bad or good? AA batteries and 9V in series and use the different voltages LED/PICAXE?

[clonetwo]

hello, so i have a post here, but now i have a different idea.
please advise me on the safety of this idea.

my old idea is here http://www.picaxeforum.co.uk/showthread.php?26434-Can-anyone-recommend-a-DC-DC-Power-step-up-converter-(2v-or-so-to-around-18v-output)/page3

My new idea is to connect 3xAA batteries in series with a 9V battery to produce 12v like so .

Can I use the 3.4 (EDIT: should be 4.5) volts from the 3xAA for PICAXE and the combined 12V for LEDs?

I have measured the voltages and I do get those voltages. But i want to run this past the pros here and make sure my head is screwed on right because this just seems to easy. Essentially no DC-DC boost.

Maybe i should have a diode somewhere?

edit. so far from what i have read this is a bad idea...

Thanks people for everything,
Regards,
Clonetwo

 

[goom]

Not a great idea in my view. The capacity of the AA's are a lot more than the 9V, so you will end up trying to force current through a dead 9v battery which could be bad news. Much better to have all batteries in series of the same capacity.
Perhaps you can arrange the LED's in a series/parallel configuration which can be driven by (say) just the 3 xAA's. If you can be more explicit regarding he number and type of LED's, then you will likely receive some useful suggestions.

 

[inglewoodpete]

Can I use the 3.4 volts from the 3xAA for PICAXE and the combined 12V for LEDs?

As an aside, I'm not sure how you can get 3.4v from 3xAA unless they are nearly flat NiMH or NiCd. Or very flat alkaline or CZn.

To contradict others, the idea can be used for low current 12v loads. However, a 9v battery is really not suitable for powering LEDs unless you only want a brief periodical flash from them.

 

[AllyCat]

Hi,

You must tell us what current your "LED" requires and for how long it is to be illuminated (e.g. in minutes per day). IIRC that earlier thread did specify only a short period of illumination, but no sensible measurement of the current (somewhere between 1.35 mA and 1.5 Amps ?).

So the idea might be possible, but the most you will usefully get from even an alkaline "PP3" is probably about 20 mA and it will typically cost as much as a stack of 8 AAs (which can deliver at least 10 times as much current and energy).

Cheers, Alan.

 

[manuka]

8 x 1.5V alkaline or 10 x 1.2V NiMH = housing hassles & bulk. The fewer battery contacts the better!
Hence just 4 x 3.2V (= 12.8V) rechargeable LiFePO4 cells may suit? These have a very flat supply voltage during service, are now pretty cheap (~US$2 each in the right places) & are ultra reliable in my experiences.

 

[eclectic]

8 x 1.5V alkaline or 10 x 1.2V NiMH = housing hassles & bulk. The fewer battery contacts the better!
Hence just 4 x 3.2V (= 12.8V) rechargeable LiFePO4 cells may suit? These have a very flat supply voltage during service, are now pretty cheap (~US$2 each in the right places) & are ultra reliable in my experiences.

Please tell us where to find the batteries and chargers......

ebay. co uk shows much higher prices.

e

 

[manuka]

Yikes - you're right ! Aside from imports (which naturally may incur p&p,customs & air security) maybe check solar garden lamps? These increasingly abandon several NiMH & use a single AA sized (14500) LiFePO4 cell - I recently bought several swish LiFePO4 powered PIR lamps for ~£7 each here in NZ.

 

[AllyCat]

Hi,

AFAIK, all the "low cost" (eg. Pound store) solar garden lights now use a single NiMH (often a tiny wire-ended button cell) with a simple boost converter; the problem is identifying the better sets which use LiFePO4s. However at least two UK suppliers do sell "spare" AAs, but only rated up to 400 mAh. Surprisingly, one is Maplin (at £2.99) but not even on the cell itself is there any reference to it actually being LiFePO4 (but they are 3.2 volts). An AA-sized cell can be good for up to about 800 mAh (any more claimed is likely an "optimistic" fake), however, I've never found any stock in the UK.

