# Voltage Adjustment - Pot vs. Diodes ??



## Dslot (Sep 2, 2007)

In the *Switch Variable Power Supply* thread, *AZSlot Racer* sez:


> ...I have mostly magnet cars and your deeper into the controller but seldom wide open, the AW cars I have still spin their tires relentlessly. I'm adding a 5k pot to dial it down for the kiddos ...


I've been planning to build Driver Stations with this  *diode-based stepper* to allow each driver to adjust the voltage going into his hand controller for best response with his particular car, and to reduce voltage for kids and learners. I'll be running mostly Tjets and JLs, old AFX Mag and non-Mag, and occasional old Tyco and current Tomy magnet set cars - no serious super magnet cars, just social fun and nostalgia).

*THE QUESTION:
Would a pot (potentiometer) be a better choice* than building the stepper? If so, *is 5K a good value* for the pot?

My power source will be old Aurora 20-volt (12VA) powerpacks - one per lane. Each pack has a 470mfd-35V capacitor across the terminals to filter the DC for cooler running. Gonna use set controllers - I've got plenty, in different ohm ratings, and will decide which ones, after I test them with the final electrical setup. (I know Parmas are smoother, but don't want to spend the money to get four or five matched ones - I've got a lot of Hong Kong painted spectator figures to buy).

One other option - use old *Aurora wheel-controllers* (rheostats) instead of stepper or pot. Anybody got any experience with that?

I am just about to order the diodes and switches for the steppers, but thought I'd make one last try to see if anybody with more electrical knowledge or experience had any thoughts about the alternatives. Thanks for any input.

-- D


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## toomanyhobbies (Mar 27, 2005)

Old wheel controllers aren't very useful in that the range of adjustment is pretty small. Any setting lower then just off full throttle (on the wheel controller) is too low to start the cars off a full stop. There is a little adjustment, just not too much. I tried this when I got back into slot racing the "first" time in the late 80's.  

You'll need more then just a pot to adjust the voltage to the track. Here's something I was looking into just this morning:

http://www.electronics123.com/s.nl/it.A/id.440/.f

It's a kit so it requires assembly and either a panel or box to mount it .

It looks like you are trying to do this on the cheap so i would think the diode controller route would be the best for you. I have no experience with the diode controller but if you try it please post with your results. 

Dominic


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## AfxToo (Aug 29, 2003)

It depends on what you plan to do over the long term. When you look at pots and rheostats you need to look at the maximum current rating. The Tomy wall wart puts out around .3 amps which allows a wider range of pots to be used. But if you ever want to upgrade your power supply you'll find out very quickly that the current draw for any magnet car is beyond the max current rating off most pots you'll find in commonly used ranges. The max current rating will be the differentiator. You'll find rheostats that can handle > 3 amps but they are very large physically and very low resistance rating, like 2-8 ohms.

On the other hand, it's fairly easy to find high power rectifier diodes (like Schottky rectifiers) that can handle considerably more forward current (20A-60A). Even the inexpensive rectifier diodes that Radio Shack sells are capable of handling a lot more current than that 5K pot. The downside of diodes is that you only get 0.65V steps and not a linear variability. Bottom line, check the max current rating first and never run it constantly at the max rating.

That 470 uF (microfarad) capacitor is not giving you much more than noise filtering. For ripple reduction I would go up to a 4700 uF 35V capacitor or two 2200 uF 35V capacitors connected in parallel (with matching polarity), whichever is cheaper.


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## Dslot (Sep 2, 2007)

toomanyhobbies said:


> Old wheel controllers aren't very useful in that the range of adjustment is pretty small. Any setting lower then just off full throttle (on the wheel controller) is too low to start the cars off a full stop. There is a little adjustment, just not too much. I tried this when I got back into slot racing the "first" time in the late 80's.


Just out of curiosity, did you try putting a lower-value resistor in parallel with the wheel controller? Seems like that would expand the range, but I'm not absolutely sure. My knowledge of electrics and electronics is mighty small (but growing).



> It looks like you are trying to do this on the cheap


 Yes, indeed. Ja, oui, da.



> so i would think the diode controller route would be the best for you. I have no experience with the diode controller but if you try it please post with your results.


 Will do. Thanks for the help, Dominic.

-- D


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## Dslot (Sep 2, 2007)

AfxToo said:


> It depends on what you plan to do over the long term ... The Tomy wall wart puts out around .3 amps which allows a wider range of pots to be used. But if you ever want to upgrade your power supply you'll find out very quickly that the current draw for any magnet car is beyond the max current rating off most pots ...


 Thanks, AFXToo, for the thoughtful and detailed replies.

