Critical to the design of my 99 Ford F150 Project is a good voltage regulator. For example, Using a linear regulator chip such as 8705 is a typical solution. However, in environments such as automotive it is critical to consider design requirements. Especially, in some applications such as ECU (engine control unit) with its time critical requirements as timing of injectors and ignition. So, voltage regulators requires careful considerations is some applications.
Linear Voltage Regulators
As shown above, 78XX series of regulators are Linear Regulators. That is, they have control circuits that regulate the output down from a higher voltage to a more constant lower voltage. So, this regulation of voltage helps keep attached devices happy. How? Well, by keeping the output voltage more constant it helps the attached devices stay within specifications of supply voltage
Higher to Lower Voltages
Simple, that is what they do! So, design for the circuit is simple. However, it is critical to consider the power consumption. Ok. just think of this type of regulator as a big variable resistor. Simply put, it takes high voltages down to a consistent lower voltage with some resistance to the change of that voltage in regards to current requirements. Simple right, the linear regulators whole job is to resist the change in output voltage regardless of current! Well, at least that is what it attempts to do.
Remember, it can only do this within specifications it is designed to handle. So, look up the spec sheets and do not expect it to do more than what it is designed to do! Ok, so there are improvements that can be done to allow for this regulator to handle even higher current. Fundamentally, this is called a “shunt” regulator as it acts like a valve. However, that comes at a cost the more it has to resist the input voltage the more heat it will produce. Remember Ohm’s Law, V = I * R Also, the power components of that such as P = V * I. See here for more. Fundamentally, more voltage difference the regulator has between input to output voltage the more power it has to dissipate as heat! So, that is why I tend to use switching voltage regulators.
- Simplicity – Easy to build into circuits.
- Common place – I see these used everywhere
- Ripple rejection – They help filter out the ripple that can exist on AC filtered DC circuits.
- Adjustable versions are easy to implement compared to adjustable switching regulators.
- Efficiency – vary wide range depending on how much regulation difference from input to output.
- Heat generation – they get hot especially with wide range between input to output.
- No boost only shunt aka buck.
Switching Voltage Regulators
So, what is a switching regulator? Well, simply put it is a voltage controlled oscillator. The whole design around it is to use a inductive and or capacitive device to switch up or down the voltage. Simularily that is what a transformer does. To clarify, I am talking about DC to DC switching regulators. However, even ac ones work about the same. Typically, there is an IC controller chip and some sort of power MOSFET to do the switching. Currently, I find myself using preconfigure ones that work like the above linear voltage regulators.
Similar to linear regulators they simply drop the voltage from higher to lower voltages. Input ranges can be much higher however due to the nature of the regulator. Critically, so can efficiency as they can be anywhere from 75 to upper 90s in efficiency. Here are some example regulators I have used:
- Handles 5 to 30 volts input
- 1 amp output
- Approaches 93 percent efficient. Especially, on input voltage of < 10 volts.
- Input 6.8 to 28 volts.
- 500 mA output
- Low ripple and high efficiency.
Same as buck regulators but designed to increase the output voltages. Wide range of designs here are some examples:
- K7812-500R3-LB – Can be + or – 12v output voltages
- LT8551 – Really adds current out of phase with initial chip to increase output capacity current.
- MIC4575 buck boost chip
Switching Regulator Complexity
The sky’s the limit, that is the best way I can say it for may circuits read the datasheets and see what is possible. Overall, just about anything is possible. However the complexity can be just off the wall. Unless you need complexity then it is better to sick to some other approaches in design. Overall, the design should come from what is needed and not just what can be done. So, as with any design. Think about the end goals and only design as much as is needed. I know, I have built some crazy complex regulator circuits in the past.
Op Amp Regulator
Ok, so maybe this should be obvious to everyone. That is, Op Amps are regulators. Yes, the heart of almost any regulator is actually an op amp with a reference voltage as the key to it success. Sure, maybe I should have placed this first in the list of regulators. However, I wanted to get the most common ones out of the way first. So, all the stuff above really uses these crazy devices to achieve the required goals.
Different Op Amps
One really critical thing here is how to choose op amps.
- What voltages are input and output?
- If close to “rail” is needed remember to pick “rail to rail” op amps.
- LM324 has over 1 volt offset to rail voltage and so it must be kept will above this or it will not be able to regulate and clip.
