12-5v regulator for smartphone charging

[Hermnnn]

Hello all, just joined, I've browsed for a while (sooo many distractions ) and Googled this site, but couldn't find a reference to this. Apologies if I'm just ignorant (which is quite possible).

I want to make a simple cable to charge my HTC Desire smartphone from my motorbike battery. So I need to regulate nominal 12v down to 5v at 1A

I came across this switchmode regulator which seemingly needs no other components.

I am naively thinking I can just solder a fused flylead onto the supply pins, and a micro-usb cable onto the output pins, without any additional capacitors etc. Is this right? It seems too simple.

I'd really appreciate advice from someone who actually knows something about this stuff, rather than relying on my diy hack limited knowledge and cooking my phone.

Thanks.

 

[Reloadron]

There are several ways to go about building your own smart phone / cell phone charger but all things considered it is both cheaper and easier in most cases to just buy an automotive version that plugs into the 12 volt cigarette lighter or 12 volt accessory socket. For a bike it is just a matter of hacking the power cord and getting 12 volts.

While a DC to DC converter as you linked to will work a common approach is to use a LM7805 plain old 5 volt voltage regulator. However, I would still just buy a simple charger as I mentioned.

Just My Take
Ron

 

[Sceadwian]

Motorbike? Aren't those typically 6 volt systems?

 

[Mike_2545]

Motorbike? Aren't those typically 6 volt systems?

No, they are 12v

 

[Sceadwian]

That wasn't really a question Mike, many motor bikes do use a 6 volt battery. What motorbike is the OP using?

 

[Hermnnn]

Thanks all. The bike is 12 volt.

I have already broken apart a cig lighter plug and soldered in a 12 volt supply lead and a usb lead on the 5v output, but I'm not really that happy with it. It's rather bulky, and looks a little fragile for sustained vibration on a bike. I was hoping to find something more compact and robust, so the less solder joins the better. I was thinking one of these regulators could just be soldered to the leads, bedded in silicon or something in a small watertight box (or even just heatshrinked inline in the cable), and we are done (the supply will be fused at the battery of course).

A couple of questions I've come up with from further browsing here...

It seems it would be technically better to add a capacitor across the output-common to reduce ripple, but is that really necessary just to charge a phone? The phone can regulate it's power draw internally, so I woud guess it has some power filtering built in anyway.

These regulators don't seem to have any voltage protection on the output. Is it possible that the regulator could fail and put 12v (up to 14v in reality) on the output (most probably frying my phone)? Can anyone advise a simple solution to overvoltage protection?

EDIT: Sorry folks, should have Googled first. It looks like the phone has overvoltage protection built in anyway. So am I right in saying: if the output ripple of the OKI-78SR I linked to above is acceptable, it is the only component I need to provide 5 volts to charge my phone?

 

[Reloadron]

OK if you want to roll your own this is the actual data sheet for the DC to DC converter you initially linked to. Buried in there you will see the following:

Recommended Input Filtering
The user must assure that the input source has low AC impedance to provide
dynamic stability and that the input supply has little or no inductive content,
including long distributed wiring to a remote power supply. The converter will
operate with no additional external capacitance if these conditions are met.
APPLICATION NOTES
For best performance, we recommend installing a low-ESR capacitor
immediately adjacent to the converter’s input terminals. The capacitor should
be a ceramic type such as the Murata GRM32 series or a polymer type. Initial
suggested capacitor values are 10 to 22 μF, rated at twice the expected maximum
input voltage. Make sure that the input terminals do not go below the
undervoltage shutdown voltage at all times. More input bulk capacitance may
be added in parallel (either electrolytic or tantalum) if needed.


Recommended Output Filtering
The converter will achieve its rated output ripple and noise with no additional
external capacitor. However, the user may install more external output capacitance
to reduce the ripple even further or for improved dynamic response.
Again, use low-ESR ceramic (Murata GRM32 series) or polymer capacitors.
Initial values of 10 to 47 μF may be tried, either single or multiple capacitors in
parallel. Mount these close to the converter. Measure the output ripple under
your load conditions.
Use only as much capacitance as required to achieve your ripple and noise
objectives. Excessive capacitance can make step load recovery sluggish or
possibly introduce instability. Do not exceed the maximum rated output capacitance
listed in the specifications.

Given a choice for your application I would likely place a .1 uF and 10 uF 25 volt rated cap in parallel at both the input and output. This considering motorbike power is likely quite noisy. The caps are small and cheap. I would also fuse the unit at about 2 amps.

