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Author Topic: Power supply modification questions for a specific application  (Read 8088 times)
N2NJO
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« on: September 03, 2012, 06:10:03 AM »

This "power supply" is actually what is called a "transistorized throttle" for a model railroad.

 The original design I modified to substitute a old school xformer, bridge & filter cap for a switching power supply for this model railroad "throttle. It entire unit consists of a single enclosure with two separate "throttles" inside which are just identical, duplicate circuits. This has to use separate, independent power supplies due to the application. One won't work to power both "cabs".

 The problem is a path that I'm not sure what to do with. It was originally connected to one leg of the output of the xformer. Since there is no xformer anymore, I don't know if I should remove it altogether, or try to connect it to the negative side of the output of the switching supply which I haven't tried yet.

Here is the circuit (click on the pic for a full size photo);


 The circuit in question is the C3 cap off the lower output of the xformer. I assume it is some type of drain or stabilizing function. I have tried the "throttle" w/o it connected and it appears to operate properly.
 BTW, if you look at the lower left, that was the optional design I followed, replacing those original components, including that fuse with what appeared to be the better design for 'short' protection.
« Last Edit: September 03, 2012, 06:17:43 AM by N2NJO » Logged
AC5UP
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« Reply #1 on: September 03, 2012, 06:37:46 AM »

Best guess is C3 was intended as noise suppression or to dampen any spikes that might appear on the base of TR1.

OTOH, it will supply a modest 60Hz AC signal across R12 from the lower winding of the transformer to the center tap. With R12 at 1 megohm there isn't much loading so even a cap as small as .022 mike will flow some juice. Years ago I studied electronic throttle controls for model railroading and learned pulse width modulation schemes were desirable as they allowed for better low speed operation. A smooth DC throttle had to be opened to a certain point before a motor would start and in some cases that was at a faster speed than desired. With a pulsed throttle the motor would see enough voltage to turn but at a reduced duty cycle so it would turn, pause, turn, pause, etc and that allowed the loco to crawl very slowly.

Makes me wonder if the AC on the base of TR1 is intended to wobbulate the throttle at low speed. (?)

If that was true, with the throttle in its original configuration and a 'scope across the track one might observe some AC ripple at the lowest throttle setting that's overcome by DC as the throttle is advanced.
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W8JI
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« Reply #2 on: September 03, 2012, 07:21:53 PM »


 The circuit in question is the C3 cap off the lower output of the xformer. I assume it is some type of drain or stabilizing function. I have tried the "throttle" w/o it connected and it appears to operate properly.
 BTW, if you look at the lower left, that was the optional design I followed, replacing those original components, including that fuse with what appeared to be the better design for 'short' protection.

Since I have model trains I can tell you the idea behind that, but it isn't particularly well-designed. To promote low-speed starting torque, and to overcome track-to-wheel resistance and wheel-to-contact resistances,  most power supplies are DC pulsed. In this case they used a Darlington of TR1 and TR2 to pass the regulated filtered voltage in 50/60 Hz DC pulses. The throttle control controls pulse width and peak voltage by changing AC bias on TR1.

The trigger is the current flowing through C3. Capacitor C3 turns the base-emitter junction of TR1 off and on, the width of the on-time and level of DC peak voltage is controlled by the impedance from the 2.2k resistor to the negative buss rail.

If you leave C3 off, you will have no pulses and no output unless you pull the 2.2K resistor positive.

As drawn, the circuit is not complete, but that is how it would work. I'm also not sure the 135 k ohms of C3 reactance is adequate, because I haven't looked at the minimum beta of the Darlington. Off the cuff, it looks too small.

I personally would not use a circuit like that, although the concept is good. It is desirable to run pulsed DC to non-DCC controlled locomotives, because it helps maintain connections and helps maintain torque. But the basic design is sloppy and crude, and is a mix of 50/60 HZ pulse and variable DC. If I were to design one, I would use an 18V source (or whatever maximum allowed motor voltage is for the models I had) and use a true pulse width modulation of the DC.

