Converting SMPS from 12v to 3v

[kolbep]

Hi All,
Just wondering,
I have a 12v SMPS similar to this type:


I need to change the output voltage to 3v (to operate a device that normally runs on 2 x 1.5v AA Cells (Very low current, the AA Cells typically run the device 24x7 for 3 months)
What is the best solution for this :
 1) Put a LM317 or 7803, or other regulator,
 2) Change the feedback resistors (where the potentiometer is) to lower the voltage.   

If I go for option 2, do you think the 12v powersupply would still be stable if I adjusted it like that down to 3v?

Thanks

[TimNJ]

2) Change the feedback resistors (where the potentiometer is) to lower the voltage.   

If I go for option 2, do you think the 12v powersupply would still be stable if I adjusted it like that down to 3v?

Probably won't work at all, and if it does, the safe power capability will probably be something like 3/12 = 25%. If the power supply is 100W, originally 12V @ 8.33, you can't expect to get 100W @ 3V = 33.3A. This would be significantly more stressful on the output rectifiers. (I^2*Rdson for MOSFETs and  Vf * If for Schottky diodes).

If it uses a TL431 reference + error amplifier, the reference voltage is 2.5V. Typical optocoupler LED forward voltage is 1.0V. So, with a 2.5V reference, you are generally limited to about ~4V minimum voltage. There are 1.2V and 1.25V types though.

You can try adjusting the output voltage down as low as it can possibly go (while maintaining good regulation, stability, etc.) as a "pre regulator" and then knock down the remaining voltage with LM317 or similar. But, if it's very low current anyway, might not matter much.

[langwadt]

find a random USB charger and add a regulator, that big supply is going to be terribly inefficient if you only need low current

[T3sl4co1l]

If the load is low current, as it sounds to be, then a linear regulator like LM317, 78M33 etc. will do, yes .  It won't be particularly efficient, not so much because of the regulator itself but more because you've got a massive (20W+?) power supply just idling away in the background.  So, mind the "parasite" power draw.

Tim

[Ian.M]

If 2x AA batteries last over 3 months, the average load current cant be over 1.3mA.   Its patently ridiculous to use such a large power supply for a load that's a small fraction of the PSU's no load quiescent consumption.   

Which way to go hangs on how peaky the loads current consumption is.  If it has sustained high drain periods, it will need a supply that can accommodate that, and if the supply isn't 3V output, a reasonably efficient regulator to drop its output to 3V.   

Alternatively, if the load current isn't so peaky, or if the peaks are of short enough duration that they can be supplied by a reasonably size reservoir capacitor, so the supply only has to provide the average load current, the best option may be a capacitive dropper  to reliably get a few mA from the mains without gross inefficiency, feeding a bridge rectifier followed by a low power isolated DC-DC converter module to provide the safety isolation essential for powering a gadget originally intended for only battery supply.

[magic]

Don't mod the PSU. Any major adjustment is not as simple as changing TL431 feedback resistors and totally not worth the effort for such low power load.

This is the sort of problems you will encounter:
https://www.eevblog.com/forum/projects/modding-switchmode-psus-for-different-output-voltage-and-why-it-sucks/
https://www.eevblog.com/forum/projects/smps-always-trying-to-restart/

[enut11]

Neither is a good option.
Given that dry cell output is not regulated anyway, what about a phone charger (5v) with 3 or 4 rectifier diodes in series with the output. This will give you about 3v.

[DavidAlfa]

I've modded smps before, it's easy but you can't go too low, or the secondary circuits will stop working.
Ex. 12 to 16V, or 12 to 8V might be possible, but not 3V.
Anyways, for a very low current, a big smps will waste a lot of power.
My gas heater ignition uses batteries, it's either crap or gone bad, they only last a month.
I took a very small 12V 1A charger, opened it up and added a DC/DC converter to lower the voltage to 1.5V, 1 year later it's still working great.
The ignition uses a decent amount current, so I used the DC step down converter, but in your case, if very little power is needed, I'd use a simple LDO to lower the output voltage.
But not such huge smps for 1.3mA, it's completely overkill.

For little money, you can get a small 3.3V mains adapter like this one, cheap and easy:
https://a.aliexpress.com/_v20PVk
If there's enough space, you can embed the 5.5mm connector to it, which is a very common connector for DC converters.

[BrokenYugo]

I don't trust cheap AC adapters. I like the idea of regulating down a USB phone charger. Safe 5V USB supplies are far easier to come by, many people have a few kicking around that were included with old cell phones. 3.3 or 3.0V LDO, a couple caps, a load resistor for stability, and you've got an ideal battery eliminator. Or just a few 1N400x or similar would probably be good enough.

[magic]

I wouldn't trust the series diodes "regulator" any more than a cheap AC adapter.
They have the annoying tendency to increase output voltage under light load
Build the resistor-zener shunt regulator if you can't live without a truly dumb PSU.

[mariush]

If your device runs on 2 AA batteries, it's gonna support a wide range of input voltages.

The device is probably designed to accept Ni-MH alkaline batteries which will be almost 1.65v when fully charged and maybe 1.1v when discharged enough to no longer supply enough energy, and will probably also work with rechargeable AA batteries which are 1.35v when fully charged and around 1v when discharged.

