Digital Multimeter LCD Display Not Turning On: Voltage Measurement & LCD Specs

Hot_Amir wrote:

The LCD display of my digital multimeter does not turn on. I changed the battery, unfortunately it didn't work. Instructions for testing and measuring the voltage and the specifications of the LCD bases

Hi User 4348391,

Sorry to hear about your issue. Let's try to diagnose this problem together.

First, I want to clarify that fixing a malfunctioning digital multimeter might require some basic electronics repair skills, as it involves testing different internal components.

To start the diagnosis, here are a few things you can try:

1. Check the Fuse: Many digital multimeters have a fuse that protects it from over-voltage. It's possible that this fuse has blown. If that's the case, replacing it might solve your issue.

2. Inspect the Circuit Board: Open the multimeter and inspect the circuit board for any visible damage such as burnt components, cracked solder joints, or loose connections.

3. Check the LCD Connection: Ensure that the connection between the LCD display and the circuit board is intact. A loose or damaged connection can cause the display to malfunction.

4. Measure the Voltage: Use another multimeter to measure the voltage reaching the LCD display.

For this, you'll need to locate the points on the board where the power supply connects to the display. The voltage should match the specifications of the display.

However, without knowing the exact model of your multimeter, I can't give you specific instructions for testing the voltage.

Can you please provide the model of your multimeter? Also, were there any events leading up to the malfunction (like exposure to water, a fall, etc.)?

Hot_Amir wrote:

How to test health backlight LCD multimeter? Backlight base number? How to test backlight health with voltage?

Hi User 4348391,

Testing the backlight of the LCD display in your multimeter involves a few steps:

1. Inspect visually: The simplest way to check the health of the backlight is to turn it on in a dark room. If it's working, you should see a uniform light coming out from behind the display. If some parts are dark or the light is very dim, it's likely that the backlight is failing.

2. Check the Voltage Supply: Use another multimeter to check the voltage going to the backlight. The exact voltage will depend on the specific multimeter you're using, but it's usually between 3 to 5 volts for most LCD screens.

3. Check the LEDs: If your multimeter's backlight is an LED type, which is most common these days, you can test the health of individual LEDs. This would require de-soldering the LEDs from the board and testing them with a power supply. If an LED doesn't light up with the correct voltage (usually around 2-3.5V for white LEDs), it's bad.

However, please note that this is a relatively advanced procedure and requires some experience in electronics repair.

To give you more accurate instructions, could you please tell me the model of your multimeter? Also, what kind of symptoms does the backlight show? Does it not turn on at all, or is it just dim?

Hot_Amir wrote:

not The LCD does not turn on. It belongs to digital multimeter model DT9205A. How can I test the health of this LCD? From which part does the input voltage + VC come to the LCD?

Hi User 4348391,

Thank you for providing the model of your multimeter, this makes it easier to guide you.

The DT9205A Digital Multimeter uses a standard LCD display with a common anode and cathode for the backlight. Here's how to test it:

1. Visual Inspection: The first step is to visually inspect the LCD and its connectors for any signs of physical damage, corrosion, or loose connectors.

2. Test the LCD Connection: Disconnect the LCD from the PCB (printed circuit board), then re-connect it. Sometimes, this simple act can resolve the issue if it was due to a loose connection.

3. Check the Voltage Supply to the LCD: The LCD display in the DT9205A is commonly driven by a +3V supply. To test this, locate the pin marked as VC+ on the PCB where the LCD is connected. Use another multimeter to measure the voltage between this pin and ground. Normally, you should read around +3V.

If you do not get this reading, it indicates a power supply problem on the PCB and you will need to trace back the power supply line to find the issue.

4. Test the LCD Itself: If you have a variable DC power supply, you can also apply a +3V signal directly to the VC+ and V0 (ground) of the LCD to see if it turns on.

Please remember, these tests should be performed with care because any wrong connections can further damage the LCD or the PCB.

Do you have experience working with PCBs and handling electronic components?

Hot_Amir wrote:

Please explain more and simply: The standard LCD display uses a common anode and cathode for the backlight.

Sure, I'd be happy to explain that.

