Power & Heat

Power & Heat

Often a beginner will only look at the current rating to see if a power component is good enough but that is only half the story.

Theory

Don’t shy away from this first bit, I know there are equations but they are of the most simple kind. Power is expressed in Watts and is calculated from the product of voltage and current:-

W = E * I

Where W is the power in Watts, I is the current flowing and E is the voltage across the device.

However, as we know ohms law (E = I*R) we can also express this relationship by including resistance and eliminating either E or I. For example substitute for I * R for E gives us:-

W = I * R * I or as we have I * I this is I squared so we have:-

W= I2 * R

If you substitute E/R for I you get:-

W = E * E/R again there is an E * E giving us E squared so we get:-

W = E2/R

Remember that in any calculation E is in volts, R is in ohms and I is in amps. This is especially important when you have the current in a much more useful unit like mA. Suppose you have 4mA flowing through a 470 ohm resistor then 4mA squared is not 16mA it is 16uA * 470 = 7.52 mW.

What is power.

Power is defined in Physics as the ability to do work. In this context this means either to physically move something or generate heat. It is the heat generation properties of power that mainly concerns us here. When current is flowing through something heat is being generated, and this heat has to do somewhere. It is spread throughout the material and we say it is being dissipated. The up shot is that the material gets hotter, or experiences a temperature rise. The heat causing the rise in temperature flows into the environment and an equilibrium is reached. The amount of heat that flows depends on the temperature of the environment or ambient temperature and the thermal resistance between the material and the environment. In fact is is very similar to electricity with temperature being equivalent to voltage, current being equivalent to heat and thermal resistance being equivalent to resistance.

Why should I care?

You care because you can calculate if something will get too hot and burn out in advance of it happening. The key to this is the thermal resistance, it’s units are in “Degrees Centigrade per Watt” (0C / watt). If one end of you thermal resistance is anchored at the ambient temperature the other end gets hotter, the temperature it get to is dependent on the power dissipated and the thermal resistance. In an electronic system there are two key thermal resistances, the resistance between the actual thing getting hot and the component’s case, and the thermal resistance between the case and the ambient. You can reduce the latter by applying a heat sink but no amount of heat sink is going to affect the former. In fact manufacturers of devices often use the concept of an infinite heat sink in getting their headline figures. That means a heat sink so big that the case temperature and the ambient temperature are the same thing.

Another reason to care is that the average life time of a component is dependent upon it’s temperature. As a rule of thumb:-

Every 10 0C reduction in temperature doubles the expected life of a component.

Power & Heat

Reducing power by design

Reducing the power in a circuit means reducing voltage or current. Hence the current move away from 5V devices to 3V3 or even 1V8 and lower. Modern micro controller systems often have a 3V3 interface with a 1V2 core, requiring two supply voltages but dissipating less power.

Switching

That is also why FETs are much preferred nowadays over transistors for switching large currents. Where as a transistor can have a saturation voltage of between 0.9V and 2V when it is fully on a FET can have a very low “On resistance”. Typically a FET’s On resistance is in the order of 0.1R (ohms) so for 1A flowing down it it will only dissipate 1 * 1 * 0.1 = 100mW. Where as a transistor with 0.9V saturation voltage switching the same current will dissipate 0.9 * 1 = 900mW so in this case there is nine times less power being dissipated or wasted than in a transistor. Some FETs can push this ON resistance down even further to values like 0.01R or lower.

One mistake beginners often make is to think if they are switching say a 50W load they are going to dissipate 50W in the switch, this is not the case. You only dissipate the power in the switch given by the current down it and either the series resistance or the saturation voltage across it. So with a good FET you can switch 50W of power and only dissipate a few milliwatts in the switch.

Regulation

Power supply regulation can be done using a series regulator and such three pin regulators are very common. Suppose you have a 5V regulator being supplied by a 12V supply. This means that the regulator has to “drop” 12 - 5 = 7V across it. If the circuit is taking 500mA that is a power of 3.5W, all that is wasted in heat and has to be removed from the regulator, a large heat sink being necessary. However, using switching regulators does not require such a voltage drop and their efficiency is around 90% so the heat dissipated in them is much smaller and is related only to the load current, so for the same example the actual circuit is taking 5V at 500mA so the power is 2.5W. A switch mode power supply is 90% efficient so only 10% of this power is dissipated giving 0.1 * 2.5 = 250mW of heat in the power regulator. Compare this with the 3.5W of a conventional series regulator.

House keeping

Pull up resistors are usually a higher value than pull down resistors so if you have the choice use them in stead. The savings are not so dramatic but when you have a lot it all adds up. Also instead of using a 1K pull up how about using a 10K, that is using only one tenth of the power.

LEDs are normally run at 20mA but sometimes this can be too bright, 10mA is often more than bright enough for a non multiplexing LED.