Bipolar power supply

[jplebre]

Hey guys

need a bipolar power supply for a project and was wondering if this would work ok.
Also, not sure about the second pair of caps (the 100nF ones) not too sure about their function.

For the values and voltage that we are talking, would it be a good procedure to add bleed resistors to the caps? noticed all pro audio stuff (good stuff!) i've worked on had ones (thank you guys, I can now spot these things! )

[amptramp]

The 100 nF capacitors are usually ceramic or flim types that retain a low impedance into high frequencies.  You cannot count on an electrolytic to handle high frequencies and most electrolytics act like inductors above audio frequencies.

Bleeder resistors at the output are used to prevent the output voltage from remaining high once the unit has been turned off.  Some regulators are specified to be used with a reverse-biased diode connected from output to input to avoid reversing the voltage across the internal power transistor of the regulator.  This would be a better idea as it addresses a possible failure mode.  Bleeders are also used to ensure voltages do not remain at an unsafe level once the unit is switched off, but 15 volts is not usually a safety issue.  You may not be able to control what amount of capacitance is connected to the output at the load, so regulator protection via diodes may be useful.

[Pablo1234]

I would also suggest that you use 2 1100uF caps instead of 1 2200uF, it reduces the ESR and spreads out the ripple current. this will make them last longer and preform better.

[jplebre]

As simple as putting 1 diode parallel with each regulator, cathode pointing to the input? the 4001 would do as we are talking of 15v right?

@Pablo weird, I can only find 1200uF caps with higher ESR :S but it does make sense about the ripple current:
http://uk.farnell.com/panasonic/eeutp1v122/capacitor-radial-35v-1200uf/dp/1890573
http://uk.farnell.com/panasonic/eeutp1v222/capacitor-radial-35v-2200uf/dp/1890583

[Pablo1234]

resistance in parallel reduces the total resistance. its not about the esr per cap its about how much its reduced by putting 2 caps in parallel.

0.029 ohms x 2 in parallel is 0.0145 or you could add 3 and get 0.009

[Pablo1234]

Also I don't recommend using $2-3 caps for power supply designs, its just a wast of money.
http://uk.farnell.com/panasonic/eeufc1v122/capacitor-1200uf-35v/dp/9692320

[jplebre]

noted, will keep expensive caps for audio paths

[jplebre]

Alright, update PSU with suggestion from the folks here:

The new (cheaper) and improved design

Another thought on the 100nF cap. If we are worried about the higher frequencies couldn't we use an inductor? or am I missing something essential on why it shouldn't be there?



Edited to reflect a change: D5 had polarity reversed.

Thanks
J

[amptramp]

Reverse D5.  You want it to conduct only when the output is more negative than the rectified output.

[DavenPaget]

I believe you want a schematic that is verified ?

I have already tried this with Rubycon 2200uf Normal range i guess , but it's okay with the ripple and ESR
@Pablo1234
2x Bad caps can cause cascade failure .
Anyway , the regulator is sitting there to regulate and eliminate a fair bit of noise , ripple and ESR .
Forgot your basics ?

[jplebre]

I've seen those extra diodes there as well. Are they extra protection against polarity reversal?
Wouldn't they make the Vreg diodes redundant as well?

[DavenPaget]

I've seen those extra diodes there as well. Are they extra protection against polarity reversal?
Wouldn't they make the Vreg diodes redundant as well?

I bet they're there for a feedback path i guess ?

