[Mr Modifier]

I'm thinking ahead here to when I start a portable rally track build next year.

I'm going to need a power supply and I would like to be able to dial the power down as necessary.

What do all you rally track builders do for power?

I see images of routed wooden tracks with banks of very muscular and expensive variable power supplies and it looks like a bit of overkill for rallying. I take it nobody is going to run a 40k motor in a scooby on twisty tracks.

I have found a cheapo variable power supply on evilbay that has a digital display of voltage from 0-15v and pushes our 2 amps for £20 all in. Is 2 amps man enough? Can I do better for the same money?

I do have a spare Scalextric power supply for the new APB (15V 4Amps) so the alternative would be to fit some form of variable voltage reduction thingy but it would be nice to be able to tell what the exact setting was so I could replicate it when needed. I know nothing about electronics but I am happy to solder in fine detail.

All advice gratefully received.

 

[300SLR]

2 amps is man enough for a "18k" motor.

A cheaper option is to use the 12 way switch and 11 diodes to adjust the output from the 15V 4Amps supply you already have. The bit's will only cost around 3 pounds, you'll have the extra current available if needed (that'll probably be OK for most motors up to about 40k, which sounds plenty for your needs!). That'll allow you to dial the voltage in 12 steps, mark the switch positions and you can replicate the exact setting.

 

[Lloyd.L]

Hi, I use a PSRT3 that I got from Pendles some time ago. The specification is shown on their website for you to compare. So far the output is more than adequate for my cars but they are 1/43 and dont pull too much current.

Having the adjustable voltage has proved essential on a hilly and winding rally track, only a couple of volts sometimes making the difference between a difficult drive and an enjoyable one.

Regards, Lloyd

 

[AAWSCC]

I'd go for the one you have spotted.
I use a train, Ugh! don't like using that word, transformer I got from Gaugemaster. Havn't got a clue what it's rated at, but the ones I use for the club track are only 2A. Never had any trouble.

 

[Mr Modifier]

OK - thanks for the info.

Lloyd.L I wasn't sure whether the option to vary voltage was essential so thanks or confirming. Don't really want to spend another £60 when I have almost got the solution.

300SLR - I like the idea of a 12 way rotary switch. I would be quite happy to fit the switch into a panel and connect up the existing 4amp transformer BUT I haven't got a clue what diodes I would need to buy for what voltages.

I guess I would never want to drop the voltage below 6 or 7 and would want the highest setting to be straight through. So the other 11 settings could range in 0.7volt increments from 7.3v up t 14.3v. Do you know what diodes I would need? (you see I would have assumed resistors so that shows how little I know!!!)

 

[carsy]

QUOTE (Mr Modifier @ 18 Dec 2011, 06:04) <{POST_SNAPBACK}>OK - thanks for the info.

Lloyd.L I wasn't sure whether the option to vary voltage was essential so thanks or confirming. Don't really want to spend another £60 when I have almost got the solution.

300SLR - I like the idea of a 12 way rotary switch. I would be quite happy to fit the switch into a panel and connect up the existing 4amp transformer BUT I haven't got a clue what diodes I would need to buy for what voltages.

I guess I would never want to drop the voltage below 6 or 7 and would want the highest setting to be straight through. So the other 11 settings could range in 0.7volt increments from 7.3v up t 14.3v. Do you know what diodes I would need? (you see I would have assumed resistors so that shows how little I know!!!)

This site shows the parts you need for a rotary switch and how to make it.

http://web.me.com/pmarchand/Slots/page21/p...e58/page58.html

Phil

 

[Mr Modifier]

Sorry AAWSCC - was typing my response when yours came in.

£20 doesn't seem bad if it's going to be strong enough for club use. I was going to volunteer the stage for the Wye Valley club as the club is less than an hour's drive from me - but only if the track would handle what the SlotRally GB competitors are known to run.

Is 2 amps enough or will 4 amps and the 12 way switch be a better bet?

Thanks Carsy - I will go look now.

 

[300SLR]

QUOTE (Mr Modifier @ 17 Dec 2011, 20:04) <{POST_SNAPBACK}>300SLR - I like the idea of a 12 way rotary switch. I would be quite happy to fit the switch into a panel and connect up the existing 4amp transformer BUT I haven't got a clue what diodes I would need to buy for what voltages.

I guess I would never want to drop the voltage below 6 or 7 and would want the highest setting to be straight through. So the other 11 settings could range in 0.7volt increments from 7.3v up t 14.3v. Do you know what diodes I would need? (you see I would have assumed resistors so that shows how little I know!!!)
Yes those numbers are about right. (You could use resistors, but diodes give a more consistent voltage drop with different motors)

3 amp rectifier diodes part number 1n5401 to 1n5406 (doesn't matter which) will do nicely. They are about 39p each from Maplin, or 10p each from Rapid Electronics.
If you wanted a lower voltage you could use 2 diodes rather than one at a step - each step you did that would give you another 0.7v drop.

EDIT
Carsy's reply came in while I was typing. The link he's provided is the same thing with a lower current diode option - 1 amp 1n4001. You'd probably get away with the 1 amp diodes, but it's probably better to spend a few pennies more and get a diode that's definitely man enough for the job.

 

[Mr Modifier]

I think I am getting this now.

As the diodes bridge the connectors can I assume the first position is always going to be straight through? Or should I bridge the first two with copper wire? I guess it will be obvious when I get the switch so sorry if that's a stupid question.

I was unaware that diodes drop 0.7v over 10ma so my choice of voltage range is a tad spooky!!

Will 3amp diodes be enough? (given the power supply is 4amps so probably peaks over 4 amps as I expect that is a constant current rating)

The website address supplied by Carsy mentions a 3amp and a 6 amp diode with a small price difference.

 

[300SLR]

The most common type of switches have one terminal near the middle , it connects in turn to each of 12 terminals round the outside, so you just connect the 11 diodes between the 12 termnals (making sure you've left the gap in the right place and the diodes are all the same way round) and take a wire from the correct end. You can work out which the correct end is, or just swap it over to the other end if you guess wrong. (It'll do no harm if you connect the wrong end, it just won't work)

Rectifier diode are designed to take huge overlaods for short periods, slot cars only take maximum current for short periods, so 3A will be OK for the sort of motors you are talking about.

The voltage drop across a diode goes up with current, they are usually speced at about 1 volt max at the full rated current and typically 0.6 v at low currents. The voltage drops as the diode warms up, so its difficult to give an exact figure. The motors you are talking about would probably not take much over an amp when up to speed, so 0.7 is about right.

 

[Mr Modifier]

Brill.

300SLR thanks so much for your help

I went to Rapid's website and ordered the switch, 12 diodes (a spare in case a leg breaks off when I'm making the switch!), a snazzy aluminium knob with an arrow and a digital multimeter (always wanted one - now's my excuse to check the output voltages!). A tenner in total plus a fiver for delivery. Not bad at all. Cheaper than the evilbay option and I get a multimeter thrown in. The meter will be handy for testing track connections and lighting circuits on tracks and in cars.

Good result. Can't wait to try it out