In-Vehicle Voltage Supply From Cigar Lighter / Accessory Sockets

This article documents an investigation into voltage levels provided by an automotive cigar lighter socket. These sockets are nowadays used to power a wide variety of devices. It is, in fact, rare for them to be used for their original purpose and many are provided purely as accessory sockets. The voltages provided by these sockets can, of course, affect the functionality of any connected device. In Prosig’s case we were studying the effects on one of our P8000 data acquisition systems. These systems use an external 12V supply and, when used in a vehicle, these systems are often powered from the cigar lighter/accessory socket.

Data acquisition systems, such as the P8000, are often required to operate during hot or cold engine starts. During cranking the vehicle’s on-board electronics will divert power to the starter motor as required. This is often achieved by means of switching off unnecessary parts of the vehicle’s electrical system. This is why one often experiences the audio system being turned off or internal lighting going out during engine cranking. The cigar lighter or accessory socket is also sometimes affected in this way.

The Prosig P8000 systems have a variety of power options. The most common of which is to be powered by a 12 volt external source. Normally, this would be the in-vehicle power supply. However, the power from the in-car cigar lighter or even directly from the battery terminals is anything but steady. So, the Prosig engineer used the P8000 itself to measure the voltage from the in-car cigar lighter/accessory socket.

The measured voltage supply is shown in Figure 1. Initially, the engine is off. After approximately 7 seconds the vehicle is started, as can be seen the voltage drops to 8 volts before returning to a normal charging level, some 30 seconds later (13.8 volts) in the case of this vehicle.

Figure 1: Raw supply from socket during engine start

Drops in voltage like this would adversely affect any devices running from the vehicles power supply. These affects may or may not manifest themselves immediately and may not be obvious in nature or cause. That is, the system may not actually stop working, but the integrity of the captured data may be compromised.

When using Prosig equipment, it is recommended that a Prosig Power Pack is used. The Prosig Power Pack can be fitted internally or externally and consists of a small lightweight power buffer. This will smooth out and, where possible, correct for loss of input voltage.

The voltage supply during engine starting with a Prosig Power Pack fitted can be seen in Figure 2. Here the engine start occurs at around 21.8 seconds. A much less significant voltage drop can be seen.

Figure 2: Supply with Power Pack fitted during engine start

After adjusting the time base of the signals we can overlay them. The difference between the raw supply and that provided when using the Prosig Power Pack are shown in Figure 3. Figure 4 shows a more detailed view of the initial voltage drop. The blue trace is the raw output from the socket as in Figure 1 above and the green trace is the supply with a Prosig Power Pack fitted.

Figure 3: Comparison of supply with and without Power Pack
Figure 4: Detailed view of voltage drop with and without the Power Pack

It can be seen that the Prosig Power Pack greatly reduces the magnitude of the initial voltage drop. Also note the faster rise back to full voltage after engine firing . This ensures smooth operation of any connected devices.

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Prosig are experts in the measurement and monitoring of noise and vibration. They provide data capture and analysis systems for a wide range of applications with particular focus on noise & vibration, NVH and acoustics for the automotive, aerospace and power generation Industries. The company designs and develops its own products and its engineers have decades of experience in solving real-world noise and vibration problems for major organizations throughout the world.
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