Re check your attachment: FET right side up, pins toward you, L to R , G-D S, tab with hole -D.
In your test, did the motor run? With + V applied to S, source, connecting G via 51 oh
m Rs, to +V should turn FETs off, grounding turns FETs on.
Sense R ? depends on what schematic you are refering to.

 

As Bernard states the pin out is G D S. Also your picture is of a NFET (note the direction of the little arrow). Don't know if this would cause you to hook it up backwards or not. If I read the data sheet correctly on the chip you are using the maximum duty cycle is 44%. That means the motor will run at about 5 volts at its fastest. Maybe this is ok? Make sure you have the big diode across the FETs or they will pop.

 

Looks like I have swopped the tabs around, I have mine as Gate, Source, Drain not Gate, Drain and then Source...

So I will swop them over Bernard and get straight back to you... Thank you...

Ronv the image was something I quickly found on the net to try and explain what I was doing, Didn't realise I had grabbed the NFET not the PFET

The datasheet didn't show the pin outs so I looked on line and obviously got sent down the wrong path... :-(

Where did you see the Duty Cycle information, only I cannot see where it mentions it...

What size diode should I be using?

What is diode D3 in your circuit being used as Ronv, why use a low drop power schottky rectifier... you show a STPS3030CT in line with the motor is this what you are recommending that I use? It looks like yours is possibly a 30A rated one, would I need one rated for maximum load?

 

She popped in style, I have never had flames coming from a circuit before...

Only tried it briefly, before the circuit burst into flames obviously I had already damaged the components when I had reversed the connections...

Largest diode I have here is a P600G not sure if that is suitable...

So obviously I need to go back to the drawing board as I am very surprised that she blow so easily, but only the one IRF4905 blow so dramatically but does this mean that they are not man enough for the job?

Is it pulling more current than I have anticipated on startup?

Should I use 4 x IRF4905????

Is the diode to prevent back EMF? only you have it drawn on the schematic Ronv opposite to how I would have expected it? or is it because it's a low drop power schottky rectifier that they work differently...

 

Starting current may be in the order of 240 A, depending on other V losses in battery & wiring. I'm not really sure how many FETS in parallel that are really needed; seammed that 4 was close, I used 3 for a 250 W motor, & wished fore more. Think if I were going to power a starter motor now it would be rated for 1.5 X stall current even if 1 would carry running load. " Some" say that snubbing diode should be same rating as load current. A dual 30A diode, leaning heavily on peak rating may work.
The experience I've had with starter motor driven cable reels [ 48" wide, flange 28" dia, with 2" friction drive wheel, approx 200 lb ] was a wild ride picking up 1/2 in cable at 10 to 30 mph. Lost a glove or two but never an arm. I later made ball bearing front end plates for the motors which trippled the life. Where was OSHA back in the 1950s?

 

I may be mistaken about the duty cycle. I saw it under the comparator section. But you have the circuit built. Will it go from 0 to 100%?
The diode needs to be rated for 50% of the maximum current in the motor.
We are thinking the start current of your motor may be 200 amps with the run current down around your 67 number.
The diode is to clamp the inductive kick from the motor when it is turned off. When the FET turns off the motor inductance wants to keep current flowing so the voltage "kicks" negative. The diode keeps it from going below ground.
I just read your very first post again. Do you only need this to rewind your spool after a run? If so maybe there are other ways to do it.
If not we probably need lots of FETs since we don't know how long it takes for the motor current to drop back to 67 amps.

 

I was using two fets but they popped instantly I am going to give 6 a try on the next run, connected in parallel on a very large heat sink...do not ever wish to reverse the motor have no need for it... Just require speed control nothing else... As you can see from the scope at max I have an equal mark to space ratio...

Initially I wanted to reverse the motor to feed the line out instead of pulling the line and back feeding the circuit with spikes... but there may be a way of protecting the circuit from such spikes...

So the motor on full load is rated at 67A, but I am currently not even getting close to that how can I calculate what she is pulling on initial startup...

Why a dual diode, what is the advantage with that over a normal diode?

 

If you didn't have the diode that would also cause it to go. The one you have is way to small. Something like this on a large heat sink might do it:
http://www.mouser.com/ds/2/427/100bgq10-49888.pdf
So at max you do have 44% duty cycle. If you could find a 6 volt battery you might try that.
We might be better off switching the ground side. NFETs are more robust so you might not need so many, but you didn't sound like you wanted to do that. Any reason?

 

Would this do the job, just placed an order for one...

STTH6002CW ST Dual 30A 200V 22ns TO-247 Ultrafast Common Cathode Diode

No reason, will give it a try just not something I have ever done so seemed alien to me...

So 44% duty cycle is equal mark to space ratio?

Not able to find the diode you specified only one I can obtain is in Hong Kong, so will take weeks to arrive...

How can we get round this?

 

No reason, will give it a try just not something I have ever done so seemed alien to me...

Making the case motor frame positive is nothing new, automobiles used to be wired that way.
Max.

 

Did they, I never new that...

What is the reason for doing it that way?

 

IIRC it was to do with helping prevent corrosion at frame connections, or so the reasoning went?
Also it was back when a DC generator was fitted instead of Alternator, as is now.
Max.

 

Right on, Max; 6 V + ground mixed with 100 A alternator with - gnd. 6 V starter motors for reel which stayed after 12V - gnd. systems came along. Boy did they hum on 12 V, really 14V as thc trucks were running when reel was in use. Gradually were changed over to 12 V motors. Adding a large, small value resistor in series with motor, guess about 50 m ohms, really helped in taming the beast. Dont remember construction, but was made from # 9 bailing wire & ran hot.

 

Disclaimer- I have not built this.

Take a look at this one. It switches the case of the starter and has what seems to be a pretty good current limit.
Still seems like overkill to send a 1 horsepower motor to do a one bunny job.

 

So I now have the new IRF4905 so will be using 6 of them in the next trial run, and I now have the STTH6002CW ST Dual 30A 200V 22ns TO-247 Ultrafast Common Cathode Diode will that do the job and save the IRF4905...

How should I connect it as it's 3 pin, and don't wish for it to go bang on me...

I know it seems like over kill, but it's the standard way of doing things except I wish to tweak it...

It's much better than having to run as fast as you can holding a length of string, which does not appear natural and does not train the bird in any way in fact it's the bird training you...

 

Just teasing..

Your diode probably isn't big enough, but you can give it a try. Do you have things on a big heat sink?
Where do you buy parts? Maybe we can make better suggestions if we know what you can get.
On the diodes. Tie the 2 anodes (A1, A2) together- they go to ground. The cathode to the FET.

 

Not got too many choices, I mainly have to use ebay, Farnell and Rs do not happily supply you without you being a business... Nearest trade counter is over an hours drive away... maplins is useless never got the parts you require or the stock available... All other parts on ebay are from Hong Kong and standard delivery is a month... So I'm a tad stuck, plus parts are so expensive these days...

 

Do you have an auto wrecking yard or used auto parts seller near you? Car alternators have six high value diodes inside them, usually mounted to a PCB board. Three of the diodes have the outer case as positive and the other three have the outer case negative. Heres a generic link to what they look like, http://i01.i.aliimg.com/photo/v0/11703097/Alternator_Diode.jpg

 

All scrap yards round here have gone... hence any parts required I am having to search ebay for... long gone have the days where you could nip to the local scrap yard and have a mooch around to see what they have...

Health and safety put a stop to all that, so now you have to search for the part you require online...

http://www.asm-autos.co.uk/

 

Attached is the updated circuit, hence the largest heatsink I have to hand...