I have managed to get a stps2045ct Schottky diode, looking at the data sheet these are a dual diode, am I right in connecting A1 + A2 power in and K power out, if I connect it this way using all 3 pins does it then make it a 20 amp ?

Thanks

 

I'm pretty sure they are not intended to work that way. There may be slight variances in the forward voltage drop that would cause one of the diodes to carry the bulk of the current.

Having said that, since both diodes are made on a single die, they re likely to be very close in actual forward voltage. Given that your load is 12-15 amps, There is a possibility, but no guarantee, that it will work.

Be aware though, the package will be dissipating near 10 watts. You will need a fairly decent heat sink. Something rated at about 5degrees/W or better.


All in all, I do not think you should pr0ceed unless you are okay that the diode might fail. Perhaps a SBL3040PT or a NTST30100CTG would be better choices.

 

Thanks for the info, talking of heatsinks can any of these components that have to bolt onto a heatsink run to cold if the heatsink is to big, or does it work the colder that run the better

 

The cooler the better, in almost every case. As long as it is not cooler than the minimum operating temperature.

 

And a heat sink can only cool down to ambient temperature, not below. By ambient I mean the fluid passing over the surfaces of the heat sink, into which the heat is dissipated. This is often room air.

 

Keeping on heatsinks, what are the mosfet's that have no, what do you call it, top part with a bolthole in, what cools those down ??

Thanks

 

Small packaged MOSFETs exist and they are not meant to connect to a heat sink. I suppose they are not considered "power" mosfets and are being used for signal processing, not power. You need to consider the power dissipation specification for any particular package. The MOSFET makers like to quote huge amperages but in many cases the package itself cannot support that high current.

 

Thanks for that, when I ordered the mosfets needed for the project I ordered the wrong one to start with, I ordered PSMN017-30EL instead of PSMN017-30PL, these have the bolthole, looking at the data sheet the only difference I can see is 1 is a SOT78 and the other a SOT226 what ever that means ?

Thanks

 

The ones like your PSMN017-30EL, use a metal spring clip to hold them to the heat sink. This is one of the types used;

http://www.altronics.com.au/index.asp?area=item&id=H7295

 

If you are feeling brave, you can solder the MOSFET to a copper tab with a mounting hole in it and then bolt it to a heat sink.

That mounting clamp is almost as expensive as the MOSFET. It's probably best to buy the right MOSFET.

Keeping to the heat sink theme, it will be dissipating as much as 5W and has a junction to case thermal resistance of about 3.3 K/W so in order to keep the junction reasonably cool (65C) you'll need a heat sink rated at about 5 K/W.
This One or one like it would do nicely.

 

I have this:



It was massive, ive cut it down, Im going to drill a hole through it where the red dot is at the bottom, I also have a clip, I hope that will do the trick.
I also have been having a play, I mounted the Schottky diode and the LED, that works well, when the polarity is reversed there is no power at all on the board so would this work, ive taken away switch 1 and am going to (if it will work) put a 40 amp relay in there, the badly drawn wires are for the coil side of the relay

 

Well, if you are using that circuit, you don't need an enormous diode. One side of the one you bought ought to do it. I am assuming the 40 amp relay will switch the power feed to terminal 1, right?


Heat sink looks fine, just make sure the metal plate on the back of the MOSFET makes full and complete contact with it. A little thermal compound might be in order.

 

Hi, I have just used 1 side of the diode and yes the relay Im planning, if it will work, as a switch between J3 & J1, Ive just got to make sure Im not going to get a coil collapse voltage spike