Putting 5V through a 6V bulb?

Almost certainly a dumb question.

I have 6V cycle lights which came with a chunky 4.5Ah lead acid battery.

Now I can get a replacement for under £10 (or more - not sure what the difference is although those targeted at security alarms seem to carry a premium).

However there are USB backup devices which offer 3.2Ah or more at 5v at a fraction of the weight.

So I had the passing thought that running 6v lights at 5v might give a reasonable light level and much lightness.

However I have no real grasp of what the effect of under volting a DC light could be - in fact 12V car systems are probably running at 13-14V at times so the whole idea may be a non starter.

Almost certainly not worth buying a 5V battery (although I already have a backup battery for phone/tablet) so I was just idly wondering.....

Cheers

Dave R

Reply to
David
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A big reduction in light output

Yep, alternator output is typically 14-14.5v, so you should see close to that at the bulbs at all times the engine's running.

Reply to
Adrian

Presuming E over I still equals R, I think you are just going to get a dull orangey glow that won't be much use to anybody.

Years ago, I built a contraption around my hub-dynamo which gave me lights, signalling and a battery back-up which cut in when stationary. Nowadays, I think I'd do it completely differently and use LEDs and rechargeable AA cells.

Nick

Reply to
Nick Odell

While you /can/ do it, bulb systems have been almost completely wiped out by LEDs these days - you get more light output for less battery usage and not much money (unless you want to). So retrofitting to a bulb system seems to be to be a waste of time.

The one thing bulbs are still good at is non-directional lighting (eg all round rear light visibility), but clusters of LEDs or reflectors do a decent job at approaching this.

If you insist, though, one thing you could try is replacing the bulb with a lower voltage. IIRC, back in the day it was common for 2-cell torches to take either a 3.5V or a 2.5V bulb - the latter would be brighter but take more battery and burn out more quickly. So maybe you could run a 4.5V bulb in your lights?

Theo (wondering whether torch bulbs will go the way of the valve and the photographic film...)

Reply to
Theo Markettos

Oy! Less of the doomsaying about photographic film if you don't mind! Today I took my 35mm SLR in for an overhaul.

Nick

Reply to
Nick Odell

A chunky 4.5AH 6V battery implies something like a 10W incandescent halogen bulb, so a running current of 1.7A and a startup surge of several times that. I would be surprised if a USB backup battery would supply that, and a 6V bulb on 5V will be not very bright and rather orangey. In any event, the claimed capacity of a USB battery is likely to be a lie.

If you don't hang about, LIDL have some LED lights for £6 at the moment, and they're pretty good. If they're the rechargeable ones get two sets.

Cheers

Reply to
Syd Rumpo

Yes for filament bulbs, though not quite as much as you would think because at the lower temperature the resistance will also be lower, so you get more than 5/6ths of the current, so the power dissipated will be more than the 69% predicted by a simple minded calculation. The light will be less "white".

For LED maybe no perceptible difference.

The "USB" power packs may well be regulated to provide no more than about 1 amp.

Reply to
newshound

This raises quite a few questions.

How many lights are you powering (presumably 2?) and what is the power of each bulb? If you work out the total power and divide by 6, that will tell you how many amps of current you need. How many amps can this USB device deliver? You say that it's 3.2 Ah - but that's the capacity, not the current. For example, it may be able to deliver 3.2 amps for a period of hour, or 0.32 amps for 10 hours - or something in between. If it can't deliver the current required by the lamps, you're going to get a very dim glow at best.

Assuming that it *can* deliver the required current, reducing the voltage from 6 to 5 means that the bulbs will only run at 69% of their rated power - because power is proportional to V squared. Worse than that - because the lamps will run a lot cooler, the colour temperature of the light emitted will change, with white light becoming yellower, and more of the radiation being in the infra-red part of the spectrum and less visible light.

In summary - a pretty bad idea!

Reply to
Roger Mills

And spinning-rust harddrives.

Reply to
Graham.

The lamp resistance isn't fixed though, it varies significantly with temperature. Light output is roughly V^3.4, so for a 6V lamp on 5V you'd get about 54% of the 'rated' light. Power goes at roughly V^1.6 so you'd use about 75% of the rated power. So about half the light for three quarters the power.

S'true.

Indeed.

Cheers

Reply to
Syd Rumpo

5 X 1.2v NiCd or NiMh cells will give you a very decent 6V.

Lithium polymer sadly will do 7-8v on full charge for a 2 cell.

But LiFePo is about 3v per cell so two of those....

Reply to
The Natural Philosopher

Well I guess the answer is, it very much depends on the light source. If you are going to use filament bulbs, then the light might well be a bit dimmer than on 6V, but if you are saying the lights ran on 4.5V, then I'd hazzard a guess they will be a little brighter and last less long. I thought these days most cycle stuff used LED lights, and as these are current limited internally you will notice no difference as the extra disipation by the current limiting system would be able to cope I'm sure.

Brian

Reply to
Brian Gaff

Which reminded me of the last time I used my film SLR's, for a roll of Kodachrome 64, which got sent via Kodak UK (and maybe via Kodak Switzerland) to the last processing facility in the USA.

I know it was sent only a few days before the final deadline, a quick google suggests this would have been the end of 2010 but I seem to recall the UK / Euro cutoff might have been a month or two earlier.

Reply to
The Other Mike

Hmm....didn't know they had rechargeable.

{Nips out the the bike bits stock to check}

Crivit ones I just bought (for another bike) are rechargeable. Hadn't even clocked that.

I still like my old CatEye twin head lamps though - outdated and non-LED as they are.

Sadly I think our local Lidl is out of stock now - will check though.

Anyway, proper lead acid battery it is.

Cheers

Dave R

Reply to
David

You should have taken it to be euithanised ;-)

Reply to
whisky-dave

Further check shows previous Crivit light supports recharging but didn't come with charger or rechargeable batteries.

However I now have a charger.

Looks like a double result :-)

Reply to
David

So put it through one of these for a stable 6v, just 99p and a few minutes tweaking the output pot.

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Reply to
The Other Mike

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