Lima / Hornby Dummy HST Question

[Sir TophamHatt]

I have a dummy (power car) HST from the Lima days.

Have just replaced the bulb for an LED (no real need to as I never really use it).

 

On the rear end [of the power car] there's one of those black with a silver ring things (can't remember the name!) a diode. 

 

Is it required now I've changed the light to an LED?

Makes a slight buzzing noise.

Edited April 22, 2020 by Sir TophamHatt

[Yardman]

Yes its a diode. Leave it out and you'll destroy the LED. Make sure you connect the LED up the right way round. Nothing should be buzzing. LED's are not designed to take reverse voltages.

[Sir TophamHatt]

Oh!

I just connected the LED any old way. It lit up. The power car buzzed before, I thought this was just the bulb. It's not very loud at all. I have it set up:

 

Front pick up - LED - 1k resistor - diode - rear pick up.

 

Sorry, my electrical knowledge is very poor - not sure what reverse voltages are.

[pheaton]

when the power car changes direction the polarity of the voltage will go reverse, so +. will become - and vice versa, LEDS cannot handle this and because you only have a resistor on one side of it the diode will prevent any voltage going the wrong way into the LED.

[DCB]

Is it DCC
If not put a bulb back in. They have a much greater voltage range and  don't burn out instantly when connected  wrongly .   You will need a resistor for the LED (if it does not have one as in a "12" volt LED, and if you have tested it already you probably need a new LED.

[Sir TophamHatt]

It's the oldest lima tooling and a dummy car, so no motor, no DCC. No running at all. Just a static model that I want the tail lights lit all the time.

 

Because I run a DCC layout, the lights are on instantly when I put it on the track. I read it was better to change the bulb for an LED (less current draw, brighter?).

 

I'm not sure what's so different as I tested the LED by simply holding one prong to one track and the other prong (with a resistor pre-wired) to the other track and it lit up. It was fine. I tested three I have ore-wired with resistors the same way.

 

I can't see how a diode fits into this. I'm sure I could switch the LED either way before and it didn't blow.

 

The aim is to have the dummy power car lot with tail lights on all the time whenever it's on the track. It'll just be in a siding.

Edited April 22, 2020 by Sir TophamHatt

[DK123GWR]

If the LED not working then you could try turning it around. This is why:

 

The important thing to remember is that a diode will only allow current to flow in one direction. A light emmitting diode (LED) is the same, so it will only light up when current is flowing in the correct direction through the LED. A DCC track provides an AC power supply, where the direction of the current changes direction several times per second. This means that when you tested your LED against the track, it will flicker, turning on when the current is flowing in the correct direction and off when it is flowing in the wrong direction (you will not be able to see this since the frequency of a DCC supply can be up to 10kHz, so the current changes direction 10,000 times each second).

However, in your model the LED is being wired in series with a diode. As already explained, a diode will only allow current to flow through the circuit in one direction. This means that the diode will only allow current to pass through the power car when one of the rails is positive, but not the other. If your LED is wired in the opposite direction, then the only current flowing to the LED will be in the wrong direction, so the LED will block that current and will not light up. Turning either the LED or the diode around would solve this issue,

 

HOWEVER even if this is the problem, and you manage to get the LED to light up, it will not last for very long. The AC current will damage the LED and cause its life to be very, very short. It is very possible that you have wired the LED the correct way, but it has already broken (I would suggest that this is actually the most likely cause of the problem, since a buzz would indicate that current is flowing through the power car). You have two (maybe three) options in order to resolve this problem. The first option is to switch back to the bulb (bulbs allow current to flow in both directions, so will work on an AC power supply). The second is to install a DCC decoder, which will convert the supply from AC to DC. The third is to install a rectifier (described here as a 'diode matrix') of your own creation. A circuit diagram for this is provided in the article, although given your apparent lack of confidence with electricity this may not be the best option for you.

 

Please could you describe how the lights operated originally when the power car was using a DC power supply? Did the lights light up when the train was going in one direction or both? If it was only one direction, and there is nothing at all other than the lighting system connected to the circuit within the power car, I would recomend removing the original diode when carrying out any of the three options above, since this will prevent a continuous power supply to the bulb. Please also note that if you choose options 2 or 3, you may need to replace the LED if it is indeed damaged.

 

I hope this helps, and I hope that I haven't made too many mistakes. I'm taking an A-level in Physics but I there are a lot of areas around DCC electronics which we don't cover (or at the very least haven't covered yet) so I am relying on my understanding of the basic principles combined with a little advice from Professor Google, rather than any firm knowledge.

As a result of this, please correct me if it turns out that I'm talking out of my gluteal aperture!

Edited April 22, 2020 by DK123GWR

[Sir TophamHatt]

I think these answers have been over engineered...

 

The LED works fine when I put the loco on the track. It's working. No where have I said it doesn't.

 

I never had the power/motor car. Always just the dummy.

The bulb turned on when I put it on the track. No control over it.

 

It always buzzed, even with the old bulb. If I rocked it, the bulb would light up brighter or dimmer - I guess because more/less contact with the track had been made through the wheels.