73 Tom

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G3RZP
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« Reply #3 on: September 04, 2012, 01:38:02 AM »

I seem to remember a scheme that provided, in addition to the variable pulse width DC, something like 50 or 100kHz to the track to 'burn through' the oxide. A capacitor across the motor kept the HF outof the motor, although one would expect the armature impedance to be high enough to prevent damage.

Not a problem you get with live steam.....
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N2NJO
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« Reply #4 on: September 04, 2012, 04:00:42 AM »

I took a closer look at the parts list and for the C3 Peter Thorne (the author) states:
Quote
This cap injects a small pulse into the throttle to improve slow running. Increasing it to .047uF doubles the pulse to 2 volts, but may cause creep with some motors.

Quote
I personally would not use a circuit like that, although the concept is good. It is desirable to run pulsed DC to non-DCC controlled locomotives, because it helps maintain connections and helps maintain torque. But the basic design is sloppy and crude, and is a mix of 50/60 HZ pulse and variable DC. If I were to design one, I would use an 18V source (or whatever maximum allowed motor voltage is for the models I had) and use a true pulse width modulation of the DC.
This is a older design from the early 90's. I know it isn't the best design based on current practice, but other than that, the question is, what would attaching that lead to the negative side of a 19v switched PS do?

Please note (as stated) I used the LM350K version of the regulator circuit for my throttles, not the 2N6058 that is shown in the top circuit (if that matters).
« Last Edit: September 04, 2012, 04:17:19 AM by N2NJO » Logged
AC5UP
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« Reply #5 on: September 04, 2012, 04:37:39 AM »

Since there is no AC without the transformer it doesn't matter if there is a C3, but you won't hurt anything by connecting what used to be the transformer end to DC ground. C3 will then dampen any voltage change on the base of TR1 and that's a good thing in case the potentiometer used for speed control becomes scratchy.

BTW:  The LM350K offers exactly the same function as the 2N6058 circuit. It provides a stable voltage to the throttle circuit. The LM350K is a more sophisticated regulator and will self protect to the point where it's almost indestructible. If it becomes overloaded or too hot it will shut down before it breaks. The 2N6058 isn't that smart.   Tongue
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N2NJO
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« Reply #6 on: September 04, 2012, 05:00:06 AM »

Quote
The LM350K is a more sophisticated regulator and will self protect to the point where it's almost indestructible. If it becomes overloaded or too hot it will shut down before it breaks. The 2N6058 isn't that smart.
Which is why I choose that version.  Wink

I now understand I need some type of 'timer' circuit to produce the missing AC pulse for slow speed operation. Something like this:
http://home.cogeco.ca/~rpaisley4/LM555.html#14
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W8JI
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« Reply #7 on: September 04, 2012, 06:48:00 AM »

Since there is no AC without the transformer it doesn't matter if there is a C3, but you won't hurt anything by connecting what used to be the transformer end to DC ground. C3 will then dampen any voltage change on the base of TR1 and that's a good thing in case the potentiometer used for speed control becomes scratchy.

C3 won't do anything meaningful. There is so much mechanical and electrical inertia in the system that scratchy will not hurt a thing.

Actually, scratchy would help....as would UNfiltered DC.

The last thing I would do with a non-DCC locomotive is run filtered pure DC to the track.   

Quote
BTW:  The LM350K offers exactly the same function as the 2N6058 circuit. It provides a stable voltage to the throttle circuit.


If we wanted to run a stable speed control, the requirement would be a current regulated source. This is because resistance varies and we want the same ultimate voltage across the motor.

That, however, creates a mechanical load issue with the locomotive. A current regulated source will slow or speed the locomotive more as the mechanical load changes. It corrects track and contact resistances.

So in one case, with filtered regulated DC, we hurt immunity to the very common contact resistance issues. In the other case with current regulation, if there are tight curves or hills, we increase the speed change with mechanical loading changes.

The best system is a regulated constant voltage pulse of variable pulse width, from zero to 100% or whatever results in maximum allowed speed. The worse system is a fully filtered dc supply.

The very last thing I would do is filter the track supply to pure DC. That's just asking for starting and speed control issues at low throttle. That's why the author of that circuit injected AC into the system after he filtered and regulated the voltage.

