LED Tester Circuit Diagram for 3 LEDs

Description

It can be acknowledged that these miniature electronic lamps are convenient, and they possess a lifespan approaching half a century. Approximately 40 years after Nick Holanyak developed the initial LED, they have become virtually indispensable. Any self-respecting electronics enthusiast habitually keeps a few within their junk box. However, prior to utilizing LEDs, it’s advisable to examine them closely. With an LED tester, this can even be accomplished in darkness!

LEDs are presently available in a variety of shapes and colors. Some types feature transparent, colorless packages, while others incorporate colored plastic housings. Numerous contemporary LEDs require reduced currents compared to older models. Certain types produce a considerable amount of light when supplied with sufficient current. When working with used LEDs from the junk box, there’s a potential that the lead designations are no longer discernible.

(If the leads haven’t been trimmed, the shorter lead is typically the cathode lead and the longer lead is the anode lead.) If several LEDs are employed in a display where they all operate with identical currents, it’s natural to desire uniform brightness across all the LEDs. Nevertheless, this isn’t always the case, even with LEDs of the same type. To minimize unnecessary soldering work, it’s prudent to assess the LEDs first. This is the function of the LED tester described here.

LED Tester Circuit

This circuit is designed to evaluate up to three LEDs simultaneously, connected in series. Increasing this capacity is easily achievable by utilizing a higher supply voltage. In such a scenario, approximately 2.7 volts would be required for each additional LED. The Zener diodes are integrated into the circuit, allowing for the evaluation of one or two LEDs. Furthermore, the Zener diodes ensure that even if one or more LEDs are defective or connected with reverse polarity, the remaining LEDs will operate normally. This facilitates the identification of suspect LEDs.

If the tester is expanded to accommodate more LEDs, an additional Zener diode should be added for each LED position. The current that flows through the LEDs is maintained reasonably constant by FET T1, independent of the number of LEDs being tested. The FET is utilized as a constant-current source to simplify the circuit. A drawback of this approach is the relatively large tolerance range of FET characteristics. The version used here is type B, enabling current adjustment using potentiometer P1 across a range of 1–7 mA. If greater current is needed, a BF254C can be employed, but a higher supply voltage will also be required. For example, two 9-V batteries connected in series or power from a mains adapter can be used. However, certain LEDs have a maximum rated current of only 5 mA. Therefore, testing should always begin at the lowest current setting for P1.

The brightness of the LEDs readily indicates whether increased current is needed. If an LED doesn't illuminate, it might be defective or connected incorrectly. Reduce the current to its minimum level before reversing or replacing any LEDs. If the polarity of the terminals on the LED tester is labeled, the cathode and anode leads of the tested LEDs can be easily identified. To facilitate easy LED swapping, an IC socket can be utilized as a test socket. The selected Zener diodes were chosen to make the tester suitable for red, yellow and green LEDs.

Red LEDs have a forward voltage between 1.6 V and 1.8 V. The forward voltage for yellow LEDs is approximately 1.9 V, while for green LEDs, it can reach as high as 2 V. If testing modern blue or white LEDs is desired, the Zener diodes must be replaced with types possessing a voltage of 4.7 V or 5.1 V. The supply voltage will also need to be increased accordingly – for instance, by connecting two 9-V batteries in series.

Circuit diagram:

This circuit can be used to test up to three LEDs at once, connected in series. You can easily increase that number by using a higher supply voltage. If you do so, you should allow 2.7 V for each additional LED. The Zener diodes are included in the circuit so it can also be used to test one or two LEDs. Another benefit of the Zeners is that even if one or more of the LEDs are defective or connected with reverse polarity, the remaining ones will light up normally. That makes it easy to spot suspect LEDs.

If you extend the tester to handle more LEDs, you must add another Zener diode for each LED position. The test current that ? ows through the LEDs is held reasonably constant by FET T1, independent of the number of LEDs being tested. The FET is used as a constant-current source to keep the circuit as simple as possible. The drawback of this approach is that the tolerance range of FET characteristics is especially large. The type used here even has three versions: A, B and C.

We used the B version here so the current through the LEDs can be adjusted using potentiometer P1 over the range of 1–7 mA. If you need more current, you can use a BF254C instead, but then you will also need a higher supply voltage. For example, you can connect two 9-V batteries in series or power the circuit from a mains adapter. However, some LEDs have a maximum rated current of only 5 mA. You should thus always start testing at the lowest current by setting P1 to maximum resistance.

You can easily see from the brightness whether you need more current. If an LED does not light up, it may be defective or connected the wrong way round. Reduce the current to the minimum level before reversing or replacing any LEDs. If you label the polarity of the terminals on the LED tester, you can easily mark the cathode and anode leads of the tested LEDs. To make it easy to swap the LEDs, you can use an IC socket as a test socket. The selected Zener diodes were chosen to make the tester suitable for red, yellow and green LEDs.

Red LEDs have a forward voltage of 1.6 V to 1.8 V. The value for yellow LEDs is around 1.9 V, and with green LEDs the forward voltage can be as high as 2 V. If you also want to test modern blue or white LEDs, you will have to replace the Zener diodes with types having a voltage of 4.7 V or 5.1 V. The supply voltage will also have to be increased accordingly – for example, by connecting two 9-V batteries in series.