You never can tell where a hobby will take you, but I doubt I'll ever need any more output than my old Aurora doorstop packs provide. (If I'm working Ohm's Law correctly, that's about .6 amp at 20V, per lane.) My main interest is T-jets. My layouts are small and temporary, for get-togethers with boomer-guy friends. The layouts are not suited to hi-performance magnet cars, and I'm not very interested in maximum performance and serious competition. If I have a car that doesn't perform very well after it's cleaned and lubed and obvious problems corrected, I just stick it in a lower racing class and let it have fun with its fellow underachievers.



> The downside of diodes is that you only get 0.65V steps and not a linear variability.


 I'm not sure whether you're concerned that .65V is too little or too much (i.e. not enough total range with 10-11 diodes, or not enough fine resolution with .65V steps). Unless I'm missing something important, a 7V range seems adequate (20V down to 13) and I suspect that the coarse resolution and non-linearity would be adequate for duffers like me and my friends. I want to be able to say, 'For this racing class, you might want to set the response knob to somewhere around five,' and then let each driver run some test laps to tweak it up or down to suit his own taste in response vs. speed. 



> That 470 uF (microfarad) capacitor is not giving you much more than noise filtering. For ripple reduction I would go up to a 4700 uF 35V capacitor or two 2200 uF 35V capacitors connected in parallel (with matching polarity)


 Yes, I discovered they turn into little M80 firecrackers if you're careless about the polarity. That 470 number is just blind following of a recommendation from years back. The cars do run a bit faster with it, so I guess it's doing _some_ good - mostly I want to keep them cooler and smoother. Capacitors are a bit mysterious to me. I know they fill and dump voltage. I guess they filter DC by filling up during the high parts of the cycle and dumping during the lows, smoothing out the curve, eh?

I'm assuming that noise is random small spikes in voltage, while ripple is the cyclic 0-to-Max voltage variation in full-wave-rectified DC, yes? Sorry for the ignorance, but could you please explain what each does to the car motor. 

-- D


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## Grandcheapskate (Jan 5, 2006)

Dslot said:


> My power source will be old Aurora 20-volt (12VA) powerpacks - one per lane. Each pack has a 470mfd-35V capacitor across the terminals to filter the DC for cooler running.
> -- D


 I am also currently running my four lane track on four Aurora power packs (one per lane), with a fifth available and reserved if I ever get back to using the Aurora electric lap counter.

But my question is about the capacitor. Is this capacitor simply connected across the terminal screws of the power pack? And all the other wiring remains the same? I assume this is done to reduce the amount of AC ripple?

Thanks...Joe


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## AfxToo (Aug 29, 2003)

Yes, a capacitor would be placed across the power supply output, matching the polarity of the power supply. Caps will try to keep the voltage at a steady DC value when the voltage would otherwise change. If the power supply voltage drops suddenly the cap will try to compensate for the drop by converting electrical charge in the cap into voltage. The net effect is the voltage across the power supply stays more constant than it would have without the cap. The amount of compensation the cap can provide is directly related to its capacitance. Larger capacitance values can provide more compensation. AC ripple is a long duration effect since it is tied to the low frequency household AC source (60 Hz North America, 50 Hz Europe). To compensate for ripple you need a larger capacitor because it needs to compensate for longer deviations in the output voltage. In general, caps designed to compensate for lower frequency devaitions tend to be physically larger and higher capacitance value.


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## Grandcheapskate (Jan 5, 2006)

AfxToo said:


> Yes, a capacitor would be placed across the power supply output, matching the polarity of the power supply. Caps will try to keep the voltage at a steady DC value when the voltage would otherwise change. If the power supply voltage drops suddenly the cap will try to compensate for the drop by converting electrical charge in the cap into voltage. The net effect is the voltage across the power supply stays more constant than it would have without the cap. The amount of compensation the cap can provide is directly related to its capacitance. Larger capacitance values can provide more compensation.


AFXToo,
Thanks for that explaination. From previous posts and Dslot's experiance, I understand that a capacitor can blow up like a firecracker if wired incorrectly (or fails?). I guess the size of the exposion is proportional to the size of the capacitor. But, if the cost does not get too high, it sounds like it would be worth wiring in some capacitors if you are going to stick with the old power packs. Maybe put them into a plastic box? Might be a good use for all those plastic battery boxes I've gotten from the battery sets.

Joe


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## toomanyhobbies (Mar 27, 2005)

Dslot said:


> Just out of curiosity, did you try putting a lower-value resistor in parallel with the wheel controller? Seems like that would expand the range, but I'm not absolutely sure. My knowledge of electrics and electronics is mighty small (but growing).
> -- D


No, not on the controller but I did try resistors in series with a wall wart at first. I tried all kind of Frankenstein experiments in the basement back then. You think I would have known better with4 kids growing up at the time!  I wound up using 4 dc model railroad transformers (one for each lane) that i had I had around the house, both rheostat and electronic, to control the voltage to the track. None of mine had enough amps to race seriously though.


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