The tiny USB-AA Soshine charger (SC-F3) probably still can be obtained from Fastech, but I believe any Lithium cells ordered from them now get deleted (and refunded) because of air cargo (and our Royal Mail) restrictions. There are a few China/HK sellers on ebay (search for LiFePO4 14500) which will probably arrive eventually, but prices have risen, and I see that my "favourite" seller no longer lists them. The HobbyKing website has some nice LiFePO4 packs (even the UK warehouse) but not in cylindrical format.

Cheers, Alan.

 

[clonetwo]

Not a great idea in my view. The capacity of the AA's are a lot more than the 9V, so you will end up trying to force current through a dead 9v battery which could be bad news. Much better to have all batteries in series of the same capacity.
Perhaps you can arrange the LED's in a series/parallel configuration which can be driven by (say) just the 3 xAA's. If you can be more explicit regarding he number and type of LED's, then you will likely receive some useful suggestions.

do you think that if i used the PICAXE 08M2 to detect the dead 9V(or low voltage of the 9V) and then not allow the LEDs to turn on as well as shutdown the PICAXE if there is a dead 9V.

Do you think that would be safe? or is this idea something I SHOULD NEVER CONSIDER because of fire and or leaking battery risks?

Thanks!
Clonetwo
The LEDS will be turned on with a 5v Relay Via PICAXE control just long enough to blink 5 times within 4 seconds then shut off and not be used again until next day or week. Maybe used 3 times a day or one time a week. The PICAXE running on the 4.5V would be on full time 24/hours.

Im using 9 LEDs from an 300LED SMD 5050 LED strip rated at 12V with built in resisters. the strip can be cut into 3 led sections. this is common for house or car lighting effects. I bought it on ebay. here http://www.ebay.com/itm/5M-3528-5050-SMD-300-600-LED-Strip-Lighting-12V-FLEXIBLE-DIY-6-Colors-Wholesale-/181435517609?pt=US_String_Lights_Fairy_Lights&var=480407303324&hash=item2a3e664aa9

 

[goom]

Clonetwo,
I invited, as others have also done, to provide more details regarding the driven LED's. Without knowing the quantity, type and wiring configuration, it is impossible to give any meaningful comments or advice.

 

[manuka]

Goom- sign-I heartily second this plea. AllyCat-NiMH for cheapie solar lamps, but LiFePO4 are emerging.

But battery wise check this "2 for 6 quid" 14500 LiFePO4 listing. Although more costly than similar I've bought in from Asia,this outlet gives free p&p. In fact these cells look identical to some great performers I've here, so their capacity is probably more like 600mAh. Stan

 

[AllyCat]

Hi,

@manuka: Yes indeed, but they're only rated at 250 mAh*, which Maplin do here for £2.49 each (in many of their shops, or click-and-collect for orders below their free shipping threshold). Also unbranded, but I can confirm that their similar 400 mA versions (£2.99) are close to specification. There are some (unknown) "branded" 700 mAh cells on ebay from France, but at £10 a pair I'm not sure they're worth a punt. Even at 700 mAh they still have a lower energy content that a single AA NiMH and little more than a pair of AAAs.

*EDIT: I now see that seller also offers a 400 mA version at £6.95 a pair.

In fact these cells look identical to some great performers I've here, so their capacity is probably more like 600mAh.

Hmm. Some of the "fakers" are quite good now and "blue" is not too difficult to copy.

However, this is all said with sadness, because I have several projects which actually need to use a single 3+ volts AA cell.

@clonetwo:

do you think that if i used the PICAXE 08M2 to detect the dead 9V(or low voltage of the 9V) and then not allow the LEDs to turn on as well as shutdown the PICAXE if there is a dead 9V.

How do you propose the PICaxe to measure the voltage across the 9 volt battery without (the resistor divider chain) draining the battery even faster than the LEDs will do?

Im using 9 LEDs from an 300LED SMD 5050 LED strip rated at 12V with built in resisters.

Those listings still don't appear to quote the current required for any particular length (or number) of LEDs, [Edit] but I have now found a data sheet for the "3528". That specifies 24 watts for 300 LEDs, wired three in series for each resistor, so each triplet would use 20 mA (i.e. 2 Amps / 100). That would make 9 LEDs draw about 60 mA, perhaps just possible (very briefly) from a PP3 battery. But the only configuration I would consider is four (or three) NiMH AA rechargeables and a 7.2 volt (or 8.4 if you can find one) rechargeable PP3. LEDs need a reasonably constant supply voltage (over the life of the batteries) which you won't achieve with Alkalines (or even worse the horrid "Heavy Duty" Carbon Zincs).

