Reducing voltage from 3 V DC to 1.5 V DC - Page 1
Forum section: electronics / beginners

author: pin23
author_rank: Newbie
timestamp: March 19, 2022, 03:04:35 pm
content: I have an electric toothbrush that uses one 1.5v battery. Rechargeable battery does not work well being at 1.2 volt. I want to reduce the voltage from 3 V DC to 1.5 V DC from a DC power supply I will need 2 resistors of the same type. I had an electronic lesson decades ago. I have the formula to calculate the resistors but i don't know how much mA I must use. I assume it would be like 10-15-20 mA. I wouldn't not want the resistors to reduce the flow of the current. What do you think the current would be to power a 1.5V toothbrush.I used 15mA for my calculation. 1.5V/.015A= 100 ohm resistor.How many Ohm value should I use?Thank you

author: Ian.M
author_rank: Super Contributor
timestamp: March 19, 2022, 03:11:44 pm
content: Wont work. An electric toothbrush contains a fairly powerful vibration motor which will draw somewhere between high hundreds of mA and several Amps. Assuming you want a stiff enough supply not to degrade the motor performance you'll need ten times more standing current in the potential divider. That's not a voltage reducer, its a portable heater!Also, unless the DC power supply is medical grade, its unsafe to use for a toothbrush due to the high risk of electrocution.

author: rstofer
author_rank: Super Contributor
timestamp: March 19, 2022, 03:12:27 pm
content: Texas Instruments builds a controller chip for toothbrushes (who knew?) and they're talking about 5 Amps continuous and 8 Amps peak current. They allow up to 5.5V input voltage.There is no reason to believe that your toothbrush uses such a controller but there is similarly no reason to suspect that the current is down in the mA range. Come up with a way to measure it!https://www.ti.com/lit/ug/tiduaz7/tiduaz7.pdf

author: tunk
author_rank: Super Contributor
timestamp: March 19, 2022, 06:03:39 pm
content: Measure how much current it draws, and then do your calcuations again.I assume that you by an "1.5v battery" mean an alkaline cell?If so, a NiMh cell should be better at providing high currents, e.g.:https://lygte-info.dk/review/batteries2012/Ikea%20AA%20UK.htmlhttps://lygte-info.dk/review/batteries2012/Ikea%20Ladda%20AA%202450mAh%20%28White%29%20UK.html

author: Zero999
author_rank: Super Contributor
timestamp: March 19, 2022, 06:23:43 pm
content: Why not simply use a 1.5V PSU?

author: mariush
author_rank: Super Contributor
timestamp: March 19, 2022, 07:22:14 pm
content: The first question is why 3v? Seems like you chose this value because you have a power supply which outputs that value.However, power supplies are very cheap, don't get stuck on a specific voltage. Would make way more sense to use a 5v power supply and use a micro-USB / usb type c connector, and you could simply use your phone charger or any usb charger to power your device.Or you could put a barrel jack connector and plug any barrel jack of same diameter.Here's an example :https://www.ebay.com/itm/322709169282?hash=item4b22f72482:g:gMQAAOSwJJVZrAg6These step down converters can output as little as 1v and up to 3A of current so once you set them at 1.5v, hopefully the currrent's enough for your toothbrush motor.They're also small enough to fit in the AA battery compartment - they're 17 x 11 x 4mm (length, width, height) while a AA battery is 48mm long, 14.5 mm wide so the PCB would fit inside that 14.5 mm of diameter.You can solder the wires from a 5v ... 12v power adapter directly to the pcb, or put a usb connector , or a barrel jack ... or for example, you could take an existing usb cable and cut one end and solder the wires. Set the output to 1.5v using that potentiometer (and once you set the potentiometer to the right position, you can use a multimeter to measure the resistance and then replace the potentiometer with a resistor of that value)

author: pin23
author_rank: Newbie
timestamp: March 19, 2022, 08:20:29 pm
content: OK thank youI will use a step down voltage regulator like the LM2596S. I already have the power supply.

author: pin23
author_rank: Newbie
timestamp: March 19, 2022, 08:21:41 pm
content: OK thank to all, problem solved.I will use a step down voltage regulator like the LM2596S or a 1.5 volt PSU. The voltage regulator is very cheap, $1.25 on Ebay. i have the DC connectors to hook it up.

author: pin23
author_rank: Newbie
timestamp: March 19, 2022, 08:25:06 pm
content: I don't want rechargeable batteries. I don't want the hassle to recharge them and they could stop working while i am brushing my teeth, or being less efficient as the charge goes down.

