Electronics > Projects, Designs, and Technical Stuff
Increasing LM317 current
clay1905:
Presently I'm trying to construct a power supply for a portable valve radio. The original power transformers' secondary went open.
The schematic I've been following is here:
https://www.vintage-radio.com/projects/battery-set-psu.html
It's also below, but not mine and I can't give credit.
There's a catch, isn't there? Not quite enough current for the filaments. After about 5 to 7 minutes the radio fades away. At first I blamed some aging component in the radio, but when I burnt my finger on the LM317, I realised almost, but not quite enough current was being passed by the regulator, and it was gradually warming up until the thermal protection cuts in.
So, for a learner, a massive brain fart occurred. My schematic for increasing current capacity is included below. It's simple really. Just use two regulators in a parallel set-up with rather small resistances to ensure they stay pretty well balanced.
So why wouldn't this work? Nothing is ever this easy, and I'm not seeing anything like this idea on the web, so where has my thinking gone wrong?
Your help in understanding would be appreciated.
I've labeled the little balancing resistances as being 0.5 Ohms each, but that's just a nominal value. The schematic isn't intended to contain all the precise details just yet.
Thanks,
Clay.
Gyro:
What sort of heatsink do yo have on the LM317?
rdl:
Have you measured the actual current your circuit is supplying? Do you have a heatsink on the LM317?
Paralleling 3 terminal regulators is not a good idea. Two possible options, switch to LM350 or LM338 if you just need a little more current or if you need a lot more use a PNP transistor to boost current (look in a datasheet for the circuit).
Ian.M:
Your resistor divider is chosen for 7.5V DC out, yet you say you want 1.4V DC out . . . .
Also, you cant add current sharing resistors to LM317 as regulators like that as they will cause the output voltage of your circuit to drop by 1.5V per amp of load current!
Furthermore the input voltage is grossly excessive so its no wonder the regulator is overheating and going into thermal limiting.
IMHO it needs a complete redesign, e.g. take a toroidal transformer with a single secondary, then add an overwind to get a low voltage secondary for a much lower voltage at the regulator input + use a proper LDO so you don't need as much voltage drop across it, and also choose one rated for the required load current. Set it up with a resistor dummy load for the max. current required, then adjust the overwind turns count for just enough input voltage to the LDO to eliminate ripple break-through, when running from a Variac (or fixed buck autotransformer) at 90% of your line voltage, so its got enough margin for brownouts when fed direct from the mains. N.B. the LDO is still going to need a decent sized heatsink . . .
2N3055:
--- Quote from: Ian.M on May 18, 2020, 11:57:12 am ---Your resistor divider is chosen for 7.5V DC out, yet you say you want 1.4V DC out . . . .
Also, you cant add current sharing resistors to LM317 as regulators like that as they will cause the output voltage of your circuit to drop by 1.5V per amp of load current!
Furthermore the input voltage is grossly excessive so its no wonder the regulator is overheating and going into thermal limiting.
IMHO it needs a complete redesign, e.g. take a toroidal transformer with a single secondary, then add an overwind to get a low voltage secondary for a much lower voltage at the regulator input + use a proper LDO so you don't need as much voltage drop across it, and also choose one rated for the required load current. Set it up with a resistor dummy load for the max. current required, then adjust the overwind turns count for just enough input voltage to the LDO to eliminate ripple break-through, when running from a Variac (or fixed buck autotransformer) at 90% of your line voltage, so its got enough margin for brownouts when fed direct from the mains. N.B. the LDO is still going to need a decent sized heatsink . . .
--- End quote ---
His divider is configured for 1.4 V.
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