Cheers, Alan.

PS: Why are you using a relay? Almost any NPN transistor (e.g. BC548) can switch 60 mA, 12 volts.

 

[nekomatic]

Yes indeed, but they're only rated at 250 mAh*, which Maplin do here for £2.49 each (in many of their shops, or click-and-collect for orders below their free shipping threshold). Also unbranded, but I can confirm that their similar 400 mA versions (£2.99) are close to specification.

What's the order code for those Maplin cells please? I can't find them on the website...

 

[eclectic]

What's the order code for those Maplin cells please? I can't find them on the website...

Are these them?

http://www.maplin.co.uk/p/replacement-250mah-lithium-aa-battery-for-solar-fairy-lights-1-pack-n73jk
http://www.maplin.co.uk/p/replacement-400mah-lithium-aa-battery-for-solar-fairy-lights-1-pack-n74jk

But note the warnings about charging.

 

[AllyCat]

Hi,

Yes, quite a "secret", you have to search for something like "Replacement Solar Lithium" to find the Maplin codes N73JK (250mAh) and N74JK (400mAh). They're not only unbranded but virtually unmarked and the first one I got (from a shop) was "loose" and completely flat until charged. But they do deliver 3.2 volts so are LiFePO4s.

"Batteries Plus" also sell 250mAh and 400mAh LiFePO4 and, for the first time I've looked, they appear to have stock of 600 mAh versions (£3.45). But the others are rather more expensive (than Maplin), and there's a shipping fee (£1.95).

Also, an interesting listing on ebay for "Top Quality 500 mAh", UK stock (£6.39 a pair). The listing actually shows a discharge test where they're virtually stone dead at 450 mAh. But compared with some of the Chinese Ahr ratings for batteries, that is exceptionally good.

However, sadly the time (only earlier this year) has gone when I could buy two branded "700 mAh" LiFePO4s with 2 x "placeholder" AAs from HK for just over £3.

Cheers, Alan.

 

[nekomatic]

Thanks for the info! I may consider those for some PICAXE circuits...

 

[manuka]

Sadly the time (only earlier this year) has gone when I could buy two branded "700 mAh" LiFePO4s with 2 x "placeholder" AAs from HK for just over £3.

Yes- I enjoyed similar...

All may not be quite lost for LiFePO4 imports however. As Li regs are specific about CHARGED cells/batteries an air modeller mate is pondering importing LiFePO4 cells (not battery packs) that are deliberately (& boldly stated as) DISCHARGED.

Naturally you wouldn't do that with Li-ion/Li-poly, but LiFePO4 are near inert when "discharged" down to ~2V. My local experiences with assorted "dead ones" (14500 AAs from solar lamps) show they'll bounce back to life nicely when fully charged by the end user. Stan.

 

[clonetwo]

First of all, you need to figure out your led strip type and the quantity of LEDs.
The common led strips we can find in the market have two types, SMD 3528 and SMD 5050.
SMD3528 inside package one light-emitting chip, while the SMD5050 inside package three light-emitting chip package.
and it will consume 0.08W per light-emitting chip.
In theory, one smd 3528 LED will comsume 0.08watt while one smd 5050 LED is of 0.24watt.
To calculate the total power consumption of your led strips, just multiply the quantity of LEDs by the wattage of specific type of LED.
For instance, one reel (16.4ft) of SMD 5050 LED Strips 300 LEDs will consume 300X0.24=72 Watt,
in this case, you need a more powerful transformer than 72 watt, such as 90 watt transformer.
By analogy, you can figure out how many wattages your LED transformer need to be.

thanks for replys,
okay i have found some info on the smd 5050 leds.
http://www.ledlightsworld.com/page.html?id=32
300 PCS, DC 12V, <6Amp, <72 watt.
I'm only using 9 LEDS from this strip so i can calculate this.
9PCS, DC 12V, <.18 Amp < 2.16 watt
@AllyCat
I agree getting the voltage would waist battery. maybe a switch/relay in the divider chain would prevent that I wont go that rout.
also the SMD3528 inside package one light-emitting chip, while the SMD5050 inside package three light-emitting chip package.