Can a DC to DC converter fail and short? Yes, as can an everyday regulator like the LM7805 calling exactly what would happen is difficult but a fuse is your new best friend. Actually I would start with a 1 amp fuse and move up not to exceed a 2 amp fuse as in 1, 1.5, and 2.

You can pick up a cheap 1 3/4" X 1 3/4" little project board from Radio Shack if you are in the US or any parts house outside the US which is way more than enough room.

Anyway, those are my thoughts on the project. Other members may jump in with better ideas. Likely I would have just used an LM7805 regulator with the caps I mentioned. Either way should work fine.

Before I forget, also in the data sheet:

Output Fusing
The converter is extensively protected against current, voltage and temperature
extremes. However your output application circuit may need additional protection.
In the extremely unlikely event of output circuit failure, excessive voltage
could be applied to your circuit. Consider using an appropriate fuse in series
with the output.

Just My Take
Ron

 

[Hermnnn]

Thanks Ron, appreciate the advice.

The main reason I am avoiding the 7805 is that I understand they generate heat and need a heatsink along with other external components. I now plan on mounting the regulator inside a waterproof box up on my handlebars where space will be limited, and the phone will also be inside the box which itself generates too much heat. If it was a single component for simplicity I'd be happy but I take your advice and accept that it may not be possible.

I'll again show my ignorance - fuse protection. My limited understanding of electronics tells me that a fuse will blow on high current, not high voltage. How therefore does a fuse protect the circuit from over-voltage (given that a higher voltage will equate to a lower current for the same watts)?

I would naturally include a fuse on the power supply to protect from short to ground (either through a cable being damaged or a component failure), but if the regulator just allowed 12 volts through to the output I can't see how the fuse will help.

Sorry to keep asking questions!

 

[Reloadron]

It doesn't work quite the way you see it. Now I don't know what the typical charge current actually is but let's just for explanation purposes assume it is .4 amp (400 mA). The voltage is 5 volts to the load and the load draws 400 mA maximum. What that tells us is that the load presents a DC resistance of about 5 / .4 = 12.5 ohms. Now the DC resistance of the load is not going to change, it is for all practical purposes a fixed value. So let's say we fuse the load side with a 500 mA (.5 amp) fuse. If the regulator shorts we can expect actually 12 plus volts to the load. Remember, the load is a fixed 12.5 ohms. So what happens is now we have 12 volts / 12.5 ohms = .96 amps (960 mA) well exceeding almost by twice our fuse. The trick and what we hope for is the fuse blows before the load cooks which it should.

My cell is a Blackberry Storm (which I dislike but it was free) that I charge in my truck all the time. I have only had one charger in years fail and it failed open and when I opened it up it was actually high side fused. I forget the details but devices like this seldom fail. Not in my experience anyway. You can fuse both up and downstream as adding a few fuses won't hurt anything.

I just have no clue what your normal charge current would be for your phone.

Ron

 

[Hermnnn]

It doesn't work quite the way you see it. Now I don't know what the typical charge current actually is but let's just for explanation purposes assume it is .4 amp (400 mA). The voltage is 5 volts to the load and the load draws 400 mA maximum. What that tells us is that the load presents a DC resistance of about 5 / .4 = 12.5 ohms. Now the DC resistance of the load is not going to change, it is for all practical purposes a fixed value. So let's say we fuse the load side with a 500 mA (.5 amp) fuse. If the regulator shorts we can expect actually 12 plus volts to the load. Remember, the load is a fixed 12.5 ohms. So what happens is now we have 12 volts / 12.5 ohms = .96 amps (960 mA) well exceeding almost by twice our fuse. The trick and what we hope for is the fuse blows before the load cooks which it should.

My cell is a Blackberry Storm (which I dislike but it was free) that I charge in my truck all the time. I have only had one charger in years fail and it failed open and when I opened it up it was actually high side fused. I forget the details but devices like this seldom fail. Not in my experience anyway. You can fuse both up and downstream as adding a few fuses won't hurt anything.

I just have no clue what your normal charge current would be for your phone.

Ron

Thanks for that explanation, I uderstand how it works now, which is always a good thing.

I had a good look at a car charger that I pulled apart and there is no fuse protection in it at all, so I guess whatever I build won't be any more risky than using one of them. Since the phone apparently self-protects for overvoltage I guess it's not an issue anyway.

The specs state 1A at full charge for the Desire. While playing with the dis-assembled car charger I cut a charge cable in half and put a terminal strip in the middle so I could see what was going on (mainly to check I had the right voltages from my hacked charger before I plugged it into the phone). I will check actual charge current using that, although I won't get time to look at it for a while now.

 

[Reloadron]

Whenever, the forum isn't going anywhere that I know of. Let me know what you find as I would be curious.

Ron