I need to get 6 LM3914s into bar mode using a NE555, but of course the 555 won’t go close enough to the rail to meet the 3914s VCC -20mv requirements.

So I need to know if using this MOSFET relay will do the trick. The load will be whatever 6 3914s in parallel offer.

PVN012APBF

https://www.infineon.com/dgdl/pvn012apbf.pdf?fileId=5546d462533600a401535683f9ec2950

The reason I am asking about this particular relay is I have 2 on hand.

At least a good guess, before the experiments begin.

I know I should just use a mechanical relay but space is very limited, even for those small dip types.

 

If the LM3914s are cascaded then there are special arrangements for the mode pin in dot mode. Check the data sheet.

 

Post your circuit then we can see what's happening...

 

the 555 won’t go close enough to the rail to meet the 3914s VCC -20mv requirements.

Try a pull-up resistor (say 1k) from pin 3 to the +ve rail.

 

Try a pull-up resistor (say 1k) from pin 3 to the +ve rail.

It needs to be within 200mV of the positive rail. I don't know how far you can pull up a '555 output.

 

I don't have a 555 to hand, but if its output is a totem-pole, as in Fig 2 of this datasheet, I think that amount of pull-up should be possible?

 

AlbertHall…

No, they are all operating independently.

And the data sheet says 20mv not 200mv so I have to say tying directly to the rail is really my only choice, the data sheet doesn’t even say that using any kind of logic will work, and considering getting within 20mv of the rail is the reason.

From the data sheet…

“Taking into account comparator gain and variation in the 100mV reference level, pin 9 should be no more than 20mV below V+ for bar mode and more than 200mV below V+ (or open circuit) for dot mode. In most applications, pin 9 is either open (dot mode) or tied to V+ (bar mode). In bar mode, pin 9 should be connected directly to pin 3. Large currents drawn from the power supply (LED current, for example) should not share this path so that large IR drops are avoided.”


Alec_t…

Already tried that, went all the way down to 470 ohms

Thanks guys, but I guess I will just bite the bullet and use a mechanical relay, because my gut is telling me even a MOS transistor won’t work.