Harpo said:
mrphotodude said:
i dont see a diode across the 220r in the schematic though.
Thats why this protection diode is missing.
Datasheets from various chipbuilders advise to protect the regulator from discharging caps. The one diode you had in the wrong direction protects against the 330uF and at least one 220uF in parallel. (BTW no need for the 1N400
7 rating @1000V, a 1N4002 up to 1N4004 will be easier to source and have enough safety margin). The diode to protect against the discharging 10uF at the adj.pin is missing. Have a look at the datasheet (maybe National LM317, pg.5/6) again.
IMHO the schematic is the problem. At the risk of suggesting you start over, I suggest you start over. This circuit has a lot of problems. It is almost certain to fail soon. A Phantom supply has big caps charged up to relatively high voltages that store enough energy to vaporize the guts of your 317 if you don't protect it.
1. Use LM 317HV It has a fighting chance of surviving.
2. 317 regulator requires 10 mA minimum load. Use 121 Ohm 1% instead of 220 Ohm
3. 10K adjustment pot will be almost impossible to adjust, and will run way hot --dissipating 1/3 watt. Use 1K pot in series with 4K42 1% 1/2 watt or two 22K1 1% 1/4 watt in series. That restricts the adjustment range to approx 47 to 57 Volts instead 1.25 Volts to 58 Volts. The fixed resistors will drift less than the pot, and everything will stay cool.
4. Use 100uF/63V instead of 10uF for adjustment filter. 317's have rising/peaking Midband noise with 10uF.
5. With 100uF cap above you
MUST use a protection diode from output to adjustment terminal. The bigger cap is sure to destroy the 317 upon turn-off when the 10uF cap will only probably destroy the 317.
6. Make sure that there is a protection diode from input to output of the regulator.
7. Add a 100K resistor in parallel with each input filter cap to bleed them off and make sure that they share the voltage equally. Without them, one cap can be at 25 V and the other at 75 Volts depending on cap leakage.
8. To fully protect the regulator, add a 56V 5 Watt zener diode between input and output of the regulator to clamp the I/O voltage to safe level. If you use a regular 317 instead of the 317HV, make the Zener 36Volts. The clamp is much more important with the lower voltage 317 since an accidental output short to ground will poof the regulator. Once the Phantom supply is boxed up and the only access to the regulator is thru the 6K81 phantom resistors, then the chance of a short is minimal, so I would consider the clamp optional, unless the input voltage gets too high. With a regular 317, you don't have much wiggle room. If the voltage at the input exceeds 40 Volts above the output --that's only 92 Volts with a 52 Volt output, the regulator will fail.
9. I recommend reducing the 220 Ohm output series resistor to 200 Ohm. 52V with 200 Ohm + 2 x 6K81 = good universal supply that will properly power just about any microphone correctly.
10. The 10K resistor (dissipating 1/4 watt!) and LED should not be at the junction of the 220 (now 200) Ohm and 6K81 resistors. The LED draws more current (5mA) than almost every mic and will cause a 1 V voltage drop across the 200 Ohm resistor. Move the LED and its resistor(s) to the other side of the 200 Ohm resistor. Use a 1/2 watt 10K or 2 1/4 watt 4K99 Resistors in series to dissipate the heat from the resistors.
11. Switch connection (on the schematic) is correct. When the switch is off it discharges the final phantom filter thru the 200 Ohm series resistor. On your layout you substituted a SPST switch that doesn't do the discharge. See edit below.
12. The 6.81 K resistors should be 1/2 watt and matched better than 0.4% according to the DIN Phantom specification.
The absolute value is not critical, so just find two 6K81 1% 1/2 watt metal film resistors that are within a couple of Ohms of each other. When driving power thirsty mics, the resistors can dissipate 1/4 watt or so and you don't want to run resistors more than 1/2 to 2/3 their power rating, especially if you don't want their value to change.
13. Layout concerns:
a. Connect 121 Ohm resistor via a separate trace to the output terminal of the 317.
b. The output of the regulator should have its own trace that also connects to the 317 output terminal.
c. The regulator reference is determined by the connection of the bottom resistor (44K2 + 1K pot) to the output common. For lowest ripple and noise, connect the bottom of that resistor string via a separate trace into the negative pad of the output filter cap (330uF/63V). Also the 100uF (was 10uF) reference filter must be in parallel with the "bottom" resistor string and its negative terminal must connect to the same spot as the resistor string. Usually the best way to do that is to connect it across the resistor string and then take a single trace from the resistor string bottom to the output cap negative pad.
d. make sure the output terminal connects directly to the positive pad of the 330uF/63V output cap and then continues on from the other side of that pad to the switch.
Following the above tips will result in a much better supply than the original schematic would have yielded.
Edit: e. layout of switch terminals: you did not follow the schematic of the switch connections. With your SPST switch, the LED and 10k are left to bleed off the 220uF/63 V final filter cap. It will stop bleeding off at about 2 volts or whatever the Vf of the LED is. Better to use a SPDT switch and toggle the 200 Ohm resistor and the relocated 10K + LED between +52V and Ground.