Edited April 22, 2020 by Sir TophamHatt

[DK123GWR]

If it's just buzzing, and it was buzzing even with the bulb, what exactly are you worried about? Unless there are other problems then you could leave it. Are you sure that the buzz is coming from the dummy, not something else such as the controller?

I would still reccomend going back to the bulb or installing some sort of rectifier (whether homemade or in a decoder) and removing the diode (don't do this unless you do change something else though - it would only be worth it to ensure that the rectfier's output is smooth) in order to prolong the life of the LED.

[John ks]

A LED (is a diode that emits light) will have a forward & reverse voltage

Diodes can have reverse Voltage ratings of hundreds of volts whereas a LED reverse can be as low as 4 Volts

 

The forward voltage is the voltage drop across the LED when it is conducting

 

Connect a suitable resistor in series with the LED, power it with DC (correct polarity) & it will work for a long time

 

If you reverse the DC polarity or connect to AC or DCC then the reverse voltage is important.

If you exceed the reverse voltage then you can damage/destroy the LED

 

Even with a resistor in series when the polarity is reversed you get full voltage across the LED

(the current(I) is 0A so the voltage drop across the resistor will be 0V (From V=I*R))

 

The first drawing is a lamp & diode

 

The next 2 are a LED & resistor & work fine if you don’t reverse the polarity

 

In the last 2 I have added a diode to protect the LED from reverse voltage

(The reverse voltage across the LED cannot exceed the forward voltage of the diode which is less than 1V)

 

 

John

[Sir TophamHatt]

So in order to not reverse the polarity, is this as simple as making sure it's the same way on the track all the time?

 

How do I know if the current position isn't the wrong way?

[DK123GWR]

 

50 minutes ago, Sir TophamHatt said:

So in order to not reverse the polarity, is this as simple as making sure it's the same way on the track all the time?

No. With DCC the polarity changes thoudands of times each second, and is not controlled by which way the dummy is placed onto the track. You need to use diodes to ensure that no current is able to flow the wrong way through the LED and damaging it. I feel like the answer to the original question, if everything is working correctly, will be:

 

18 hours ago, Sir TophamHatt said:

Is it [the diode] required now I've changed the light to an LED?

 

You didn't before (with a bulb), but you do need it now.

[Pete the Elaner]

John's diagram was probably the best reply.

 

LEDs are not necessarily brighter than bulbs. They are more efficient, which allows them to run a lot cooler. A bulb can handle a lot more power, which gives it brightness. LEDs are better for showing light in a certain direction.

 

The diode's role has changed. When used with a bulb, it was to allow it to work in 1 direction only. The LED now takes care of that, but when using AC (or DCC) the diode now allows current to flow in the reverse direction to the LED, to prolong its life.

[knitpick]

Just a thought.  the power to the rails is DCC and therefore is AC.  There is a thing called "mains hum" which some people can hear - it's the effect of the buzz from the AC mains supply at 50 Cycles per second (cps).  Actually most people can hear the hum id they go near high power main items - such as transformers at power stations etc.  DCC will also be capable of producing a buzz or hum albeit at a much higher frequency than the mains hum.  A capacitor across the LED leads will smooth the DCC voltage fluctuations and thus should reduce and may eliminate your buzz.  

[Il Grifone]

On AC a capacitor is conductive and will not smooth anything. To do this it requires a diode in series with the supply to convert the supply to DC.  The lighting circuit should not buzz however. This is either something vibrating when excited by the alternating waveform or something arcing and should ideally be traced and eliminated.

 

The advantages of using LEDs are:- 1. lower current consumption (not really important here) 2. More constant brightness with voltage variations (not relevant with DCC) and lower heat dissipation (it is quite likely that a continuously illuminated bulb will melt the plastic).

Edited April 23, 2020 by Il Grifone

[knitpick]

13 hours ago, Il Grifone said:

On AC a capacitor is conductive and will not smooth anything. To do this it requires a diode in series with the supply to convert the supply to DC.  The lighting circuit should not buzz however. This is either something vibrating when excited by the alternating waveform or something arcing and should ideally be traced and eliminated.

 

The advantages of using LEDs are:- 1. lower current consumption (not really important here) 2. More constant brightness with voltage variations (not relevant with DCC) and lower heat dissipation (it is quite likely that a continuously illuminated bulb will melt the plastic).

I agree that over AC the capacitor will not smooth.  This is why I suggested placing it across the LED leads - the LED will require a DC feed.  Hence the "standard - non LED" diode in the circuit along with the LED.  The Standard diode being to protect the LEED from reverse voltage and thus effectively converting the AC to DC by blocking one half of the AC wave form.

[Il Grifone]

On 24/04/2020 at 10:44, knitpick said:

I agree that over AC the capacitor will not smooth.  This is why I suggested placing it across the LED leads - the LED will require a DC feed.  Hence the "standard - non LED" diode in the circuit along with the LED.  The Standard diode being to protect the LEED from reverse voltage and thus effectively converting the AC to DC by blocking one half of the AC wave form.

 

Yes, I wasn't considering the diode. I would be inclined to put the diode (or better, four as a bridge rectifier) in series with the LED and then put the capacitor across the LED (it would probably need to be an electrolytic and they don't like reverse voltage any more than LEDs and don't just die quietly - it is only around 600mV, but why risk it?).