Batteries, heat and wood

[RichHox]

Hi All,

Apologies if this topic has come up before (if so please point me in the right direction)

So I have been building lamps from hardwoods and this has been using mains power and feeding the cable through a hole inside etc. I want to build a battery operated one, and I'm making some progress. My main concern is that batteries are more likely to create heat and combust than mains cable.

My question is:
Does anyone have any recommended finishes or easy methods to give some heat protection to wood? This would be a 38-40mm wide hole approx 110mm in length inside the wooden block, so a spray would be ideal. Heat resistant paint comes up in some searches, has anyone tried this?

Thanks in advance for any help!

 

[Sandyn]

If you design the wiring correctly, there is no more risk with DC power. With modern LED lamps, the power is only about 5 Watts, In addition, you can put re-settable fuses in series to protect against short circuit. If designed correctly, the lamp will be safe. Is this for sale or personal use? what battery voltsge and what type of bulb?

 

[RichHox]

If you design the wiring correctly, there is no more risk with DC power. With modern LED lamps, the power is only about 5 Watts, In addition, you can put re-settable fuses in series to protect against short circuit. If designed correctly, the lamp will be safe. Is this for sale or personal use? what battery voltsge and what type of bulb?

Thanks for the reply.

It is a 12v 5w LED lamp and the battery is 8xAA in series so just under 12v. I have been looking at resistors to see if I can increase the load to make the lamp dimmer. I burnt out a resister because I believe the resistor I had isn't rated for the correct wattage, which made me consider heat proofing! (Still doing research in resistors as we speak!)

It would be for sale which is why I want to make sure I've got all my angles covered, I've yet to go over any BStandard regs etc.

 

[Sandyn]

Having a series resistor isn't a very efficient way of dropping the voltage. All you do is waste power and generate heat which is what you are trying to avoid. You would be better just going for a lower wattage LED lamp. What voltage gives you the correct light level? If you know that, you can calculate a better LED wattage. If you want variable brightness, you would be better having a clever switching arrangement to change the voltage this avoids having any regulation which will add cost and reduce efficiency. I would definitely add a re-settable fuse and mount it with the battery. They are tripped by either excessive current or excessive heat. The wiring needs to be correctly rated and the correct standard, You also need to take care of excessive heat around the LED bulb, Are you using MR16 holders? You need to protect the user from hot surfaces and lots of other safety issues. Will this be produced in volume or just bespoke lamps?

 

[TheTiddles]

You’ve nothing to worry about with that arrangement unless you do something really wrong. But as mentioned using a potential divider to dim the lamp is a bit of a waste.
Aidan

 

[Sideways]

You can get an complete LED light that runs of a 5V USB charger (not included) for about £8
These chargers for phones, ipads, etc are ubiquitous and cheap.
If you can get a complete lamp for so little, you can probably find an LED with a driver chip for small money
Build it like that and you could always plug in and power it off a rechargeable USB power bank if you wanted to go mobile ....
If you want to stay battery powered, LED's only need a few volts. Use one lithium rechargeable cell instead of the stack of AA's.
The LED should last effectively forever so no need to design for the LED to be replaceable.

 

[RichHox]

thanks for all the replies and info! It appears I need to look more at the electrics than the wood side of all of this!

See the picture of one of my other lamps, this is the style that I'm going for. So I have a 12v screw bulb at 4w with a filament, which is the best I can find currently with a filament. MR16 and similar would be SO much easier, but just wont work for the aesthetic im attempting without some form of glass cage or optical effect.

The lamp is actually 4w and the battery runs at 11v. I know that the desired brightness works with a 22-33 ohm resistor. I used a 33 ohm resistor and it lasted for nearly 13 hours compared to 5 hours without anything added... but since then all my fuses have been burning up so I think if I go down that path I would need a 4w resistor. The switches sound like a great idea, will look into that a bit further.

Thanks!

 

[minilathe22]

DImming LED lights does not work very well using an inline variable resistor, which does work well with traditional bulbs. LED dimmable circuits tend to be PWM based, and the cheaper ones might give a perceivable flickering which can be annoying, but much more energy efficient.

In terms of the batteries, I would try and make a little air space around them and also a few small vent holes, so heat cannot build up at all. But as others have said, focus on making sure its put together well and you are unlikely to have issues.

 

[RichHox]

DImming LED lights does not work very well using an inline variable resistor, which does work well with traditional bulbs. LED dimmable circuits tend to be PWM based, and the cheaper ones might give a perceivable flickering which can be annoying, but much more energy efficient.

In terms of the batteries, I would try and make a little air space around them and also a few small vent holes, so heat cannot build up at all. But as others have said, focus on making sure its put together well and you are unlikely to have issues.

Yeh I'm with you on the PWM side of things. I was surprised with this bulb that using resistors that effectively halved the light output and it still hasnt flickered at all!

The varistor seems like a no-go because from what ive read up a varistor works on creating heat to use up the excess voltage. Similar i guess to a resistor, except that it seems like for resistors it is more of a by-product.

 

[Robbo60]

Education time (for me) If you are making & selling electrical items, do they not have to be certified by some official body? e.g. CE as was?

 

[MorrisWoodman12]

@RichHox Just a few comments. A Varistor is completely the wrong component for what you are trying to do: it is a surge protection device and totally unsuitable.
A resistor will drop voltage so your LEDs run at lower power. The math is quite simple. You need to know the voltage and current at which you want to run your LEDs and from that you can work out resistor values and power ratings.

Let's say that you know the LED voltage (Vled) and current (Iled).

Then the resistor value............................R = (Vbat - Vled)/Iled

and the power rating................................P = Iled^2 * R
I would choose a power rating that is at least half as high again as the calculation to help the resistor run cooler.

