injectors) = 2.8 amperage draw. Will the driver not apply any resistance to the circuit and allow a 2.8 amp draw for peak, and then apply resistance to the circuit for a lower amp draw for hold?

And if my above assumption is correct, then how much amperage would the driver allow in the hold stage for a 5 ohm injector? If it stays with the 4/1 ratio, then the hold stage would be 2.8 divided by 4 = .7 amperage draw.

 

Monkman33, I think you are over-complicating this, but you are on the right track.

Since the FJO injector driver is wired in series with the injector, really all it is accomplishing is adding resistance to this circuit after the peak stage. Think of it like this, straight battery power is used to open the injector (peak stage), and then on the fly, a resister is added into the circuit to allow less amperage to flow in the circuit like how many people use resistors already.

Since the peak and hold injectors are designed to work on a 12V system, then the manufacturer sets the resistance of the injector to control how much amperage flows in the circuit. So like I said above, let's assume the "straight battery power" that is used for the peak stage is anywhere from 12-14V.

14V divided by 5 ohms of resistance = 2.8 amps

12V divided by 5 ohms of resistance = 2.4 amps

So with battery power anywhere between 12 and 14V, the most peak amperage that can flow is 2.4-2.8 amps.

So to sum it up and assure you in any of your doubts, the manufacturers design injectors so that you can't damage them by flowing too much amperage through them with their resistance (ohm rating).

 

I sent the following email to FJO tech support. I'm pretty surprised, I sent the email last night (Friday) at 11:45 pm and got a reply this morning at around 11:00am.

LOW IMPEDANCE INJECTOR DRIVER Q

I am interested in this product and have a few questions. In your product description, it states, "Reproduces ECU injector pulses in PEAK & HOLD format (4A/1A)". How can I expect the driver to react when using an injector that will not draw a current of 4 amps in the circuit?

For example, considering a 14V operating voltage, if I use an injector with an impedance of 5 ohms, 14 volts divided by 5 ohms of resistance would mean this circuit would draw 2.8 amps.

Would the driver apply no resistance to the circuit and allow 2.8 amps to flow for the peak stage? And for the hold stage, would the driver apply resistance to the circuit to achieve a lower current flow? If this is the case, how much current would be allowed to flow for the hold stage? Is it set to hold the injector open with 1 amp, or would it maintain the 4:1 ratio and only allow .7 amps?


Onto the next question. How long does the driver let peak current flow until switching to the hold current? Is the current dropped after a set time? The reason I ask this, is because when using very large injectors for very low loads and duty cycles (idle and low load driving), the injectors will be open for a very short amount of Ms's. For example, if your driver is set to drop to hold current after 2 milliseconds of peak operation, and at idle the injector is only open for 1 millisecond, then would only peak current be applied in this situation? This will be for a ported turbo rotary, so if idle conditions can be improved with more precise control, that would be a nice benefit.

And this was the reply:

The KID0404 uses a complex algorithym to determine when it transittions from peak to hold mode. I hope you can appreciate that we do not publish details on the inner workings of our products as this is considered proprietary. However, I can say that the device will drive higher impedance injectors more effectively than most ECU's. The device does not do a 4:1 current limit but rather 4A and 1A independantly because limiting the current is not function of applying inline resistance. Obviously if the injector you are using doesn't draw 4A then it becomes the current limiter and not the driver. Unlike other drivers or some ECU's, the KID0404 has a very sophisticated method of determining when to transition from Peak to Hold. The design also takes into consideration that you are switching a coil on and off and not just a resistive load. They are different and this is where the KID0404 can really make a difference when tuning low duty cycles. This applies to low and high impedance injectors. The box was designed for racing applications that require high flow injectors, yet still offer tunability at very low load conditions. Because the KID0404 can drive 4 injectors independantly it is possible to mix injector types (low/high impedance). KID0404 MSRP $149.95

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