I also found some info on wiki. http://en.wikipedia.org/wiki/SMD_LED_Module

I'm gonna try some transistors again, it seemed that when i used a 2n4401 NPN vs the relay the relay gave way more light from the LEDS. might have something to do with voltage drop from the diode in the transistor and my battery s, perhaps?

I need to use batteries that can be obtained at any store and not cost alot so im dinking around with DC-DC boosts and caps trying to get the LEDS bright as i can.

Thanks guys, I'm gonna consider this a complete topic. but feel free to give me any advice, I'm so new i will not be offended at even the simplest of insights regarding electronics.
Regards & Groovey, http://www.youtube.com/watch?v=Q2DCr5D8eZw
Clonetwo

 

[westaust55]

I'm gonna try some transistors again, it seemed that when i used a 2N4401 NPN vs the relay the relay gave way more light from the LEDS. might have something to do with voltage drop from the diode in the transistor and my battery s, perhaps?

From the 2N4401 datasheet, with the transistor operating in the linear region the gain at 150 mA (you calculate a load of 180 mA) will be a minimum of 100 (Hfe) however although not stated in the Fairchild datasheet, the collector to emitter voltage (Vce) can be around 0.7 Volts or more.
If you drive the transistor into saturation the the collector to emitter voltage (Vce(sat)) drops to around 0.4 volts so more voltage remaining for the driven circuit. However in saturation the transistor gain will reduce to around 10 (Hfe(sat) as determined by the datasheet figures such as IC = 150 mA; IB = 15 mA; (gain = 150/15 = 10). So with a load of 180 mA you will need to drive the transistor with at least 18 mA.
That is getting very close to the recommended limit (20mA) for a PICAXE IO pin.
The PICAXE (assumed to be at 5 Vdc) to transistor base resistor would need to be something like;
(Vcc– 0.2) – Vbe / Amps = (5 – 0.2) – 0.8 (typ) / 0.018 = .222 Ohms so use 220 Ohms.

By comparison the relay with good contacts will present only a few mV of volt drop hence the LEDs appear brighter for the same LED and series resistor combination.

Based upon the transistor base current requirement, I would recommend you look for an alternative to the 2N4401 or a Darlington transistor but then also needing a higher voltage.

 

[BeanieBots]

I agree with westy's comments and sums but would NOT recommend using a darlington transistor.
There are plenty of transistors that could do the job but a darlington would give an even larger difference between relay and transistor switching.
This is probably a good project for a cheap FET such as IRL520 or similar, though many of the even cheaper bipolars would be just as good. (hfe ~ 250)

I'd stick with using a boost converter to take a simple readily available battery pack at ~5v for the PICAXE boosted to ~12v for the LEDs.
It all comes down to preference. Personally, I prefer to have high battery voltage and drop down for the PICAXE because it is often more efficient and I have plenty of 12v pb batteries.
If your 12v load requirement is low and you want readily available "AA" or similar, and not too many of them, then boost is better.
I'd also go for a FET switch rather than bipolar or relay to reduce the overall load current on a battery based system just as a matter of course.

 

[westaust55]

Yes a single transistor is better than a Darlington which is effectively 2 transistors.

You can find some Low Vce(sat) transistors such as a collection from On Semiconductors:
http://www.onsemi.com/PowerSolutions/parametrics.do?id=808

Here is an NPN transistor with Vce(sat) of the order of 0.15V (max 0.3V) and Hfe(sat) of around 20 at 200mA collector current.
It can be a case of finding a transistor (or two) that will achieve the goal and then finding finding a seller with them in stock.

 

[fernando_g]

"I'd stick with using a boost converter to take a simple readily available battery pack at ~5v for the PICAXE boosted to ~12v for the LEDs."

This is the approach I would also take.
Even if right now you are not proficient with Switch Mode supplies, a simple circuit like this is an easy way to become familiar with them.
And it is a skill set that is valuable for any electronics practitioner.