Any Electronics Wizzards out there?

[Ray Marinaccio]

Hi,
I put a 3 volt motor From a portable CD player in a tyco power truck. Now I need to reduce the
voltage inside the engine so it uses the same powerpack, 18 volts. A
resistor may be all that's needed. BUT, I want a circut that uses the
first 5V for constant intensity lighting and power for other electronic
circuts, then what's left to control the motor speed using the rest of
the 5-18V to regulate the 0-3V motor voltage proportionaly. Sounds easy
enough. BUT, what if the other powerpack you're using is a 0-12V? Any
ideas?
If I can make this circut work and make it small enough, I can use
those matchbox size RC car motors in an N scale engine.You can get the
motors at Radio Shack.

Ray

 

[ezdays]

Ray,

First of all, welcome to The-Gauge. I'm just down the hill from you in a round-about way, Wickenburg. We used to go through Dewey all the time when we went to Prescott. To be sure, there's a train show in Prescott on the 16th and we will be there. They guy tells me they have 100 tables and they're all sold out.

Anyway, with you problem, if you try to drop 5 volts across a "constant intensity" light, there is no way you can get that and still have a variable voltage left to run the 3 volt motor. You could use a reistor to drop a proportional amount of voltage across it, but you'd have to know how much current the motor draws so you can size the resistor for the right wattage. Too big and it might not fit, too small and it will get hot and burn up. If you use the lamp in series like a resistor, it will vary in intensity along with the voltage from the power pack.

That's the best I can do with the information at hand.

Don

 

[Tyson Rayles]

One other thing, are you sure that motor has the proper torque to power the loco and still be able to pull any cars? Most of those type of motors are high on speed but VERY low on torque.

 

[ezdays]

Tyson,

Good point. I could probably design a circuit that would do what Ray wants, maybe not exactly the way he want to do it, but I don't advise it, plus he could wind up with a truck that couldn't even pull itself. It would be easier, and in the long run a lot cheaper to just go buy one of those LL Gp-38's that Model Expo has on sale this week for $14 and use the motor from one of those.

Don

 

[Ray Marinaccio]

Thanks for the welcome .
This is the first time on a forum.
This unit was originally intended to be a battery poered loco to pull a track cleaning car. But while testing it I accidently overshot the first notch on the throttle and the little bugger popped a wheelie and did a backflip, that got me thinking about other uses. Oh. I failed to mention I regeared it too.
You are right these motors don't have much torque. this one only draws 200ma under load. I dug up a 5W-75 ohm resistor which is more than enough.
We went to the show in Prescott last year, I was impressed. It was the first show we went to since we moved here from western Pa. a year and a half ago.
As far as the L/L GP38-2s I've got about a half dozen or so didn't pay more than a couple bucks each. Don't get me wrong I'm not bashing them, well actually I do, kit-bashing that is.

 

[jon-monon]

Welcome to the gauge, Ray! Glad you joined us as your first forum This is a fun place with great people.

Would something like this work? The constant lighting circuit would have to live between this circuit and "the rails". I understand the motor might not be viable, but I thought the circuit could be usefull with other motors. I have a couple of 5V motors that are small and would pull a Mack truck (the only question being what scale mack truck )

 

[Ray Marinaccio]

I can work with that, thanks

 

[sumpter250]

OK, I understand that the three volt zener diodes will permit a variable voltage across the motor up to three volts, at which point the zener will fire and shunt all additional current around the motor. What are the non-zener diodes, one in each leg used for?
A standard diode pack for constant intensity lighting could be used in series between the track and the motor/zener.

 

[sumpter250]

Took a minute but here's the modified drawing to show what I mean. This assumes the use of 1.5V bulbs.

 

[jon-monon]

As you may know, the zenners break down and conduct when they are reverse biased (the arrow end gets more postive than the other end) by more than the rated capacity, 3V in this case. So they work trying to pump current through them backwards, and up to 3V, they don't do anything.

But if you forward bias one, it conducts like a regular diode, so with two "back to back" with out the extra diodes, one is always forward biased and it would essentially short the track. So these are blocking diodes.

Now that I think about it, you might just wire the two zenners in series, then parallel the pair across the motor. One acts as a zenner, the other is forward biased and just conducts.

Sumpthin' like dat.

 

[Ray Marinaccio]

I was thinking the first zener diode circut would make a good power supply for some 3V smoke generaters I took out of some of some old battery opperated dollarstore sets. Just put it in series with the motor.

 

[ezdays]

I havent worked with zeners that much, but I really think a zener is not going to work if there is no resistor in the circuit to take up the excess voltage. If the rail voltage was say 12 volts, in order to just drop 3 volts across the zener the other 9 volts has to go somewhere.

And no, putting a zener is series will not get you 3 volts across the motor, it will simply act like any other diode in the circuit. If you need a constant 3 volts, I would use a regulator.

Don

 

[jon-monon]

A zener is a voltage regulator. You can think about it as an automatic variable resistor. Forward biased, it behaves like any other diode, virtually shorted, like a very low resistor. Reverse biased, it will act like any other reverse biased diode, virtually open, like a very large resistor, until the breakdown voltage is reached. Once breakdown has been reached, it begins to conduct, like a resistor, and the value of the "resistance" varies accordingly to keep the voltage across if constant. The voltage across a zener can not excede it's rated breakdown voltage without destroying the diode. So, when paralleled to the motor, the voltage across the motor cannot excede breakdown of the zener.

