variable voltage regulation

[analognow]

I'm using a LM317 with 24vdc in and my desired vout is +/- 6v. My load is the filaments of 2 tubes at 300ma parallal or 150ma in series.

For the filaments in parallal, the LM resistor adjust is fixed at 300k., and the reference resistor is 10k. The filaments light up fine, but make a lot of buzz, even with additional C filtering.

When the filaments are in series, the LM adjust resistor is 4.7k, and the reference resistor is 200R. When I turn on the circuit, the filaments light up for approximately 1 minute, then fade, with the initial voltage at 6.5v.

While still in a series load connection, when I adjusted the adjust resistor to get 10-14v output, the filaments become really bright at the higher voltage and stay lit up. I have no experience with adjustable regulators, having worked mostly with the fixed 7805 etc. types.

What's going on here with the difference in regulator output voltage behavior between a series versus a parallel connection of the load?

 

[IanP]

The values of R1 (resistor between Vout and Vadj pins) and R2 (resistor between Vadj and GND) that you selected do not make sense ..
The recommended value for R1 is 120Ω>R1>240Ω

Lets say R1=220Ω.
With this in mind, for 6V output the value of R2 should be around 820Ω ..

As you operate of 24Vdc the IC will become very hot. It should be mounted on a substantial heatsink using mica (or similar) insulating washer and bush (the IC’s tab is connected to the Vout pin) ..

I assume that you have input and output capacitors in place (C1, C2, C4 – see attached picture), you can also consider C3 and D6 – they should address the ripple issue ..

Rgds,
IanP

 

[nashwan]

hi
u can drive ur filament with lm317 connected as current regulatore
with that connection no inrush current trought filament... that increase
life time of filaments

here is simple circuit for drive 2 filaments with current abt 150mA

R1 is 8.2 1/2w

 

[analognow]

Thanks for the tips, guys. I'll get to work on it and let you know what's happening.