Help/advice/guidance translating schematics onto a pcb

[Brian Roth]

It is a bit odd that the VPR Alliance spec says 5 mA max for the 48V rail while the "official" IEC standard mentions up to 7 mA (10mA max) for any given mic or other device. The VPA spec ignores the existance of "hungry" mics. However, most mics draw 5 mA or less.

Keep in mind we have two "branches" for the 48 V rail after it passes through the phantom switch.

The primary path is through the pair of 6k8 resistors which provide power to the mic. The absolute worst case current would be approx 14 mA with a fault condition where XLR pins 1, 2 and 3 were all shorted together (totally crapped out mic cable!).

[Sidebar: The 6k8 resistors will dissipate around 0.3 Watts under the bad fault condition so need to be rated at least 0.5 Watts. I've seen more than a few consoles that had been in service for many years which had random 6k8/0.25 Watt resistors "browned" due the presence of a bad cable at some point.]

The other 48V path in the proposed design (and the CAPI schemo) only makes the lamp illuminate. In this case (10K in series with the LED which will have approx. 2 Volt drop across it), approx. 4.6 mA will flow with the lamp "on". If no phantom flows to a mic, the module will draw 4.6 mA. A hungry mic drawing 10 mA means the total module draw will be 14.6-ish mA. A boogered mic cable (worst case condition) raises the total to approx 18.6 mA. Note that the LED will still light even with the boogered cable.

Rambling answer to @djmiggymigz IMHO, the 4.6 mA draw in the CAPI design may be too high to account for a wimpy 48V power supply and whatever. As has been mentioned in this thread, a high efficiency LED (and a higher series resistance) should be a good solution to CYA.

Bri

PS, for jollies I looked in the JKL catalog for 48V incandescent bulbs. Found one that draws 53 mA! lol

 

[Cqwet Dbdfte]

Don't be afraid of using 6.8k wire-wound resistors, budget and space permitting. 3W are 1/2" long.
Not as good as a choke, but trying to;-)

 

[Cqwet Dbdfte]

Adding a green LED in series with supply would indicate that the load is applied. If that is of any value.
A weak current from LED cathode to ground could give some indication of power applied.
A typical LED will drop out about 1.8V some types over 3V.
Not sure if the voltage loss is tolerable.
The intensity of the LED would indicate load current, opens and shorts.

 

[Brian Roth]

Adding a green LED in series with supply would indicate that the load is applied. If that is of any value.
A weak current from LED cathode to ground could give some indication of power applied.
A typical LED will drop out about 1.8V some types over 3V.
Not sure if the voltage loss is tolerable.
The intensity of the LED would indicate load current, opens and shorts.

Unless I'm missing something, the LED in series won't illuminate unless there is a load from the mic. Hence, it won't indicate the presence of 48V until a mic is plugged in, which seems to defeat the purpose of the indicator .

Bri

 

[Alva Goldbook]

I learned on Eagle, but moved over to KiCad. There's a few easier programs you can use for simple stuff like this. Learning how to generate gerbers on something like KiCad can be quite the undertaking, it's not something you learn over night, but if you dig electronics, you might as well start now. Watch some how tos on Youtube and just go for it. Send your gerber off to a board house and see how it turns out. The best way to learn is by doing. And expect it not to go right the first time. Happens to all of us. That's why we prototype things. I can do up a small order of boards for something simple for like 20 bucks. So if it doesn't work, no biggie, fix your errors and try again. The important point is to take the plunge!

 

[Cqwet Dbdfte]

Unless I'm missing something, the LED in series won't illuminate unless there is a load from the mic. Hence, it won't indicate the presence of 48V until a mic is plugged in, which seems to defeat the purpose of the indicator .

Bri

You missed my remark of "a weak current from the cathode".
Add a high value resistor to to ground, indicate 48V on, then plugging in a load would show a different intensity.
Cheers

 

[djmiggymigz]

a high efficiency LED

Okay so when I'm looking at the specs/datasheets for LED's in regards to getting a "high efficiency" LED, I should be looking at the Forward Current correct? I.e if an LED states a forward current of 20mA, that's more or less the current that it will be drawing?

Thus, looking for a high efficiency one would look something like a Forward Current of around 5-10mA?

 

[Cqwet Dbdfte]

Okay so when I'm looking at the specs/datasheets for LED's in regards to getting a "high efficiency" LED, I should be looking at the Forward Current correct? I.e if an LED states a forward current of 20mA, that's more or less the current that it will be drawing?

Thus, looking for a high efficiency one would look something like a Forward Current of around 5-10mA?

Stated current is the value corresponding to stated light output.
The LED will draw as much current as you let it. Or until it burns up, if too much.
Efficiency is typically stated as mcd, millicandela, when associated with a mA number would roughly give the efficiency.
WP710A10LZGCK is rated 1100 mcd and 2.65V, @ 2mA.

 

[djmiggymigz]

Efficiency is typically stated as mcd

Ohhh okay I completely glanced over that number/spec,, didn't know what it meant! So is efficiency determined by the combo of mcd, Forward current, as well as forward current?

Specific example: 20 mcd, 1.9V, 20mA = ??? I think 20mA is a lot for my specific case as well as maybe the 1.9V.

The LED will draw as much current as you let it

Is this determined by the series resistor prior to the LED??

 

[Cqwet Dbdfte]

Yes, LEDs are current driven devices, forward voltage does not change much.
A resistor will easily set the current.

 

[djmiggymigz]

Okay back to the newbie questions that I'll look into on my own but also going to ask here!

Does the resistor -> current relationship work something like this: The series R only lets a specific amount of current/V thru it. Meaning depending on the R value, the LED is only getting a calculated portion of the current being fed to the resistor. Then, since it has to go somewhere, the rest of the current is fed back down the main path towards the mic pins?

Essentially, depending on the LED I get, I can alter the series resistor value to let through as little current as possible (to the LED), like just enough for the light to light up well. And thus, minimize the amount of current being taken away from the actual Phantom power being fed to the mic?