Math Formula to calculate DC power supply capacitor value?

[Nigel Goodwin]

WHAT is O/C ?

Open Circuit

 

[MrAl]

Yes. It comes from the basic formula relating current, time, capacitance and voltage change:

C = I * dT / dV

where C is the needed capacitance in farads, I is the load current in amps, dT is the time during which the capacitor is discharging in between being recharged through the rectifier in seconds, and dV is the voltage droop during the discharge period, i.e., the allowable peak-to-peak ripple, in volts.

Where have i seen that formula before?
Oh yeah, it was in the post JUST before yours

NO ONE uses chokes anymore to smooth out power supply ripple; those were common back in the valve era, when voltages were very high and load currents very low. For modern low-voltage, high-current supplies, they just add unnecessary weight and cost.

Chokes are not used for smoothing out power supply ripple voltage, but they are used to smooth out power supply ripple current. If you read the post just before yours you would have seen that the main reason for the choke was to smooth out the current peaks so the diodes dont see as much of a peak which can go really high in a capacitor only filter. That also causes harmonic problems on the input line.
If you want to see the profound difference, do a simulation with and without the choke.
Also, this is one of the reasons swinging chokes came about. To keep the efficiency higher the choke is made to be non linear.

Bleeder resistors, too, are a relic of the valve era, and were used largely for safety reasons. On low voltage supplies, they're not necessary. I haven't used a bleeder resistor or a smoothing choke in a power supply design in more than fifty years.

What kind of power supply.
Do you actually believe that you are the only one on earth that designs power supplies?

 

[Nigel Goodwin]

If you read the post just before yours you would have seen that the main reason for the choke was to smooth out the current peaks so the diodes dont see as much of a peak which can go really high in a capacitor only filter.

Except chokes haven't been used for many decades in PSU's, as I've repeatedly stated they were used in valve equipment (and still are) for the obvious reasons, but pretty well in NOTHING else since (again for the obvious reasons).

You claimed previously that:

The peak current in the diodes is always high with capacitor filters, so with high current supplies like this a choke is usually added to help smooth out the peaks.

Can you site any significant quantities of PSU's using massively expensive and huge sized chokes in such a way?.

We've basically got an OP who doesn't know what he's doing, or even what he wants, and from the confusing little he's mentioned I suspect he's not particularly after a very high current PSU at all (as he mentions a modest power amplifier, and I'm dubious he's got a 75A transformer).

 

[rjenkinsgb]

They are possibly confusing the smoothing used on some high frequency switched mode PSUs with that used on 50/60Hz systems.

Many SMPSUs do use inductors as part of the smoothing (and schematics show these), but at high frequencies they are typically only a few turns of very heavy wire and would do nothing whatsoever in a low frequency system.

It is a totally different application to low-frequency transformer - rectifier - capacitor - regulator type setups and of no relevance whatsoever to those.

 

[gary350]

What kind of power supply.
Do you actually believe that you are the only one on earth that designs power supplies?

ALL types of power supplies.

http://www.vias.org/crowhurstba/crowhurst_basic_audio_vol3_025.html

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[MrAl]

Except chokes haven't been used for many decades in PSU's, as I've repeatedly stated they were used in valve equipment (and still are) for the obvious reasons, but pretty well in NOTHING else since (again for the obvious reasons).

You claimed previously that:



Can you site any significant quantities of PSU's using massively expensive and huge sized chokes in such a way?.

We've basically got an OP who doesn't know what he's doing, or even what he wants, and from the confusing little he's mentioned I suspect he's not particularly after a very high current PSU at all (as he mentions a modest power amplifier, and I'm dubious he's got a 75A transformer).

Hello again,

Well it just surprises me that people would claim things that they cant possibly know for sure about. I myself can be sure because when i worked in the industry we worked on sine converters from maybe 100watts up to maybe 30 kilowatts and the larger ones had to have swinging input chokes to avoid problems with peak diode current and power line harmonics. There were no 'valves' involved as this was in the 1980's and early 1990's. Significant quantities, this was a manufacturing company not a one man operation, and we worked with places like Sandia Labs and the US and other militaries who ordered a lot of solar converters and high power AC/AC frequency converters for aircraft and other uses.
I dont think it takes significant quantities though to show that the ideas works and can be used, although there are other ways too of course. The other ways are usually a lot more complex, but i leave that to the choice of the end user.

The way i see it is if you can claim that nobody used this technique since the days of 'valves' then you had better have worked in every company under the sun from that point on because if you miss just one company then you'd be wrong. At least you'd have to have every company report back to you about what they were using in their designs. Again, miss one company, just one, and you'd be wrong.

Are there other reasons why he/she may not want to use an inductor that might be large and expensive? Sure, and there may be reasons why they might and there could be may reasons either way. But none of them are because the theory behind the reason for using them is wrong, and that's really want i am arguing here, along with the fact that companies have used this technique not that long ago.
Another point is that they are usually used when the current is significant, not for your typical 1 or 1.5 amp wall wart.
Another interesting point here is that even small wall warts have built in leakage inductance, so they dont need a separate inductor. That has multiple functions one of them being current limit.