[Pablo1234]

Anyway , the regulator is sitting there to regulate and eliminate a fair bit of noise , ripple and ESR .

yes after the Regulator, the ripple current is what makes Capacitors fail an having 2 is a standard. Rule of thumb is 2 caps in parallel are always better than one big cap. On a side note useing 2 capacitors for filtering is also useful. Polyester and polypropylene have inverse Thermal Coefficients, it makes them thermally stable.

also the Regulator isn't the best at Ripple Rejection(14) f = 120Hz, VI = 18.5V to 28.5V 54.0 70.0 – dB

and the diods are useful
Operation With a Load Common to a Voltage of Opposite Polarity
+VI mA78xx +VO −VO 1N4001 or Equivalent Reverse-Bias Protection VI mA78xx +VO mA7800 SERIES

Figure 5. Regulated Dual Supply
In many cases, a regulator powers a load that is not connected to ground but, instead, is connected to a voltage
source of opposite polarity (e.g., operational amplifiers, level-shifting circuits, etc.). In these cases, a clamp diode
should be connected to the regulator output as shown in Figure 6. This protects the regulator from output polarity
reversals during startup and short-circuit operation.

Figure 6. Output Polarity-Reversal-Protection Circuit
Occasionally, the input voltage to the regulator can collapse faster than the output voltage. This can occur, for
example, when the input supply is crowbarred during an output overvoltage condition. If the output voltage is
greater than approximately 7 V, the emitter-base junction of the series-pass element (internal or external) could
break down and be damaged. To prevent this, a diode shunt can be used as shown in Figure 7.

[jplebre]

Alright added that to the schemo and layout as well



As for the layout, should I not worry about ground pour?
the calculators gave me a trace width smaller than the one I used so I should be fine there.

[Pablo1234]

I always use ground pours on my power supply's, its not really required but it don't hurt either.

your 7915t isn't the correct pin-out, check the data sheet.

[DavenPaget]

Anyway , the regulator is sitting there to regulate and eliminate a fair bit of noise , ripple and ESR .

yes after the Regulator, the ripple current is what makes Capacitors fail an having 2 is a standard. Rule of thumb is 2 caps in parallel are always better than one big cap. On a side note useing 2 capacitors for filtering is also useful. Polyester and polypropylene have inverse Thermal Coefficients, it makes them thermally stable.

also the Regulator isn't the best at Ripple Rejection(14) f = 120Hz, VI = 18.5V to 28.5V 54.0 70.0 – dB

and the diods are useful
Operation With a Load Common to a Voltage of Opposite Polarity
+VI mA78xx +VO −VO 1N4001 or Equivalent Reverse-Bias Protection VI mA78xx +VO mA7800 SERIES

Figure 5. Regulated Dual Supply
In many cases, a regulator powers a load that is not connected to ground but, instead, is connected to a voltage
source of opposite polarity (e.g., operational amplifiers, level-shifting circuits, etc.). In these cases, a clamp diode
should be connected to the regulator output as shown in Figure 6. This protects the regulator from output polarity
reversals during startup and short-circuit operation.

Figure 6. Output Polarity-Reversal-Protection Circuit
Occasionally, the input voltage to the regulator can collapse faster than the output voltage. This can occur, for
example, when the input supply is crowbarred during an output overvoltage condition. If the output voltage is
greater than approximately 7 V, the emitter-base junction of the series-pass element (internal or external) could
break down and be damaged. To prevent this, a diode shunt can be used as shown in Figure 7.

Yeah i went to the datasheet and it's a clamp diode and a output protection diode and i thought it was a feedback loop diode ==

[jplebre]

I always use ground pours on my power supply's, its not really required but it don't hurt either.

your 7915t isn't the correct pin-out, check the data sheet.

Damn! I thought that the TO220 would have the same pinout regardless!
Well spotted thank you you avoided a hell of a smoke and headscratching

[Pablo1234]

ya that's always a pain with transistors, knowing the pin-out from one chip to another can get confusing sometimes.

Also when I make my power supply's I always do dual sided boards, that way the regulators have thermal pads right from the copper, in the case of the 7915 the pad is the input voltage not ground. this also gives you a larger ground plain, verry helpful when doing ground plain wiring instead of star grounding.

[Pablo1234]

this is a dual layer mockup of what you should end up with.

[egasimus]

Minor thread hijack: What do you think of this: http://www.musicfromouterspace.com/analogsynth/WALLWARTSUPPLY/WALLWARTSUPPLY.php
It's, basically, a bipolar PSU without a center-tapped transformer. Is it any good?