Husnen Rupani:
Congratulations on owning the DC Pracbox and thank you for your purchase. The purpose of this video is to give you a detailed induction of the DC Pracbox, including product and user safety. This should also help you get the most out of the product. We will discuss each part or a group of parts and cover what they are, their purpose, how to use them, what can go wrong with them, and how to avoid that. We will cover these components in this video. You may click on the links to jump to a specific section if your video player allows you to do so.

Let’s begin by talking about the power supply. This product comes with a built-in power supply with the voltage output that ranges from 1.25 volts to 25 volts with the maximum current of 5 amps. It’s used to supply voltage and current of the components to see the response to the power. The dial here is used to vary the voltage output. The toggle switch controls the current when the switch is off, no current will flow, and the pilot lamp indicates that the current is enabled. The fuse holder has a 3 amp M205 fuse for circuit protection. The red terminal is the positive side of the supply and the black terminal is negative.

This power supply has built-in overcurrent protection, so nothing should happen to them except the blown fuse. However, the component of the circuit that the power is being supplied to can be affected if the component specifications aren’t followed. The best way to avoid that problem is to identify the voltage and current specifications of the component or the circuit. We also suggest that you keep the toggle switch off when connecting the circuit and set the voltage before turning the toggle switch on.

Power resistors. This pracbox has five power resistors, 39 ohms 25 watts, 2 ohms 50 watts, 5 ohms 50 watts, 10 ohms 50 watts, and 20 ohms 50 watt resistors. These are the components you will be using the most when conducting training since they can take a high amount of current without being damaged. Each resistor has two terminals. Both terminals will need to be connected to a circuit for the resistor to work.

There are two things to remember with power resistors. One, they will get hot when the current flows through them, and two, if the specifications aren’t followed, they can eventually burn and damage the equipment. You and your learners should be aware of the heat that these components will generate and should avoid touching them. As for keeping the resistors safe from burning, we recommend two things. One, identify the maximum voltage that can be applied to them and keep it considerably below that number. For example, a 10 ohm 50 watt resistor can only take 22.4 volts max. I got this number by using this formula. You should keep the voltage below this value, let’s say 15 volts. The other thing we suggest is to only supply the power to these components or any components for less than two minutes at a time.

Transformer. The DC Pracbox comes with a center tapped input to output transformer. You can use this transformer to teach the learners about insulation resistance and continuity testing. On the primary side of the transformer, the top and the bottom terminals are connected to the full winding and the middle terminal is the center tap, which means half the winding. On the secondary side of the transformer, the two top terminals are connected to one output winding and the bottom two terminals are connected to the other output winding. The primary and both secondary windings are electrically insulated and so are both the secondary windings from each other. Transformers are AC devices and will only behave like an inductor or a resistor if connected to the DC supply, which means they will heat up and perhaps burn eventually. To avoid this, we recommend not to power the transformer at all.

LED lamp. We have installed a 12-volt red LED lamp in this pracbox. He can use this lamp for a visual demonstration of the effects of change in voltage, current and resistance. Since this LED lamp is directional, the positive side of the circuit should be connected to its red terminal and the negative side should be connected to the black. Reversing the terminals won’t damage it. However, increasing the supply to this lamp over 12 volts could and the best way to avoid it is to keep the voltage at 12 or less.

Fuse holder. This is an M205 250 volts 10 amps fuse holder. M205 means that this unit will fit a fuse that has five millimeter diameter and is 20 millimeters long. You can use it to train your learners about the use of circuit protection devices and how to install them in a circuit. The fuse holder can be opened by unscrewing the cap, install the fuse holder inside and screw the cap on. This type of circuit protection device connects in series with the circuit and both of its terminals must be connected for it to function. If the ratings of 250 volts and 10 amps are exceeded, the fuse holder will get damaged. We have limited the power supply to 5 amps max to avoid this problem.

Color-coded resistors. This pracbox comes with seven color-coded carbon resistors, 1 kilo ohm, 1 watt; 150 ohms, 1 watt; 470 ohms, 1 watt; 1 mega ohm, 0.6 watt; 33 kilo ohms, 0.25 watts; 180 kilo ohms, 0.6 watts; and 4.7 kilo ohms, 0.25 watts. These components are ideal to train your learners about identifying resistance values of four in five band color-coded resistors, measuring resistance and understanding tolerances. You can either use these resistors individually or in a combination to train your learners about series, parallel or combination resistances.

Both terminals of these resistors must be connected for them to function. Since these resistors have such low power rating, they can easily burn if these ratings were exceeded. They weren’t installed with the intent of powering them, so the best way to avoid any mishaps is to not apply power to them at all. However, if you wish to train your learners in very small currents, please identify the maximum voltage that can be applied to them and keep it considerably below that number. For example, 150 ohm, 1 watt resistor can only take 12.2 volts max, which I got from using this formula, so keep the value below that, let’s say, 8 volts.

Thermistors. The DC Pracbox has two thermistors, a positive temperature coefficient or PTC, and a negative temperature coefficient or NTC. The PTC thermistor used here is around 50 to 70 ohms at room temperature and NTC is around 110 to 130 ohms. The DC circuits unit of competency expects the learners to be introduced to thermistors and that’s the reason why they’re installed here. Using the ohmmeter, measure the terminals of these thermistors as if there were normal resistors. With the temperature change, they will change their resistances. The NTC used here is more sensitive to the temperature changes.

