The distance the point decoder

[paul_garwood]

Please will the strength of the impulse be affected by the distance the point decoder is from the point, I ask this because I have my decoders about 4 & 5 feet away from the points and I'm having problems getting them to thow. I'm using Railmaster with r8247 decoders and Peco pl11 decoders.

[Chrissaf]

Not particularly.

 

Are you using the correct wiring colour scheme? With PECO point motors it is the Green wire that goes on the Hornby R8247 common C terminal. Hornby use the black wire for that common function. Are you using suitable interconnect wiring, i.e not too thin.

 

If your PECO PL-11 point motors are wired correctly [Green common & thick enough] then read on......but I should point out that others appear [only anecdotal evidence] to use PL-11 surface point motors with the Hornby R8247 Accessory Decoders with very few reported issues.

 

The R8247 pulse outputs are IMO very weak. I just could not get my R8247s to work with PECO PL-10W point motors. The W being the higher coil impedance version. I have no experience of the PECO PL-11, but the issue you are seeing might be similar to the issue I had. My solution was to use the output of the R8247 to operate a relay, then use the relay contacts to operate a traditional power supply / CDU wiring solution.

 

There is a recent thread describing such a solution using very cheap ready made relay boards from China via eBay. Although these particular linked relay boards are Chinese 'eight relay' boards ... they are sold by a UK seller. The current listed price is £6.39 inclusive of delivery. At this price you cannot even buy the relays on their own as components. How the Chinese manufacturers can sell them so cheap is a mystery. Note that on the eBay page you have to use the 'pull down' box to select the 8 relay option. The boards are actually marketed as Arduino & Pi project boards, so I suspect that they are made and sold in large numbers, hence the low price. This also explains the +5 volt operating supply voltage.

 

The recent thread describing the 'eight relay' 5 volt board solution.

https://www.hornby.com/uk-en/forum/post/view/topic_id/34656/?p=4

 

Note the post is written for Hornby point motors and use the Hornby colour wiring scheme. The PECO wiring colour scheme is shown below:

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[96RAF]

It is odd that the R8247 output seems so weak when it has 4 x 4700μF capacitors, yet my Gaugemaster CDU only has 2 x 2200μF capacitors as do several other commercial CDUs on the market.

 

You could try increasing the R8247 pulse length from 100mS default to say 500mS by entering value 50 in CVs 3 to 6 or using RM graphic setup interface  to enter the length. Definitely do not exceed 800mS. It may be wise to check your point motor paperwork in case Peco sets a lower pulse length limit.

 

Another thing to take account of the longer the feed wires are the thicker the gauge they need to be. The limiting factor being the size of the terminals on the R8247 and access to direct solder pads on the solenoid.

[paul_garwood]

Thank you so much for this, please is it ok to use the power from the Railmaster to connect to the cdu?

[Chrissaf]

No

 

The issue with the R8247 low power is that the CDUs inside the R8247 are not getting a high enough voltage charge from the DCC supply. After rectification of the DCC Bi-polar voltage you only get about 13 volts to charge the CDU capacitors.

 

This is the reason that external commercial CDUs with their own DC Analogue higher voltage power supplies perform so much better than the embedded versions within the R8247.

 

The external CDU needs a completely separate and appropriately specified power supply. Either 16 volts AC which generates a 20 volt DC supply after rectification, or a 19 Volt DC supply as its input power source. If you rectify the DCC supply and then feed that into an external CDU all you are doing is replicating the issue affecting the R8247. The external CDU needs a much higher voltage input source.

 

Neither can you connect a CDU directly to the output of the R8247, you must use an interface such as the relays and an external power source as described in the linked thread.

 

Rationale for the above statement.

 

Hornby's DCC controllers output DCC as a Bi-polar square-wave with a 'Peak to Peak' voltage of 28.8 volts. Thus the peak half cycle voltage is 14.4 volts. Pass this through a bridge rectifier which loses 1.2 volts and you get a pseudo DC voltage of 13.2 volts max.

 

Bi-polar DCC voltages are not Sinusoidal and therefore are not stated as RMS voltage values.

 

An external 16 volt AC power source is a Sinusoidal wave-form thus it is the industry standard to always state Sinusoidal voltages as an RMS value.

 

The conversion factor to convert RMS to Peak is 1.414, so a power source quoted as 16 volts will have a peak voltage of 16 x 1.414 = 22.6 volts which is 45.2 volts 'Peak to Peak' [compared to DCC Bi-polar which is 28.8 volts peak to peak].

 

Pass this 22.6 volt peak voltage Sinusoidal waveform through a bridge rectifier (loses 1.2 volts) and you get a pseudo DC voltage of 21.4 volts [only if a high degree of capacitor smoothing is included to smooth out the AC ripple troughs]. One would normally expect to measure this voltage with a meter as being nearer to 19 to 20 volts when no or minimal smoothing is deployed.

 

Thus the recommendation for the external CDU is to feed it with either 16 volts AC or 19 to 20 volts DC from a completely separate power supply.

 

 

[Chrissaf]

It is odd that the R8247 output seems so weak when it has 4 x 4700μF capacitors, yet my Gaugemaster CDU only has 2 x 2200μF capacitors as do several other commercial CDUs on the market.

 

This battery analogy might help explain why the µF capacity of the capacitors only have marginal effect on CDU power.

 

Take a 12 volt bulb and connect it across two 6 volt Lantern batteries wired in series to provide 12 volts. The bulb will glow with a particularly brightness. Now take that same bulb and wire it across a 12 volt Lead Acid car battery .... does it glow any brighter .... no.

 

The bulb takes as much current from the battery as it needs. Connecting it to a battery capable of providing hundreds of Amps makes absolutely no difference to the bulb brightness.

 

Now a capacitor can be considered like a battery, it holds a charge, that charge delivers a current when connected to something that is drawing on that current such as a Solenoid point motor.

 

Now just for the sake of discussion, let us assume that the Solenoid wants to draw 2 Amps and that the 2 x 2,200µF capacitors are able to supply those 2 Amps [the two 6 volt lantern battery analogy]. The Solenoid is totally satisfied with this arrangement.

 

If you now increase the capacitance to 4 x 4,700µF capacitors [the 12 volt car battery analogy]. Yes they can store more charge and have the potential to deliver more current, but the Solenoid is still satisfied with the 2 Amps it needs. So increasing capacitor values past a certain level can only have marginal effect. In the same way that the 12 volt bulb does not glow any brighter with a higher current battery.

 

What will make the 12 volt light bulb glow brighter is increasing the voltage. Increasing the voltage forces the bulb to draw more current, thus the combination of increased voltage and current means more power is delivered and a brighter bulb results.

 

This analogy also applies to the CDU, if you want any CDU to deliver more power to the Solenoid, then better results are obtained by increasing the capacitor charging voltage than you get by increasing the µF value. Which is the premise for my previous reply above.

 

[96RAF]

I get it Chris.

A bigger car 12v battery will likely not start a 24v lorry any more than a small 12v battery will.