LM3914 confusion

Thread Starter

VernonLS

Joined Oct 9, 2019
33
I built a circuit using the LM3914 chip in the PDIP configuration (18 pin). When I started testing I quickly found that my +5V power was being sucked down. When I checked the circuit I found that between the +5 rail and ground (return) I had only about 15 ohms and that there was only about 15 ohms between the rails depending on which way I connected the ohmeter leads. Then I found that the actual chip had the same results. At first I thought i had a bad chip,but checking more chips produced the same result. With the plus ohmeter lead on pin 3 which is + supply voltage and the negative ohmeter lead on pins 4 or 2 I had only the 15 ohms. With the leads reversed I had a high ohm reading which is what I would have expected to find in both directions. I checked the chip description and from what I can make of it I have it connected directly, but 15 ohms is way too low. This would imply that it draws about 300 ma of current when the tech sheet says it should draw 3-5 ma.

What am I doing wrong, not understanding. I attach the LM3914 data sheet in hopes that someone can help. thanks, vern
 

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Thread Starter

VernonLS

Joined Oct 9, 2019
33
What is the make and model number of your ohmmeter?
Is it a digital meter or an analog meter?
This is an old Micronta "Range Doubler" analog multimeter from the early 70s. I should have said in my earlier post that in the circuit I have pin 3 connected to +5, pin 4 to return and pin 2 to return. The meter readings were taken with the IC freestanding on my work bench, not in circuit. thanks, vern
 

RPLaJeunesse

Joined Jul 29, 2018
62
When measuring between V+ and V- of an IC think of it as a diode. As such making a "resistance" reading is illogical, any ohmmeter assumes a linear device and a diode certainly isn't. With V+ lower than V- the diode conducts, and the voltage clamps at about 0.7V for low mA range currents. With V+ higher than V- the diode does not conduct. If the difference gets high enough (3V, maybe less?) the internals start working and the supply current should jump up towards 5-6mA. FWIW I would not test the device without proper supply bypassing (0.1uF or so), and all the REFx and Rhi/Rlo pins connected as shown in figure 1 of the data sheet. (FYI the Harbor Frt cheapo DMMs are decent and useful, only a few dollars with a coupon. I have two on my bench, and can measure V and I simultaneously.)
 

Thread Starter

VernonLS

Joined Oct 9, 2019
33
When measuring between V+ and V- of an IC think of it as a diode. As such making a "resistance" reading is illogical, any ohmmeter assumes a linear device and a diode certainly isn't. With V+ lower than V- the diode conducts, and the voltage clamps at about 0.7V for low mA range currents. With V+ higher than V- the diode does not conduct. If the difference gets high enough (3V, maybe less?) the internals start working and the supply current should jump up towards 5-6mA. FWIW I would not test the device without proper supply bypassing (0.1uF or so), and all the REFx and Rhi/Rlo pins connected as shown in figure 1 of the data sheet. (FYI the Harbor Frt cheapo DMMs are decent and useful, only a few dollars with a coupon. I have two on my bench, and can measure V and I simultaneously.)
Well, it is a nice thought to blame it on the meter and diode action, but the original malfunction was noticed when I found that the +5V to return rail on the circuit board dropped to nothing when the chip was in place. I know I have +5 connected to pin 3 and pins 2 and 4 are connected to the return. Is there something I don't understand about how to connect this IC? It seems straight forward, but obviously something is wrong.
 

RPLaJeunesse

Joined Jul 29, 2018
62
I don't blame the meter, it's just that your description of its results are not really helpful to me. Perhaps you could post the full (LM3914 section) schematic for your board, and pictures top & bottom with the chip in place? These would help in understanding what you've accomplished and what you might have overlooked in describing the situation. Also, what are you using for the 5V supply? If it "drops to nothing" what current does it limit at?
 

MrChips

Joined Oct 2, 2009
19,383
Firstly, we are not blaming the meter. However, your test with the meter is invalid. Here is why.

1) An ohmmeter is useful for measuring resistances. We all know that. So read on.
2) An ohmmeter is useful for measuring a PN junction such as a semiconductor diode. Yes, we know that too. But read on.
3) An ohmmeter is not useful for measuring an IC. Really? That is because we do not know the circuit topology in an IC and we don't know how the IC behaves when we apply a test voltage.

4) Now here is the kicker. Get a known diode (such as a 1N4001) and measure the resistance in both directions. If you have an analog meter, take readings in both directions. If you also have a digital meter, take readings in both directions and use the resistance range and the diode test range.

What do I mean by both directions? Measure with the red and black test leads connected to the diode one way. Then interchange the red and black test leads. Pay attention to the results. We expect the diode to register a low resistance when forward biased and very high resistance when reversed biased. So what? We already know this. But wait. With the black lead connected to the anode of the diode and the red lead connected to the cathode we see low resistance. What? That's correct. Your analog meter is showing a backwards diode. That is because the way the battery is designed in the meter the current flows out of the black lead. This is the opposite when using a digital multimeter. Did you know that? You learn something new every day on AAC!

So what does this mean when you test the LM3914? It means that you are correctly registering a high resistance when correctly biased and low resistance when incorrectly biased. The ohmmeter test across VCC and GND makes the IC look like a reversed biased PN junction. What is going on here? What you are measuring as a low resistance is the leakage current across the IC substrate that looks like a PN junction. Your test results are absolutely normal.

OK. So the ohmmeter test is invalid. But that does not explain why the LM3914 kills the power supply.
What it means is that you have to look somewhere else for the fault. How good is the power supply?
Connect a 1kΩ resistor across your power VCC and GND. Can the supply provide 5mA without the voltage dropping?
 

Thread Starter

VernonLS

Joined Oct 9, 2019
33
I don't blame the meter, it's just that your description of its results are not really helpful to me. Perhaps you could post the full (LM3914 section) schematic for your board, and pictures top & bottom with the chip in place? These would help in understanding what you've accomplished and what you might have overlooked in describing the situation. Also, what are you using for the 5V supply? If it "drops to nothing" what current does it limit at?
OK, This morning I cannot reproduce what happened last night, so I am going to blame my results on a loose connection or my fatigue. However, I did determine that the red lead which is on my old VOM when it is switched to read ohms is negative with respect to the black lead and the voltage is about 1.4V on all the ohm scales except the Rx10K where the voltage jumps to about 8.1V. The polarity was the opposite of what I expected and does explain why the LM3914 appeared to have only 15 ohms between power and ground as the diode effect would explain that as one person previously offered. My apologies if I appeared rude in my answer.

I thank you all for your inputs.

vern
 
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