LM3914 LED output question...

[eTech]

LM3914 LED outputs connected to LEDs, single supply operating at 5v dc.
When operating in dot mode, each LED lights then turns off when the next higher voltage level is detected.
Question:

When the LED is off, the output pin seems to float? That is, there is only a tiny ( in the tens of mv range) random fluctuating on the pin.
indicating it is in an "open" state. How is that accomplished? I'm missing something simple I'm sure, but why doesn't the pin rise to 5v?

thanks

eT

 

[MrAl]

Hi,

It's been a while since i worked with that chip, but in general you do not need to change the voltage of a pin to provide a function. Instead, sometimes just breaking the current is enough. This is accomplished with an open collector output. When the internal transistor is turned off, the current stops, the LED goes out.
This is used for many circuits.

 

[alec_t]

The outputs are controlled current sources so are effectively 'open' when the current is zero.

 

[4pyros]

The LM3914 stops sinking current.

 

[eTech]

The outputs are controlled current sources so are effectively 'open' when the current is zero.

Hi

So if the cathode of an LED is connected to the open collector of an NPN(emitter to gnd), and anode to 5v, what voltage should be measured at the cathode, if the transistor is off?

 

[alec_t]

If the meter is connected between cathode and ground then, depending on the impedance of the meter, the voltage will be slightly less than 5V.

 

[eTech]

If the meter is connected between cathode and ground then, depending on the impedance of the meter, the voltage will be slightly less than 5V.

That's what I thought, but it isn't, it appears to be floating...

Is it tri-stated maybe?

 

[alec_t]

When you say 'floating', what does your meter display? What range is the meter set to?

Edit: I forgot you said the display is 'mV'. Are you measuring from LED cathode to ground, or across the LED?

 

[eTech]

Meter set to millivolt range...
That is autoranging and in millivolt range...

 

[eTech]

When you say 'floating', what does your meter display? What range is the meter set to?

Edit: I forgot you said the display is 'mV'. Are you measuring from LED cathode to ground, or across the LED?

Measuring from Cathode to ground

 

[alec_t]

In that case the meter impedance is high enough to prevent the LED conducting significantly, so the voltage developed across the meter impedance by the miniscule LED current is negligible.

 

[audioguru]

Maybe the meter is set to AC and the "milli-volt fluctuations" are mains hum picked up by the meter's leads.
The LED might be generating a fluctuating voltage from lamps flickering nearby (it becomes a solar panel).

 

[eTech]

Ok...pilot error...

Thanks guys...

eT