deriving phantom power

[Johnny1234]

Is it bad practice or dangerous to derive 48 volt with a circuit like this?

 

[merlin]

And what are you gonna do when the mic tries to suck 5mA out of it?

 

[Johnny1234]

I just went through a few calculation and it seems I won't ever get 20ma to flow through the load.
Oh well.

 

[Johnny1234]

Would this be advisable?

 

[JohnRoberts]

is your 220V source safe? Is it DC?

Have you accounted for power dissipation in the dropping element? 150 V x 0.02 A is 3 Watts.

JR

 

[Johnny1234]

is your 220V source safe? Is it DC?

Have you accounted for power dissipation in the dropping element? 150 V x 0.02 A is 3 Watts.

JR

Hi John, Thanks for you reply. The source is 220 vdc unregulated. No I didn't account for the 3 watts of power dissipation. Will increase resistor to 100k at 1/2 watt. That will be almost double the power dissipation. Again many thanks.

 

[Johnny1234]

Hi John, Thanks for you reply. The source is 220 vdc unregulated. No I didn't account for the 3 watts of power dissipation. Will increase resistor to 100k at 1/2 watt. That will be almost double the power dissipation. Again many thanks.

I'm wrong again that only leave me with 1.7ma. Is there a more eloquent way to do this?

 

[Johnny1234]

Would using the old Trident regulator circuit be an option?

 

[JohnRoberts]

a metal TO3 3055 would easily handle the 3W but I am pretty sure it won't work with 150V across it.

Look for different parts.

JR

 

[Rob Flinn]

Personally I would just build another supply with a different transformer so the input to the regulator is a lower voltage. Certain fault conditions with your circuit would prove catastrophic for the mic.

 

[Johnny1234]

ManyThanks Rob, That was my original question. Don't want catastrophic for the mic.

 

[beatnik]

there are small SMPS modules from MeanWell and such that are just over 10 bucks.

Im not a big fan of SMPS but for phantom who cares... save the cost of a special power transformer or wasting time designing additional circuits

 

[Johnny1234]

Thanks Beatnik, I am going to look into that.

 

[Matt Syson]

The obvious problem with switchers is that they are NOISY. Not necessarily 50/60 or even 100/120Hz but HF (50KHz plus AND possibly other artifacts.
Don't forget that if it is running a 'normal' phantom powered mic then pin 1 of an XLR (Normally chassis for safety) is grounded so your supply needs to be 'floating' or be very aware of what sharing the 'ground' will actually do.

 

[Johnny1234]

The obvious problem with switchers is that they are NOISY. Not necessarily 50/60 or even 100/120Hz but HF (50KHz plus AND possibly other artifacts.
Don't forget that if it is running a 'normal' phantom powered mic then pin 1 of an XLR (Normally chassis for safety) is grounded so your supply needs to be 'floating' or be very aware of what sharing the 'ground' will actually do.

Where would I tie the negative terminal of the switching supply to.

 

[JohnRoberts]

The - of a 48V phantom supply ultimately connects to pin one of the XLR. The +48 feeds XLR pins two and three through 6.81k build out resistors.

On/off switches and simple RC filters can go between the PS and the two resistors feeding the XLR jack.

edit- BTW you can cheat and look at console/mixer schematics for ideas. /edit
JR

 

[Matt Syson]

It is also withwhile remembering that many switchers have weird 'hiccup mode' interuptions (noise) if they do not have sufficient current load to feed so your extra bits you will need start to get quite complicated.

 

[emrr]

The small MeanWell work well for this. I’ve posted specifics somewhere here.

 

[RogerAF]

Use a SMPS to drop the 220VDC down to 52VDC, then use a clean linear regulator to get your 48VDC for the phantom supply. My favorite regulator for higher voltages is the old LM723 in floating configuration.