electronic speed control

Hi

my next project hopefully will be a large 5" gauge diesel, powered by12/24 volt batterys. I notice most people use a ready made speed controlers but they are very expensive, i`ve seen speed controllers for golf buggys or electric wheelchairs on ebay and they seem to fit the criteria volts /amps and they are very cheap. Does any one have experience of theses controllers?

regards

Dave

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

No reason why buggy controllers shouldn't work, it's esactly the job they are meant for.

I made a sophisticated controller for a few quid with remote control from a wired handset.

If you have basic electronic skills, most modern speed controllers use a rapid square wave with a varianble duty cycle to switch a high-current MOSFET.

A simple speed controller could be made from a heavy duty logic level MOSFET, a 555 timer IC, two switches (on off and reverse), a potentiometer and a handful of other components on veroboard for no more than £5-£10. The biggest issue would be preventing damage caused by switching direction while in motion.

Neil

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.

It's amazing what you can get these days.

This little beastie will handle 86A continuous with 10V on the gate and has a drain-source reverse biased shottky diode built in - for £1.17 - but I agree worth including some big discrete ones for protection..

There are even better ones, but even that could be the heart of a good basic controller.

I might do a little bit of playing in eagle and look at a simple microcontroller-free design.

Neil

P.S. This guy is controlling a 12V 400W motor without needing a heatsink: link shame he hasn't got a schematic.

Edited By Stub Mandrel on 30/04/2013 21:53:59

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

Andrew,

I don't doubt you, but why do they draw the body diode with a line that bends at the ends?

I know the rating is with the junction at 25C (unlikely scenario) but using other than the headline figure could confuse more?

Neil

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 59x62=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

than ks for the replies, much appriciated

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

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

interesting, not really thought about resistors, i assumed an electronic version would be more efficient and reliable. i can see a heavy duty resistor would be more reliable, what about acceleration? would there be a jump or surge as i go faster? switching between resistor value`s.

If i was running at low speeds would i be wasting a lot of battery power?

my limited use of resitor boards was many years ago with model boats and they were a bit brutal with very little control at low speed.

cheers

Dave

I use resistors to control the speed on the power feed on my vertical milling machine, it uses a car windscreen wiper motor, as I have it, it only has 3 speeds, but that suits me fine. There seems little or no loss of power at low speed, the input voltage is 17V. Ian S C

I use a resistor speed control on the power feed of my vertical milling machine, it is 3 speed, a windscreen wiper motor, with an input voltage of 17V, works OK for me, been using it for about 15 years. Ian S C

Hi Dave ,

I purchased a 1200w unit from monster scooters thru ebay and fitted it to a golf trike for my nephews little one and it has been fine for near 12 months .

This unit did not have reverse but a relay fixed this .

I think they sell units with this feature but shop around .

They use a twist grip throttle but im not sure if they are a variable resistor or some form of rotary encoder as i bought the throttle unit at the same time and didn't bother to play with it .

I payed around $100 au delivered and this was at least 1/3 the cost of the original unit so i was happy !

No connection to them just a happy customer and for lower powered units i have a local supplier that is also good value .

Ian