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SuperCookie78

Are step down DC converters on ebay trustworthy?

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Posted · Original PosterOP

I am planning on putting together some emergency usb power banks that will run off of C or D cell batteries. I know if it use a battery holder like this i can get a 6v output. But then stepping that down to 5v without draining power (like a resistor would) is hard for me. I am not into hobby electronics and my physics class only dealt with simple circuits that only included capacitors resistors and inductors. I was planing on using this to step down the 6v to 5v to charge usb devices. But i don't know if i should trust random electronics from ebay.

 

 

And yes i know there are rechargeable portable batteries. We have some but whenever we lose power we they're all dead. We don't charge them before a storm because losing power from them is infrequent enough. and using d cell battery would be easy for my family to use. 


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Step down (buck) converters generally need a volt or two higher than their output, and 6V to 5V is barely enough for even the higher end buck converters. Not to mention, the batteries will drain below 6V and cause more issues.

 

I'd go for a buck/boost converter, since it accepts above and below the desired output voltage, so as the batteries drain down to like 4V or lower, it'll still be kicking out 5V.

 

One thing I'd look into though is to see if you can find one with a fixed 5V output instead of a variable one. I'd expect them to run a bit more stable.

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Posted · Original PosterOP
13 minutes ago, iamdarkyoshi said:

Step down (buck) converters generally need a volt or two higher than their output, and 6V to 5V is barely enough for even the higher end buck converters. Not to mention, the batteries will drain below 6V and cause more issues.

 

I'd go for a buck/boost converter, since it accepts above and below the desired output voltage, so as the batteries drain down to like 4V or lower, it'll still be kicking out 5V.

 

One thing I'd look into though is to see if you can find one with a fixed 5V output instead of a variable one. I'd expect them to run a bit more stable.

like this? also cloud i have a voltmeter that could deactivate a relay if the battery voltage gets to low?


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Most of the stuff on eBay will function, as long as you don't plan to use it as the maximums they advertise. Basically, the advertised values are too optimist or require you to add heatsinks or account for other things.

 

In the case of the regulator you linked to, it would function, but even though it says it can work with up to 40v and can supply up to 3A of current, I would not use it with more than around 30v and with more than 2A for long periods of time.

 

I would not use that particular dc-dc regulator because of the minimum 6v working voltage - it will be fairly inefficient with low voltages and it may even not work with such low voltage.

Also, that particular dc-dc converter uses a really cheap regulator IC that's a clone of a popular switching regulator IC which is fine, but the problem is that it runs at a low switching frequency which means it requires big inductors (the gray thing with 470 in the center) and big capacitors to output smooth voltage. To get high efficiency, these components need to be carefully picked and must meet certain specifications, but most eBay sellers simply use something they can buy in volume (and in the example above they use cheap electrolytic capacitor instead of a polymer one) so such a regulator would not be very efficient.

 

Also, keep in mind there's dc-dc converter boards which are step-down only, meaning they can only take a higher voltage and output a lower voltage and then there's dc-dc converters that are up-down or SEPIC , which can take a higher or lower voltage and output a specific voltage.

Here's an example of dc-dc regulator I would be comfortable recommending to you :

https://www.ebay.com/itm/DC-DC-Buck-Step-Down-Module-3-3V-5V-9V-12V-3A-Adjustable-Voltage-Regulator-Power/282752232419

https://www.ebay.com/itm/DC-DC-3-3V-5V-9V-12V-3A-Adjustable-Buck-Step-Down-Voltage-Regulator-Power-Module/112472497295

https://www.ebay.com/itm/DC-DC-4-2-23v-to-3V-3-3V-5V-9V-12V-24V-Buck-Step-Down-Regulator-Converter-Module/283094896140

image.png.170971b11e25f5fbd089785f69322838.png

 

You MUST set the voltage to the right value using that small potentiometer before connecting any device to it.

THe benefit of a device like this is that it also leaves the usb data wires separate, easy to access them... you only solder the voltage and ground wires to the board.

 

This is another design which requires you to place a blob of solder on a couple of pads (or short the two pads in some other way) to set the voltage to desired output voltage. 

https://www.ebay.com/itm/3A-Mini-DC-DC-Buck-Step-Down-Converter-Voltage-Regulator-1-5V-1-8V-3V-3-3V-5V/263220698605

Otherwise, it works the same way.