LCD displays are often illuminated from behind by a light source - this is known as the backlight. Backlights on LCDs are typically made up of several LED (light-emitting diode) units. LEDs, as you may know, are diodes that emit light when current passes through them, and like all diodes, they have two electrodes - an anode and a cathode.

When we say a device uses a "common anode" or "common cathode" configuration, it means that the anodes (or cathodes) of all LEDs are connected together.

In a "common anode" configuration, all the anodes are connected to the power supply voltage, and individual LEDs are turned on by grounding the cathode. In a "common cathode" configuration, it's the other way around: the cathodes are all connected to ground, and LEDs are turned on by applying voltage to the anode.

The DT9205A Digital Multimeter's LCD backlight is a common anode type, meaning the anodes of the LEDs are tied together and typically connected to a power source. To light up specific LEDs, the corresponding cathodes are grounded.

I hope this clarifies your question. Please let me know if you have any more queries.

Hot_Amir wrote:

How to test the health of the connection of carbon flat or zebra? What resistance should a line show first and last? How to test the health and voltage of the LCD startup module? The multimeter device works with a 9 volt book battery. How many volts should the health test voltages of the starter and LCD output be?

Great questions!

1. Testing Zebra strip/Carbon Flat:

The zebra strip or carbon flat connector is used to connect the LCD to the main circuit board. They are made of alternating layers of conductive and insulating rubber and rely on pressure to make a good connection.

Testing the health of a zebra strip can be a bit tricky. Ideally, you should be able to measure a very low resistance (close to 0 ohms) across the conductive layers when the strip is compressed, and a high resistance or open circuit when it's relaxed. However, these measurements can be difficult to make accurately due to the small size and flexible nature of the strip.

The most common problems are due to loss of pressure (e.g. if the clips holding the strip in place have loosened) or contamination of the conductive layers (e.g. by dust or corrosion). In many cases, simply cleaning the strip and the contact areas on the LCD and PCB, then reassembling everything can fix the issue.

2. Testing the LCD startup module:

This will depend on the specific design of your multimeter. Generally, the LCD is driven by a boost converter that steps up the battery voltage to produce the necessary voltage for the LCD operation (this can be anywhere from 3V to 5V).

You can test this by locating the output of the boost converter on the PCB (look for an inductor, a diode, and a capacitor together, which are the typical components of a boost converter) and measuring the voltage there with a multimeter.

The exact value will depend on the specific model of your LCD. If it's not within the expected range (for example, if it's too low or there's no voltage at all), then there might be a problem with the boost converter or the power supply circuit.

3. Expected Voltages:

Considering your device operates from a 9-volt battery, the boost converter should step this up to the necessary voltage for the LCD, which is typically in the range of +3 to +5 volts.

Remember, the exact value will depend on your device and the specific model of the LCD, so it's always best to check the datasheet if possible.

Please proceed with caution when performing these tests. Let me know if you need any further clarifications!

Hot_Amir wrote:

(...) e the zebra carbon fiber and take a health test with an ohmmeter? According to the number of bases, do you know my LCD model? 3-1/2 has the number of screens. In the next few hours, I will carry out the instructions you mentioned in the previous post and I will inform you of the test result for guidance.

It sounds like you're dealing with a complex issue, but let's try to address it step by step.

1. Resistance Measurement: A high resistance value could indicate either the lack of pressure (as these connections rely on pressure to form a good electrical bond) or damage to the zebra strip. It's hard to tell without a more thorough investigation.

2. Zebra Strip Replacement: Zebra strips are very particular because of their alternating conductive and insulating layers, and I would not recommend trying to replace it with a regular wire. If the zebra strip is damaged, it would be best to replace it with a new one of the same type and size.

3. Connection of Zebra Strips: Generally, each leg of the zebra strip connects to a different contact point on the LCD. So, if you're seeing a zebra strip connected to an upper and a lower contact, they are most likely different connection points.

4. Testing Zebra Strip: Indeed, to properly test the zebra strip, it would be best to remove it completely and then measure the resistance. Just remember that it relies on pressure to form a good connection, so the measurements can vary depending on how much pressure is applied.

5. LCD Model: It's difficult to identify the exact model of your LCD based on the information provided. "3-1/2" suggests that the display can show 3 full digits (from 0 to 9) and a half digit (which can only display a "1" or nothing), but this is a common feature of many LCDs.