So, you could probably be fine using a fixed output voltage 3.3v linear regulator to power the circuit, if you want to not have to deal with feedback resistors.  There are also lots of linear regulators that have a fixed output of 2.8v which is well within what 2 AA batteries could output (or you could add a basic schottky diode in series with the output of a 3.3v linear regulator to drop around 0.3..0.5v and get around 2.8v)

Keep in mind that a lot of linear regulators are not very stable with very low loads like 1-2mA you say your device consumes so a very simple way to solve that would be to add a small "power on" indicator like a red led/green led with a current limiting resistor on the output of the linear regulator to consume around 5-10mA and show that the device is powered.  A 10-15 ohm resistor in series with a led would be fine ... let's say 2.8v output, 5mA through a led with 2v drop would be  2.8v - 1 led x 2v drop = 0.05A x r  so r = 0.8/0.05 = 16 ohm so 10-15 ohm would result in led consuming a bit more than 5mA
 
Yeah, such power supply is excessive. If you don't trust phone chargers, buy a proper wall wart  designed to output 5v or more, then use a basic linear regulator to output that 2.8v..3.3v

You could also go the classic isolation transformer route... you can buy small power transformers good for maybe a couple watts, you can then use a bridge rectifier to convert the AC to DC, a small capacitor to smooth out the voltage, then your linear regulator.
You'd get clean power, cool power supply if you're worried a small phone charger like adapter would overheat or die... no worries.

Here's an example of such classic transformer (you can get them with 110v input if you're in US or other 110v region) : https://www.tme.eu/en/details/tez2.3_d_6-6v-ta40/pcb-transformers/breve-tufvassons/tez2-3-d230-6-6v-ta40/
This particular one outputs 6v AC ... a bridge rectifier will rectify it to around 8-10v DC ... add a 100-470uF 25-50v rated electrolytic capacitor to smooth the output out then use a linear regulator to get your 2.8v .. 3.3v
The transformer is only good for around 30-50mA but you say your device doesn't use that much power.

[Miyuki]

Just take any of the shelf Arduino USB to 3.3V power module
Or something simillar

[kolbep]

Ok All, sorry for the brief original post. Had to get it through before Eskom (our Electricity Non-Provider) loadshedded our area. Once that happens, because the Cellphone Towers batteries have been stolen, we loose almost all signal.
Also, between the Loadshedding periods, and getting this clients areas all ready to open this past weekend, I have been running around like  Crazy, the whole week, everything from hanging multiple chandeliers, tv installations, alarm system repairs, cctv, etc. And on top of that all my other clients.

Anyway, the customer has these Oxygen-Pro Airfreshners for their toilets :
https://www.oxygenpowered.com/content.php?lid=549

They work by having a cartridge with the smelly chemical in a center ampoule. There is a small oxygen generator that produces a small stream of oxygen directly into the ampoule, which pushes the chemical out into the sponge material in the cartridge, where it then evaporates. The Cartridges they purchase can last up to 3 months, and the 2 x AA Batteries typically for the same amount of time.

That is fine for the toilets in the main part of their restaurant, as it is open 6 days of the week, and from 10am to 11pm (at the moment, later it will hopefully be able to back to closing at 4 or 5AM, once restrictions are lifted).

Anyway, they have another part of their building that will only really be used on Friday nights and Saturday. They would like their Air Freshners to only really operate on those days, to save on battery and cartridge (if it even gets them another month or 2 on the cartridges, they will end up saving quite a bit (there are 8 of those units spread between 4 sets of Toilets, and those 3 month cartridges are not cheap).

Manually activating them means that the staff will either forget to turn them on or off. My thinking was take a Smaller 12v SMPS (the smallest one my supplier had in stock), drop the output to 3v, and use that to run the units. That will either be on a normal 7 day timer, or on the lights circuit, so that when the lights are on, the smelly things are killing the smells.

After reading your comments, I have decided on the following :
They already have 12v LED Striplights around the mirrors, and at the Top and Underneath the Stainless Steel Urinals (that is my customer for you - everything must be Bright and White, unless it is Bright and Red, and those Striplights are already powered by a 12v Switchmode powersupply, which is on the light circuit.
Since that is already in place, I can just take a LM1117 3.3v module, hook it up to the existing supply, and run several of the Oxygen generators from that.

But thanks for your input, it is always good for more perspective.

[mariush]

1117 is kinda bad choice ... some versions (from some manufacturers) require electrolytic capacitors on the output with specific esr (ex between 0.1 and 1 ohm)... some versions are also max. 15-17v or something like that.
Other regulators can work with just a cheap ceramic capacitor on the output.

Knowing their needs, what you could do is have a cheap microcontroller with a clock/calendar chip and a 32768 oscillator and a cr2032 battery to keep the time, and only enable the device by powering it from battery within certain time periods.

To keep everything cheap you could have a i2c/spi display and some buttons on a separate board and you could just plug this board into a header to allow you to set up the on periods and see feedback on the display and when you're done just unplug the board.

[ledtester]

...
 That will either be on a normal 7 day timer, or on the lights circuit, so that when the lights are on, the smelly things are killing the smells.
...

Another idea: your day timer or light switch operates a mains relay module which connects the batteries to the oxygen generators.