1.5 AMP PWM WALK-AROUND THROTTLE

revised April 2014

My most popular throttle - thousands in use worldwide since 1990 - now available in kit form!

Some of you will probably be asking " Why PWM?" and "What's PWM?". The answer is efficiency. The majority of model railroad throttles produce DC output voltage which is more or less proportional to the throttle's knob position. While there is nothing wrong with this in about 80% of the throttle's upper voltage output, it tends to allow motors to heat more than they really need to at slow speeds. This is due to slow ripple voltage rise and decay times at low voltages. Sound a bit technical? I'll explain a bit without getting too deep. Oh, by the way, PWM is an acronym for Pulse Width Modulation.


The new model 852B handheld throttle is a proven performer. Economy, performance and reliability put this little marvel in a class by itself. Well suited for comfortable HO, N, and Z scale one-handed operation!

I know of no other handheld throttle in the 852B's price range that can out-perform it.


Let's look at a rule of physics. "Things like to keep doing what they're doing." If a locomotive is sitting idle with no power applied, it's actually doing something. It's sitting there. When you "give it the juice", a fair bit of power is needed to get it going. Once it gets going you can back off the throttle setting. Now it's doing something else. It's moving. To keep it moving takes less power that it takes to get it moving.

Now, the funny thing is that I have never seen a 12VDC motor start to rotate with 1 volt applied to it. It might start to spin at around 3 or 4 volts. Once it starts to spin it will "jump" to life and rotate. Now that it's rotating less power is required to keep it going. You can even slow the RPM of the motor by decreasing the voltage to it. But that same voltage setting would not be able to get the motor spinning in the first place. With conventional throttles at low voltages, something happens many times per second. The throttle produces a dirty DC wave component called "ripple". Ripple is a mixed blessing. It helps to "kick" the motor around at low speeds but, the very nature of the ripple component dictates that wasted energy at low voltages is converted to heat in the motor windings as the motor tries to overcome motor bearing friction, gear friction and the weight of the locomotive itself. You can prove this with your most stubborn locomotive. Try setting it on a piece of track and turn up your conventional throttle until it starts to creep. Then carefully back off the throttle until the locomotive "stalls". Let it sit there for about a half hour or so. Then pick it up. The safe money is that it's warm, maybe even hot! That's because the voltage you've been feeding it for the last half hour or so was not sufficient to get the motor spinning and the motor is still working to try to spin. The energy expended has been converted to heat. Wasted energy.

Pulse width modulation throttles overcome friction and inertia by "kicking" the motor with full throttle voltage for a short period of time. The voltage rises to full amplitude very rapidly and shuts down to nothing equally as fast. No slow rise and decay times mean less motor heating. But even better, the motor starts to spin immediately and PREDICTABLY. PWM throttle settings produce repeatable and predictable speed changes without jump starts and motor stall at low speeds. This means no more overcompensating on the throttle control knob in the yard when you're trying to couple to a cut gently or when you're trying to take up coupler slack.


The model 851's high impact styrene case measures only 4" X 2" X 1". The inside is packed with circuitry. No additional circuit boards need to be installed. Simply wire it to an existing 12VAC to 18VAC power source and to your tracks and you're in business!

Here's the control board. It generates the PWM control and momentum to give you absolute control of your locomotives. 

Miraculously, all the required circuitry fits inside this little box! Little, yes, but BIG on performance!

Unconditionally guaranteed for one full year from the date of purchase....try and get that from China!

The 852B is easy to connect to your layout. The 8' tether cable's wires are color coded for identification and easy hook up. 2 of the 4 wires go to your track, the other 2 go to the AC supply voltage source. Attach your own connector for added versatility!

Now, how much easier can it get?


