Howto make 10.4V out of 4xAA batteries? - Page 1
Forum section: electronics / beginners

author: OE2WHP
author_rank: Regular Contributor
timestamp: August 02, 2018, 02:50:53 pm
content: Hi,I want to build a LCD backlight into my metal detector. I already have a suitable (sizewise) backlight panel that I could use. Only problem is, its running at nominal 12-14V but the metal detector runs on 4 AA batteries. I already did some tests with the backlight and found that at 10.4V the brightness would be right at the level I'm aiming for. Current is somewhere at 2mAWhat I'm searching for is a small dc/dc converter that steps the voltage up to 10.4V, with an input voltage ranging from ~4.5V to 6V depending on battery level.Important to know, the detector runs at 12.8 - 13.4kHz, thus beeing very sensitive to EMI at that frequency range. So the converter must not make any noise there.I would be very happy if someone could point me in the right direction or give hints which converters could do the trick.Thanks

author: Buriedcode
author_rank: Super Contributor
timestamp: August 02, 2018, 05:41:28 pm
content: At <10mA a charge pump doubler could work, although many tend to have a max input of 5.5V I'm sure there are still some of the older ICL7660A types. The trouble with those is the low switching frequency - often as low as 10kHz. The more modern ones can switch into the MHz range, but might not get more than 10.4V out (5.5V doubled, minus losses of two diodes = 11v - 2*0.3V = 10.4).These tend to have lower EMI than magnetic switchers with perhaps increased ripple on the output, and often have a very low part count - some require two diodes and two caps for doubling (they were designed for inverting) others just require two caps as they are meant for doubling.I'm sure searching your local distributor you'll get a list of candidates that meets requirements.

author: OE2WHP
author_rank: Regular Contributor
timestamp: August 03, 2018, 08:33:39 am
content: Thanks for your post.any suggestions for a converter that does not work below or near 14kHz and where the output can be adjusted to stable 10.4V regardless of the battery level (within reasonable range)Could I use this one?:http://www.ti.com/product/lm27313/descriptionThanks

author: Giaime
author_rank: Regular Contributor
timestamp: August 03, 2018, 12:37:57 pm
content: Quote from: OE2WHP on August 03, 2018, 08:33:39 amLooks good!Look for converter ICs that DON'T go in "pulse skipping mode" at low loads, but allow for DCM operation. This one you linked seems ok in this regard.This is important because pulse skipping could lead to a skipping frequency near your "no noise" band.

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2018, 01:58:36 pm
content: You don't want a constant out voltage converter, but a constant current output, to drive LEDs.http://www.ti.com/lit/ds/symlink/lm3410.pdfhttp://www.mouser.com/pdfDocs/STMicro_LCD_BACKLIGHT_A.pdf

author: Giaime
author_rank: Regular Contributor
timestamp: August 03, 2018, 02:11:14 pm
content: Quote from: Hero999 on August 03, 2018, 01:58:36 pmSome LCDs have built-in series resistor for the backlight... OP should check if this is true or not.Can you share the datasheet of the LCD?

author: OE2WHP
author_rank: Regular Contributor
timestamp: August 04, 2018, 09:39:40 am
content: Sorry, no data sheet. It's basically 4 NSCW335T Nichia side LEDs and a resistor all in series and one diode anti parallel to the LEDs.http://www.fairchip.com/nichia/lcd-backlighting/side-emitting-type/335-0-8t-series/1153-nscw335t-nichia-side-view-white-led.html

author: Zero999
author_rank: Super Contributor
timestamp: August 04, 2018, 12:55:40 pm
content: Quote from: OE2WHP on August 04, 2018, 09:39:40 amIs it not possible to reconfigure the LEDs, so they're in parallel and change the resistor value?Even if it has a built-in series resistor, the fact that you want a forward current of 2mA, which is presumably much lower than what it's designed for, means the built-in series resistor won't do much, so a constant current supply is still the way to go.

author: OE2WHP
author_rank: Regular Contributor
timestamp: August 04, 2018, 03:05:45 pm
content: I could shorten the resistor but reconfiguring the LEDs is not possible. They are on a flexprint which is glued into a foil/plastic sandwich.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 04, 2018, 03:09:24 pm
content: Hi,if you want to avoid RFI caused by switching spikes there was a Jim Williams application note from Linear Tech where he used an IC having adjustable slow-risetime waveforms. That could help, maybe. You will probably need extra filtering on the in- and output sides on top of that. Much luck !

author: Buriedcode
author_rank: Super Contributor
timestamp: August 04, 2018, 03:21:51 pm
content: There are plenty of boost converters specifically for driving LED strings - the only real difference is the feedback voltage is lower so one uses a current sense resistor effectively making it a constant current boost. (normally 1.2-1.25V, but often CC boosts have a 0.3-0.6V feedback threshold).Example:https://www.mouser.com/ds/2/115/ZXLD1615-93645.pdfHowever, the reason I suggested a charge pump was the lower EMI. Even with shielded inductors, careful layout, switchers still tend to radiate more interference than charge pumps. A simple (high frequency) boost converter would probably be the most efficient, but you will have to test it with your metal detector - just because it has a high switching frequency doesn't mean it won't interfere. In fact you could probably just buy a cheap boost "module" from amazon/ebay set it to your desired voltage, and see if it interferes. If it doesn't, then you could probably get away with almost any boost converter. If it does, then you have to be more picky.You don't have to have a constant current converter. Whilst efficiency will take a hit, using a 12V supply and a series resistor won't be that inefficient, especially considering you're going to run the LED's at 2mA - often switchers will have a quiescent current of 2mA+ so it won't get to 80%+ anyway.So I say, if you can source some cheaper modules to test they might be everything you need, or at least highlight any issues.https://www.ebay.co.uk/itm/5V-USB-Input-to-12V-Output-DC-DC-Step-Up-Boost-Power-Supply-Converter-Module-G/132613091764?hash=item1ee05b31b4:m:mUxjpGHBQMGT4hfCCCUQolQ(note, I have never used these, they could be noisy trash for all I know, just an example!).

author: OE2WHP
author_rank: Regular Contributor
timestamp: August 06, 2018, 09:54:30 am
content: I found a cheap module at amazon based on a MT3608 (https://www.olimex.com/Products/Breadboarding/BB-PWR-3608/resources/MT3608.pdf).I'll give it a try.Thanks for all the good advice!

author: Microcheap
author_rank: Frequent Contributor
timestamp: August 06, 2018, 05:40:06 pm
content: There are a number off boost converters module available on ebay and aliexpress, as OE2WHP pointed.I was looking at this same subject recently and found some Dave's videos that might be of interest:https://youtu.be/-V_p1GBH4pkhttps://youtu.be/qGp82xhybs4