Electrickery question: dynamo rear lamp polarity

[Samuel D]

I have a Busch & Müller Secula Plus wired up to a Cyo Premium by random polarity. The Cyo Premium is also wired to the hub (Shutter Precision SV-9) with random polarity. There are no markings on or instructions with any of these things, so, like last winter, I wired them randomly.

• Both lamps stay off until I switch on the front lamp, whereupon both illuminate brightly. Good.
• When I stop riding, the lamps go to standlight. Good.
• When I switch off the front lamp, the front goes off but the rear standlight stays on until the capacitor goes flat. Okay but I would prefer the rear to go off too.

I have always wondered if this is correct behaviour, but not enough to reverse the polarity at the dynamo and/or the rear lamp to find out (a bit of a faff with the way I’ve connected things, hence my question). Does anyone happen to know?

[Phil W]

Sounds about right. The current is AC so polarity is irrelevant.

[Kim]

Yes, that's correct behaviour.  Some rear lamps incorporate a switch to discharge their standlight capacitor (eg. the Toplight Line Plus.)

Reversing the polarity of the neutron flow isn't going to change that, anyway.  It'll either do nothing (because there isn't a path through the frame between the rear light and dynamo), cause the entire system to stop working (because there is, and it's now short-circuiting your cable run), or if you're really lucky (because there's an intermittent path through the brake levers), it might give you random electric shocks when you apply the brakes.

[Kim]

Sounds about right. The current is AC so polarity is irrelevant.

To a point, Lord Cooper.  If you have lights and dynamos that ground one terminal to the frame, then you have to be consistent with that to avoid short circuits.

AIUI Shutter Precision hubs are isolated (my SD8 seems to be, anyway).  Cyos are, too.  Rear lights are still quite likely to ground a terminal.  With a single grounding point, the polarity doesn't matter.



FWIW, B&M convention is that the wire with the white stripe is the one that's connected to the frame.  This should connect to the terminal with an earth symbol on lights and dyamos thus marked, and with the outer conductor of Schmidt-style coaxial cable if it's used.

[Samuel D]

Thank you, patient Kim.

[Phil W]

Sounds about right. The current is AC so polarity is irrelevant.

To a point, Lord Cooper.  If you have lights and dynamos that ground one terminal to the frame, then you have to be consistent with that to avoid short circuits.

Which is not the situation stated in the OP.

[Kim]

Sounds about right. The current is AC so polarity is irrelevant.

To a point, Lord Cooper.  If you have lights and dynamos that ground one terminal to the frame, then you have to be consistent with that to avoid short circuits.

Which is not the situation stated in the OP.

But might be the situation of the next person who comes along and reads the thread, which is why I elaborated.

[Samuel D]

And thanks for confirming the behaviour, Phil.

[Ben T]

A rear light is DC
It doesn't seem to matter which way round the connectors go

These two statements appear to be oxymoronic.
Yet it's what I thought/appear to have observed, respectively.
Top light line plus.
Can anyone square this circle?

[Kim]

Bridge rectifier.

[grams]

Or a half-wave rectifier (a single diode) will do, and often does.

(you can even connect an LED directly across AC and it'll light on half of each cycle. With a suitably sized resistor in series you can even connect one across mains AC...)

[Kim]

Or a half-wave rectifier (a single diode) will do, and often does.

Unless you feed it DC, which seemed to be what Ben T was hinting at...

[Ben T]

Bridge rectifier.

Ah, so the bridge rectifier is in the rear light rather than the front one?
I thought it was the front light that had all the circuitry in it and outputted DC but could be wrong. It would make sense if I was.

[Kim]

Bridge rectifier.

Ah, so the bridge rectifier is in the rear light rather than the front one?

They'll both have one.  (Or similar circuitry to achieve the same effect.)  In the interests of compatibility with traditional tungsten lighting, the supply to the rear light is the same AC as from the dynamo.  They'd work fine wired in parallel (though some rear lights rely on the operation of a front light for voltage limiting, so that's not really a good idea).

I thought it was the front light that had all the circuitry in it and outputted DC but could be wrong. It would make sense if I was.

One of the systems does work this way (Supernova?), with a single standlight capacitor in the front lamp, and an off switch that works both together, but it's the exception, not the rule.


(Since the first thing the lights do is rectify to DC internally, they will often work fine from a DC supply of an appropriate voltage and polarity.  The trick is working out what the safe limits are.)