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7805 Regulator-based USB?
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7805 Regulator-based USB?

by Snipeye on Thu Feb 25, 2010 10:08 am

Alright, here's the deal - I'm a NOOB at electronics. I kinda know what things do, but for the most part, I'm clueless. It's not the components that confuse me, it's mostly how they work, why you need them, etc.

Anyway, that's immaterial. I've been making a couple of 7805 based USB's, and they work perfectly... I think. I'm interested in selling them, and I know that the 7805 will fry before it gives out over 5 volts, but I'm not so sure about current. How does current work when charging something USB? Does the device charging only draw as much power as it needs, or what? Because I want(need) to be sure that nobody can claim that my device damaged their USB thing.

Last I checked, the 7805 regulator could give up to 1.5A, and this circuit will be powered by a 9v, so I don't think that will be happening, but I still need to know that my device won't give out too much current. Do I need a current regulator or what? Like I said, I'm completely clueless.
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Re: 7805 Regulator-based USB?

by Entropy on Thu Feb 25, 2010 12:24 pm

Snipeye wrote:Alright, here's the deal - I'm a NOOB at electronics. I kinda know what things do, but for the most part, I'm clueless. It's not the components that confuse me, it's mostly how they work, why you need them, etc.

Anyway, that's immaterial. I've been making a couple of 7805 based USB's, and they work perfectly... I think. I'm interested in selling them, and I know that the 7805 will fry before it gives out over 5 volts, but I'm not so sure about current. How does current work when charging something USB? Does the device charging only draw as much power as it needs, or what? Because I want(need) to be sure that nobody can claim that my device damaged their USB thing.

Last I checked, the 7805 regulator could give up to 1.5A, and this circuit will be powered by a 9v, so I don't think that will be happening, but I still need to know that my device won't give out too much current. Do I need a current regulator or what? Like I said, I'm completely clueless.

What do you mean by a "7805 based USB"? It sounds from your description what you mean is "7805 based USB charger"?

You can never guarantee that a silicon device won't "fail through" or short, but in this case, if using a 9V battery, you're not likely to ever damage the 7805 with overcurrent unless the device being charged shorts things.

However, it IS sometimes possible for a device to get damaged by a "brownout" condition where the charger can't supply enough current and the voltage drops - this is highly likely in the case of a 9v battery+7805-based USB charger. It's also highly inefficient - 9v batteries don't carry much energy (low mAh rating) and the 7805 will waste nearly half of the energy in the battery when charging.
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Re: 7805 Regulator-based USB?

by Snipeye on Thu Feb 25, 2010 6:06 pm

Well, the 7805 regulator has proven perfect so far, but what I'm worried about is the current.

is it possible that there will be too much current for an iPod, or a cell phone, and said iPod/cell phone could be damaged? If this is the case, is it possible to prevent that?
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Re: 7805 Regulator-based USB?

by zener on Thu Feb 25, 2010 11:06 pm

It is hard to give much advice without seeing the schematic. Here are some statements and a question:

The 7805 puts out (or tries to put out) 5 volts.

How many volts is the battery you are charging supposed to get charged with?

The 7805 has power and temperature limits as well as a max current. You will need to comply with those also. The power and temperature limits often come into play before the current limitation does.

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Re: 7805 Regulator-based USB?

by Entropy on Fri Feb 26, 2010 11:14 am

Snipeye wrote:Well, the 7805 regulator has proven perfect so far, but what I'm worried about is the current.

is it possible that there will be too much current for an iPod, or a cell phone, and said iPod/cell phone could be damaged? If this is the case, is it possible to prevent that?

The current capability of a charger/regulator/power supply is its maximum capability - as long as it provides voltage, the device will determine how much current is drawn.

HOWEVER: Linear regulators like the 7805 are very inefficient. Pull a lot of current through one and it will heat up a lot. This COULD cause a possibility of the regulator "failing through" - basically behaving as a short instead of a regulator. If this happened it would expose a device to full battery voltage.

But a 9 volt battery might not be able to source enough current to many devices to fry a 7805. 9vs have pretty high internal resistance.

Also, some devices can be damaged by brownout conditions - where the charger is not able to supply enough current at the design voltage, resulting in the voltage of the charger dropping. Brownout conditions are highly likely when trying to charge a lot of moden USB-chargeable devices from a 7805 + 9v battery combo - a 9v battery just can't supply enough current. I'm not sure if you'd even be able to get a noticeable charge into an iPod or cell phone - as I said before, 9v batteries don't have very high capacity, and if you try to pull 100 mA+ out of it, the battery is going to dissipate a lot of power within its internal resistance. The regulator will waste the rest.

If you're trying to sell this to possible customers, I would be more worried about people being unhappy at the charger just plain not charging devices... You might want to consider assembling kits of a well known design instead. (There are a few people in the MintyBoost section who have gone into a small business of performing assembly service for Adafruit MintyBoost kits I believe.)
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Re: 7805 Regulator-based USB?

by reox on Thu Mar 25, 2010 5:29 am

i had successfully build a power adapter for my ipod touch 1G 32GB with an 7805... Its running with 7,5V from an old Power supply from something ;) It says it could provide 330mA, so its enouth i think..
I had to use a heat sink beacuase its getting warm ;) When you try it without you will see smoke....

I just had found a LM2576T, may this one is better? originaly i wanted to build a 6V 2A power supply but i dont need it anymore so why dont try it with this one? the LM should have more current then the 7805.

p.s. dont try to put 12V on the 7805, but loading from 6 AA worked for my iPod classic 80GB, even if you play music or watch videos :)
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