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Wearable 5 V 9 A power supply? Answered

So I am making an electronic t-shirt with a very bright screen that gobbles up amps. At it's probable peak usage it will be using 9 amps (or 8.697, but I always feel safer sizable margin of error). Luckily this will only be at its peak for a few seconds (at most 10 seconds, probably less) when it starts up. Next it will use  between 4.5 to 3 amps continuously, and later will  be using closer to 1 amp. All of this is at 5 volts, and I will be stepping down from 7.4 volts. As if this wasn't hard enough this is battery powered, so good efficiency would be nice. I want to minimize the amount of power lost. Is there a power supply that can do all this and still be wearable?

In short, I need a wearable 5 volt power supply that can provide a few seconds of 9 amps and 4.5 amps continuously. 


Thanks Everybody! For people who find this in the future, here’s from saving you from having to read through all the posts…
In the end I found the answer to my problems was cutting the peak amps it needed to consume it half, bringing it down to around 5 amps. What I ended up choosing was the R-745.0P power supply. Look at the last page of the datasheet for clarification on how to wire it, and when it comes. The mpilchfamily thread talks about safety, which is important and worth reading if you want to know about it. I wasn't sure on whether or not I should go with the R-745.0P and all the supporting components or the UBEC 2-5S, which is an all in one package. I chose the R-745.0P but it’s up to you!


It all depends on how long you need it to be on for!

Probably, at most, 10 minutes at the 4.5 to 3 amp usage mark, and 3 hours at the 1 amp usage mark.

That's only 200mAh, so not too bad a load.
That transient is a killer though. Are you sure it needs that kind of start ?

I just timed the short animation I'm having play at start up, it ended up being more like 3 seconds. And I can decrease the brightness during that animation. Half as bright shouldn't hurt anything, as they are already pretty bright. After adding in the amp consumption of the other parts I'll be running it comes to 4.645 amps. Does that help?

So I looked around and found the R-745.0P power supply that I think will work for my needs. It can handle at 4 amps continuous, has a minimum input of 6.5, and an output of 5. Not to mention 95% efficiency! But looking at the datasheet I'm a little confused with the schematic on the bottom of the last page. I'm good with everything but the TTL and components associated with it. I can see that there is an NPN transistor, and a resistor connected to the base. Earlier in the datasheet it says to bring pin 1, which is directly associated with TTL, "active low (less than 0.8v) to disable the device". What I need to know is what do I connect to TTL (the battery, the microchip I'm powering with this etc.), weather or not I need the transistor and resistor, and what should the specs of transistor be (if I need it).
Or am I just better off getting something like this.

Thanks again!

Don't worry about it. Connect it to ground, and then use the old on/off switch to operate it

You definitely need a "buck" mode switching supply then. If you were to put a large supercap in with it, you might get a 3A supply to hold up long enough to pull 9A for 10Seconds.

Can the 7.4V battery supply near that current for as long ?

Have you considered how much heat will be generated by this setup?

Yeah, it's going to be a lot. I'm hoping that can be minimized though.

45 watts is as hot as an incandescent light bulb, and you want to put up against somebody's skin? I don't mean the power supply itself, but the circuit you're building is going to be putting all that power out as heat. There is a reason that wearable circuits use extremely low power LEDs. Second degree burns are not fun.

Hmmmm, you have a point there. Although it won't be running at 9 amps long enough for it to get super hot. Also as I mentioned in the post started by steveastrouk (below), I'll be lessening the brightness so power consumption is closer to 5 amps at it's peak, which is for 3 seconds. Anyway, as I understand it (correct me if I'm wrong) but LED's are more efficient at turning electricity to light, so high power led's still get hot and bright, just less hot and more bright.

Even at 3A your dealing with 27W which is still hot enough to burn you.

A 1W LED generates enough heat to cause a 1st degree burn.

Check out my analysis above. The OP appears to have that 27 W spread over at least 400 square centimeters, which means the local heating anywhere is minimal.

I'll have 210 LED's spread across a 7 by 9 inch square on the chest. Each LED consumes 60 milli-amps on full power, at 5 volts that's 0.3 watts per led. Here is the LED data sheet.
and a link to the thing that I'm using to organize the LEDs in a nice square, In the description is where I got 60mA.
I'm making this so there are 15 LEDs in a column and 14 LED's in a row. The second link should help with spacing. The LED's will be, at their closest, 7.5 mm apart from one edge to the other. However I don’t know if this is close enough to of any danger me when I wear it. Like I said before, I'm not running each LED at full power. They are RGB LED's and having them all white would be pretty boring. So I'll have plenty of them flashing red, purple, orange, yellow, etc. and only a few that are white, and probably none that are full brightness white. All of my estimates are high, and assuming a reasonable worst case scenario, so it probably won’t be too terrible. And if it does start getting too hot, I’ll turn it off and use the screen for things other than wearing. But I do appreciate your concern. I’ll be careful when building, programming, and using this.

Yes, but you really shouldn't need high-power LEDs for anything wearable. By construction, it has a small image area (somebody's torso), and will be viewed from close proximity.

Read my response to mpilchfamily below. It has a link to the datasheet for the LED and talks a little bit more about power consumption.

Thank you! It looks like a reasonable (numerically competent) analysis. The fact that you're wearing this yourself, rather than making it for someone else, means that you're accepting your own risks (always a good thing).

One way to evaluate your actual risk of burn is to compute the power per unit area, and compare that to what's required to induce a burn: typically raising the local skin temperature to 44 C or higher.
So, let's do the math...

If we assume "skin is water" (I'm a physicist, so I like simplifying assumptions), and a typical skin temperature of ~30 C, then 15 calories per square cm is needed to cause a significant burn to that square cm of skin (15 cal causes a 15 C temperature rise to 1 cm3 of water).

Now, 15 cal = 63 joules, and you're assuming full power for ~3 s, so anything over 63/3 = 21 W/cm2 would give you burns. But you've said that you're drawing ~27W or so for the whole 18 x 23 cm array (400+ cm2), or 0.065 W/cm2, if distributed uniformly. Sounds like you're fairly safe against burns: you could still get some local heating from individual LEDs driven too high, but you shouldn't have a broad hazard.

Yes you will need something like these circuits tweaked to your needs.


5 years ago

You need a down switcher power supply.

This is Steve's specialty lets hope he chimes in .