A diode is a rectifier question

in this thread

The OP appears to be asking about how to power a couple LEDs in a locomotive.  The OP indicates that the EVANS design is small and asks for help from the group

The design quickly converged to one with a bridge rectifier in it rather than just a plain diode.

Why is that?
Again I am NOT criticizing in any way, just trying to understand.

FWIW, the Evans LED module is a bridge rectifier inside that little bump of heatshrink.

Understood, as a matter of fact that is part of my confusion.  I had convinced myself that to get from AC to DC one used a bridge rectifier.

Also note that the team here on the forum, yourself included, designed a bridge rectifier solution.  In that case a simple diode was not even suggested (as I recall).  this is all why I was confused and started this thread.

Maybe it is becasue in the engine even to run just a LED clean power is better?  Or maybe it is needed because of the added functionality of the other parts in that circuit.

?

Clean power is required for many electronic functions, lighting an LED just happens not to be one of those.  You can light an LED on almost any old power.

The reason that Evans uses a bridge is to make their LED polarity insensitive.  The one little trick a bridge will perform is it will invert a DC input so it's always the same polarity on the output.  So, if you were running HO or 2-rail DC, you could light an LED with a bridge rectifier in front, and it would always be on, regardless of the track polarity.

Of course, that "trick" is how the bridge takes both halves of the AC waveform and outputs them in a single polarity.

ahh, that is another interesting reason to make the selection.

Why for instance is that where the group went to a bridge rectifier for that little board.  Is there a compelling reason in the case of the LED in an engine? 

No compelling reason I know of, I use a diode for my TMCC installs.  Since the lighting circuit is already half-wave DC in command mode, there's no benefit to the bridge rectifier.

BWRR posted:

...Why for instance is that where the group went to a bridge rectifier for that little board.  Is there a compelling reason in the case of the LED in an engine? 

Earlier in the thread GRJ noted that by using only half of the wave, the LED is pulsing 60 times per second which some people can "see" as flicker.  Using full-wave, the LED pulses 120 times per second which very few if any people can "see."  

But there's yet another type of flicker in an engine.  Because LEDs respond instantly, any loss of track power from dirty wheels, going over a switch, etc. makes the LED instantly go dark or blink albeit briefly.  This is different than incandescents where light is still emitted as the filament cools down.  The full-wave method has voltage available more of the time which increases tolerance to this variant of flicker.  The common solution is to add a capacitor or some type of energy storage to call into service when voltage is lost but as discussed in the other thread, there may not be space to deploy a suitably sized capacitor.

These are all great "reason" and help me to understand.

More re: bridges and diodes:

I bought a Williams GG1 chassis on eBay back in 2015.  As delivered, it had a Pittman can motor and one diode only.  The motor ran slowly and overheated quickly. 

When you run a DC motor on one diode, it's basically the same as running it on a DC powerpack with the "pulse" switch locked on!  This is why the powerpack manufacturers tell you NOT to leave the "pulse" switch on. 

With the Williams chassis, I wound up installing a Radio Shack  rectifier bridge, with a nice big honkin' capacitor across the motor terminals to clean up the power:

That motor now runs three times as fast, and dead cool. 

Mitch

I bought a Williams GG1 chassis on eBay back in 2015.  As delivered, it had a Pittman can motor and one diode only.  The motor ran slowly and overheated quickly. 

Sounds like the setup that was used on the GG-1 that Williams did for the TCA's 25th anniversary. That was their first GG-1.

M. Mitchell Marmel posted:

That motor now runs three times as fast, and dead cool. 

Most folks are trying to slow down their Williams diesels & electrics.

If quicker is good, then Faster=better!!  lolol Mitch! Capacitor for the light flicker as well? I-wanna-be-faster-and-cooler (like my women faster and hot, thou... but I digress, bad miglet-bad)

So, does the 1.5 volt dc battery with a button, run the whistle or bell as a plan d"?

positive to the center rail? (ty and salute)

ADCX Rob posted:

M. Mitchell Marmel posted:

That motor now runs three times as fast, and dead cool. 

Most folks are trying to slow down their Williams diesels & electrics.

That's merely a matter of adjusting the nut behind the controls. 

Remember:  Resistance is futile-if less than one ohm. 

Miggy posted:

If quicker is good, then Faster=better!!  lolol Mitch! Capacitor for the light flicker as well? I-wanna-be-faster-and-cooler (like my women faster and hot, thou... but I digress, bad miglet-bad)

Capacitors will help with light flicker as well, yes. 

Miggy posted:

So, does the 1.5 volt dc battery with a button, run the whistle or bell as a plan d"?

positive to the center rail? (ty and salute)

Not exactly.  Briefly, here's how pre/postwar whistles and horns work: 

In the tender/loco, there's a whistle/horn relay.  Under normal AC  track voltage, the relay vibrates (which is why, for example, a whistle tender will hum at rest when voltage is applied).

When the whistle/horn control is activated, a diode provides half-wave DC to the track.  This serves to close the points on the relay. 

