logic circuits run from 12 volt car batteries

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zotdoc
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logic circuits run from 12 volt car batteries

Post by zotdoc » Tue Apr 12, 2005 11:28 am

I want to use 12 volt car batteries to heat nichrome wire to shoot fireworks. I 've got that part all worked out. And will be using 12 volt relays to control each ignition. My question for the forum is about how to run a 5 volt logic circuit off the 12 volt battery:
1) anybody have a circuit schematic to reduce the 12 volts to 5 volts. Also, the nichrome ignitors will draw 3 amps for about .5 seconds for each ignition. The logic circuits will be isolated from this by the relays used to control the 12 volt current, but will there be a problem with the logic circuit power supply if it is hooked to the same automotive battery during the relatively high current draw for each firing. I could make a seperate power supply for the logic circuit but since I'll have a lot of battery power laying around already it might be easier to use it if there won't be any problems. Thanks in advance for all your help.

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sofaspud
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Re: logic circuits run from 12 volt car batteries

Post by sofaspud » Tue Apr 12, 2005 12:10 pm

You could use a separate battery to power the logic circuits.
3-amps for a half-second from a car battery is not considered a relatively high current draw. The battery was designed to do it.

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Chris Smith
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Re: logic circuits run from 12 volt car batteries

Post by Chris Smith » Tue Apr 12, 2005 12:25 pm

I would use a 9 volt battery. Since the logic circuits pull no real power, the 9 volt battery with a five volt regulator will completely isolate the two and last for a long time. For safety reasons, such as a false triggering, I would also separate the two.

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Re: logic circuits run from 12 volt car batteries

Post by Enzo » Tue Apr 12, 2005 5:09 pm

If the circuit has to be 5v, then I must assume you are using TTL logic. Parts with numbers like 7404 or maybe 74LS04. If you simply replace the parts with CMOS versions with numbers ike 74C04, then the exact same circuit can be run on the 12v. And in fact CMOS is not so voltage sensitive that it wil care if the 12v slips to 10. Or starts at 13.8 for that matter.<p>Or design the same circuit with 4000 series CMOS chips. They will not be pin for pin the same as TTL, but every TTL chip has a 4000 series equivalent.<p>You can also just get a 7805 or equivalent three terminal 5v regulator and use it to step the 12v down to 5. It would need a small heat sink.<p>But really the separate supply ideas are good ones too. A CMOS circuit and a 9v should indeed last a while.

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Re: logic circuits run from 12 volt car batteries

Post by ampedtech » Tue Apr 12, 2005 5:17 pm

go to the hobby store and get "rocket fuses". They are like a match head with two wires coming out. They use very little current and only for a moment, cause when the "match head" stuff goes the circuit is broken. To attach, fray the end of the fireworks fuse so a little of the power is showing. I thin you can figure the rest.<p>Safety b 4 fun

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Re: logic circuits run from 12 volt car batteries

Post by rshayes » Tue Apr 12, 2005 7:03 pm

During cranking, 12 volt car batteries drop less than 6 volts with a load of hundreds of amps. If the drop was 6 volts at 100 amps load, the internal resistance of the battery would be .06 ohm. The three amp load from firing a fuse would drop the battery voltage by less than .18 volts. That is probably insignificant.<p>Both CMOS and TTL logic can be operated on 5 volts. Even the old CD series can be used at that voltage. This can be obtained by using a three terminal regulator such as the 7805.<p>If the battery is not being operated in a car, its output will be quite clean, but not particularly stable. The regulator will take care of that. The main source of noise will be the relay coils, and these should have diodes on the coils to reduce transients.<p>The regulator will need at least 8 volts to function properly, unless it is one of the low dropout designs. If the battery stays above 11 volts, a series resistor dropping less than 3 volts can be placed on the input side of the regulator. A large capacitor to ground on the regulator input will provide a fair amount of filtering, and the supply rejection of the three terminal regulator will contribute more isolation.<p>I would use 12 volt relays with transistor drivers driven by CMOS circuits. If noise is really a problem, the relays can be isolated from the logic using optical isolators, but this is probably more elaborate than really needed.<p>Some attention should be paid to the ground paths. The circuits with the fuses and relay contacts can be separately connected to the battery terminals to minimize common ground paths.The ground for the relay driving circuits shold be separated as much as possible from the ground of the logic circuits. A common connection will be necessary, but this will only have to carry the base currents for the relay drivers.The logic circuits, relay drivers, and relays should probably be built as a unit, with the relay contacts coming out to connectors or contact blocks. The 12 volt line to the relays can be fused and should also have an arming switch to allow removing power to the relays. The line from the battery to the relay contacts and fuses should also have an arming switch and fuse. This give two ways to prevent firing.

