Bleed Down or LED?

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D

dantwomey

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Joined 2002
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#1
After reviewing several designs in this forum I've come up with the following:

I've seen bleed down after the CRC as seen below.
I've seen the bleed down replaced with a LED.
And I've seen a bleed down with the LED post regulator.
For the sake of simplicity I'm thinking that the LED in place of the bleed down is
the 'best' solution because it serves two purposes. It serves as a bleed down and
a power indicator?

Regards,
Dan

Bleed.jpg
 
#3
I often use an LED in that location as a power indicator even when there's no need for a bleed path. In fact, I like letting a single component serve a dual purpose in my electronic designs, not just in audio. It's not just the saving in cost, it's the elegance.

Watch the LED current level. You haven't specified the PS voltage. At 50V, a 3.3k resistor will push nearly 15mA through the LED. That's about the max current for a reasonably long life with a 5mm LED.
 
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#6
If you connect a 1 Watt resistor between the voltage regulator's output pin and ground, whose resistance in ohms is less than or equal to (38.4 * RegulatedVout), it has several effects:
  • bleeds the regulator's output voltage down to zero at power off
  • bleeds the regulator's input voltage down to ~2V ("dropout voltage") at power off; a voltage not dangerous to service personnel
  • guarantees the regulator's output current is AT LEAST 26 milliamps during operation -- thus the pass transistor inside the regulator is running at a reasonable current and operating at a comfortably high gain (gm > 1.0 siemens) during operation, thus its regulation is excellent and its output impedance is low
Of course, this assumes your transformer's ratings can accommodate an extra 26mA of current per rail, and your thermal budget won't be exceeded by the extra heat dissipation
 
#7
I've used LEDs as Standby indicator without discharging the ps caps. I simply switch off the main circuit section. I've had my power amp ps caps always fully charged and ready to go for 20 years now. It doesn't create current surges or pops at power up this way. When my neighborhood loses power is the only time the LEDs discharge the caps slooooooowly...
 
#8
NanoFarad -
With a modern high-efficiency GaN LED (blue or green), 10mA of current will result in a brightness level that will practically put out your eye. 10 mA is more in the line of less efficient LEDs like the GaP variety. If I were running a GaN led as an indicator, II would give it 1-2mA instead.
 
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#11
I hate the GaN blue LEDs too - the light from them is far too harsh and actinic. I rather like the first-generation SiC blue LEDs - their light is more of a sky blue, rather than the harsh blue of their more efficient GaN successors.. However, they take a fair bit of current to light up properly to even indicator light levels, 20 mA or so. I have a stash of SiC blue LEDs that I nabbed from a now-deceased surplus dealer. Cree made them (always eager to capitalize on their SiC expertise), and I think Liteon did as well. The advent of the far more efficient GaN LEDs blew the SiC devices out of the water, and you could get a real green LED with GaN, unlike the somewhat wimpy yellow green, which was all that one could get using GaP technology.
 
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#13
There are quite a few non-GaN blue LEDs now. InGaN is one. InGaAs is another. I'm pretty sure I've seen blue AlGaAs types too.

Adding an LED in series with the bleeder resistor is fine, but does have two drawbacks:
  • The current through the bleeder will be limited to whatever current the LED can handle safely (or while not turning into a supernova) which may not be enough for the application.
  • The forward voltage of the LED will limit the final voltage across the supply capacitors to a few volt. Given enough time the voltage will come down due to leakage through the LED, but that could take quite a while (hours). That may not be acceptable in the application.
I suppose some would argue that the LED is just another component that can fail. LEDs are pretty rugged these days, but that statement is true.

Tom
 
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#14
The SiC LEDs I nabbed were a lucky find. I pretty much knew what I had once I lit them up. Unfortunately, that particular surplus outlet went out of business a couple of years ago. The SiC LED was a brief interlude in blue LED history, overwhelmingly and rapidly displaced by the far more efficient GaN LED. The blue GaN LED made the LED light bulb possible, combined with conversion phosphors that approximate white light output.
 
#16
Fortunately bleeders are only needed to drain power supply capacitors if the power supply is not connected to the amplifier section. If the power supply is connected, the amplifier section will drain the capacitors quickly.
 
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