 

image.png.963b89a25b1e84e208eefac6b4782c66.png

 

You can use it together with a USB bracket like this :

https://www.ebay.com/itm/Dual-Mainboard-9Pin-Header-to-4Port-USB-2-0-Female-Cable-PCI-Bracket-AD/112071865294?epid=1949344881

image.png.f61537d01c0ca75487c8232e49be3408.png

 

You can remove the connectors, and you have the 4 usb wires ... red is 5v, black is ground, and the green and white wires are data. You can place resistors on those wires to tell the phone it can charge at higher than 0.5A - see the top answer and the links inside : https://electronics.stackexchange.com/questions/123172/what-is-the-ideal-way-to-handle-data-pins-d-and-d-on-a-usb-power-adapter-to-be

 

I'll add some more comment in a few minutes...i want to post this

 

 

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Posted · Original PosterOP
5 minutes ago, mariushm said:

Most of the stuff on eBay will function, as long as you don't plan to use it as the maximums they advertise. Basically, the advertised values are too optimist or require you to add heatsinks or account for other things.

 

In the case of the regulator you linked to, it would function, but even though it says it can work with up to 40v and can supply up to 3A of current, I would not use it with more than around 30v and with more than 2A for long periods of time.

 

I would not use that particular dc-dc regulator because of the minimum 6v working voltage - it will be fairly inefficient with low voltages and it may even not work with such low voltage.

Also, that particular dc-dc converter uses a really cheap regulator IC that's a clone of a popular switching regulator IC which is fine, but the problem is that it runs at a low switching frequency which means it requires big inductors (the gray thing with 470 in the center) and big capacitors to output smooth voltage. To get high efficiency, these components need to be carefully picked and must meet certain specifications, but most eBay sellers simply use something they can buy in volume (and in the example above they use cheap electrolytic capacitor instead of a polymer one) so such a regulator would not be very efficient.

 

Also, keep in mind there's dc-dc converter boards which are step-down only, meaning they can only take a higher voltage and output a lower voltage and then there's dc-dc converters that are up-down or SEPIC , which can take a higher or lower voltage and output a specific voltage.

Here's an example of dc-dc regulator I would be comfortable recommending to you :

https://www.ebay.com/itm/DC-DC-Buck-Step-Down-Module-3-3V-5V-9V-12V-3A-Adjustable-Voltage-Regulator-Power/282752232419

https://www.ebay.com/itm/DC-DC-3-3V-5V-9V-12V-3A-Adjustable-Buck-Step-Down-Voltage-Regulator-Power-Module/112472497295

https://www.ebay.com/itm/DC-DC-4-2-23v-to-3V-3-3V-5V-9V-12V-24V-Buck-Step-Down-Regulator-Converter-Module/283094896140

image.png.170971b11e25f5fbd089785f69322838.png

 

You MUST set the voltage to the right value using that small potentiometer before connecting any device to it.

THe benefit of a device like this is that it also leaves the usb data wires separate, easy to access them... you only solder the voltage and ground wires to the board.

 

This is another design which requires you to place a blob of solder on a couple of pads (or short the two pads in some other way) to set the voltage to desired output voltage. 

https://www.ebay.com/itm/3A-Mini-DC-DC-Buck-Step-Down-Converter-Voltage-Regulator-1-5V-1-8V-3V-3-3V-5V/263220698605

Otherwise, it works the same way.

 

image.png.963b89a25b1e84e208eefac6b4782c66.png

 

You can use it together with a USB bracket like this :

https://www.ebay.com/itm/Dual-Mainboard-9Pin-Header-to-4Port-USB-2-0-Female-Cable-PCI-Bracket-AD/112071865294?epid=1949344881

image.png.f61537d01c0ca75487c8232e49be3408.png

 

You can remove the connectors, and you have the 4 usb wires ... red is 5v, black is ground, and the green and white wires are data. You can place resistors on those wires to tell the phone it can charge at higher than 0.5A - see the top answer and the links inside : https://electronics.stackexchange.com/questions/123172/what-is-the-ideal-way-to-handle-data-pins-d-and-d-on-a-usb-power-adapter-to-be

 

I'll add some more comment in a few minutes...i want to post this

 

 

the ones with the potentiometer make me a bit nervous that i would hit by accident. The blob one seems better but it also only goes to to 5v and if i drain the batteries until they have 1.2v each then i am under 5v for the step down. adding more than 4 batteries would be too inconvenient. What do you think about this  or this?


