LM3914 volt meter wiring problem

Hey everybody - been gone for a while busy with tons of stuff... but I"m back and trying to add in a power level indicator (via LM3914).

I currently have the LM3914 circuit connected directly to battery. Parallel to that, I have a switch circuit via transistor that turns on 3 drivers via logic power mosfets(also parallel to 3914).

Problem is that when the drivers are off, the meter reads power level correctly, but when the drivers are turned on, the power level drops a few bars.

I'm confused as there should still be full voltage across the 3914... but it's not?

Fundamentally, is there anything wrong with running an LM3914 in parallel to a load? If not... then I'll draw up a schematic to show what I'm doing...

Thanks!

Sounds like your load is either high enough that it's dragging the supply down, or there is excessive voltage drop on your power leads. Are you still powering this from a battery?

Yes, The power source is 2 18650s in series. Total estimated current draw should be around 2200-2400mA MAX. I expect it should be more in the ballpark of 2000mA.

I THOUGHT I read somewhere that 18650s had a .5C discharge rating, but then read somewhere recently that they have 2C. Maybe it's just the newer ones that have 2C? That could be the problem...

What do you think?

In the mean time, I'll sketch up a simple diagram tonight and get it on here for reference. and I'll measure the volt drop as well to make sure that's not the problem...

Thanks!

Ok... measured voltage and drew a simple diagram.

When switch is on:
mosfet not connected to 555 timer
Vgs: 3.3
Vds: .03
Vds (when off): 3.03
LED Vf: 3.06, Iled: 650mA

You 'measure voltage' on unloaded battery, apply a load and 'measure' again, then wonder why the level sags when the battery is loaded?? It's working like it is supposed to. Wait and see how it goes down when you have run the battery down some...

Right you are. I don't know why I was bent on thinking the voltage would be the same when under load. duh. Battery fundamentals... I was previously using a desktop power supply initially... Thanks

ok - other side of the problem... the 2 constant LEDs in parallel aren't equally sharing load. They are the same components and I would expect them to behave more or less the same. Why would one mosfet sink 600mA with a 1 ohm resistor in series with the LED, when the other LED requires 0.5 ohms to sink only 500mA through the mosfet?

"behave more or less the same". Yup, more or less. manufacturing differences in the dies.

Besides those LEDs, I could try switching out different components... the LEDs all came on the same star and are binned the same, so I wouldn't expect the big difference there, but small differences do add up.

Thanks for slapping sense back into me! :thanks: