Speed limiting in analogue for a junior user

JimmyB

Semi-Retired; more time for trains.
OK, so one suggestion above is not going to work for all in one controller, the one you asked about actually... it needs to go between the raw power supply and a standalone speed control... not possible if all is in the same box (not easily).
Yes but would it work on the output line!
 

PhilP

G Scale, 7/8th's, Electronics
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Yes but would it work on the output line!
Only half the time! - The equipment is polarity dependent. :D
So it would only work for 'forwards', as the input would be polarity-reversed for going backwards..

We are back to a diode-dropper, a car sidelight bulb, or using a suitably rated 'OO' controller..

PhilP.
 

Greg Elmassian

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Yes but would it work on the output line!
On the output it gives you a fixed voltage, so the throttle knob is useless for controlling the speed of the train.

There are other potential issues but it's really unworkable for an all in one "controller" which plugs into the mains, has a speed control, and the variable DC is output from it.

The electronic unit shown is a device to produce a fixed regulated DC output, nothing more.

Greg
 

OBBherr

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Greg, thanks for the linki to your site. The voltage dropper looks like a good solution and my soldering skills should be up to it. However I can't match the pic with the diagram; what is the circular black component in the pic? Also what values do you recommend for the diodes?

Regards, Adrian
 

Greg Elmassian

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The circular component is the full wave bridge, in which 4 diodes live in the configuration shown. Note the plus, minus and the ~ (which means AC input)

Match those with the schematic. It's a bit cheaper to use the single component as opposed to the 4 diodes in the bridge, but either way works fine. In the picture, only the + has a marking, the - is opposite... the 2 others at 90 degrees from plus are the ~, normally used for AC input, but we are using it differently.

Realize the string of diodes in a "row" is where you adjust the final top voltage.

I would pick 3 amp diodes, and put a fuse after it to the track... if no fuse, you could overload the diodes and short one out, depending on the short and the max current of your power supply.





Basically if you trace the current flow, the FW bridge makes the circuit work in either polarity (forwards or backwards), and the string of diodes is the part you adjust (more or fewer diodes) to reduce the voltage... each diode is responsible for 0.7 volts drop. If you trace the circuit the current goes through 6 diodes, i.e. 6 time 0,7 = a voltage drop of 4.2 volts.

The circuit can go into either lead to the track, does not matter.

Greg
 
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OBBherr

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Thanks for that but I now have a further confusion. Where does AC come into it? I thought this circuit was to go into one of the DC track feeds from a playmobil or LGB all in one transformer/controller? Have I missed the point? Sorry if I’m being thick.

Regards, Adrian
 

OBBherr

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Skip my last question, I just reread your post and I’d missed the “normally used for AC” bit. Many thanks.

Regards, Adrian
 

PhilP

G Scale, 7/8th's, Electronics
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Adrian,
'AC' to 'DC' is what a bridge rectifier is 'normally' used for..

In this case, it just means the same circuit will work for both forwards and reverse.

PhilP.

<Edit>
And I have been away from the 'puter, so missed your reply above.. :)
 

OBBherr

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Helpful comment anyway, many thanks

RS Components here I come.

Regards, Adrian
 

Greg Elmassian

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Yeah, good question. You cannot believe how many arguments I have gotten into with people about FW bridges, often insisting that you cannot put DC through an "AC input".... it's just diodes, one way valves... but I've got lots of stories there. ;)

Anyway, the "voltage dropper" even works on DCC... I run G scale DCC and Z scale DCC, and I have a G scale flat car that has a Z scale loco on it, all DCC, and the dropper circuit reduces the 24v "G scale DCC" to the 10-12v DCC used in Z scale...

 

Dan

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If the engine is DCC then you can limit the top end for analog in some decoders. Zimo CV 57 is top motor speed, and CV5 is top speed for most digital engines.
 

OBBherr

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Diodes are on the way. Thanks to all for the advice will try to remember to feed back on the results.

Regards, Adrian
 

Greg Elmassian

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once you make the bridge, be sure to experiment with the number of diodes in the "long chain" with temporary connections until you get what you want... the bridge part and the "first" diode at each end of the "chain" can be soldered permanently the first time.

Greg
 

OBBherr

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Thanks, That’s what I was planning to do. I just discovered the Wago connectors which should make it easier to do.
 

Greg Elmassian

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Well, the connectors might cost more than the diodes! Also the diodes have no insulation on them, so you will still have bare wires.

I suggest soldering up the basic parts and then just twisting wires together for testing...

If you want to use the Wago's and the cost is ok, then get some heat shrink tubing to insulate all but the ends of the diode wires.

Greg
 

PhilP

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I know this is on the output of a controller...

But I would still advocate, fitting a fuse, as the first thing 'seen' after the controller..

PhilP.