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Old 06-03-2014, 06:19 PM #1
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Default 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!


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Old 06-03-2014, 10:32 PM #2
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Default Re: LM3914 volt meter wiring problem

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?
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Old 06-03-2014, 10:52 PM #3
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Default Re: LM3914 volt meter wiring problem

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!
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808nm 20watt - watercooled
20X 650nm Cheapy Home Depot 9led conversion - My first ever build using Rkstr driver.
18X 650nm rifle mounted custom module
- in progress
A140 445nm 1.4W 3-element glass lens | custom Quassimoto host (twin) | home-made LM317 driver | Radiant X4 LPM
A140 445nm 1.0watt custom Quassimoto host(twin) with 445/405 driver
M140 445nm 1.4watt custom everything
M140 445nm 1.4watt custom everything, mounted in walking staff
M140 445nm 1.8watt custom sink/module/driver in Ultrafire 504B with Extension Tube

405nm BluRay Player - Dead <5mw
protection
OEM Laser Systems
DBD Style 34: 190-534nm, 790-1090nm OD7+ 22% VLT
ARG Style 56: 180-532nm OD7+ 48% VLT
Generic
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Last edited by dethlore; 06-03-2014 at 10:53 PM.
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Old 06-04-2014, 06:20 AM #4
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Default Re: LM3914 volt meter wiring problem

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

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[ | | ]--------------------------------------------------
lasers
808nm 20watt - watercooled
20X 650nm Cheapy Home Depot 9led conversion - My first ever build using Rkstr driver.
18X 650nm rifle mounted custom module
- in progress
A140 445nm 1.4W 3-element glass lens | custom Quassimoto host (twin) | home-made LM317 driver | Radiant X4 LPM
A140 445nm 1.0watt custom Quassimoto host(twin) with 445/405 driver
M140 445nm 1.4watt custom everything
M140 445nm 1.4watt custom everything, mounted in walking staff
M140 445nm 1.8watt custom sink/module/driver in Ultrafire 504B with Extension Tube

405nm BluRay Player - Dead <5mw
protection
OEM Laser Systems
DBD Style 34: 190-534nm, 790-1090nm OD7+ 22% VLT
ARG Style 56: 180-532nm OD7+ 48% VLT
Generic
Cheap Chinese for Red Wavelengths
GB Closed for now Click "LASER SHIRTS" to see the latest run!
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Old 06-04-2014, 07:10 PM #5
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Default Re: LM3914 volt meter wiring problem

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...
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Old 06-04-2014, 07:33 PM #6
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Default Re: LM3914 volt meter wiring problem

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?
__________________
[ | | ]--------------------------------------------------
lasers
808nm 20watt - watercooled
20X 650nm Cheapy Home Depot 9led conversion - My first ever build using Rkstr driver.
18X 650nm rifle mounted custom module
- in progress
A140 445nm 1.4W 3-element glass lens | custom Quassimoto host (twin) | home-made LM317 driver | Radiant X4 LPM
A140 445nm 1.0watt custom Quassimoto host(twin) with 445/405 driver
M140 445nm 1.4watt custom everything
M140 445nm 1.4watt custom everything, mounted in walking staff
M140 445nm 1.8watt custom sink/module/driver in Ultrafire 504B with Extension Tube

405nm BluRay Player - Dead <5mw
protection
OEM Laser Systems
DBD Style 34: 190-534nm, 790-1090nm OD7+ 22% VLT
ARG Style 56: 180-532nm OD7+ 48% VLT
Generic
Cheap Chinese for Red Wavelengths
GB Closed for now Click "LASER SHIRTS" to see the latest run!
LASER SHIRTS

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www.oemlasersystems.com

www.reubart.com

Last edited by dethlore; 06-04-2014 at 07:46 PM.
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Old 06-04-2014, 07:48 PM #7
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Default Re: LM3914 volt meter wiring problem

"behave more or less the same". Yup, more or less. manufacturing differences in the dies.
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Old 06-04-2014, 07:52 PM #8
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Default Re: LM3914 volt meter wiring problem

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!
__________________
[ | | ]--------------------------------------------------
lasers
808nm 20watt - watercooled
20X 650nm Cheapy Home Depot 9led conversion - My first ever build using Rkstr driver.
18X 650nm rifle mounted custom module
- in progress
A140 445nm 1.4W 3-element glass lens | custom Quassimoto host (twin) | home-made LM317 driver | Radiant X4 LPM
A140 445nm 1.0watt custom Quassimoto host(twin) with 445/405 driver
M140 445nm 1.4watt custom everything
M140 445nm 1.4watt custom everything, mounted in walking staff
M140 445nm 1.8watt custom sink/module/driver in Ultrafire 504B with Extension Tube

405nm BluRay Player - Dead <5mw
protection
OEM Laser Systems
DBD Style 34: 190-534nm, 790-1090nm OD7+ 22% VLT
ARG Style 56: 180-532nm OD7+ 48% VLT
Generic
Cheap Chinese for Red Wavelengths
GB Closed for now Click "LASER SHIRTS" to see the latest run!
LASER SHIRTS

OEM Laser Systems Certified Goggles - Protect your eyes!
www.oemlasersystems.com

www.reubart.com
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