I'm looking forward to hearing about how your testing goes! Don't hesitate to ask if you have more questions.

Hot_Amir wrote:

Please more explain in this case how action I?
You can test this by locating the output of the boost converter on the PCB (look for an inductor, a diode, and a capacitor together, which are the typical components of a boost converter) and measuring the voltage there with a multimeter.

Sure thing, I can provide a bit more detail on testing the boost converter.

1. Identifying Components: Firstly, you need to identify the components which form the boost converter. Typically, a boost converter will consist of an inductor (a component that looks like a coil of wire), a diode (a small component with a line on one end signifying its polarity), and a capacitor (often cylindrical in shape), all located close to each other on the PCB.

2. Locating Output: The output of the boost converter is typically at the junction between the diode and capacitor. If you follow the traces on the PCB, they should lead from this point to the LCD module.

3. Measuring Voltage: To measure the voltage, set your multimeter to measure DC voltage. Connect the negative (black) probe of your multimeter to a ground point on the PCB (this can often be a large area of unmasked copper or a pin on the power connector), and the positive (red) probe to the output point of the boost converter that you identified earlier. The multimeter should now display the output voltage of the converter.

4. Assessing Result: What you're looking for here is a stable voltage reading that matches the expected output voltage of the converter. If the voltage is unstable, significantly lower than expected, or non-existent, then there may be a problem with the boost converter or the circuit supplying power to it.

Remember to be cautious when performing these tests to avoid short circuits or damage to the device. Let me know if you need further clarifications!

Hot_Amir wrote:

(...) 4 did not turn on, which part does it belong to? As a result, 90% of the problems are from the LCD starter IC?? A 40-pin IC is located in the lower circuit of the carbon connectors, which are covered with black glue or tar. What is the order of the numbers of its bases?
What are the IC power input pins?

It's interesting that you're seeing part of the LCD display light up when you touch the zebra strip. This could indeed be due to static electricity, or more likely, you are providing an alternative path for the electrical signal to reach the LCD display.

As for pins 7 and 14 not lighting up any part of the display, it's difficult to say without knowing the exact LCD or the IC model you are working with. These pins could be connected to segments of the display that aren't being used, or they might be meant for other functions such as control signals.

The 40-pin IC you're referring to could be the LCD controller, but without knowing the exact model or having a datasheet, it's difficult to say what the pinout would be. Manufacturers often cover ICs in black epoxy or similar substances to protect against physical damage and reverse engineering.

In general, ICs usually have their pins numbered counterclockwise from the pin marked as pin 1. However, without a datasheet or a clear marking on the IC or the PCB, it's hard to accurately identify the power input pins. Sometimes, the power pins are connected to larger traces on the PCB, or to capacitors used for power filtering, but these are not hard and fast rules.

I would recommend looking for a datasheet for the IC (if the part number is visible), or for the entire LCD module, as this would provide the most accurate information. If you can't find the information you need, please provide the LCD or IC model, and I'll do my best to help you further.

Hot_Amir wrote:

Model: DT9205A digital multimeter and printed circuit board number 9205A.2c-170825 is inserted. Made in China. I couldn't find the data sheet on the internet. Any help you can do for the IC name and data sheet.

I'm afraid finding specific information or datasheets for the components of a specific model of Chinese made digital multimeter like the DT9205A can be quite challenging, as these are often not made publicly available.

However, in a typical multimeter, the key components usually include a microcontroller (or similar IC) to process readings, an LCD driver for the display, an analog to digital converter (ADC) for measuring voltages and resistances, and often a separate IC for driving the continuity buzzer.

If there are any visible markings on the IC, that could greatly aid in identification. However, I understand this is difficult due to the black protective substance you mentioned.

I wish I could provide more specific help. If you're trying to repair the device, could you maybe share more about what the original fault is? It might be that we can help diagnose the issue without identifying every single component.

Hot_Amir wrote:

Why are you afraid to send me the data sheet? I found and printed many DT9205A datasheets on the internet. Unfortunately, none of them apply to my multimeter. There are many models of my multimeter made. If you really have the data sheet, please send it to me. I really don't know why the data sheet of my model is difficult to search and find.

Apologies for any misunderstanding earlier. When I mentioned that it's challenging to find specific information or datasheets for these components, I simply meant that manufacturers of such devices often don't publicly release detailed datasheets, especially for specific versions of a product like the DT9205A. This is particularly common with electronics manufactured in China. I didn't mean to imply that I'm withholding any information from you.

Believe me, if I had the specific datasheet you're looking for, I would gladly share it with you. Datasheets are indeed a crucial resource in electronics, they provide important details about how to use components properly and how they work.

Let's try to solve your problem from another angle. Could you describe the issue you're having with your multimeter? Have you noticed any components that look damaged, or is there a particular function of the multimeter that isn't working? Sometimes, failure modes can give us hints about the nature of the problem, even without a full datasheet.

Hot_Amir wrote:

I sent you my problem several times. Do you not keep the previous chats in memory? Do you not go through them for reply?

I'm sorry for the confusion. As a chatbot, I do maintain the context of the conversation within the same session. However, I am not able to recall information from previous sessions unless it is mentioned in the current one.

In our current conversation, you were asking for help regarding the IC name and datasheet of a specific model of a digital multimeter, DT9205A. You mentioned that you found numerous datasheets online but none of them were applicable to your specific model.

Could you please elaborate on the specific issues you're experiencing with your multimeter? This could help me provide you with a more accurate response. Are there any issues with the display? Any inconsistencies in the readings? Any particular function not working as expected?

Hot_Amir wrote:

My LCD multimeter did not turn on after replacing the battery. I tested the connections of the carbon flat. We found that the starter IC does not do its job. Is the input voltage of the LCD bases to be 7V? The battery of the multimeter is 9V. Or is it wrong that the input is 7.5V? Is the measuring range AC or DC to measure the input voltage to the LCD bases? I measured DC. Should I also measure AC?

Before we proceed, let me verify the operation of your multimeter. A multimeter is typically powered by a DC source, like a battery. The voltage input to the LCD should also be DC.

Now, regarding the LCD voltage, it can vary depending on the specific LCD used. However, 7V is in the normal range for many LCDs.

If your multimeter isn't turning on after replacing the battery, there could be several causes:
- The new battery could be faulty. Have you tested it?
- The battery connections might be poor. Have you checked these?
- There could be a short circuit somewhere in the multimeter.

You mentioned that the "starter IC" isn't doing its job. Could you explain more about this? Which IC are you referring to?

To test the input voltage to the LCD, set your meter to the DC voltage setting. If your LCD is receiving voltage but still not powering up, the issue could lie in the driver IC, or the LCD itself could be faulty.

Hot_Amir wrote:

If another multimeter model DT9205A works with a 3V battery, should I open it and measure the voltage, will the voltage measurement be the same as my multimeter model DT9205A but works with a 9V battery? Can I use that multimeter as a guide? Because the voltage of the power supply is not the same, will I get the correct answer?

The model DT9205A multimeters typically operate with a 9V battery, not a 3V one. If you have another DT9205A model that operates with a 3V battery, it's likely that this model has a voltage step-up circuit or a different design to operate at this lower voltage.

Using this 3V model to compare voltages directly with your 9V model might not be entirely accurate due to these potential design differences. However, you can observe the general operation and compare functional aspects. For example, you could check if the same IC is being used, verify similar signal paths, etc.

If you're specifically looking to verify the operating voltage of your IC or LCD, I recommend referring to the datasheet of the specific component instead, if available. That will provide the most accurate information.

I hope this helps! Let me know if you have any other questions.

Hot_Amir wrote:

If we open the digital multimeter selector, does it affect the multimeter (LCD) turning on?

Typically, the function of the selector switch in a digital multimeter is to choose the desired measurement function and range. It does not directly control the power to the LCD.

However, depending on the design of the multimeter, the selector switch might play a role in powering up the whole unit. For some models, turning the selector switch to any position other than "OFF" will complete the circuit from the battery, thereby allowing the LCD and other components to receive power.

If the selector switch is faulty or not making proper contact, it could indeed prevent the multimeter from powering on. So, while opening the selector switch itself should not directly affect the LCD power, any issues with the switch could indirectly prevent the multimeter from turning on.

Have you noticed any issues with the selector switch, such as difficulty turning or poor contact?