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For an example of SEPIC regulator (take in lower or higher voltage and output something in between), here you go:

 

https://www.ebay.com/itm/Auto-DC-Boost-Buck-Converter-Solar-Charge-Voltage-Regulator-25W-3-15V-to-0-5-30V/401088945994

 

Note that often SEPIC regulators are less efficient than just step-down or just step-up regulators. It may be the case you get more energy out of 4 batteries by using a buck regulator and discharging down to 4.8v or something like that, compared to discharging the batteries down to 4v using a SEPIC regulator.

It would make sense if you want to use rechargeable batteries, as those will be 1.35v when fully charged and around 1.1v when discharged.

 

One more thing I want to point out. Be very careful what batteries you buy.

A lot of cheaper alkaline C or D type batteries are NOT actually much higher in capacity compared to regular AA batteries.

For example, AA batteries will hold around 2800 mAh and weigh around 24g

Here's proper AA non-rechargeable batteries sorted by capacity : https://www.digikey.com/products/en/battery-products/batteries-non-rechargeable-primary/90?FV=ffe0005a%2C800002&quantity=0&ColumnSort=-33&page=1&pageSize=25

You can get 50 for 12$ : https://www.digikey.com/product-detail/en/fdk-america-inc-a-member-of-fujitsu-group/LR6-C-2S/SY628-ND/5305214

 

You'll find C batteries from ~2500mAh to 8000mAh ... if they're good batteries they should weigh around 60-70 grams each.

Here's C sorted by capacity : https://www.digikey.com/products/en/battery-products/batteries-non-rechargeable-primary/90?k=&pkeyword=&sv=0&pv412=91&pv412=94&pv412=93&sf=0&FV=800009%2Cffe0005a&quantity=&ColumnSort=-33&page=1&pageSize=25

You can get 7.5 Ah for 1$ each: https://www.digikey.com/product-detail/en/fdk-america-inc-a-member-of-fujitsu-group/LR14-F-2S/SY631-ND/5305217

 

D cells are up to 15-18 Ah and should weigh up to 140 grams each : https://www.digikey.com/products/en/battery-products/batteries-non-rechargeable-primary/90?k=&pkeyword=&sv=0&pv32=5&sf=0&FV=670005b%2C670005d%2C670005e%2Cffe0005a&quantity=&ColumnSort=-33&page=1&pageSize=25

ex 1.5$ each : https://www.digikey.com/product-detail/en/fdk-america-inc-a-member-of-fujitsu-group/LR20-F-2S/SY632-ND/5305218

 

 

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16 minutes ago, SuperCookie78 said:

the ones with the potentiometer make me a bit nervous that i would hit by accident. The blob one seems better but it also only goes to to 5v and if i drain the batteries until they have 1.2v each then i am under 5v for the step down.

The evaluation kits will be expensive.

 

You can desolder the potentiometer and replace it with two resistors.

You have the datasheet of the chip freely available , in the case of the first one with the picture it's MP2307 : https://www.monolithicpower.com/en/mp2307.html (here's datasheet )

 

The potentiometer is the R1 resistor in the schematic below, and in the actual circuit board R2 may be a slightly different value than 10k (I think it's 822 = 8200 ohm). On the circuit board, the potentiometer is probably a 100 kOhm one :

 

image.png.09af49e754c224b5644d16ceaa73c0b7.png

So you have page 7 in the datasheet :

 

image.png.9904b6ff1ef7cf6f485e355a32c6d4d5.png

 

So let's say they used a 8.2 kOhm resistor on the circuit board and you want the board to output 4.8v (phones should still charge with this voltage) ...

4.8v =  0.925 x ( R1 + 8200 ) / 8200  => R1 + 8200 = 4.8 x 8200 / 0.925 = 36408 ... => R1 = 36408-8200 = 28,208 ohm or 28.2 kOhm

28kOhm is E96 value, easy to find, 28.4 kOhm or 28.7 kOhm (E192 values) are also easy to get

 

 

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Posted · Original PosterOP
33 minutes ago, mariushm said:

The evaluation kits will be expensive.

 

You can desolder the potentiometer and replace it with two resistors.