If you have some idea of the resistor value then you can calculate the power rating you need from................P = (V^2) / R.......... where V is the voltage across your resistor.

Resistors come in preferred values so choose the nearest one to the calculated value. However the higher the power rating the less choice of resistor values.

Apologies if you knew all this beforehand: I'm pretty good at teaching my grandmother to suck eggs!
Martin

 

[Sandyn]

Pulse width controller would be the way to go. You will find suitable controllers for a couple of quid in the popular web sites.

 

[Sandyn]

Education time (for me) If you are making & selling electrical items, do they not have to be certified by some official body? e.g. CE as was?

As soon as you sell a product to the public, you become liable for ensuring the product is safe and meets the requirements of the relevant safety directive. For a battery powered lamp, it would be the Low Voltage Directive. You don't necessarily need to do a formal CE approval. It is ridiculously expensive. For prototypes, small production, typically less than 1000 and in other cases, you won't need formal approval, but all of that is irrelevant because the real requirement is that the product is safe. People make and sell things all the time on web sites, but fortunately there aren't many disasters.
I always sound like a gloom and doom person when I give advice, but anyone who makes an electrical product and sells it must seriously think about what is the worst case scenario and how can they protect against that. For a battery powered lamp, the greatest risk is causing a fire and someone getting killed. Voltage wise, 12V is safe, but 8 AA batteries stores enough energy to cause a fire.

 

[RichHox]

Thanks again.

I spent my lunch break today looking up the CE side of things. It seems that it is considered safe and professional to have a CE marking and all the conformity but it isn't necessarily a legal requirement. I do fully test my mains lamps within my abilities before I send them. So far I've only sold about 10 or so, so its still very much bespoke and small scale. I agree that the safety in the first place is the most important part, getting a CE or BS EN ISO etc is the next step.

Thanks Morris for those formulas, I feel that a resistor is the best option with the small space I have to work with, looking at the PWC and other switch/controllers seems to be PCB related and I'm not competent at electronics (obviously!). A 'simple' correctly rated resistor that doesn't create flicker and maintains a cool temperature seems like the simplest method. I plan to heat shrink the battery with whatever electronics was required so that It can be as compact and safe as possible, Plus the heat shrink would help diffuse any heat, if caused.

I also have a shoe string budget (out of work theatre technician!) so I need to limit my outgoings on experimentation so that it all balances out in the end haha

 

[TheTiddles]

Thanks again.

I spent my lunch break today looking up the CE side of things. It seems that it is considered safe and professional to have a CE marking and all the conformity but it isn't necessarily a legal requirement. I do fully test my mains lamps within my abilities before I send them. So far I've only sold about 10 or so, so its still very much bespoke and small scale. I agree that the safety in the first place is the most important part, getting a CE or BS EN ISO etc is the next step.

Thanks Morris for those formulas, I feel that a resistor is the best option with the small space I have to work with, looking at the PWC and other switch/controllers seems to be PCB related and I'm not competent at electronics (obviously!). A 'simple' correctly rated resistor that doesn't create flicker and maintains a cool temperature seems like the simplest method. I plan to heat shrink the battery with whatever electronics was required so that It can be as compact and safe as possible, Plus the heat shrink would help diffuse any heat, if caused.

I also have a shoe string budget (out of work theatre technician!) so I need to limit my outgoings on experimentation so that it all balances out in the end haha

I think you should be very careful where you are getting your information from.

Placing anything on the market (ie selling it, including a one off, prototype etc) may require a technical file and to be safe (which you provide evidence of in the technical file) and necessary documentation stating compliance with the required standards for UKCA.

Do many people sell things that should have this but don’t and get away with it, yes.

Have these things also killed people and resulted in the manufacturer going to jail, also yes.

If it is required, there is no get out for just making a few, I’m only starting out, I’m really careful etc...

Its not a hard job to comply with the law, and claiming ignorance of it is not a defence (or so I hear)

Aidan

 

[novocaine]

the only way to avoid requiring CE marking (soon to not be UKCA) on a product is to make it a prototype for for demonstration purposes (unless you are a fair ground) or make consumables.
you can self certify for certain things, this being one of them because you arn't actually making anything you are integrating products (the bulb should be CE marked, so should the batteries). you don't need the CE mark on the product but you do need a technical file (which includes a safety file). for something this simply it's really easy to bring together, find the manufacturer of all the parts you are using and collect there declarations together (back or front page of the user manual normally but you can request them too), do a little hazard identification and risk ranking, be honest here.

as already said, you will fall under the low voltage regs, I assume you aren't selling them from use in flammable atmospheres so get a copy of the regs and read through it, do a gap analysis of each requirement and record evidence for each requirement.

theres a bit more but that is going to cover your bum for the most part.

 

[RichHox]

Thanks again for all your advice.

I did more research on it last night so I feel like I've got the jist of the regs and requirements now.

 

[MorrisWoodman12]

@RichHox Two further points. You say you test your mains lights within your capabilities: I hope that means a PAT test.
You also say heatshrink will diffuse the heat. Hmmm! Maybe but mainly it will act as a thermal insulator and raise the temperature of those components under said HS. If the resistor is covered by the HS then it will need a higher rating than that specified at 25'C plus you may need to measure its temperature rise to check its suitability (and record that as evidence in your technical file)
Have fun.
Martin

 

[TheTiddles]

Just a note, again be careful where you get your information...

People are merrily telling you to comply with the LVD... have a look at the voltage range of the low voltage directive.

Also, combining certified components together does not mean your assembly is certified and the really pants part... it’s depressingly common to find certificates themselves are out of date and invalid too.

Aidan

 

[MorrisWoodman12]

Good advice Aidan.