MOSFET use

[alec_t]

could two MOSFETs replace the Darlington?

Each Darlington could be replaced by a respective MOSFET.

Does this mean: it needs a high Gate voltage (potential) (10V not 6V) to create the Field required

Essentially yes (though you might be lucky with 6V). According to the datasheet it may need a gate voltage,Vgs, as much as 4V just to get a drain current of 250uA! With Vgs = 10V the Rds is 0.4Ω, which is relatively high by today's standards and means the FET would be dissipating 0.4 x 5 = 2W minimum. With Vgs only 6V the Rds and power wastage would be greater.
Look for FETs with a lower Rds(on) rating (e.g. IRL1404, IRFZ44N). The Rds(on) ratings usually are for Vgs = 10V.

 

[alec_t]

Following up earlier suggestions to use a 555 timer rather than CMOS, here's how I'd do it:

This uses a single 555 monostable to define the hold period (adjustable), and an R-C differentiator to define the fire period (adjustable).
The relevant waveforms are:

Note that the Fire and Hold pulses are shown as 12V because an ideal 555 was modelled in the sim. In practice the voltage out from a 555 would be closer to 10V than 12V.

Edit: A 5.6Ω resistor in series with Schottky diode D2 will reduce the switch-off time of the injector pulse.

 

[audioguru]

The IRF630 Mosfet is so old that I had trouble to find its datasheet. It has a high voltage rating (in those days) of 200V which is another reason that its on-resistance is high.
The printed text in its datasheet shows that its maximum on resistance is 0.4 ohms when its gate-source voltage is 10V.
The graph in the datasheet is for a "typical" device that you cannot buy. Some are better and some are worse. You get whatever they have. The numbers for a "maximum" on-resistance device are guaranteed for any of them. The graph at room temperature shows a "typical" device conducting better than a "maximum" device.
With a gate-source voltage of only 5V then a "typical" device conducts Amps but a "maximum" device barely conducts less than 1 milliamp.

 

[alec_t]

A thought. Considering the injector coil resistance is only 0.15Ω, why do you want to use a 15V supply? That means wasting a lot of power in the dropper resistors in series with the coil.

Edit: Presumably the 15V is to give a more rapid response?

 

[audioguru]

A fuel injector is not a continuously flowing nossle so I think it spits fuel for only a moment. Its inductance slows down its opening time so they blast it with a high current so it opens fast enough.

 

[shortbus=]

Not knowing what Delphi injectors the op is using.... the 15V is probably not enough. The injectors for "common rail" diesel EFI use 50V as the standard drive voltage. They also switch on the low side, not high side like he wants to do. They (injector coils) are 'hot' on the high side, switching to ground to fire. All EFI systems I've worked on do it this way.

 

[malc9141]

1) MOSFET model: I'll try to follow this advice.

2) The Injector is a Delphi EJBR02101 Z, common in Euroland.

Its interesting you say 50 v. When these are bench tested, for servicing, 12 v is the norm. We found we needed 15 v or more with our home made circuit - this before there was the info available (Industrial Secret?) that is available now. Because there were several transistors in play, the final voltage would have been about 13 v. But it needed that, the way we had it.

It worked fine but maybe not most power-lean. As told above, we seek a new better circuit owing to creeping unreliability.

And we were using the coil between the + 15 v and ground. As advised above, it will be with better FETs. Thank you!

 

[malc9141]

§42. the 555 circuit.
Thank you. Most interesting
You say: Note that the Fire and Hold pulses are shown as 12V because an ideal 555 was modelled in the sim. In practice the voltage out from a 555 would be closer to 10V than 12V.
1) would this reduction in V mean a lower current would go thro' the FETs? Should it be remodelled slightly? We need about 4.5 A for Fire, half that for Hold.
2) The sim looks very impressive. Presumably the Fire period could be reduced if C5 was smaller.
3) Presumably the timed "Hold" pulse from the 555 can be shortened to go to Fire (near bottom of scheme) because of the C5 and smaller Trim2.
4) The different currents for Fire and Hold in the 15 v circuit are because the Vgs is different because of the voltage divider for Hold?

 

[audioguru]

§42. the 555 circuit.

You say: Note that the Fire and Hold pulses are shown as 12V because an ideal 555 was modelled in the sim. In practice the voltage out from a 555 would be closer to 10V than 12V.

The datasheet for an LM555 shows that with a 12V supply its typical output is +10.7V into the 330 and 10k series resistors. Then the gate of the Mosfet gets +10.3V.

1) would this reduction in V mean a lower current would go thro' the FETs? Should it be remodelled slightly? We need about 4.5 A for Fire, half that for Hold.

The datasheet of the modern IRF1404 Mosfet shows that with Vgs at 10V its on resistance is typically only 0.0035 ohms which is less than a piece of thick copper wire.
Since an injector is 0.15 ohms the single 5.6 ohm resistor in series provides a current of 15V/(0.0035 ohms + 0.15 ohms = 5.6 ohms)= 2.6A for hold and two Mosfets and 5.6 ohm resistors provide 5.6A for fire.

