Injector Control Options

[jharvey]

Hmm, don't you need two V signals? One for each current. What did you do for the other V signal?

[AbeFM]

Hmmm, maybe I have to look at this a bit closer. I thought one "V" was the +12V the injector sees. The other "V" was the +5V square wave drive signal which turns on the circuit. For that matter, what's "each" current? I had two injectors IN PARALLEL to get the ohmage down and the currents up. Both my "low Z" injectors were 2.3 ohms

I think I must be missing something.

Anyway, one thing doesn't make sense: How was I seeing 7+V on that 0.01 resistor? Methinks the probe didn't stay where it should have. I did flow plenty of fuel in my little test, though.

Have to look at it again, but I'm sure curious what I should look at.

[jharvey]

One thought about the 7V thing, your pics show the scope is 1X, but many probes come with 10X as an option, sometimes switchable, some times hardwired. One way to check is to measure the resistance on your probe. I seem to recall the 10X has a meg ohm in series, and 1 X is close to 0R straight wire.

Still .7V across .01R is 70 amps. Perhaps your scope ground floated. Another common problem in scopes is that the grounds will get fried. Often unexpected ground loops will burn up and damage the grounds wires leaving you with unknown impedance on the ground side of the reference. I've been annoyed enough in the past that I've salvaged a broken ground lead, and gave it a spade terminal. Then when I broke the ground later on, I could simply put another spade on it.

Can you get a trace that shows the the supply voltage, and confirm it isn't dropping to 7V when the injector is firing?

Looking at the pg 2 schematic, I see V1, V2 and V3. Can you draw a schematic of what you have put together? Paper and pencil is good for this. You mentioned MS for your drive signal, where and how is it connected? You know hip bone, and leg bone kind of information.

[AbeFM]

Well, I might have to go recheck things.... I'm definately wondering why I saw such a change between runs. I tend to trust the first, noisey signal the most.

I built it per the schematic - the V3 I ignored, the inductor was the injectors, R9? (1.2kohm) was actually a 1.0k 1% and a 220 ohm 5% in series.

I had three screw terminals, one for each injector lead (one to +12V, one to the FET), and one 'signal in' which went to the input resistors, driven from the MS's +5V ign coil out - in my case a 2n222 that should drive an LED normally.

Power was run right to the board.

I'll get some pics, I don't have the scope today, and am leaving for the weekend tomorrow, though I was also wondering about the supply being drug down as well. Again, I trust the noisey traces more.

Anyway, any other suggestions on what to measure and I'll try to work it in next time.

[jharvey]

I'd like to see a sample of the FET's input. I'm curious if it's oscillating, or running as a analog voltage, keeping the FET in a transient state.

Also did the FET get hot to the touch?

[Fred]

Did you ensure that the MS was on high Z mode with the pwm disabled? That would screw up your reading somewhat.

[AbeFM]

Both the resistor and the FET were pretty warm. Neither were heat sunk. I wouldn't want to have to hide either of them when naked while that hot, but they weren't burning up either.

I was thinking I would put a thermocouple on each when I repeat the test.

What timescale switching are you looking for?

I think my next test will be, all relative to system ground:
Ch1:Across Sense Resistor
Ch2:Trigger In
Ch3:+"12"V
Ch4:Main drive FET's Gate

Sound right?

Did you ensure that the MS was on high Z mode with the pwm disabled? That would screw up your reading somewhat.

Are you sure you read twice and commented once?

I was worried about that, so I drove the whole thing off the spark output instead.

[Delta]

Well, I might have to go recheck things.... I'm definately wondering why I saw such a change between runs. I tend to trust the first, noisey signal the most.

I built it per the schematic - the V3 I ignored, the inductor was the injectors, R9? (1.2kohm) was actually a 1.0k 1% and a 220 ohm 5% in series.

I had three screw terminals, one for each injector lead (one to +12V, one to the FET), and one 'signal in' which went to the input resistors, driven from the MS's +5V ign coil out - in my case a 2n222 that should drive an LED normally.

Power was run right to the board.

I'll get some pics, I don't have the scope today, and am leaving for the weekend tomorrow, though I was also wondering about the supply being drug down as well. Again, I trust the noisey traces more.

Anyway, any other suggestions on what to measure and I'll try to work it in next time.

Erm you can't ignore V3 and V2 CAN NOT be 12V. The circuit is meant to be V3 is battery voltage and V2 (which supplies the circuit doing the current limiting) is meant to be steady regulated 5V. So if you had 12V at the top of the two mosfets doing the dual stage current limit then you will be limiting to roughly 2.4X what it should be - if at all - not sure what the mosfets will do with 5v gate and 12V Drain coz they aren't logic level fets.


I'd like to see the mosfet driven directly - just to see if the snubber is working etc - perhaps with about 10-12ohms injectors to check that it works fine driving High Z. If it works fine - then we start looking at possible failures in the limiting driver.

[Delta]

Adding to the previous reply - I just changed the 5V source to 12V in the simulator - and it gave me

No limiting and around 6 amps for 2.3ohms
Almost no limiting and around 7.5 amps for 1.7ohms

So it might be just that but we'll see I'm sure.

[AbeFM]

Erm you can't ignore V3 and V2 CAN NOT be 12V. The circuit is meant to be V3 is battery voltage and V2 (which supplies the circuit doing the current limiting) is meant to be steady regulated 5V. So if you had 12V at the top of the two mosfets doing the dual stage current limit then you will be limiting to roughly 2.4X what it should be - if at all - not sure what the mosfets will do with 5v gate and 12V Drain coz they aren't logic level fets.


I'd like to see the mosfet driven directly - just to see if the snubber is working etc - perhaps with about 10-12ohms injectors to check that it works fine driving High Z. If it works fine - then we start looking at possible failures in the limiting driver.

Oh, god, well, that explains why I didn't get the behavior I expected! I saw pretty much what you predicted, though. I'll have to do all this testing monday or tuesday, as I get back pretty late sunday from my trip.

So to be CLEAR, please fill this out:

v1=
v2=
v3=
The opamp uses +/- what voltage?
The dual FET chip doesn't take power?
....All grounds are common, and all listed voltages in reference to that same ground?
....and the drive signal comes in where?

I guess I made more assumptions in building that from the drawing than I realized. I powered the opamp off +12V (for more swing), too.

Hrrmmmm, well, hopefully the next one works a bit better.