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Old 09-28-2007, 02:23 PM   #16
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DC voltage


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Originally Posted by J. V. View Post

Lets say you only had a 24 volt dc supply, but the lamp was 12 volts. You just connect two 12 volt lamps in series and off you go.

Sorry, but this is not at all true. Bulbs in series are not the same as batteries in series. The problem with series connections in low voltage DC applications is that the resistance of each bulb (or other load) slightly decreases the current. Higher resistance will cut the current more. So if you have three r ohm resistance bulbs in a row, they can have enough resistance that the 12V supply can't overcome the total circuit resistance (r + r + r), and none of those bulbs will light - no current. However, if you run them in parallel, the resistance of the circuit is 1/r+r+r. So the 12V potential is plenty to light all three.


Last edited by NateHanson; 09-28-2007 at 03:49 PM.
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Old 09-28-2007, 03:39 PM   #17
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DC voltage


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Originally Posted by J. V. View Post
Slakker,
Series AC & DC circuits are needed in many applications. Example would be a golf cart. Without the use of series connections where would you get a battery large enough to supply the cart.
Understood and that's why there are 2 types of circuits.. but didn't he say this was to light up a couple of small lamps? Not golf carts? And technically, both will work (assuming his equipment is sized appropriately)... I was pointing out one pratical considerations between series vs. parallel circuits when it comes to a couple of light bulbs...

Last edited by slakker; 09-28-2007 at 05:18 PM.
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Old 09-28-2007, 04:13 PM   #18
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DC voltage


Sorry NateHanson but you are totally wrong. It's obvious that you don't understand both Kirschoffs & Ohms Law. Read the attached doc.

The preferable way to do what the OP wants (assuming he is using one 12 volt battery) is to wire the lights in parallel (assuming 12 volt lights). However, if he doesn't have a 12 volt battery but instead has two 6 volt batteries (assuming the lights are 12 volts), the batteries can be wired in series & the lights wired in parallel.

Either method will not cause a problem if the wire & battery size is adequate.

So, the original question remains, what voltage (& wattage) lights are you using & what voltage/size battery (or batteries) have you got?
From what the OP says, he has 2 x 6 volt items. This means that they can be wired in series if he has a 12v source or wired in parallel if he has a 6v source. Assuming the DC source can supply enough current & assuming that the correct wire sizes are used according to the current requirements, there is no problem whatsoever using either of these methods.
Attached Files
File Type: zip Kirschoffs Law.zip (4.2 KB, 3 views)

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Last edited by elkangorito; 09-28-2007 at 04:20 PM.
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