Since the resistances of both thermistors here are so low, there is a higher chance that they will get damaged if you supply power to them directly. The intention of having them here was only to introduce the learners to temperature-dependent resistors and not for use in live circuits.

Varistor. This component is called a varistor or a voltage-dependent resistor and it has a clamping voltage of 36 volts. It’s an important component in surge or overvoltage protection, and it’s here for you to train your learners about these topics. If the voltage on them exceeds the clamping voltage, the varistor short circuits. The best way to see their working is to use an IR tester at 250 volt range and the continuity tester. These type of components have a finite life when it comes to how many voltage pulses they can handle, but this can vary vastly based on how much current was going through them. The best way to avoid any damage to these is to only connect them in dead circuits to show how they’re connected for protection and then use an IR and a continuity tester to see they’re working. The IR testers, although have very big voltage, have very limited current, which keeps the varistors safe even after multiple test cycles.

LDR. The DC Pracbox comes with a light-dependent resistor or LDR that changes its resistance based on the light it’s exposed to. The range is thousands of ohms when it’s dark and only hundreds of ohms when exposed to light. By using this component, you can show your learners how the photoresistors work, and by connecting them with other components, you can show them how they affect the circuit. There are two ways we recommend you use it in training.

One, measure the resistance with an ohmmeter with changing light intensity. And two, use it in series with LED and see the LED brightness change when the light intensity on the LDR changes. The LDR can be damaged if connected directly across a power supply and is exposed to a bright light at the same time. As the light intensity increases, its resistance decreases, which will create a very low resistive path and cause an increase in current, which the LDR may not be able to handle. They’re usually designed to be used either with control circuits or in series with other components so that the current through them is limited. And by doing that, you will ensure that these components will last you a long time.

Potentiometer. This pracbox has 1 kilo ohm, 1 watt linear potentiometer or a variable resistor. You can use this pot in series with other components to show changes in voltage drop and current when its value is changed by rotating it. It has three terminals. The two on either side are connected to the full resistance of the pot, which is 1 kilo ohms. Either one of the side terminals and the top middle terminal will show the variable resistance based on the knob position. There are two ways we recommend using it in training. One, measure its resistance with an ohmmeter while rotating the knob and two, connect it in series with the LED and see the LED brightness change when the knob is rotated.

As the knob is rotated in either direction, the resistance of the 1K pot will either increase or decrease. If the resistance was decreasing and you connect it across the power supply directly, this will create a short circuit and damage the pot. To ensure that this doesn’t happen, always connect the pot in series with another component when connecting it in a circuit.

Capacitors. There are three electrolytic capacitors in this pracbox, 2000 microfarad and 100 microfarad capacitor. All three are rated at 35 volts and are bipolar. Capacitors charge and discharge depending on how they’re connected in the circuit. You can show your learners how they can be connected for both scenarios and what their effect will be on the circuit. Each capacitor has two terminals and they will need to be connected to a circuit for them to work. Connecting them in series with another resistor will determine how long they will take to charge and discharge, and by connecting them across an LED, you can show visually how they discharge.

Usually, electrolytic capacitors have a polarity, which if reversed in a live circuit, can damage the capacitor. They can also be damaged if the voltage rating of the capacitor is exceeded. To avoid any damage due to overvoltage, we have chosen the capacitors with higher voltage ratings and in DC Pracbox version 3, we have used bipolar capacitors which remove the danger of wrong polarity altogether. However, if you own the previous versions of DC Pracbox, please follow the polarity markings on the capacitor. To make it easier for you, we have internally wired the positive terminal to the red and the negative terminal to the black. Always connect the more negative section of the circuit to the black terminal.

Nichrome wires. We have installed four nichrome wires in the DC Pracbox. Their dimensions are 0.26 millimeter square thick and a half meter long, 0.26 millimeter square and 1 meter, 0.45 millimeter square and half meter and 0.45 millimeter square and 1 meter. All conductors have some resistance and since nichrome wires are resistance wires, even the small lengths have measurable resistances, which you can use to emphasize the resistance of conductors.

Using the dimensions of these wires, the learners can calculate their expected resistance values and using an ohmmeter, identify the correct wire. These wires are also called heating wires because they heat up very quickly and at very high temperatures if connected to the supply. This can damage the entire pracbox and even cause a fire. The intention behind using these wires is only to show the resistance of conductors and they should not be connected to the supply at all to avoid any damage.

Toggle switch. This is a single pole, double throw SPDT toggle switch with 5 amp max contact rating. You can use this to control the current to various circuits or even use it to direct the current to a specific part of the circuit. The connection should be made with either of the side terminals and the bottom middle terminal because that terminal is the common. The side terminals are not connected internally. A switch should be connected in series with any component that you want to control the current to. Its contacts can internally weld shut if the current exceeds the rating, and to avoid this, always keep the current less than the maximum rating.

We have programmed the power supply to deliver just less than 5 amps, even in a short-circuit situation. However, care should be taken not to allow this to happen for longer than a few seconds. Otherwise, the switch will get damaged. Also, never connect the power supply directly across the switch.

IEC socket. The power to this pracbox is provided via the IEC socket at the back, which also comes with its own fuse for extra protection. In the event of a blown fuse, always replace with the same fuse as installed, which is a 2 amp M205 fast blow glass fuse.

We have covered all the aspects of the DC pracbox, and I hope that you now have enough information to ensure the safety of the product and the users. We’re always here for you. Please feel free to contact us if you have any questions or run into problems. We hope you and your learners enjoy using the DC pracbox. Thanks for watching.