I am building a camera system for automotive use, and need to get 7.5vdc out of a cigarette lighter. The absolute maximum current draw will be 2A (thats with a safe window added on). I cant find any ready made adapters for this voltage, and am looking to build one. I dont know very much about this type of thing, if anybody has any hints, that would be great.

Thanks

 

I think the most common approach with automotive system power supplies is to use a voltage regulator circuit. Voltage regulators will smooth out the input voltage and drop the value down to the desired voltage. Since automotive systems feed off of the alternator, there is the potential for some pretty nasty voltage spikes (Some have seen up to 60 volt spikes!!!). These voltage spikes will cause problems for circuits which are designed to run from a specific voltage range, such as your camera equipment. Thus, the use of a voltage regulator will smooth these voltage spikes into a very uniform voltage, say 7.5 volts.

There are several different types of voltage regulators. Some will regulate only positive voltage, others will only regulate negative voltage. Some are designed to output a fixed voltage, some will output an adjustable voltage. Some will only handle 500 mA, some will handle up to 5 amps. To see National Semiconductors product line for voltage regulators, check here: You'll be able to see the capabilities of each regulator.

http://www.national.com/catalog/AnalogRegulators.html

For your ouput voltage of 7.5 volts, you'll probably have to go with an adjustable regulator that is capable of delivering 2 amps (SEE LM350 part number).

The voltage regulator datasheets (obtainable from the link above) will provide operational circuits for the given regulator, as well as how to calculate the other component value to output a certain voltage.

Be advised that voltage regulators produce heat, for the amount of voltage that they are reducing from the supply. The heat is also proportional to the amount of current that you demand from the regulator. Thus, heat sinking the regulator is almost always necessary, especially if you are drawing 2 amps, and regulating the voltage from 12volts to 7.5 volts. Consult the datasheet for the heat characteristics.

So, I suppose a little studying of the datasheets, and a little design work is in store for your application. I hope that this helps.

 

Yeah, you can't use a transformer on a DC circuit.

 

So that LM350 looks like it will do the job. I cant quite figure out, however, how to calculate the values of the resistors necessary to get the proper voltage. Can anyone help me?

Thanks

 

You won't need a resistor , the regulator is a 3 pin device.. in/ground/out.
Make sure you place capacitors on the in/out pins of the regulator to ground..The value can be 220uf on the input and .01uf on the out..The caps should be as close to the regulator as possible, they are for filtering and stabilization..At 2 amp draw you should definitly use a heat sink and as Johnson said proper ventilation..

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gerty

 

But its adjustable, is it not? They dont have a fixed one for 7.5v, so dont you use resistors to set the voltage?

 

You're right, my mistake..Best I can remember you'll need a 5k pot between the ground pin to ground and a 220 ohm resistor from the ground pin to the output.. you can adjust the pot to the desired voltage then remove it from circuit and meter the value and replace with a fixed resistor..I guess I should have paid more attention to the part # ops:

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gerty

 

This is a basic circuit.

Attached Images

350.GIF (1.6 KB, 358 views)

 

Might I suggest at least 1/2 watt 240ohm resistor.

 

Why 1/2W? On the 240ohm resistor is always 1.2V, this is only 5mA.

 

The circuit shown here is that of a DC-to-DC converter module as used in a modular power supply for a stepper-driven CNC router. This converter uses the +12VDC and 0VDC outputs from a typical PC switch-mode power supply as its input, and as shown outputs +7.75VDC at about (if I remember correctly) 6 amps. It is based upon the LM723 regulator IC, and uses the additional transistors for boosting output current.

As implemented, the 2N6394 carries a small TO-220-style heatsink, and the 2N5302's and the 2N3055 are all mounted to (but electrically isolated from) a single large heatsink -- in this case, the housing of the PSU assembly. If 2N5302's are hard to find, 2N5886 can be used instead.

It's not that far from what you want, and may even be suitable as is... A complete parts list and ExpressPCB drawing for the PCB is available if you want it.