For a whistle, this delivers AC power to the whistle motor.   For a horn, this delivers DC power from the onboard battery to the horn. 

Depending on the transformer and the locomotive, you may also see a bit of additional speed as the transformer compensates for the additional voltage sucked down by the whistle motor. 

On newer electronic whistles and horns, the sensing circuit senses the half-wave DC and delivers power to the sound circuits (and/or sends DC voltage to the whistle motor).  This circuit is sensitive to which phase the half-wave is, which is why, if a whistle/horn doesn't work properly, reversing the wires to the track often helps. 

Mitch

When the whistle/horn control is activated, a diode provides half-wave DC to the track.  This serves to close the points on the relay

There is a bit more to Lionel's whistle controls.
The whistle/horn control is a complex switch. As the button is depressed, the voltage is raised by approximately 6 volts and a rectifier is inserted into the circuit. At this point the transformer is putting out half-wave DC. This causes the relay to close
As the button continues it's travel, a resistor is placed in parallel with the rectifier, which restores some of the missing half-wave. The resultant wave form has a DC bias of about .6 volts. This lower DC bias holds the relay closed.

When the whistle control is built into a transformer, that extra 6 volts comes from an extra winding on the transformer core.
Prewar and postwar Lionel external whistle controls have a choke to drop the voltage during normal running. As the whistle button is pressed, the choke is cut out of the circuit to raise the voltage.
Modern era Lionel uses an arrangement of diodes in the whistle controls with which I am familiar.

M. Mitchell Marmel posted:

ADCX Rob posted:

M. Mitchell Marmel posted:

That motor now runs three times as fast, and dead cool. 

Most folks are trying to slow down their Williams diesels & electrics.

That's merely a matter of adjusting the nut behind the controls. 

Not really... it's at the other end of the spectrum - the problem is the high speeds of the Williams diesels at the lowest setting of the ZW and other large postwar transformers - about 6 volts. They really take off.

ADCX Rob posted:

Not really... it's at the other end of the spectrum - the problem is the high speeds of the Williams diesels at the lowest setting of the ZW and other large postwar transformers - about 6 volts. They really take off.

Single motor:  Add a resistor or two.

Double motor:  Rewire the motors in series instead of parallel. 

Yer welcome. 

M. Mitchell Marmel posted:

ADCX Rob posted:

Not really... it's at the other end of the spectrum - the problem is the high speeds of the Williams diesels at the lowest setting of the ZW and other large postwar transformers - about 6 volts. They really take off.

Single motor:  Add a resistor or two.

Double motor:  Rewire the motors in series instead of parallel. 

Yer welcome. 

I've heard of that method.

^.^

M. Mitchell Marmel posted:

Single motor:  Add a resistor or two.

I suspect you haven't actually tried this. A resistor would be the totally wrong way to do this, it would work against you. As the load when up, the current goes up, and the resistor drops more of the voltage. That's exactly what you don't want to happen! I won't even get into the fact that with a reasonable load on the locomotive the resistor would be dissipating a lot of power, and thus creating a lot of heat!

If you really want to drop voltage to the motors in a much more reasonable way, use back-to-back diode pairs to reduce the voltage, they don't have all the undesirable characteristics of the resistors.

I was kinda thinking a resistor bank under the locomotive, sorta like what they do on prototype trolley cars... 

Why, is your track chilly?

According to what I have read about those Williams TCA anniversary GG-1's, they could barely pull their three passenger cars with the single rectifier (diode). The fix was to switch to a full bridge rectifier.
They did offer a kit to add a second motor.

Also, they were delivered without the capability to reverse. Williams had hoped to develop their own reversing unit, but plans did not work out. They sent e-units to those who purchased the engine.

C W Burfle posted:

According to what I have read about those Williams TCA anniversary GG-1's, they could barely pull their three passenger cars with the single rectifier (diode). The fix was to switch to a full bridge rectifier.

That, I can believe.

Also, they were delivered without the capability to reverse. Williams had hoped to develop their own reversing unit, but plans did not work out. They sent e-units to those who purchased the engine.

Fortunately, I use that particular loco for shows, where backing up isn't usually necessary. 

gunrunnerjohn posted:

Why, is your track chilly?

I hate cold track in the morning, don't you?

While I have you here, how exactly does that back-to-back diode bit work to reduce track voltage?  Wouldn't it just stop the flow of current altogether?

Mitch

M. Mitchell Marmel posted:

While I have you here, how exactly does that back-to-back diode bit work to reduce track voltage?  Wouldn't it just stop the flow of current altogether?

Nope, works perfectly and each diode pair drops around .7 volts over a wide range of currents.  This has been discussed here many times.  The diodes are wired back to back in reverse polarity, current passes either way after a .7 volt drop.

A bridge rectifier can be used in place of two diode pairs, just wire as per the green wires.  This gives you a 1.4 volt drop through the bridge.

# ( wow   Who knew that a simple button created such an intricate chain of events..... (ohms law)?? Soo if the post war whistle is not..... well,,,,, making a sound, I'll search that thread.

Thanks!