DC/DC voltage conversion

[Robin GrahamParticipant @robingraham42208]

I have a project for a charity to equip a powered wheelchair with head/tail lamps. I can do that easily enough with bike lights powered by their own batteries, but it would be neater to use the chair's motor batteries through a handlebar switch. The lamps I've been looking at are 12V LED arrays, but there's no obvious way to tap the 24V motor battery halfway. So – any ideas for a voltage regulator which doesn't dump too many valuable Coulombs?

Rob

[Robin GrahamParticipant @robingraham42208]

[Trevor Drabble 1Participant @trevordrabble1]

Robin , Would suggest you look at the voltage regulators from Component Shop .

Trevor .

[duncan webster 1Participant @duncanwebster1]

Put 2 (identical) lamps in series, this is as efficient as it gets, or use a 24v/12v converter off ebay, which are still pretty good

[Paul LousickParticipant @paullousick59116]

Tapping 12v from a 24v battery will shorten its life compared to using the full voltage across the output terminals. (Learnt the hard way with expensive batteries in a big RV vehicle). Electronic voltage regulator/converters from ebay are cheap. Some will supply a constant output voltage (12v) from a variable DC input voltage.

Paul.

[Speedy Builder5Participant @speedybuilder5]

Use 24v LED lamps. Most lorries are 24V ? or use something like this:-
**LINK**
BobH

[MuzzerParticipant @muzzer]

Posted by Paul Lousick on 08/09/2017 05:15:46:

Tapping 12v from a 24v battery will shorten its life compared to using the full voltage across the output terminals. (Learnt the hard way with expensive batteries in a big RV vehicle). Electronic voltage regulator/converters from ebay are cheap. Some will supply a constant output voltage (12v) from a variable DC input voltage.

Paul.

+1. Tapping is not a solution.

Think about how the 12V tap is supposed to magically recharge itself. It can't, unless you significantly overcharge the whole string by more than the amount you removed from the bottom string. And that overcharge will gas the top string. No winners.

Murray

[John HaineParticipant @johnhaine32865]

What you need is a 24 v input 12 v output switched mode power supply. eBay item no. 192007762971 looks about right.

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by John Haine on 08/09/2017 08:38:38:

What you need is a 24 v input 12 v output switched mode power supply. eBay item no. 192007762971 looks about right.

That's probably going to be a bit over the top at 10 A maximum output.

Robin, first you need to know what the current draw is for your LED array. Then choose a DC/DC converter that will handle about twice as much for a good safety margin. I wouldn't worry about getting the best efficiency as the wheelchair motor will consume far more energy than the LEDs.

It might be worth looking at bicycle lamps to find something ready packaged and waterproof.

Russell

[Martin KyteParticipant @martinkyte99762]

If you are using LED's without their own current control the way to go is switching current regulators. Such as the XP Power range

**LINK**

These are essentially DC to DC converters but use output current as the feedback parameter. If you use a dimmable type which can be programmed with a single resistor you end up with a Led Driver that will handle input voltages from between 7 to 30 Volts for the LDU08 and LDU24 versions and can simply set the brightness of your lamps.

regards Martin

[Perko7Participant @perko7]

+1 for Duncan Websters suggestion, simple is better, and no electronics to go 'pphhht' when someone touches a wire to the wrong spot.

[Brian OldfordParticipant @brianoldford70365]

24v LED automotive lights are readily available.

[John HaineParticipant @johnhaine32865]

Posted by Russell Eberhardt on 08/09/2017 09:03:49:

Posted by John Haine on 08/09/2017 08:38:38:

What you need is a 24 v input 12 v output switched mode power supply. eBay item no. 192007762971 looks about right.

That's probably going to be a bit over the top at 10 A maximum output.

It's a box, sealed, protected, flying leads that could be wired into terminal blocks, it does exactly what the OP asked for, and it's £12.99. What's not to like? Why does it matter that it can handle 10A max?

[colin hawesParticipant @colinhawes85982]

I look after a Freerider mobility scooter which does use one of its pair of 12v series batteries to provide 12v for its pcb and lights. Colin

[Paul LousickParticipant @paullousick59116]

Using one of the two 12v batteries for lights will work but you will find that one of them fails way before the other. Each battery needs a different carrgiing rate and if charged together, one will either be over (or under) charged. Low wattage LED's may not be as much of a problem because they do not draw much power when used for a short time.

Edited By Paul Lousick on 08/09/2017 11:15:47

[Robin GrahamParticipant @robingraham42208]

Thanks for all your advice chaps. Duncan's suggestion of putting the lamps in series seems the obvious answer – Gawd, why didn't I think of that? Partly because of of innate stupidity, partly because I signed up for the charity (Remap) because I wanted challenges to keep the neurons working, and totally overthought the problem. Simple is good!

Thanks also to others for links to voltage regulators. It's amazing how cheap these devices are! I hadn't realised.

Thanks again, Robin

[John HaineParticipant @johnhaine32865]

Wiring them in series is fine if they have the same current demand. If not the one with the lowest will have more volts across it and the other less. As I assume the rear lights will be red and the front white you probably can't guarantee this.

[Nick HulmeParticipant @nickhulme30114]

Posted by John Haine on 09/09/2017 09:14:46:

Wiring them in series is fine if they have the same current demand. If not the one with the lowest will have more volts across it and the other less. As I assume the rear lights will be red and the front white you probably can't guarantee this.

2 rears in series, 2 fronts in series, with possibly – 2 LH indicators in series, 2 RH indicators in series, 2 brakes in series.

What's not to be guaranteed?

You're looking for far too complex a solution when a simple one is too close for you to focus on!

[Robin GrahamParticipant @robingraham42208]

Yes, I was thinking to wire two series pairs (2 headlights and 2 tail lights) in parallel, so John's worry about mismatched current draw is perhaps unfounded. Nevertheless it is a good point – at least to me, and will be filed away in the remaining brain cells. I'm not too good on non-linear devices, but it's by doing and asking that one learns,

Rob.

[John HaineParticipant @johnhaine32865]

Posted by Nick Hulme on 11/09/2017 22:59:54:

Posted by John Haine on 09/09/2017 09:14:46:

Wiring them in series is fine if they have the same current demand. If not the one with the lowest will have more volts across it and the other less. As I assume the rear lights will be red and the front white you probably can't guarantee this.

2 rears in series, 2 fronts in series, with possibly – 2 LH indicators in series, 2 RH indicators in series, 2 brakes in series.

What's not to be guaranteed?

You're looking for far too complex a solution when a simple one is too close for you to focus on!

Since I've been doing electrical engineering for a living for the last 45 years I think I have some qualifications here. Perhaps you could explain exactly how your rather complicated arrangement will deal with the issue of unequal current demand? All I suggested was that if you have 12V lights the simple thing to do is to organise a 12V supply using a rather low cost and efficient power supply to drop the battery volts from 24 to 12. Then you don't need to worry.

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