hst light

[mr.postman]

my hornby intercity hst directional light on the power car has gone,can anyone tell me where to get a replacement,and is it possible to fit a l.e.d instead? also i am thinking of getting a couple of pullman coaches with the working table lamps, will these

 

work on standard dc or are they for dcc only? cheers everyone!

[JLBA]

Right, your lights gone. Firtsly, try to think of where the light may have gone to! Deos it have friends, or family! Whats happened to make it go in the first place? Has there been any recent family arguments, or disagreements, or anything? If you do think

 

that it definatly has gone, make sure you search the whole house before you report it missing. It will be very embarrasing if you do report it missing and then have it turn up after hiding in a cupboard!! If it deos turn up make sure you let it know how much

 

worry and concern it has caused!

Lets know how you get on though!!

JLBA

[Bulleidboy]

I can't help you with regard the HST light - I would guess Hornby spares, but someone will know the exact answer. With regards the Pullman carriages, you can run them on dc and dcc. On dc the lights will come on as you turn the power up, and go off as

 

you turn the power down. On dcc as the track power is on all the time, the lights will remain on as long as the carriage remains on the track.

[bocaj]

The Pullman coaches will work fine on DC and DCC, aslong as you clean the track a wheels so there is full eletrical pickups, otherwise thye might flicker. They look better on DCC I find though, as they stay lit up always as there is constent power going

 

through the track, but they will work fine on DC.

[Class421]

The HST light will most likely be available from one of the spares sites. However, it is easily possible to replace it with an LED, just ensure you install in the right way round. You could also put in a rear light as well as there is quite a bit of space.

 

The LED's will have to be fairly bright though as I did it with 'standard' ones from maplins and they weren't really bright enough

[oorail]

You can easily fit an LED instead. If you want directional lighting, you can use a standard LED connected to a resister (220 or 330 Ohm should work, the larger the resistor the dimmer the LED will be). I used an LED connected to a 220 Ohm resistor and

 

a rectifying diode on mine. You connect one side of the LED to one side of the motor, and the other side to the resister.

 

motor ========= (pin) LED (pin) ------ resistor ------- rectifying diode ----- motor

 

You can test it before putting it into

 

the HST chassis by touching it against the track with power applied. The above circuit should give you a constant LED light, unlike the regular bulbs that go from dim to bright as you apply more power to the controller.

 

I actually posted a video on

 

how to do this on my YouTube channel (same as my username here).

 

mr.postman said:

 

my hornby intercity hst directional light on the power car has gone,can anyone tell me where to get a replacement,and is it possible to fit a l.e.d instead?

 

also i am thinking of getting a couple of pullman coaches with the working table lamps, will these work on standard dc or are they for dcc only? cheers everyone!

[mr.postman]

Oh many thanx 00rail you have answerd my question perfectly! thats just what i am after cheers!

[Class421]

One thing to add to 00rail, you don't require the rectifying diode as an LED is a diode!

[poliss]

LEDs hate reverse voltage.

[oorail]

Yes LEDs are diodes, but you still need the rectifying diode to protect the LED from reverse voltage. I think the typical 3mm and 5mm LEDs have a reverse voltage tolerance of around 5V. Since the average controller can kick out upwards of 12V, then the

 

rectifying diode makes sense unless you want to keep replacing LEDs or run your locomotives in one direction! :)

[Colin Keightley]

Or you could install rear lighting at the same time, simply putting your rear LED cathode to anode and vice versa to your forward light means no rectifying diode is required as the reversed LED prevents reverse voltage being applied to other LED

[Jimbo1707820979]

Hello All. I'm rather puzzled by the comments about LED's not liking reverse voltage. I have several older locos (some Hornby, some Lima) on which I have fitted LED headlights, drawing current from the tracks via the motor. I have run these many times in forward and reverse but have not experienced any burnt out LED's.  Have I just been tempting providence ?  🤔

[walkingthedog]

I have street and yard LED lights on my layout. They are powered by an old controller. In forward certain lights come on, in reverse other lights come on. None of the LED's have been affected by reverse voltage when the other lights are on. Perhaps I have been lucky.  😀

[RB51]

@ Colin K.

 

Welcome to the forum.

 

This query was raised in Feb 2012. I'd have thought it unlikely that the OP will be back now to see your advice.

 

R-

 

P.s. You will be restricted to two posts for the first 24 hours of your time on here. This is an anti-spam measure - you are not doing anything wrong.

 

Any images you upload will be held for moderation by Admin. (not a Moderator) and this can take a few minutes in the working week and longer at weekends.

 

Also, if you could refrain from using the blue button with the white arrow in it that would be appreciated. It repeats the whole post you are referencing including any images and is not usually necessary.