You have the datasheet of the chip freely available , in the case of the first one with the picture it's MP2307 : https://www.monolithicpower.com/en/mp2307.html (here's datasheet )

 

The potentiometer is the R1 resistor in the schematic below, and in the actual circuit board R2 may be a slightly different value than 10k (I think it's 822 = 8200 ohm). On the circuit board, the potentiometer is probably a 100 kOhm one :

 

image.png.09af49e754c224b5644d16ceaa73c0b7.png

So you have page 7 in the datasheet :

 

image.png.9904b6ff1ef7cf6f485e355a32c6d4d5.png

 

So let's say they used a 8.2 kOhm resistor on the circuit board and you want the board to output 4.8v (phones should still charge with this voltage) ...

4.8v =  0.925 x ( R1 + 8200 ) / 8200  => R1 + 8200 = 4.8 x 8200 / 0.925 = 36408 ... => R1 = 36408-8200 = 28,208 ohm or 28.2 kOhm

28kOhm is E96 value, easy to find, 28.4 kOhm or 28.7 kOhm (E192 values) are also easy to get

 

 

what about enabling faster charging. is there an easy way for me to short the data lines or something so that any device (apple, samsung, etc.) can charge at at least 1amp?


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Even the fake ones are good enough for 5V 2A but don't trust there raiting nor the chip they tell they use.

If you need fixed voltage just buy a 5v version and not those adjustable boards.

 

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oh... another option would be to just buy a low capacity usb power bank, but which has a good reputation for handling various phones well.

and replace the lithium battery with your alkaline batteries.

You can have groups of 3 in series to get up to 4.5v and then have two of these groups in parallel for more capacity. A simple diode with very low voltage drop would prevent the power bank from trying to top up the batteries and the internal chip will be designed to handle 3v..4.2v and 4.5v is not that far off.

 

Here's a 13 GBP (~16$) power bank from a reputable company : https://www.amazon.co.uk/Anker-Portable-bar-Sized-High-Speed-Technology/dp/B00P8SY7HQ/

Seems like it may be using two batteries in series, for up to 2 x 4.2v  ... if that's the case it should be safe to have 6D cells in series for a total of 9v ... when nearly discharged the batteries should be around 1v..1.1v, or around 6v ... which is about the same as the cutoff voltage for lithium batteries, 3v.

Even if the batteries are in parallel, you could have groups of 3 D cells in series, then have these groups in parallel to increase capacity.

 

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My experience with cheap buck converters has been very negative. In the sense that I had two literally catch on fire.

 

Even though the XLSemi IC on it was supposed to have thermal safeties and the like, the moment the diode on the board failed and created an open feedback loop, the IC incinerated itself and the rest of the board along with it. The second board right next to it also suffered catastrophic failure, though it was hard to say whether it was due to the first board catching on fire.

 

Both boards were being run at about 20% of their rated performance (in amps) to supply power to some LED strips, so they weren't even being pushed hard. The main issue with most of the cheap boards you can get on sites like EBay, AliExpress and kin seems to be that they lack even the most basic safeties, and even nice luxuries like in or output filtering.

 

It's much more safe to go with a Recom, TRACO or similar DC-DC converter module from a reputable site like Digikey, Mouser and such. With those modules you know at least that they have been designed by competent EEs and there's a whole datasheet to go with it that's even correct (usually) :)

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Posted · Original PosterOP
3 hours ago, hackkitten said:

My experience with cheap buck converters has been very negative. In the sense that I had two literally catch on fire.

 

Even though the XLSemi IC on it was supposed to have thermal safeties and the like, the moment the diode on the board failed and created an open feedback loop, the IC incinerated itself and the rest of the board along with it. The second board right next to it also suffered catastrophic failure, though it was hard to say whether it was due to the first board catching on fire.

 

Both boards were being run at about 20% of their rated performance (in amps) to supply power to some LED strips, so they weren't even being pushed hard. The main issue with most of the cheap boards you can get on sites like EBay, AliExpress and kin seems to be that they lack even the most basic safeties, and even nice luxuries like in or output filtering.

 

It's much more safe to go with a Recom, TRACO or similar DC-DC converter module from a reputable site like Digikey, Mouser and such. With those modules you know at least that they have been designed by competent EEs and there's a whole datasheet to go with it that's even correct (usually) :)

I think i am going with a design from texas instruments simple switcher. I can specify high efficiency and simple design.  


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