I recently drove Q2 on...


I assumed that the body diode on the mosfet would take care of any flyback that the small solenoid would produce (attached to RY1)
On inspection it looks as though R13 has burned out, there is less than 100R on it now.
edit : the source has shorted to the drain which really could mean anything, but i believe it was down to no diode protection on the solenoid. It s possible (on closer inspection) that there was bad solder joints or even that the 12v lead was snapping on the joint. However tried RY2 side of the fet with the diode across the solenoid and seems to have done the trick.
The circuit worked fine without load, is this (probably) a case of no diode protection?
Although There is a series diode after RY1 (because its attached to another 12v switch).
Also can the fet's drains be tied together as to give more amperage?

 

Depends on the body diode specs, not all MOSFETs characterized or
designed with fast body diodes. You have to consult datasheet.

An article discussing this, attached.


Regards, Dana.

 

I assumed that the body diode on the mosfet would take care of any flyback that the small solenoid would produce

You need a diode across the solenoid coil.
With or without a diode I don't see how R13 would burn out.

 

I assumed that the body diode on the mosfet would take care of any flyback that the small solenoid would produce (attached to RY1)

It's not wise to make such assumptions. Not all MOSFETs have avalanche rated body diodes.

 

You need a diode across the solenoid coil.
With or without a diode I don't see how R13 would burn out.

Yes I did think that a tad unlikely - prob down to some bad joints, On that note does the placement of the diode in the circuit make a difference to its effectiveness? What i mean to say is this is a high side driver, so really i only have one wire going to the solenoid, the rest of the machine is a common ground with a 12v Pb battery. I have put a diode across the solenoid terminals, but this being the only 12v appliance would it make a difference if i had the diode on the battery terminals? FYI its a little 7hp generator

 

The MOSFET substrate diode, does not provide the flyback function, as the flyback voltage is in the reverse diode direction (negative at the P-MOSFET drain).
You need a diode across the coil to ground or from the MOSFET drain to ground (anode to ground).

 

It's not wise to make such assumptions. Not all MOSFETs have avalanche rated body diodes.

...learning everyday, thanks

 

The MOSFET substrate diode, does not provide the flyback function, as the flyback voltage is in the reverse diode direction (negative at the P-MOSFET drain).
You need a diode across the coil to ground or from the MOSFET drain to ground (anode to ground).

So what if i have a uC(5v) sharing ground with 12v, do i assume this is a bad thing for the uC if the transient can reach the common ground?
i.e. diode from drain to ground rather than across coil

 

hi,
The most likely cause of R13 burning out is that you used a 10R instead of 10K.
E

 

hi,
The most likely cause of R13 burning out is that you used a 10R instead of 10K.
E

the thought had crossed my mind, but im sure thats not the case, i replaced the resistor and the case is that the source had shorted to gate.
edit
Also just replace fuse in DMM to take amps off solenoid
flab
ber
gasted
2.2A and who knows if thats even correct with this old thing.
That maybe explain why the 4.9A rated mosfet died!

 

hi,
Applying 12V across a 10k resistor will not burn out the resistor, its only 14.4mWatts.!
If the FET has developed an internal short it most likely the 10K was the wrong value, too low and the excess Drain to Source current has blown the FET.
E

 

hi,
Applying 12V across a 10k resistor will not burn out the resistor, its only 14.4mWatts.!
If the FET has developed an internal short it most likely the 10K was the wrong value, too low and the excess Drain to Source current has blown the FET.
E

That would sound like whats happened. anyway i had better luck with the other half of the fet (with the diode in place)

 

anyway i had better luck with the other half of the fet

Your usage of FET (and it should be capitalized) is ambiguous. There are probably dozens of different types of Field Effect Transistors. In your case, you're probably using a MOSFET.

 

Your usage of FET (and it should be capitalized) is ambiguous. There are probably dozens of different types of Field Effect Transistors. In your case, you're probably using a MOSFET.

Noted. thank you, yes it is a MOSFET

 

So what if i have a uC(5v) sharing ground with 12v, do i assume this is a bad thing for the uC if the transient can reach the common ground?
i.e. diode from drain to ground rather than across coil

The diode suppresses the voltage transient to be just the forward drop of the diode (about 0.7V)
The current transient to ground is equal to the solenoid current, so that likely wouldn't cause any problem, but connecting the diode directly across the solenoid will eliminate that as a concern.

 

The diode suppresses the voltage transient to be just the forward drop of the diode (about 0.7V)
The current transient to ground is equal to the solenoid current, so that likely wouldn't cause any problem, but connecting the diode directly across the solenoid will eliminate that as a concern.

Nice one crutschow. I think overall thats going to be the best way to implement it - nearest the source and it can take care of the starter motor bemf. It would have been nice to implement it on the protoboard but looking at it why bother when its just a single diode to take care of two problems.

Thanks everyone for your help. And now for something different.

 

this is my ignition read circuit:



I dont know what happened to the transistor but some how it turned out to be PNP(should be NPN)
However the idea is that ignition 'spark' is fed (from right) into 50K pot, which fires base of transistor, which feeds the uC with a pulse. The 10K is to pull the line down during pulses.
Now the reason i went with this is because it was easy to implement and it works - albeit sometimes a bit of fiddling with the pot to get the pulse reading on cue.
Ive looked at the wire wrap around the HT lead but that posed more challenges than I could handle and when it started reading when the radio was turned on/off that was the final straw.
As i was saying this seems to work fairly accurately but can anyone point out any better idea?
I also tried clamping the signal but that didn't seem to work. (maybe tho due to duff components) I would revisit the idea if mentioned here.
The other thing to note is that the ignition spark is fairly tame, although it still produces 22KVish its has a trigger coil, (transistor) that actually fires the primary, this is where im getting the pulse from. not 300 or 400V.

 

Did you provide the correct partial schematic? Your showing a BJT part number and symbol but calling it a mosfet. If your dealing with the primary of an ignition coil, when you talk about "the primary", after the first spark there is ~400V on the primary, an induction coil works both ways in other words it no longer has the original 12V on it after it makes a spark.

Or I am simply missing something the others here are seeing in this, hence their way of answering you.

 

Or I am simply missing something the others here are seeing in this, hence their way of answering you.

Sorry shortbus, Im getting things in a twist.

Originally was discussing The MOSFET but I then moved onto another circuit just before your post (POST#16). Really sorry should have started another thread - so your right talking about MOSFET, that was the original post but i asked in my last post about transistor.
The ignition i refer to is for single cylinder generators, it does not require voltage to produce a spark - a magneto ignition i think they call it - magnet on a flywheel passes the coil.....

 

Really sorry should have started another thread

You could ask the Mods to shift it?