author: pqass
author_rank: Super Contributor
timestamp: March 19, 2022, 08:27:10 pm
content: Quote from: pin23 on March 19, 2022, 03:04:35 pm3V power supply + 2 diodes (any 1N400x where x=1..7) in series = approx. 1.5V output.

author: langwadt
author_rank: Super Contributor
timestamp: March 19, 2022, 08:43:22 pm
content: Quote from: Ian.M on March 19, 2022, 03:11:44 pmyeh, sticking something connected to a cheap supply with unknown certifications and grabbing a metal faucet is living on the edge

author: pqass
author_rank: Super Contributor
timestamp: March 19, 2022, 08:58:20 pm
content: Quote from: langwadt on March 19, 2022, 08:43:22 pmYou do have a point. But the toothbrush should be water resistant and the bit that goes in your mouth is plastic.OP just needs to make sure he has his socks and slippers on and his free-hand occupied in his PJs.

author: pin23
author_rank: Newbie
timestamp: March 19, 2022, 11:02:15 pm
content: The voltage regulator is fed by a 12v PSU. I am not worry at all but your answer made my day. Still laughing writing this.

author: themadhippy
author_rank: Super Contributor
timestamp: March 19, 2022, 11:14:02 pm
content: Quoteso wot ,the important bit is whether the 12v is isolated from the mains supply or not

author: Ian.M
author_rank: Super Contributor
timestamp: March 19, 2022, 11:35:18 pm
content: Quote from: themadhippy on March 19, 2022, 11:14:02 pmFixed that for you. Any PSU out of China not manufactured to a 1st world company's design complying with the stringent UL or TÜV safety standards, with ongoing factory inspection by 1st world engineering qualified 'boots on the ground', has a significant risk of *NOT* having adequate isolation. If its no-name or fake branded, the odds its dangerous are much much higher!

author: langwadt
author_rank: Super Contributor
timestamp: March 19, 2022, 11:57:49 pm
content: Quote from: pin23 on March 19, 2022, 11:02:15 pmhttps://youtu.be/w-p3GXXFfQware you sure if you psu is similar to the good or the bad one ...

author: Ian.M
author_rank: Super Contributor
timestamp: March 20, 2022, 08:52:23 am
content: +1 for Langwalt's video link to one of DiodeGoneWild's USB 'charger' PSU teardowns. Many tear down dangerous PSUs, but few dissect the transformer with such attention to safety details. Also, its a straight A/B comparison between the dangerous PSU and a most likely genuine Samsung one in the same form factor so you get to see how it *SHOULD* be built.His accent is less intelligible than those of us used to BBC 'received pronunciation', or regional British English, or most American accents would prefer, but please persevere and watch right through as the presentation is good and the content excellent.

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2022, 09:26:57 am
content: This power supply will do the job. It will handle the surge current. Add a 5A slow blow fuse for fire protechtion.https://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/1904243

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 20, 2022, 12:59:19 pm
content: What kind of 1.5V battery does it use? If it's an AA battery it cant put out more than 1A. If it draw 1A the battery would be dead in about 12 minutes. Also if it draw 1A the NiMH battery would have higher voltage at that level of current draw as compared to an alkaline battery.

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2022, 02:19:23 pm
content: My previous post was tongue in cheek, being ridiculously overkill.Here are some more sensible options:https://www.dvdoverseas.com/seven-star-ss104-universal-power-adapter-1-5v-12v-500mahttps://www.poweradaptorsuk.co.uk/2000ma-2a-15v3v45v6v75v9v12v-24w-universal-regulated-dcdc-in-car-power-adaptor-eagle-8-p.asphttps://www.tlc-direct.co.uk/Products/NMNA338.htmlhttps://www.amazon.co.uk/REGULATED-Mains-Adapter-Supply-Charger/dp/B000L99ZQK

author: m3vuv
author_rank: Super Contributor
timestamp: March 20, 2022, 03:00:29 pm
content: just get a regular toothbrush,or a toilet brush! lol.

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 20, 2022, 04:25:50 pm
content: There are version of electric toothbrush that run on AC, rechargeable battery.

author: srb1954
author_rank: Super Contributor
timestamp: March 20, 2022, 04:36:49 pm
content: Quote from: Ian.M on March 20, 2022, 08:52:23 amIf you still can't understand DiodeGoneWild's accent then check out bigclivedotcom's channel on YT. He also does some tear downs, narrated in a sonorous Scottish accent, of dodgy Chinesium power supplies.