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jbuturff 
Member - Posts: 4
Member spacespace
Joined: February 19, 2007
Location: United States
Posted: February 19, 2007 at 9:10 PM / IP Logged  

I was thinking about installing a usb port or two in my car for charging various devices (MP3 player, phone, bluetooth headset, etc).  I found this nice article ( http://www.utdallas.edu/~jbt015000/cgi-bin/blosxom.cgi/2004/03/07) and was going to follow this. 

I figure a second opinion would probably be good.  Any experts here see any reasons I should not go this route?  I'm fairly inexperienced at dealing with mobile electronics, so I figured this was a good start....

Thanks,

Jeff

Hymer 
Silver - Posts: 695
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Joined: November 20, 2004
Location: United States
Posted: February 20, 2007 at 12:15 PM / IP Logged  
Looks to me like i will work just fine... as long as everything regulates at 5v...
Tire Proz Stillwater Mn
High End Restyling and Comlete Repair
z1172 
Member - Posts: 1
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Joined: October 13, 2006
Location: United States
Posted: February 24, 2007 at 1:16 AM / IP Logged  

That's pretty cool....makes me want to try it....though probably not in a cup holder.

Hey, just curious but what happens to that transistor in the article if you spill coke on it?

Or you try to pull more than the 1A its rated for through the circuit?

I'm not an engineer but my guess is heat/melting/potentially fire, if things get crazy.

1998chevy1500 
Copper - Posts: 66
Copper spacespace
Joined: March 12, 2003
Location: United States
Posted: February 25, 2007 at 1:59 AM / IP Logged  
You shouldn't have any trouble. The LM7805 voltage regulator will usually take any input voltage up to around 30v I believe and regulate it down to a 5v output. The only problem you may run into is it heating up, because while they are usually rated for around 1Amp of current they can only handle around 500mA without a heatsink mounted on them to keep them cool. Depending on what you are hooking up you still shouldn't have any trouble as most USB stuff won't draw that much current. Also most 7805s have a thermal shutdown built in should the current draw become too high, but at that point the head sink will be hot enough to melt plastic and give you a pretty good burn.
My only recommendation to add to the circuit is to add a small capacitor between the input terminal and ground and another between the output and ground to help regulate out any voltage spikes that may occur. This defiantly isn't necessary but it doesn't hurt. I would say something like 0.33uF on the input side and 0.1uF on the output side. Really you could use 0.33uF on both if you get them in a package.
Also keep in mind that 7805 is a series of chips, and a general name at that, many different companies make a 7805 or something equivalent. What you are looking for in general is a 5v voltage regulator.
Good luck
Conrad
jbuturff 
Member - Posts: 4
Member spacespace
Joined: February 19, 2007
Location: United States
Posted: March 05, 2007 at 7:40 AM / IP Logged  

Er, um, yes, that LM7805 does get HOT, doesn't it?    OUCH!

I wired up the basic circuit, and before putting it in my car just hooked it up to a 9V battery to check the outputs with a multimeter.  Without anything even hooked to the USB port, the LM7805 pretty much fried my finger.  Doing some basic Googling shows that it's normal to generate some heat on this thing, but is that much heat normal?  I sure don't want to put that into my car if it's gonna melt my center console...

1998chevy1500 
Copper - Posts: 66
Copper spacespace
Joined: March 12, 2003
Location: United States
Posted: March 05, 2007 at 7:55 AM / IP Logged  
With no load it shouldn't get that hot esp if you are only using a 9v battery to test with, you should only need a heat-sink if you are drawing more than 500mA. Chances are you have it wired up backwards. Double check your wiring and the pins on the chip.
jbuturff 
Member - Posts: 4
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Joined: February 19, 2007
Location: United States
Posted: March 05, 2007 at 8:15 AM / IP Logged  
Another test this morning shows that it's only getting that hot when I put the multimeter on it.  Could that really be drawing enough load to heat it like that??
1998chevy1500 
Copper - Posts: 66
Copper spacespace
Joined: March 12, 2003
Location: United States
Posted: March 06, 2007 at 9:52 AM / IP Logged  
jbuturff wrote:
Another test this morning shows that it's only getting that hot when I put the multimeter on it. Could that really be drawing enough load to heat it like that??
Not unless you either have a bad DMM or are trying to measure current. Like I said you probably have it wired up backwards, what are you measuring for your Vin and Vout with respect to ground? What about under say a 100 ohm load?
I've used the 7805 many times before and when wired up correctly I have put upwards of 10-15 TTL chips on it without it even getting remotely warm. I've also used the TO-92 package version of it to power a uController and a few transistors with it never getting perceivably warm. Assuming that the one you got is the TO-220 package with the heat sink on it, you do know that the head sink is wired to ground. If you short the heat sink to either Vin or Vout you will have problems. Lastly, though highly unlikely you may have gotten a bad chip, though with the QC that most manufactures have I doubt thats the case.
jbuturff 
Member - Posts: 4
Member spacespace
Joined: February 19, 2007
Location: United States
Posted: March 06, 2007 at 10:07 AM / IP Logged  

It's definitely wired up correctly, the Vin and Vout are showing correctly.  Also, I don't have a heatsink, just a standalone 7805 soldered to 4 wires:  one for Vin, two grounds (one to supply, one to USB ground), and one Vout to the USB +5V lead.  

I will run some more tests this afternoon when I get a chance.  Thanks for all your help. 

1998chevy1500 
Copper - Posts: 66
Copper spacespace
Joined: March 12, 2003
Location: United States
Posted: March 07, 2007 at 8:12 AM / IP Logged  
Did you use any capacitors with the 7805 or is it just the 7805 by itself? If you used some electrolytic caps, keep in mind that they are polarized(a +and- side).
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