Injector Control Options

[Delta]

While staring at the circuit this morning I had a small revelation - is it possible the P type mosfet has drain and source mixed up in your circuit? I turned the P type mosfet around and ran a simulation and the current held just under 5A - so thats not far away from where you are.

[AbeFM]

It's certainly possible. To be honest, I've avoided FETs since I never wanted to bother learning another schematic, and I had friends bothering me while I put the circuit together.

Just to be clear... the source always has the arrow? Seems so. Anyway, sadly I woke up late, didn't bring the circuit in with me. I might go home during lunch to go pick it up though. I want to swap out for bigger phones anyways.

I mean, resistors.

[jharvey]

Source only points in on N channel.

Most uses for MOSFET are N channel enhancement mode. That means turns on with a positive voltage relative to the drain, and normally closed. If used as a high side driver rather then a low side, you'll see an N channel enhancement mode.

Good read here

http://en.wikipedia.org/wiki/Mosfet

[AbeFM]

While staring at the circuit this morning I had a small revelation - is it possible the P type mosfet has drain and source mixed up in your circuit? I turned the P type mosfet around and ran a simulation and the current held just under 5A - so thats not far away from where you are.

So far, so good, unfortunately. 1.215k ohms where it calls for 1.2k, but the op-amp, all mosfets and so far resistors check out. So far everything looks solid. I can still try bigger values, though.

[Delta]

Just to make sure we're on the same page with the N/P mosfet pair - The P type is MEANT to be upside down - I probably should have labeled drain and source on each. Basically the N mosfet has source side connected to the ground side of the circuit and the P mosfet has the source side conneced to the +ve rail directly. The mosfets both have diodes in reverse across them so if its in backwards it will not turn off - which is the idea with the P type - it stays on long enough to peak, then it turns off. The slight drop in amps rather than big drop looks exactly like P mosfet upside down. *shrug*

If its all connected properly like that - then yeah lets try the 0.1 1k or 0.25 2.5k combination. If its STILL not working properly then the 39k and 18k are the wrong values - although I can't understand how that could be, 5V/18k||39k * 100 = 0.0405V so it should peak at 4Amps and 5V/39k * 100 = 0.0128V so it should hold the injector at 1.28amps so that the voltage across the 0.01ohm is the same.

[AbeFM]

That's good you said that, since my next question was going to be "did you know the IC has diodes built into it".

Anyway, I'll give that a shot. My folks are visiting for a few days, so I'm likely to work on whatever project interests my dad the most: Pulling my motor/pistons, rebuilding a tranny, playing on the lathe, or working on this circuit. I might even go so far as to mess with the ignition on the bike.

I wonder what normal families do when they get together?

[AbeFM]

Question, how critical is the 10:1 ratio? Does that just make 40mv into 4 amps, so 11.2:1 would simply put in 12% more current?

[Delta]

You mean the 10,000:1 ratio? Basically the circuit is an opamp assisted current mirror. So yep if the ratio of the resistors is 10,000:1 then what ever the current is down the left side, down the right side it should be 10000 times greater. So if the values are all out in opposite directions by 6% (5% for the 1W/5W resistor and 1% for the other) then yeah you'll have approx 10650:1 or 9430:1. So 40mV might become 4.3Amps or 3.7Amps.

[Delta]

That's good you said that, since my next question was going to be "did you know the IC has diodes built into it".

Anyway, I'll give that a shot. My folks are visiting for a few days, so I'm likely to work on whatever project interests my dad the most: Pulling my motor/pistons, rebuilding a tranny, playing on the lathe, or working on this circuit. I might even go so far as to mess with the ignition on the bike.

I wonder what normal families do when they get together?

No such thing as a normal family Sounds like fun to me

[Delta]

Just wondering if there has been any further trials? I assume not or it would be in here hahha, but thought I'd ask anyway. How was the time with your family?