Hub dynamo with capacitor in light

[Stroud Active]

I've got a hub dynamo powering front and rear lights with a capacitor to keep the lights on for a few minutes after stopping.

The system failed and I traced it to the front light, which I've replaced. The old light was a Kenda Plus by Spanninga. The new front light is a Busch + Müller Lumotec Lyt T senso plus.
With the new set up then the front light works and it stays on at reduced power when I stop pedalling. The rear light seems to turn off straight away when I stop.

I'm not sure how the system used to work. Was there a single capacitor in the front light with the rear light piggy backing off its control, or was the rear light independent, with just the power from the front light to charge it?

[simonineaston]

Are you certain that the rear lamp used to enjoy a standlight feature?

[Stroud Active]

Yes, that's why I'm asking now, because it no longer does it. The rear light used to stay on for several minutes when I stopped.

I'm wondering if the rear light has its own capacitor inside it. I'll ride around for longer and see what happens.

[Bmblbzzz]

Yes, a standlight is powered off that light's own capacitor, or sometimes its own storage battery. It won't be powered off the front light. But are you sure the rear light is working when it should be on normally? It's possible there's a different fault, eg a broken wire from the front light to rear light, which would mean the rear was never getting any power so not coming on at all.

[Stroud Active]

The rear light is working fine when the front wheel is turning. I'm going to hope that it just needs charging up for longer. I'll take it out for 10 minutes later on as a test.

[tatanab]

It would be handy to know what the rear light is.

[PH]

Yes, try it on a decent run, it takes a bit to re-charge a capacitor if it's not been used in a while. I don't know how long, but more than a quick spin on a workstand.
Do some of them need to be the right polarity? I remember having one where the capacitor would only charge with the spade connectors one way round. I followed someone with a dim rear light, which turned out to be just the stand light, which then didn't stay on, I think it turned out to be a bad earth.

[robc02]

As far as I know, most rear lights for use with generators have their own standlight capacitor. Supernova are an exception. their rear light is fed dc from the front light rectifier / standlight circuitry.

I have had a number of standlights that have progressively failed. In each case the standby period has become gradually shorter until it is non-existant. This is almost certainly due to the capacitor failing. One of them was replaced under warranty, the others I have ignored either because the light was mechanically damaged / cracked so it was due for replacement, or it got so little use it was hardly worth the bother.

Electrolytic capacitors are known to suffer in this way, especially if left unused for extended periods. I have had a couple of "broken down" electrical items (unrelated to bikes) that have been completely rejuventaed by replacing all of the electrolytic capacitors.

As mentioned by another poster, simply re-using the device can sometimes restore some of the function, but the real answer is to replace the capacitor. Unfortunately they are normally surface mounted so are not that easy to replace, but it can be done with a bit of patience. In an ideal world, these would be user serviceable items, but there is little chance of that when a lot of manufacturers don't even think you should be able to change a battery yourself!

[rjb]

Disconnect the front light and try running the rear light only spinning the wheel by hand. check to see if the standlight capacitor then charges up sufficiently to power the rear light when stationary.
report back with your findings.

[slowster]

Disconnect the front light and try running the rear light only spinning the wheel by hand. check to see if the standlight capacitor then charges up sufficiently to power the rear light when stationary.
report back with your findings.

The majority of rear dynamo lights are not suitable for direct connection to the dynamo, because they mostly have no inbuilt over-voltage protection and rely on the front light to limit the voltage. I think some or all Busch & Mueller rear lights do have over-voltage protection, but it is still advised that they are supplied via the front light.

I suspect PH is right and it just needs a few miles on the road to get the capacitor charged. I had exactly that experience with a new B&M Toplight, the standlight function of which did not work when first installed and tested by spinning the front wheel and then only riding a very short ride. After a few more miles it was fine.

[Bmblbzzz]

That is unlikely to work. The majority, if not all, hub dynamos power the rear light through the front light, not directly. If you tried connecting the rear light directly to the dynamo output, you would be feeding unregulated AC into an LED and either nothing would happen or, more likely, you'd melt something.

[simonineaston]

As an aside, I was amused to read of a rear lamp (B&M) that detects a sharp drop in the supply from the dynamo and interprets the drop as the cyclist slowing down and boosts the lamp brightness to simulate a brake light - How Fun!!

[Stroud Active]

I've taken the back light apart. Inside are 4 diodes and an LED. i.e: a rectifier. There's no capacitor so it has no ability to hold charge. I believe the old front light was running as the master and feeding AC when the wheel was spinning and DC from capacitor when stopped. I'll read the specs of the old and new front light.

The old front light was a Spanninga Kendo+ with an XDA label on it https://spanninga.com/product/kendo/
The rear light looks like one of these which is also a Spanninga unit, a Pixeo: https://spanninga.com/product/pixeo/

The description doesn't eliminate all doubt but it does suggest that the safe-stop function is relying on the front light.
"The dynamo version of the PIXEO features the innovative Safe-Stop function. This function makes the light burn for 4 extra minutes after the bike stops and increases the safety for you as a cyclist tremendously."

"* The dynamo version of the PIXEO is connected to the dynamo (3W) via the headlamp."

The new front light is one of these https://www.bikester.co.uk/busch-muelle ... 23459.html

I've seen this Pixeo XS on Amazon and I think you can see a capacitor through the lens: https://www.amazon.co.uk/dp/B007MJTKIA? ... yp_imgToDp

[Norman H]

How have you connected the rear light?

Your B+M front light should have a dedicated connection for a rear light. As others have said if you simply connect the rear light in parallel to the front light the safe-stop function will not work.

[Bmblbzzz]

As an aside, I was amused to read of a rear lamp (B&M) that detects a sharp drop in the supply from the dynamo and interprets the drop as the cyclist slowing down and boosts the lamp brightness to simulate a brake light - How Fun!!

Is that how it works? I thought it used an accelerometer but this way is indeed much more fun!