No experience, Dave, but they should be OK. I don't know the laden weight of your loco + rolling stock, but it is probably something like a 2-person golf buggy, and (provided it doesn't derail and set off across country) the rolling resistance on rails will be much less than on a golf course, perhaps with sloping sections.

Andy

There was such a design in ME about 20 years ago. I've got the remains somewhere. FETs blow easily so don't forget the shottky diodes not ordinary diodes.

Not according to the datasheet it won't, but you have to know how to 'read' the datasheet to get the real story.

By the way the drain-source body diode isn't a Schottky diode, it's an ordinary pn junction diode. But it's a pretty good one, and the MOSFET is avalanche rated, so there's no need to add external diodes.

Regards,

Andrew

Neil,

The diode symbol shown is what I would use for a transient suppressor diode. So I assume it's an attempt to show that the device is avalanche rated, and that the diode can play a useful role rather than just being an annoying parasitic component. As I'm sure you're aware the Schottky effect occurs at a metal-semiconductor interface, whereas the datasheet talks about a pn junction diode, which is what you'd expect from a conventional MOSFET structure.

As Neil correctly says, the ratings are normally given at a rather unrealistic 25°C. For those that are curious consider the following. From the datasheet a continuous rated current of 86A and a Rds(on) of 8mohms gives a device dissipation of 59W. Now the 'thermal resistance' junction to ambient is 62°C/W, so the junction will be at 59×62=3658 degrees above ambient. Clearly the magic smoke will have long since gone! If we assume an ambient of 25°C and the maximum junction temperature of 175°C, to dissipate 59W we need a heatsink with a 'thermal resistance' of about 1.4°C/W taking into account the 1.14°C junction to case 'thermal resistance'. For air cooled, no forced flow, that's a pretty big heatsink. It's actually worse than calculated since Rds(on) is strongly temperature dependent and as soon as the device heats up it will dissipate more power and so on (see Fig10 in the datasheet).

The slightly sneaky part of the datasheet is the 86A continuous under maximum ratings. See the little note in brackets afterwards? Silicon limited? In other words that's the limit of the MOSFET conduction channel on the die. The next, lower, specification of 75A is case limited. So the upper limit is taking the case beyond it's rated current, even though the die itself is capable. This is reflected in Fig9 of the datasheet. In practise the device will not immediately fail if you pass more than 75A through it, but the longer term reliability will suffer.

Confusing isn't it!

Regards,

Andrew

Just out of curiosity why do you want an electronic control system ??

Three or four levels of shunt resistor and series / parallel connection of motors will do it all . This is what full size electric locomotives used for a hundred years or so – only in relatively recent times have more complex electronic systems been used .

Use a drum selector switch as your primary control and you could have all the fun of driving a small engine in same way as a real one .

A very good series of articles on building an electric locomotive with this type of control appeared late 60's or early 70's ME .

MikeW

Just out of curiosity why do you want an electronic control system ??

Three or four levels of shunt resistor and series / parallel connection of motors will do it all . This is what full size electric locomotives used for a hundred years or so – only in relatively recent times have more complex electronic systems been used .

Use a drum selector switch as your primary control and you could have all the fun of driving a small engine in same way as a real one .

A very good series of articles on building an electric locomotive with this type of control appeared late 60's or early 70's ME .

MikeW

Cheaper, smaller, less heat dissipation, more reliable?

Andrew

He said it. More effcient too, and for my engine which only has a 7Ah battery that makes sense.

Neil

He said it. More effcient too, and for my engine which only has a 7Ah battery that makes sense.

Neil

If you have to deal with huge loads and 24/7 working then look at lathe backgear systems

Electrics are almost removed from the equation

No overheating, no fires, and non stop hard work from your equipment

It's nice to have an electrical solution but sometimes mechanical cant be beaten