- Current requirements should not exceed 20 to 40 mA.
- There are ways to buffer this but design around this is critical to success.
- Positive and Ground or Positive and Negative.
- It can matter when driving buffer devices and with overall circuit design.
- Overly aggressive slew rates can cause circuits to oscillate.
- Slightly less expensive op amps can make better regulators due to feedback and oscillation opportunity of the differential inputs of op amp. Also, capacitors can help and hurt too. Good design makes requires thinking about rejection rates.
Essentially, at the heart of every regulator is an operational amplifier. Look below, at the schematic of a simple yet effective operational amp.
Basic Op Amp Regulator Design
So, this is a regulator. Furthermore, it is a linear regulator much like the 78XX series of regulators. Yet, all it is an op amp. Wow, when I saw this circuit for the first time I did not think much about it. Ok, so why not just buy a 78XX series and be done with it. Yet I has a place see the virtual ground below:
First, This is not my schematic it is from here. Not that there is really anything wrong with it. Yes, it is a little hard to read. Black and Blue make terrible partners in such a design, but whatever. The key, it is the same concept as my design of the voltage follower above. Critical notes, the supply to this HAS to be regulated at 5v Especially, the part that creates the voltage divider. So, does that make sense? If the voltage supplied to the divider fluctuates so does the + pin voltage; and therefore, so does the output!
As I mentioned before, the virtual ground circuit came from a Wideband O2 Controller. Ok, that stuff will be coming in the future too. Overall, op amps the framework for so much of the circuits it all becomes a blur after a while.
Virtual Ground Regulators
Wow, what a concept but designing one for the first time was a little complex; Or at least, can become complex. Let’s start with the most basic regulator ever and then add a little different reference voltage and make it have some gain to achieve basically the same thing. So, like all designs. Complexity should only be done if there is a reason. Anyway, below is the circuit. Next, I will explain it.
Over The Top Virtual Ground
Yup; I know, keep it simple stupid. However, let me explain. Everything has a purpose. For example, TIP125 and TIP45 listed above buffer the output. That is, they give the op amp the ability to provide more current. Yes, there are two for a reason. PNP Transistor TIP 125, it can sink the current to ground. NPN Transistor TIP41 sources the current to the load. So, the combination of both allows this circuit to both source and sink current. Now yes, the original circuit did that as well but only up to 40 mA. So, the above circuit can source and sink current up to 2 amps.
Also, the voltage reference is not a voltage divider but instead a resistor R1 and a diode D1. Why, because it has better rejection to changes in supply voltage. Simple to use a diode with a .6 forward voltage drop. R2 and R3 make up the gain part of the circuit to get the output to grow from .6 volt ref to whatever is needed. As designed, it is 2.5 volts based on the formula Vo = .6*(1+R2/R3). After some research, I found this site. It simplified the calculations as follows.
- Make R3 1000*ref == 1000*.6 or 600 ohm.
- R2 = 1000(Vout – .6) == R2 = 1000(2.5-.6) => 1.9 K
See, it all makes sense now. Oh, the real circuit uses TIP125 and TIP120 transistors. The reason, because my schematic did not look as nice with the TIP120 resistor on it. It looked like this:
See, it looks horrible. So, I chose to use the TIP41 as a representation of what I was attempting to do as understanding the PNP and NPN is critical to this particular circuit design. That is, for this circuit to work it requires both a PNP and a NPN transistor to buffer the output. Without both, the circuit cannot source and sink current. I know because I tried to build it without both and it would not do both without both in the circuit. Oh; some other things too, these are darlington transistors and they are from the same series. In other words, a matched pair. Best results happen with match pairs in a circuit like this type. Similar to amplifier design, A matched pair have similar responses to input so mirror each other well in a circuit.
Ok Great, Why?
Yes, my explanation got a bit long winded. So, here it is short and sweet. I never do anything that simple. My end goal for the 99 Ford is to have eight wideband o2 sensors on it. Yes, eight and the controller is going to have to source up to eight virtual grounds. See, that was not to bad was it. Now, granted that is the future end goal. For now and one o2 I could simply use an op amp. See the original design here.
Below, are all the schematics used in this article. I know I will be referring and adding to this article in the future. Hope it helps, and have fun!