 

[MrAl]

ALL types of power supplies.

http://www.vias.org/crowhurstba/crowhurst_basic_audio_vol3_025.html

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Hi,

Nice pics

Yes, all types of power supplies, and it surprises me that anyone can ever claim that they know every possible design in use today or yesterday. I am actually arguing theory here though, so i dont really care as much about the time frame just about if it works or not.

Designs are also knocked sometimes before they becomes popular. I could quote my experience in this area too but i'll hold off on that for now

 

[audioguru]

The book was made in 1959 and talks about the PLATE supply voltage of an antique vacuum tubes amplifier.
In post #11 Gary talks about a 70W per channel stereo amplifier (using vacuum tubes?) powered from +24V/-24V. That is impossible for a solid state amplifier unless the speakers are 4 ohms and the outputs are extremely distorted squarewaves. 70W at low distortion into 8 ohms is a sinewave that is 23.7V RMS which is 67V peak-to-peak. The amplifier will have losses of about 6V so the supply must be about 67V + 6V= 73V total or about 73/2= +36.5V/-36.5V.
Maybe a modern bridged amplifier can drive 8 ohm speakers to 70W at low distortion with his +24V/-24V power supply.

 

[Nigel Goodwin]

The way i see it is if you can claim that nobody used this technique since the days of 'valves' then you had better have worked in every company under the sun from that point on because if you miss just one company then you'd be wrong. At least you'd have to have every company report back to you about what they were using in their designs. Again, miss one company, just one, and you'd be wrong.

So you're talking about one small niche market, and inverters, not even PSU's.

I'll ask you again, can you point to even one manufacturer using choke smoothed PSU's in modern electronics?.

 

[gary350]

The book was made in 1959 and talks about the PLATE supply voltage of an antique vacuum tubes amplifier.
In post #11 Gary talks about a 70W per channel stereo amplifier (using vacuum tubes?) powered from +24V/-24V. That is impossible for a solid state amplifier unless the speakers are 4 ohms and the outputs are extremely distorted squarewaves. 70W at low distortion into 8 ohms is a sinewave that is 23.7V RMS which is 67V peak-to-peak. The amplifier will have losses of about 6V so the supply must be about 67V + 6V= 73V total or about 73/2= +36.5V/-36.5V.
Maybe a modern bridged amplifier can drive 8 ohm speakers to 70W at low distortion with his +24V/-24V power supply.

I said nothing about vacuum tubes unless I mentioned building an amplifier in the past. I gave no details on the 70w stereo amp it was one I bought on ebay from China I have not looked at it for maybe 7 years but 2 days ago I looked and it has a built in power supply circuit all it needs is a transformer to provide 24vac to each side. Dynaco 60 would be nice but i never listen to music much these days there is a 99.999999999% chance I will never build another music amp of any kind I have no use for one.

I worked in industry 40 years everything these days is computer controlled. The controllers all have power supplies, some are built in, some are external. With all the equipment running in a factory power 3 phase power lines are full of static, power supplies are full of filters to kept that static out of the computer controllers. I repaired, circuit boards, mother boards, power supplies, many different kinds of welders, packing machines, extruders, robots, laser cutters, water jets, machining centers, onens, you name it & I repaired on it, and everything in the building. Some very large 300 hp 3ph AC electric motors have capacitors to correct phase power factor. When old equipment was junked I stripped it for electronic parts that is why have have so much industrial stuff it was all free so why not take it home.

 

[JonSea]

"Please tell me every conceivable thing about every conceivable type of power supply which may or may not have to be high current or high voltage for an application I am unsure about, won't explain and that I have no interest in building anyway."

I think that about sums up this thread.

 

[MrAl]

So you're talking about one small niche market, and inverters, not even PSU's.

I'll ask you again, can you point to even one manufacturer using choke smoothed PSU's in modern electronics?.

Hello again,

Small niche market? That's just not an accurate summation of what i told you already.

I dont have to point to anything, even not that the industry i worked in used them all the time, which they did, and it certainly was long after the 'valve' era. In spite of that i have pointed already but you cant accept it.
I told you that in theory we CAN use one, i dont care if nobody on earth uses one today or yesterday or tomorrow.

Now you seem to have changed your point of view also. Before it was 'valves', and now it's 'modern'.
'Modern' is a relative term not absolute, and certainly not as absolute as the 'valve' era. If i build one tomorrow, is that modern enough for ya

FYI, converters, AC to AC, AC to DC, DC to AC, DC to DC, are all part of the power supply industry.
I happen to have two that have switcher front ends and linear back ends. Still a power supply.

You're trying to find a way to try to say that it's impossible to build a power supply with a choke input filter. I dont see how you could ever prove something as silly as that. Do we actually want to do that though? It's up to the individual, and more so in the hobby forum. I've read about people designing computers with all TTL logic chips just because they felt like doing it.

It's always your choice though too, you dont have to use a choke input filter if you dont want to, now or any time in the future.

 

[Nigel Goodwin]

You're trying to find a way to try to say that it's impossible to build a power supply with a choke input filter.

