"Power On" Led using bleeding resistor. An Issue ?

[coolnose]

Hi Everybody

well, as stated in the header I was wondering if I could use the bleeding resistor in PSU to hook up a "power on" Led, fading as the caps discharge.

Any issues with that ?

Cheers,

Max

[DigitalJunkie]

That should work alright.

[Mooly]

No issues as long as the LED max current is observed.

[coolnose]

Gentlemen,

thanks a lot for your input.

My calculations using the "standard" LED values as 3,3 V and something like 25mA max, resulted in 10mA with my rail voltages. Should do fine.

Another question: Any particular reason bleeding the caps ? Longevity ? Risks when working on the amp (would bleed them "manually" anyway in this case) ?

Wouldn't leaving the caps under charge limit the surge current on next turn on ?

Cheers,

Max

[Eva]

When the voltage on the capacitors starts to become potentially dangerous, say more than 50 volts, bleeding is a good idea. It's of little use when the amplifier is finished and placed in a clsoed box, but it becomes very useful for development and servicing because it may save you the work of manyally discharging all supply capacitors before making any modification.

No assumptions can be made about keeping charge for next turn on because you don't know when it's going to happen and capacitors tend to slowly self discharge anyway. Inrush current should be kept under control even for fully discharged capacitors.

[AndrewT]

I looked at using a bank of 12V or 24V low wattage bulbs as bleeders.
At rated voltage they draw rated current.
When cold the resistance is very much lower.
At start up they will draw more bleed current but this very quickly falls as final PSU voltage is approached.
At shut down, they initially draw rated current to start discharging the caps. As the voltage falls, the bulb resistance also falls and they draw proportionally more current than simple ohms law predicts. It is a sort of CCS type bleeder.
Good for drawing more current when the PSU voltage has fallen to <half charged voltage.

If using a LED to monitor PSU voltage, I like to use a Zener + resistor + LED in series so that change in LED brightness is much more noticeable with major changes in PSU voltage.

[jrenkin]

Any schematic showing how this is applied?

[Mooly]

Originally Posted by coolnose

Another question: Any particular reason bleeding the caps ? Longevity ? Risks when working on the amp (would bleed them "manually" anyway in this case) ?
Wouldn't leaving the caps under charge limit the surge current on next turn on ?

Cheers,

Max

Would have to see the circuit to comment on that one.
Most caps discharge naturally into the circuitry they supply anyway.
Lifespan of the cap is unaltered by just remaining charged... it's the AC ripple current and consequent heating that shortens there life (mainly).
Caps that have been used at high voltage (Eva will have come across this) for many months or years have a tendency when removed from equipment and placed on the shelf to gradually "build up" charge to quite a high % of the original voltage it was run at. That catches the unwary

[coolnose]

Originally Posted by Mooly

Would have to see the circuit to comment on that one.

Nothing fancy, ist actually the PSU board as by cviller in the F5 thread, will be using it for Aleph 30... running on 18 V toroids... so I assume using the bleeder cap/LED is most likely pimping my amp.. but hey, so what ?

Originally Posted by Mooly

Most caps discharge naturally into the circuitry they supply anyway.Lifespan of the cap is unaltered by just remaining charged... it's the AC ripple current and consequent heating that shortens there life (mainly).Caps that have been used at high voltage (Eva will have come across this) for many months or years have a tendency when removed from equipment and placed on the shelf to gradually "build up" charge to quite a high % of the original voltage it was run at. That catches the unwary

Thanks for the insight into the subject "caps lifespan", learned something again. As for building up charges: I feel like the lil' boy that was told NOT to pee on the electrical cow fence....but of course at that time nobody told me...

[coolnose]

Originally Posted by Eva

When the voltage on the capacitors starts to become potentially dangerous, say more than 50 volts, bleeding is a good idea. It's of little use when the amplifier is finished and placed in a clsoed box, but it becomes very useful for development and servicing because it may save you the work of manyally discharging all supply capacitors before making any modification.

No assumptions can be made about keeping charge for next turn on because you don't know when it's going to happen and capacitors tend to slowly self discharge anyway. Inrush current should be kept under control even for fully discharged capacitors.

Eva,

thanks for you answer. As you stated no need for bleeding as I am on 22 V rails, but will do ... makes it "cleaner" to me.

Cheers,

Max