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Re: logic circuits run from 12 volt car batteries

Post by zotdoc » Wed Apr 13, 2005 9:31 am

Thanks to everyone. This is by far the best forum for general electronics I've come across. A couple more questions though;
1. Would any signal transistor do to trigger the 12 volt relay coils from the logic circuits, or do I need to buy power transistors, I have a lot of 3904's laying around.<p>2. Is there anything I need to do to limit current flow from the battery to the logic circuits other than a fuse?
Thanks again.

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sofaspud
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Re: logic circuits run from 12 volt car batteries

Post by sofaspud » Wed Apr 13, 2005 11:16 am

1. How much current will flow through the energized relay coil? If I'm visualizing the circuit correctly, you'll need a transistor that can handle the same amount of current. A 3904 is good to about 200mA as I recall.<p>2. My guess is the normal resistance of the logic circuit will set the amount current drawn. No separate limiter needed.

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Re: logic circuits run from 12 volt car batteries

Post by zotdoc » Wed Apr 13, 2005 11:40 am

I am using 12 volt 35 amp automotive relays from seimans, but I don't now how much current the coils draw. 8 AA bateries in series were used to check each relay and they closed the contacts OK.

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sofaspud
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Re: logic circuits run from 12 volt car batteries

Post by sofaspud » Wed Apr 13, 2005 11:51 am

You have a DVM? Measure the resistance of the relay coil. Then divide 12 by that number. That answer should give you the amperage.<p>Also, I took a look at the 3904 datasheet and that 200mA figure is a continuous rating. It can be exceeded dependent upon the amount of time the transistor is switched on.<p>[ April 13, 2005: Message edited by: sofaspud ]</p>

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Chris Smith
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Re: logic circuits run from 12 volt car batteries

Post by Chris Smith » Wed Apr 13, 2005 12:16 pm

Car relays dont draw much current. Bosch type ice cube relays usually draw less than 100 mills.

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Re: logic circuits run from 12 volt car batteries

Post by josmith » Wed Apr 13, 2005 3:42 pm

Interfacing logic with transistors can be a challange in itself. A ttl low is often above the .7 volt threshold that will keep the transistor on. If you use the (good) idea of cmos, a low is around 1/3 of supply. Make sure you have somthing that works before you go ahead and build them all.

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Re: logic circuits run from 12 volt car batteries

Post by rshayes » Wed Apr 13, 2005 10:46 pm

I would use something a little heavier than 2N3904s for relay drivers. The data sheet ratings are often based on high stress levels and favorable cooling conditions. That high current rating may be based on a junction temperature of over 150 degrees centigrade (302 degrees farenheit) with the case cooled to 25 degrees centigrade (77 degrees farenheit). The interior of a metal box out in the sun on a warm day is probably much more than 77 degrees farenheit.<p>Unfortunately there are not very many alternatives between a TO-92 and a TO-220. The TO-220 part will still probably cost less than the relay that it is driving.

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Re: logic circuits run from 12 volt car batteries

Post by zotdoc » Wed Apr 20, 2005 5:29 am

I have a follow up question for this project. I hope it doesn't come off as too ignorant. I want to interface a 5 volt logic system with twelve volt (coil) automotive relays. Is it correct to hook the base of the transistor to the five volt logic output and the collector and emitter to a seperate twelve volt circuit? If this is the right way, do I need to hook the "grounds" for both systems (5 & 12 volt) together?

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Re: logic circuits run from 12 volt car batteries

Post by dyarker » Wed Apr 20, 2005 7:40 am

answer: Yes and yes. There are some layout considerations so igniter current isn't flowing through the logic common. But 5V and 12V commons do need to be connected at one point for transistor base current to return through the emitter to the logic section. No return path, no transistor turn on, no ignition.<p>If for example, you make a circuit board with logic on the left side, and power transistors on the right, with a common trace running across the bottom. Then the common wires from the two supplies could be connected to the trace near the center, or the 5V common on the left and 12V common on the right. It is still one common, but the heavy igniter current is not flowing past logic ICs. If you see what I mean.<p>Cheers,
Dale Y

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