comment on this simple psu circuit

[AndrewT]

Hi,

average voltage after the 1st cap: .9 x Vsecondary

I think this applies to a choke regulated PSU. It certainly does not apply to a capacitor input PSU.

peak voltage after the 1st cap: 1.41 x Vsecondary.

applies to all capacitor input PSUs.

It is very normal for capacitor input supplies to use the peak voltage value and design for it's consequences.

Your 9Vac transformer will develop 12.7Vpk before the rectifier.
A bridge will take out 1.4V to 1.6V leaving 11.3 to 11.1Vpk.

From this you need to subtract the ripple to find the minimum voltage into the regulator. If your caps are small then the ripple must be higher. Lets put in 1Vpp of ripple leaving about 10.1V (in the waveform trough) into the regulator. You have just 2.1V drop across the regulator. The average voltage drop is half the ripple above this i.e. 2.1V +0.5V=2.6V. Pd=2.6V*2.6A~=7W.

Unfortunately you have to now look at the peak voltage when the transformer is delivering the current you are actually using and re-do the calculations for that and check your regulator dissipation.

Then re-calculate for the maximum mains supply voltage and also calculate for the minimum mains supply voltage. This is the senario that may rule out the 9Vac transformer. 10Vac would probably do the job.
Where do you find a 10Vac transformer? Add a few turns to the secondary of a toroid if you must. But the jump to 12Vac might kill your regulator.

BTW the DC current out of your supply needs to be factored by two (2) to give you the nescessary AC current in your tranformer to take account the various factors that determine the VA rating you really need. So 2.6Adc needs 5.2Aac resulting in a 10Vac 52VA transformer or transformer winding.

[N-Channel]

Jarthel,

If those induators are in the value ranges specified by the National nomographs in the datasheet, then, yes, they will do.

The LM267X are available in the standard 5-pin TO-220 thru-hole mount, a 5-pin surface-mount version. This is basically the TO-220 with a much smaller metal tab for direct soldering to the main pc board and pins bent for surface mounting. I have used the surface mount version because they eliminate the need for the thru-hole drilling step of the fabrication process, and are easier to work with.

In answer to your heatsinking question, yes, you can use them (most of the time) without the need for a heatsink. My most efficient LM2677 ckt ran so cool and efficient, that the only time the chip got warm was when it picked up heat from my finger when I touched it.

If you are still having trouble with sampling from National, let me know, and I will sample a few from them, and send them to you in Australia. There's more than one way to skin a cat.

Steve

[jarthel]

Originally posted by N-Channel
Jarthel,

If those induators are in the value ranges specified by the National nomographs in the datasheet, then, yes, they will do.

The LM267X are available in the standard 5-pin TO-220 thru-hole mount, a 5-pin surface-mount version. This is basically the TO-220 with a much smaller metal tab for direct soldering to the main pc board and pins bent for surface mounting. I have used the surface mount version because they eliminate the need for the thru-hole drilling step of the fabrication process, and are easier to work with.

In answer to your heatsinking question, yes, you can use them (most of the time) without the need for a heatsink. My most efficient LM2677 ckt ran so cool and efficient, that the only time the chip got warm was when it picked up heat from my finger when I touched it.

If you are still having trouble with sampling from National, let me know, and I will sample a few from them, and send them to you in Australia. There's more than one way to skin a cat.

Steve

1. I got some samples. thank you for telling me and thank you for the offer. It seems some parts are free shipping while others are not.

2. In regards to chokes, I assume I can't use chokes like common mode/RF and others in the circuit. right? I can only use "power chokes"? Looking at the 2 biggest electronic place in the country, the best I can find are SMT parts.

3. Do I need to make my own PCB?

4. for parts substitution: for example, the suggested Panasonic FC (560uF 50V) is not available locally. I can use a different manufacturer? I assume the ESR and ripple must be same or at least close to each other?

[AndrewT]

Hi,

for parts substitution: for example, the suggested Panasonic FC (560uF 50V) is not available locally. I can use a different manufacturer? I assume the ESR and ripple must be same or at least close to each other?

for an LDO type regulator this is quite important. See the data sheet for details of stability.
A conventional regulator is much more tolerant of the output capacitor.