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H-Bridges

Given a bag of random electronic parts icluding some bipolar transistors, how would you go about building a low-power h-bridge circuit to examine its behavior in detail and become familiar with failure modes? It is always a good idea to build a low-power test circuit before building the actual high-power circuit. It is also interesting to see how these components will fail. This low-power work is intended to familiarize you with the different classes of transistors, teach you about switching power electronics, and generally to gain some insight into the issues related to h-bridge design. This work is also in preparation for building high speed IGBT h-bridge DRSSTC drivers.

Shown above is a $5 bag of transistors, something that can easily be found on E-Bay. If you are reading this, you probably already have many such bags at your very fingertips. Getting a little closer in the image below, we can see the bulk purchase of fifty assorted bipolar transistors.

Figuring out what you have should be easy. Grab a transistor and google the part number online. Eventually, you will become frustrated because vendors want to charge you for each datasheet that you want to view. You will get tired of moving from computer to parts and back again. There must be a better way.

For the purpose of this experiment (and many future projects like it), all you really need to know is if the bipolar transistor is N-channel or P-channel. In other words, is it NPN or PNP? Sometimes it can be difficult to read the label off of the part. Other parts may not even have labels.

No worries. There is an easier way. Get out the DVM and switch it to diode mode. Since these transistors are generally silicon diodes, when the internal diode conducts it will have a 0.7 volt drop between the emitter and the base. There are just a few simple things you need to remember in order to sort out the designations:

The direction of current determines if the diode faces into the transistor or in the opposite direction.

When the diode faces into the transistor, the type is PNP. If N is negative and P is positive, then designation PNP shows the voltage dropping off as the current goes into the transistor. This is because the Ps are on the outside of the designation.

When the diode faces out of the transistor, the type is NPN. If N is negative and P is positive, then designation NPN shows the voltage dropping off as the current leaves the transistor. This is because the Ns are on the outside of the designation.

Below are some of the more common pin layouts for the different kinds of transistors:

Let's go back to that bag of transistors. You may have to probe around with the DVM until you find the two terminals that have the diode. In a case in which you already know what the pins are, you can use the following trick to determine if the transistor is NPN or PNP.

Set the DVM to diode mode. Hold a transistor so that the label faces you with the leads pointing down. Spread the wires from the transistor and put the DVM red probe onto the base. Put the black probe onto the emitter. Look for a diode validity check or a reasonable bias voltage reading. If the DVM reads '0' or 'no connection', reverse the input leads and check again. If you find a valid diode in which the current enters the base and exits the emitter, then the current is coming out of the transistor. Therefore, the transistor is PNP. If you find a valid diode in which the current enters the emitter and exits the base, then the transistor is NPN. Remember that current comes out from the red probe.

You can see that the diode pictured above reads almost 0.7 volts.

Now let us talk about the h-bridge itself.

At the heart of the DRSSTC is a high frequency, high power h-bridge. The idea behind the h-bridge is to switch a DC square wave pulse at the resonant frequency of the primary current in the Tesla coil. There are many design options available, but the most efficient power delivery system is the h-bridge itself.

The h-bridge is often used as a DC motor control circuit with robotics and forms the core technology for switching DC power systems. The power supply is a typical computer is a DC switching power supply using an h-bridge to accomplish its functions. The h-bridge allows electronic control of the voltage polarity and current flow direction of an attached two terminal device.

The basic idea is to suspend the device so that if any two of the diagonal switches conduct, the motor will turn. If the oppose diagonal switches conduct, the motor will turn in the opposite direction. If the top two switches are ON, the motor terminals are clamped to high voltage. If the bottom two switches are ON, the motor terminals are clamped to low voltage. Both of these cases are useful because the motor will fight in order to stay still under these conditions. Notice that if the two switches on the right side are ON, the h-bridge will burn up. Similarly, if the two switches on the left side are ON, the h-bridge will burn up.

Shown below is a compact way in which to wire a bipolar transistor h-bridge. If you need to put a motor reversing circuit into a small space, this is the way to do it.  

"The Beam Reference Library"

Shown in the image below, the upper left green LED and the lower right red LED indicate that 20V power is active on the h-bridge. With the upper right PNP transistor base pulled down and the lower left NPN transistor base pulled up, current flows from right to left and makes the midline green LED glow. With the upper right PNP transistor base pulled up and the lower left NPN transistor pulled down, current flows from left to right and makes the midline red LED glow.

By default, the circuit pulls the bases of each transistor so that no current will flow. In this case, a 20K pull resistor is used with a 20V bus, and one milliAmp is used to keep the bipolar transistor OFF. A 2.2K resistor is used to override the default pull resistor. The negative 2.2K test lead is a green wire, and the positive 2.2K test lead is the yellow wire. When working with BJT transistors, it is important to limit the amount of current that can go into the base of the transistor. For example, if you attempt to activate one of the transistor base terminals without using and override resistor, the BJT will just conduct all of the available current through the base -- quickly burning up the component. It is necessary to keep in mind that BJT transistors are current based devices and not voltage based devices, such as FETs.

 

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