 

If you haven't already done so please take a moment to read the Forum Guidelines a link for which can be found in a panel on the right hand side of this screen.

 

[96RAF]

I have had buffer leds running off the DCC track fr years with only a resistor in circuit. No failures to date.

[Chrissaf]

One has to look at the datasheet for the LED in question to see what its electrical maxima are.

.

Take this example datasheet pulled at random off the internet for a White LED.

.

/media/tinymce_upload/170f23243370c0883b028d35367a37c0.jpg

.

Note the yellow highlighted title stating values are Maximum. This particular datasheet states the maximum reverse voltage [ Vr ] as being 5 volts. It also states (blue highlight) the maximum reverse current supported is 50uA at 5 volts [ Vr ].

.

Being a white LED, the forward bias voltage is 3 volts with a max forward current of 20mA (yellow highlight).

.

Note how the LED is extremely sensitive to current maxima (50uA) when applied with a reversed supply voltage, compared to the forward bias condition (20mA). Therefore there is potential for LED damage if reverse voltages are maintained over periods of time.

.

Note: DCC track voltage alternates at 7,000 cycles per second, thus the reverse voltages are brief and not sustained on a DCC supply. But that doesn't mean that reverse polarity diodes shouldn't be designed in for ultimate (best engineering practice) protection - see further below.

.

Now assuming a current limiting resistor is also in series with the LED, a forward current passed through an LED will cause a voltage drop across the LED. As the current increases the voltage will rise until the Vf value (3 volts in this case) is reached. As long as the protection resistor is in the circuit, the voltage across the LED will stabilise at this Vf 3 volt value, with any voltage in excess of 3 volts being dropped across the resistor.

.

If the voltage across the LED is reversed. Then if it did exceed 5 volts [ Vr ] there is potential for the LED to be damaged (based upon the specifications in this particular LED datasheet). Therefore it is good engineering practice to be 'belt n braces' and include reverse polarity protection. Either by including a forward biased rectification diode in series with the LED so a reverse voltage can never be applied, or the inclusion a reversely biased rectification diode directly across the LED. It is only LEDs that can potentially be affected by these 'reverse voltages'. Rectification diodes are completely immune to these reverse voltages. This is due to the chemical make up of an LED vs a normal silicon type rectification diode.

.

This is the main reason why DC supply voltages (rather than AC) are usually recommended for LED power sources. If it should become necessary to power the LED with an alternating power source (DCC supply for example), then I would always recommend without hesitation that a protection rectification diode is included with the LED circuit to protect the LED on the alternative (opposite polarity) half cycle of the AC supply. If only to provide best 'engineering practice' protection.

.

Now consider a circuit where two LEDs are wired in parallel, but in reverse to each other. Such that one LED comes on when the polarity is one way and the other LED comes on when the applied supply polarity is reversed.

.

This is, in essence, what Colin Keightley is describing in his first forum post above.

.

This could potentially be the situation that WTD has documented with his description of different LEDs coming on when the repurposed DC Analogue controller is either in forward or reverse direction. This parallel connectivity could also be replicated in a loco where LEDs are wired across each other to come on with different track polarities to represent forward and reverse direction loco movement i.e red and white directional lighting (when wired for analogue and not including a DCC decoder).

.

The LED that is on will generate a voltage across it that will stabilise at the Vf value [3 volts in the example data sheet]. Now if this LED is directly wired across another LED in reverse (i.e sharing the same protection resistor). The forward bias Vf voltage [3 volts] means that the reverse voltage across its sister LED can never exceed this 3 volt value, thus the Vr maximum value of 5 volts is never reached. Therefore the Vf value of each LED protects the other. Note that Red LEDs typically have a Vf value between 1.7 & 2.1 volts, but this doesn't affect the parallel reverse operation just described as the Red LED Vf values are still less than the Vr value of the other LED.

.

TIP.....If building a buffer with red LEDs that pick up the power directly off a DCC track. Use two Red LEDs, but wire them in parallel with opposite polarity but sharing the same protection resistor. In that configuration the Vf voltage of each red LED is protecting the other. Because DCC track voltage is alternating, both LEDs will still light up.

 

[Jimbo1707820979]

Chris, I think you really enjoyed writing this. Right up your street !  😆 Thanks anyway.

[Chrissaf]

As you say, right up my street and well in my personal comfort zone. I doubt that average forum readers would get the same enjoyment that I got producing it. Hence why it has not been added to my 'boiler-plate' index document for linking to in future replies. Unless one has an understanding of how to read component data-sheets, I daresay even my simplified exampled description will still go over the heads of many.