Griff Posted March 31, 2006 Share Posted March 31, 2006 Electricity isn't my thing. How can I get a 6~7.2V low power feed supply from the 12V accessory socket on the car? Cheers! It's not pink, it's medium red violet...! Link to comment Share on other sites More sharing options...
Myles Posted March 31, 2006 Share Posted March 31, 2006 Resistance in series. You have a big resistance in line with the socket (duff connection for example). Project Scope-Creep is live... Alcester Racing 7's Equipe - 🙆🏻™ Alcester-Racing-Sevens.com Link to comment Share on other sites More sharing options...
Griff Posted March 31, 2006 Author Share Posted March 31, 2006 I'd thought about that, but what size would I need and how would this cope with alternator voltage fluctuations (assuming that these are big enough to cause problems)? It's not pink, it's medium red violet...! Link to comment Share on other sites More sharing options...
Myles Posted March 31, 2006 Share Posted March 31, 2006 The extra resistance shares the voltage with the device in the socket (lighter or whatever) The sum of the voltages will add up to whatever the alternator is putting out - so if the alt. voltage varies, so will the voltage at the the socket. Basic electrical theory tells you that the voltage across the whole assembly is fixed by the alternator and that the current through the circuit is the same at each point (assuming a simple series circuit). Current (in Amps) x Resistance (Ohms) = volts across that widget in the circuit. Add the voltages up across each of the widgets in the circuit (the unwanted resistance - whatever the source), the item in the socket etc. and you will end up with the voltage put out by the alternator. Kirchoff or something IIRC. When I say 'the voltage across each widget' - I mean *directly* across the widget (not explaining this too well...) A---xBx----yCy------D The voltage across the whole circuit (A-D) = voltage across B (measured at the x's) + voltage across C (measured at the y's). Current through B and C *has* to be the same, so if B has a huge resistance, the current (and voltage -> V=IR) in C will be low. If C is your socket.... Project Scope-Creep is live... Alcester Racing 7's Equipe - 🙆🏻™ Alcester-Racing-Sevens.com Edited by - Myles on 31 Mar 2006 07:37:14 Link to comment Share on other sites More sharing options...
davef Posted March 31, 2006 Share Posted March 31, 2006 Griff, what are you powering that requires 6-7.2 V? What amperage? The answer to these will help decide the solution. For instance Maplin sell an adaptor with stepped variable output voltage (inc 6V) @1000mA (L17AR) Dave Link to comment Share on other sites More sharing options...
Sheds Moderator Posted March 31, 2006 Share Posted March 31, 2006 I'd build a voltage regulator. I used to have one such circuit with discrete components but these days you can get a chip that allows you to vary the OP voltage by changing resistor values in a simple circuit. If all this is beyond your ken then either the manufacturer for an adaptor or somewhere like Maplin. I imagine that the 6-7.2V thing is a straightforward consumer item, they prob sell an adaptor for a car. Link to comment Share on other sites More sharing options...
Griff Posted March 31, 2006 Author Share Posted March 31, 2006 Dave - it's a condenser microphone that normally takes power off the camcorder except mine doesn't have a power out socket to drive it (the camcorder & microphone are both Sony by the way). It's not pink, it's medium red violet...! Link to comment Share on other sites More sharing options...
davef Posted March 31, 2006 Share Posted March 31, 2006 OK, so I think the Maplin adaptor will be more than sufficient. Dave Link to comment Share on other sites More sharing options...
Chris W Posted March 31, 2006 Share Posted March 31, 2006 I'd go with batteredoldsupersport and knock up a simple voltage regulator using a dedicated chip. It's so simple to do and all you need is the regulator chip and a couple of capacitors and resistors. The regulator chip for your application is the LM317 (100mA version @ 33p) which can supply any output voltage bewteen 1.2v and 37v. see here. This could be put together in a very tiny box as small as a matchbox. (Maplin also have boxes). You could of course use a ready-built regulator and the only real penalty (apart from being a few quid dearer) would be size as it would be somewhat larger than a home-made version. That may not be a problem if it's stuffed under the dash somewhere. Using a resistor is not recommended (sorry Myles) because all devices draw a variable current as they operate and so the supply voltage will vary in a similar way as more or less voltage is lost across the resistor. This will cause a supply variation which will show itself as noise on the system as well as causing potentially erratic operation. Chris 2003 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
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