Presumably, in this application, the operator would understand the limitations of the circuit. Once the breakdown voltage is reached, there is no reason to increase the voltage (throttle). On the surface, it might seem OK to open the throttle all the way and leave it that way for a long time. But in practice, the zener is producing heat, and the power pack is being loaded to it's current limiting capacity to keep it at breakdown voltage, so it might not be good for the supply or the diode, depending on the ratings of each and the design of the supply. If I were to use one of these zener circuits, I would just stop increasing throttle when there is no perceivable speed increase, and maybe back off a bit at that point. You could just as easily just monitor (track) voltage, but the zener offers protection to the motor if junior cranks the throttle, or we have a little cockpit error ourselves.

I should also note, across the motor, you will be slightly over the breakdown voltage of the zener, as we have a slight PN junction voltage drop across the forward biased diode. This is normally .7 volts, and could be accomidated for with a lower voltage zener. DC motors usually are OK with 10% or 20% or so over voltage. On a 3 V motor, this may be getting close to the limits for safe operation.

The advatage of a voltage regulator IC, is that it won't load the power supply. The drawback is, they are not designed to operate below their rated output voltage, and may be unpredictable. Also the small ones don't handle much current or heat before they current limit or shut down.

Whatever circuit is used, it would be very desirable to experiment with caution before comitting to it and running the loco like mad, unless of course you have loads of money You can get a decent, cheap digital volt meter for $10 nowdays, and everyone should have one. I have 3 or 4 as well as expensive ones, and the cheapies are fine.

Zener Tutorial

 

[ezdays]

Jon,

Nice explaination, and I don't disagree, but I still wouldn't use a zener in any circuit that doesn't have a dropping risistor in it. Like you said, anything over the zener voltage is dissapated in heat, anything below it, the zener might just as well not be there.

Andy you're right about the regulator. To be sure, you need at least 1.5 volts more in then you get out.

Both devices have their place, dependent on what you need to do. Ray's original request was to take a variable 0-18 volt supply, take the first five volts and apply it to the lighting circuit and regulate the rest down to 0-3 volts for the motor. Tough to do with just a few zeners or regulators. Something is going to get pretty hot, a zener, a resistor or a regulator. Let's say you are drawing 100 mA. Five volts across the lamp, 3 across the motor. That leaves about one watt to burn up somewhere. Not much until you try to pack it all into an engine housing.

Don

 

[Ray Marinaccio]

This is the little critter that sparked this thread

 

[Ray Marinaccio]

A look at the other side. I put it in an old C-430 with that 75 ohm 5W resistor and a little extra weight. It's still a Tyco but it's fun to play with. It can actually pull some cars up my 2 percent grade at a realistic speed.

Anyway back to the subject. How hot do you think an ajustable regulator would get. We might be melting the loco shells without a big heatsink which will probably rule out using it in N scale applications.
What about a triac? Weren't those used in some of the early command control recievers? I have a power supply I built for my 027 trains that uses a triac and a potenteometer { excuse the spelling }. Could the variable 0-18V be used to regulate the triac's output from 0-3V in either direction? Depending on the track potarity.

 

[jon-monon]

You're right, Don, heat could be a problem if the operator does not excercise control of the applied voltage, keeping the voltage across the motor at or near 3 VDC max. The zener would serve primarily as motor protection. It looks like he does have a heck of a heat sink in this case, but that just buys time. I'm also concerned about the power pack itself. A current limit resistor might just be the key. I might play around with some of these ideas with my 5V motors one day.

Ray, I believe triacs are usually used as switches? I think a regulator would produce less heat than the zener diode, and if you get a regulator with a heat sink tab on it (TO-220), you should be able to insulate it and bolt it to the power truck. I don't know if you can find a variable regulator that goes down to 0 VDC, which can be "adjsuted" by varying the input voltage. If you used a fixed 3 VDC regulator IC (actually two, one +, one -), it would regulate at 3 VDC max., as input exceeds 3.x volts, before that, it may be linear, or it may be unstable. It would require experimentation, but may prove to be linear, and a very good low cost solution. Maybe Don has some ideas on a variable reg., or a current limit resistor for the zener? Nothing is lost if you experiment with it and have fun doing it.

I like the power truck. I see a speeder! or a Baby Boxcab! or maybe a little gas-electric!

One final thought, if you just want it to work, and the electronics is not fun for you, you could just monitor track voltage. Nice thought stimulating thread, guys!

 

[Ray Marinaccio]

A look at the other side. I put it in an old C-430 with that 75 ohm 5W resistor and a little extra weight. It's still a Tyco but it's fun to play with. It can actually pull some cars up my 2 percent grade at a realistic speed.

Anyway back to the subject. How hot do you think an ajustable regulator would get. We might be melting the loco shells without a big heatsink which will probably rule out using it in N scale applications.
What about a triac? Weren't those used in some of the early command control recievers? I have a power supply I built for my 027 trains that uses a triac and a potenteometer { excuse the spelling }. Could the variable 0-18V be used to regulate the triac's output from 0-3V in either direction? Depending on the track potarity.
__________________

 

[Paul Davis]

Seeing you said you wanted to run electonics of the 5V then you'll need a bridge rectifier (or 4 diodes to make a bridge rectifier) otherwise the 5V will flip polarity every time you change direction which at the lesat will stop the electonics from working when backwards and at worse destroy them.


If heat really is a problem then you could probably set up simple pulse width modulation using an dual op-amp and a fet or two to give a maximum of 3V to the motor at full voltage without any element disipating the extra voltage.

 

[jon-monon]

Originally posted by Paul Davis
If heat really is a problem then you could probably set up simple pulse width modulation using an dual op-amp and a fet or two to give a maximum of 3V to the motor at full voltage without any element disipating the extra voltage.

Now that's the right way to control a motor! Less heat, higher torque at lower speeds.