2) The sim looks very impressive. Presumably the Fire period could be reduced if C5 was smaller.

correct.

3) Presumably the timed "Hold" pulse from the 555 can be shortened to go to Fire (near bottom of scheme) because of the C5 and smaller Trim2.

C5 with trim2 and R3 shorten the pulse from the 555.

4) The different currents for Fire and Hold in the 15 v circuit are because the Vgs is different because of the voltage divider for Hold?

No.
Hold uses a single Mosfet and 5.6 ohm resistor. Fire has both Mosfets and 5.6 ohm resistors for twice the current.

 

[alec_t]

The trimmers T1 and T2 are adjustable to allow both the Fire and Hold periods to be shortened (or lengthened) independently as required.

Edit: The minimum effective Fire period is limited not by the Trimmer/capacitor but by the rate of rise of current in the inductor (coil). That rate can be increased only by reducing the inductance (not possible without using a different injector) or increasing the supply voltage above 15V.

Edit2: With both trimmers set to minimise pulse width here is what should be attainable with C5 as is.

 

[alec_t]

So far, fixed timing for the injector has been considered, but in practice won't it need to be varied dynamically according to engine speed? If so, a microcontroller would be more appropriate than the above circuits for the timing function.

 

[malc9141]

First of all, many thanks to everyone for all this help. But esp to alec_t. This circuit is a breakthrough for me. (My colleague who built the original is now most intrigued and now promises to build "his circuit" but which has been the story for > 2 years).
Responding to 49 - 51:
*Of course - two circuits give twice the current.
*Interesting about the voltage - I know this, but only qualitatively - your quantitative data is brilliant. I think you have it right, and from experience (@ 13 V) we need about 0.7 ms to Fire. So your set-up is well in. Lengthening Hold is easy.
*Fixed timing: No; all we do is have a Pot on your Hold variable resistor near the 555, and turn this for more power (it works OK).
We are interested in running at a particular engine speed because aircraft props are so designed. We seek more torque about a small rev range.

These Posts have taken a huge weight of my mind. I am truly grateful. I plan to put it together and ...we'll see. Fingers crossed.

 

[malc9141]

Just adding (Post 50). When we began we found 12 V would not Fire (with our circuit) but 25 V (two small wet batteries in series) would. This 25 V will Fire given a shorter Time Interval than with the 15 V. As you predict. But I'm talking about fractions of a ms.

 

[malc9141]

On the circuit, post 42, could I put an LED between R4 and R5 ?

On the original circuit, a flashing LED is very useful.

 

[alec_t]

could I put an LED between R4 and R5 ?

Not the best place to put one. Here's a revised circuit with the LED (D3, your colour choice) connected to the 555 output via a current-limiting resistor R10. Note also added components C6 and R9. C6 and C2 should be connected close to the in and out pins, respectively, of the 7812 regulator. R9 helps to shorten the tails of the Fire and Hold pulses while keeping the FET drain voltage below its rated maximum.


Edit: The circuit shows the FETs as IRF1404, but many other suitable (Rds_on <50mV, Vds >=40V) FETs are available.

 

[Mosaic]

Glad to see the OP is getting results, I was tied up on another project this week.
The switching time of the FETS can be improved by using a low side driver chip to amplify the gate current as the 555 drive current is limited. This might improve the time by 50 to a 100 nanoseconds depending on the total gate charge of the MOSFETs used.
Charge Q= As (amp seconds), thus s= Q/A; Total Gate Charge over avg Current. More current = less time.
An MCP1407 driver chip comes to mind.

 

[malc9141]

Have circuit but timing not working. Dry joint? Looking.

 

[Mosaic]

I take it you built the same circuit in the schematic?
1) You can trace backwards. Verify that the 555 isn't putting out the pulse. 2) Then verify if the Capacitor C4 isn't doing a charge/discharge cycle.

If (1) is good then the prob is in the area of the gate drive of the MOSFET or the MOST fet connections (Drain /source switched)
If 1 is bad and 2 is good then check the other 555 connections and then swap the capacitor and then the 555 if the cap swap doesn't fix it.. However, I have never seen a bad 555 yet.

 

[malc9141]

1) Yes
2) I think it'll be OK. Don't have quite the same "feel" for the 555 as I developed for the 4013 and 4093 but I'll report.
Thanx for interest tho'

 

[malc9141]

Hello:
The 555 circuit is beautiful. I've delayed replying because I've been "shut out" of the Mech Eng workshop while they deal with real customers. So I bought a finicky Salea device and confirmed Alec_t's timing (if not voltage). I made a little 555 oscillator (circuit found on the Web) to run at "1500 rpm". The Yellow LED flickers.

I show the picture.

But the reason I write is:
On the schematic, the RST is linked to the 12 V input, but not to the timer circuit. Is that how it works???? The timed output ENDS and the RST just comes back into play?