Not at all, it's perfectly possible - I'm just saying (along with everyone else except you) - that's it's not been done since the valve era (as the reasons no longer exist).

Do you consider giving advise that it's 'usual' to use a choke filter on a PSU to a complete beginner, when it certainly isn't, and you can't even point to any such example post valve days.

 

[MrAl]

Not at all, it's perfectly possible - I'm just saying (along with everyone else except you) - that's it's not been done since the valve era (as the reasons no longer exist).

Do you consider giving advise that it's 'usual' to use a choke filter on a PSU to a complete beginner, when it certainly isn't, and you can't even point to any such example post valve days.

Hello there,

Thanks for the very reasonable reply

Probably not, but it depends and also i have a lot of reserves for beginners. For example, a 50 amp power supply? Not a beginner project i dont think.

You're saying "post valve days" but then you dont seem to accept early 1990's data as post valve days. To me valves were back in the 1950's or maybe 1960's early 1970's.

I should also point out that some of the power supplies were as big as most peoples' clothes closets, and even had doors on them. The transformers sometimes weighed in at over 100 pounds each, and there could easily be three of them in one unit.
Maybe a bit surprising is that the input power choke did not have to be extremely large, and that was because it was a non linear choke commonly referred to as a "swinging choke". The size could be small because the inductance was purposely designed so that it would decrease with higher and higher DC current, and that would of course lower the permeability and thus the inductance.

Here's a couple interesting links, one a little on a swinging choke and one for use in audio i think...
https://www.abb.com/cawp/seitp202/890f9dbebe73607a85256e2f00744396.aspx
https://www.diyaudio.com/forums/tubes-valves/46985-swinging-choke.html

 

[gary350]

There must be a lot of different types of power supplies that I don't know about and maybe don't need to know about. It seems the power supplies I have been building are fine for the projects that I build. I am just trying to learn if I need to build power supplies, different or better. 1000uf capacitor per amp is probably good for everything I do at the moment.

I am a pack rat when it comes to salvaging parts. I noticed a microwave oven that I took apart has a ferrite donut tube over the AC cord plus inside the microwave oven the 2 AC wires each connect to a choke coil. I don't know what that does but maybe my power supplies need that.

 

[Nigel Goodwin]

I noticed a microwave oven that I took apart has a ferrite donut tube over the AC cord plus inside the microwave oven the 2 AC wires each connect to a choke coil. I don't know what that does but maybe my power supplies need that.

It's to reduce interference IN and interference OUT, and it only has a limited effect.

If you feel the need to add one, feel free, but for almost all purposes it won't have any effect.

 

[Nigel Goodwin]

You're saying "post valve days" but then you dont seem to accept early 1990's data as post valve days. To me valves were back in the 1950's or maybe 1960's early 1970's.

I wouldn't go as far as 70's, only barely.

But again, you're not making any sensible point for choke power supplies in anything other then extremely niche systems, nothing mainstream at all, and certainly not for anything under discussion in this thread. Neither of the two links were at all relevant, and one was even to valve amps.

 

[MrAl]

I wouldn't go as far as 70's, only barely.

But again, you're not making any sensible point for choke power supplies in anything other then extremely niche systems, nothing mainstream at all, and certainly not for anything under discussion in this thread. Neither of the two links were at all relevant, and one was even to valve amps.

Hi again,

Well ok, what would you suggest then for a 'universal' power supply with a four diode bridge and puts out 50 amps?
Line voltage 120vac let's say 60Hz.
Also, we still have the small wall warts with leakage inductance built right in. That's equivalent to an inductor and transformer combination. That is VERY universal.

If i get a chance i will do some simulations but i cant be sure when.

 

[MrAl]

There must be a lot of different types of power supplies that I don't know about and maybe don't need to know about. It seems the power supplies I have been building are fine for the projects that I build. I am just trying to learn if I need to build power supplies, different or better. 1000uf capacitor per amp is probably good for everything I do at the moment.

I am a pack rat when it comes to salvaging parts. I noticed a microwave oven that I took apart has a ferrite donut tube over the AC cord plus inside the microwave oven the 2 AC wires each connect to a choke coil. I don't know what that does but maybe my power supplies need that.

Hi,

Oh yes, i got a transformer from a mic oven a while back, removed the secondary, but never got around to rewinding the secondary for low voltage that i wanted (like 12v or something).
BTW, that's yet another application that has leakage inductance built RIGHT into the transformer itself, and that's equivalent to a separate inductor without having to need the separate inductor.

 

[gary350]

You're saying "post valve days" but then you dont seem to accept early 1990's data as post valve days. To me valves were back in the 1950's or maybe 1960's early 1970's.

I worked in a TV repair shop part time 1970 new TVs came out with new tubes they were called, compactrons. Crazy idea put 2 complete tubes inside 1 glass body I assume to save space tube sockets were mounted on printed circuit boards. That only lasted 3 years then circuit boards were plug in with transistors. To repair a TV unplug the bad circuit board and replace it. Tubes were gone from TVs by the end of 1973 except for the 1B3GT high voltage tube. OH.....almost forgot picture tube is a tube also.