It is generally not a good idea for 2 reasons:
1) There's a tremendous amount of electrical noise in a car's electrical system and the voltage can drop significantly when starting. The noise can create significant spikes to well over 12V (actually the normal voltage is considered more like 13.8V).
2) The Stamp's regulator has to dissipate 13.8V-5V = 8.8V as heat and it doesn't have any significant heatsink. The regulator will shut down if it gets too warm. Even 2 LEDs can create enough load to do this with an essentially 9V drop needed (20ma x 9V = 180mw ... way more than the regulator can handle without a heatsink).
It's better to have a noise filter (inductor/capacitor) followed by an intermediate voltage regulator (like 8V) that has a heatsink.
I wouldnt Robban, Stamps are 12 volt tolerant but a cars electrical system is a scary place. Even though the recommended voltage requirements on most stamps is 5-15 volts a charging system routinely runs as high as 14.2-14.5 volts which puts you at the ragged edge of the power supply rating and over it for some stamps. Then you have the question of what kind of aftermarket parts etc are installed in a car, even something as simple as a relay or solenoid can generate a very short·term kickback of a much higher voltage. (older nitrous oxide solenoids do not come with clamping diodes and I've seen those generate 70 volt spikes·of about 2ms)
So that being said, invest in a voltage regulator at the very least, 7805 is a inexpensive 5 volt part available at radio shack, cant remember if they have 7810's there or not to go for the middle of the range. There is also a choke/inductor (part no 273-102) that they sell that has always been very effective for me as a noise filter in automotive applications. Then a cap in there somewhere would help too.
At the minimum a regulator, prefferably a regulator, cap, and choke for me.
ChrisP,
I have posted a similar question in a different thread, but you posed a point that I have not encountered. You mentioned a choke/inductor (part no 273-102) for your automotive application.
May I ask, how does the part function? Is it like a capacitor? Or, not?
I have found very little information on the Internet on this part; it seems that it is used for RF interference. Is that correct?
Does it have any affect on electrical interference?
Hey all, I want to make a 12v automotive project too. Could I simply get an in-car cell phone charger that outputs ~12v, would that cut it? I was at an electronics surplus shop and they have bins of those things for $1 a pop!
Franklin, ·· The honest answer is that I'm really not that knowledgeable on inductors and chokes. The only thing I can tell you for sure is that the inductor/choke I listed above seems to be very well matched to automotive applications for removing alternator and typical automotive interfence/electrical electrical noise. Low current, only good for a couple of amps but have worked fantasticaly for me in both automotive and motorcycle applications running SX processors.
My very limited and possibly very incorrect understanding is that the wire coil around the core generates a magnetic field causing it to act as a block to noise. Type,l rating, construction all seem to effect how well they work at specific frequency's. I'll try to remember to ask the boss for a better answer at work tomorrow.
Jef,
I honestly would think it would depend on the charger, voltage/current rating, how well it was filtered etc etc.
Do you happen to have any more info on the choke and is anything else additional required for it? Just wire it in series? Input or output side or does it not matter?
I'll be wiring up a quick 7812 power supply for a couple bullet cameras and may add this in. I tried to google wiring an RF choke but couldn't find anything solid.
The choke goes in the positive power lead close to the regulator input (within a few inches). There should be a power on/off switch and a fuse between the choke and whereever you connect the whole thing to the car's electrical system. You should have a capacitor across the input of the regulator, actually two ... a 0.1uF ceramic capacitor and a 470uF to 1000uF electrolytic capacitor rated at maybe 25V. Sometimes you can buy the choke and electrolytic capacitor as a complete noise filter package (www.tessco.com/products/displayProductInfo.do?sku=84130&eventPage=1).
Robban
Posts: 124
Comments
1) There's a tremendous amount of electrical noise in a car's electrical system and the voltage can drop significantly when starting. The noise can create significant spikes to well over 12V (actually the normal voltage is considered more like 13.8V).
2) The Stamp's regulator has to dissipate 13.8V-5V = 8.8V as heat and it doesn't have any significant heatsink. The regulator will shut down if it gets too warm. Even 2 LEDs can create enough load to do this with an essentially 9V drop needed (20ma x 9V = 180mw ... way more than the regulator can handle without a heatsink).
It's better to have a noise filter (inductor/capacitor) followed by an intermediate voltage regulator (like 8V) that has a heatsink.
So that being said, invest in a voltage regulator at the very least, 7805 is a inexpensive 5 volt part available at radio shack, cant remember if they have 7810's there or not to go for the middle of the range. There is also a choke/inductor (part no 273-102) that they sell that has always been very effective for me as a noise filter in automotive applications. Then a cap in there somewhere would help too.
At the minimum a regulator, prefferably a regulator, cap, and choke for me.
Hope this was some help
Chris
Post Edited (ChrisP) : 3/17/2007 1:37:09 PM GMT
I have posted a similar question in a different thread, but you posed a point that I have not encountered. You mentioned a choke/inductor (part no 273-102) for your automotive application.
May I ask, how does the part function? Is it like a capacitor? Or, not?
I have found very little information on the Internet on this part; it seems that it is used for RF interference. Is that correct?
Does it have any affect on electrical interference?
Thanks.
·· The honest answer is that I'm really not that knowledgeable on inductors and chokes. The only thing I can tell you for sure is that the inductor/choke I listed above seems to be very well matched to automotive applications for removing alternator and typical automotive interfence/electrical electrical noise. Low current, only good for a couple of amps but have worked fantasticaly for me in both automotive and motorcycle applications running SX processors.
My very limited and possibly very incorrect understanding is that the wire coil around the core generates a magnetic field causing it to act as a block to noise. Type,l rating, construction all seem to effect how well they work at specific frequency's. I'll try to remember to ask the boss for a better answer at work tomorrow.
Jef,
I honestly would think it would depend on the charger, voltage/current rating, how well it was filtered etc etc.
Chris
I'll be wiring up a quick 7812 power supply for a couple bullet cameras and may add this in. I tried to google wiring an RF choke but couldn't find anything solid.