73 Tom
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N2NJO
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« Reply #8 on: September 04, 2012, 07:08:26 AM »

Ok, so I need this insertion of AC to make the rest of his circuit work. This addition of a 555 timer was the suggestion. I assume I would use pin 3 from the chip to replace that feed thru C3 which wouldn't be needed anymore.

Again, I understand this is far from a ideal design I would just like to get it working whth waht I have for the time being.
DCC throttles can not be used with DC locomotives (I already though about that route) and I can't see investing in two new walk around DC only throttles at this point.
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G3RZP
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« Reply #9 on: September 04, 2012, 08:34:29 AM »

Was this why in the old days, we use rectified but unfiltered DC - allowed slightly better control with just a rheostat?
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N2NJO
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« Reply #10 on: September 05, 2012, 04:12:28 AM »

I looked into a couple of replacements and one spefied that only a AC PS could be used fore their "throttle" telling me they are doing the same as this, using unfiltered AC off the xformer for slow speed 'creep' operation.
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AA4PB
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« Reply #11 on: September 05, 2012, 05:59:47 AM »

I have no experience with model trains, but have built controllers for pan/tilt mechanisms. With PIX chips and FETs it easy to control speed using a constant voltage, constant current, by pulsing a DC motor and varying the pulse duty cycle to change the speed. It allows you to get nearly full torque at slow speeds.

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W8JI
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« Reply #12 on: September 05, 2012, 03:43:54 PM »

Was this why in the old days, we use rectified but unfiltered DC - allowed slightly better control with just a rheostat?

Yes, every good controller I have ever seen uses unfiltered or pulsed DC.

The pulse makes the motor require more voltage to run, and that burns through bad connections. It also vibrates the motor at low voltages letting it run better at low speed, although with a little growl.

I ran a bunch of tests here because I have hundreds of feet of track to clean, and really old stock that has old connections on wheels.

http://www.w8ji.com/train_real_and_models.htm

A PWM supply would probably be great. I'm not sure what the optimum pulse rate would be, but my bet is pretty low frequency. There was a distinct advantage to a little low pitch vibration shaking the motor armature on my real old junk.

73 Tom     



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KB3HG
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« Reply #13 on: September 11, 2012, 06:47:36 AM »

Now you have an interesting point. PWM , Pulse Wave Modulating, an introduction of AC for low torque, Hmmm... I tried sawtooth and triangle waveform pulses and a summing circuit with a DC level back in the 80's. That worked OK. I like steam engines.  One of these days I'll get the HO stuff out again. I have to have a big layout for my 4-8-8-4 Big Boy , but now the newer stuff has different controllers that can be had.

JI saw you lay out looks nice. 

Tom Kb3hg
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AC5UP
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« Reply #14 on: September 11, 2012, 08:05:57 AM »

Years ago I did some reading about the legend and lore of the Baldwin Big Boy and the ongoing debate over whether it was the largest locomotive ever built in the US, the world, or the Milky Way.........

If you're ever in Scranton, PA, forget the visit to Dunder-Mifflin and reserve a day for a walk through Steamtown.

http://www.studiosclavi.com/graph/depticinese/steamtown/up_4012_2.jpg

Steamtown is the home of one very, very big boy formerly known to the Union Pacific as #4012. For those wondering why Scranton for a national park devoted to steam locomotion, the answer is about a thousand feet under the rails. One of the world's largest deposits of high quality hard rock anthracite is on the south side of Wilkes Barre near Ashley and Nanticoke. If there's one thing a big boy likes it's an abundant supply of anthracite and Scranton has some of the best ever mined.........

BTW: When UP started running the big boys on coal runs between Wyoming and SoCal the engineers discovered two issues with them: Previously it was SOP to accelerate when going into an uphill climb for added momentum. Do that too often or too enthusiastically with a big boy and the loco could literally hammer the track hard enough to bed the rails into the gravel or break ties & rail joints. The other had to do with snapping couplers. As a train starts from a full stop the slack in each coupler is progressively taken up from front to back... Click, Click, Click... If the loco accelerates too quickly each successive coupler sees a faster 'snap' until the couplers toward the back of the train start to fail. Loaded gondola cars were never meant to go from zero to five in 20 milliseconds but a big boy had the moxie to make them try...  Cool
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