A SHORT PRIMER ON PWM

Most of you will be familiar with the concept of proportional voltage throttles. But, what about PWM?. Take a look at the voltage pulse diagrams below. When you start at the low end of the PWM control knob, output voltage will rise from 0VDC (the blue dashed line) to full output voltage (the red dashed line). This happens 60 times per second for this throttle design.  The first wave is 10% pulse duration. This means that full DC voltage is present for 10% of one sixtieth of one second. The pulse duration is actually less at the bottom end of the control setting, but 10 % is a nice place to start to explain what's happening here. At the 10% setting the locomotive will be traveling at typical slow yard speeds. Turn it up to 25% and the speed increases. The voltage doesn't increase, only the amount of "on" time does. 50% and 90% settings increase the speed of the locomotive proportionally. The throttle design presented here varies the pulse duration from 0% (no voltage) to 100% (which is pure DC) at full setting.

Real locomotives use PWM speed control methods to turn their motors as do most heavy industrial DC motors. Even VCRs, tape decks and CD players use some form of PWM control to control motor speeds. Sadly, it seems that poorly designed, cheap, proportional voltage control systems proliferate in the world of model railroading.

If you have never experienced the pleasure of absolute control in the operation of a locomotive powered by a PWM throttle, you're truly missing out on one of the better aspects of model railroading. It's kind of ironic that people will spend upwards of $100 on the purchase of a quality locomotive (just one? are you kidding?) and refuse to dish out a few extra bucks to realize the full potential of their new acquistion with a great throttle.


WANT TO BUILD IT YOURSELF?

THERE'S FINALLY A KIT!

Yes, I'm pleased to announce that the new 852B circuit is replacing the old 851. The new circuit has been designed with kit builders in mind and in the process of migrating the design I've managed to simplify construction. A complete kit of parts will be available mid MAY 2011. 

But... if you're bent on "scratch building" the new revised 852B schematic diagram is below.

This throttle will fit into a 2" X 4" X 1" ABS box as shown in the photograph. It will dish out 1.5 amps at up to 12 volts indefinitely. I won't get into construction details. The schematic below shows all you need to build one these throttles. 

A neat thing about the LM317T  is that when they overheat due to excess loading, they shut down and stay shut down until the overload is removed from their output. This means that fuses and light bulbs are not needed on the output. The output regulator needs to be attached to a steel or aluminum heat sink as it will operate very hot and shut down unless the heat is carried away from the component. These regulators are virtually "bullet proof"... ideal for model railroading!

The direction change switch is a DPDT center off switch with at least 3A contact rating. The center off function is nice if you need an "emergency" brake. It also assures that the train comes to a stop before changing direction.

POWER 12-18VAC are the AC power inputs. Be sure to select a power transformer of supplying at least 1/2 amp more current than the 852B is capable of delivering. This will allow the transformer to withstand throttle overload conditions without burning it's windings. By the way, this is called "de-rating" a product selection. That means you're not expecting the transformer to operate near it's limits. The transformer voltage can be anywhere from 12 to 18 VAC.  

PLEASE install the power transformer carefully and safely. If you are unsure of electrical safety practices, refer this installation to QUALIFIED personnel.


852B KIT DETAILS

The 852B kit comes with all the parts you'll need to build a complete 852B. The kit is universal for all scales and "scale specific" programming resistors for Z (7.6VDC output), N (9VDC output) and HO (12VDC output) are all included. All you have to provide are the tools and solder. This is a fun project and a great way to save a few bucks as well as a great fund raiser for clubs! Try selling a few assembled kits at your next club open house or swap meet!

The instruction set is very comprehensive and is targeted to intermediate kit builders. A novice may require some guidance. Kits take 1 hour to assemble.


 


PRICING SCHEDULE

 

MODEL DESCRIPTION QTY PRICE EACH - USD
852BA-(SCALE - Z - N -HO) ASSEMBLED & TESTED 852B PWM THROTTLE 1-3 $45.00
    4-8 $43.00
    9-25 $39.00
852BK KIT FORM 852B PWM THROTTLE 1-3 $32.00
    4-8 $30.00
    9-25 $27.50

FOR QUANTITIES IN EXCESS OF THOSE LISTED PLEASE CONTACT ME

 


SHIPPING EXTRA

To order this item, send an e-mail to me and I'll supply you with details.


 
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