# dc power supply capacitor values

Discussion in 'The Projects Forum' started by InnocentOfTheWorld, Dec 25, 2010.

1. ### InnocentOfTheWorld Thread Starter Member

Apr 15, 2010
47
1
Hello,
Q#1-Capacitors C1 and C2 ain't available in market what value capacitor should i use for filter part instead of these.
Q#2-I am giving this circuit an input of 25V(rms),4A......i did this in circuit maker although i am getting variable voltage their in graph at output but how i will know the output current it can supply...
Q#3-Required output is from 1.2 to 17 volt capable of supplying 800mA...Will this give me the required results.
Q#4-Constant output of 5, 2, and 9 volts, capable of supplying 1A.( Should i use
the same circuit by varying variable resistance and get 5,7and 9 or should i use
some other IC....if other which one and how??? )

Last edited: Dec 26, 2010
2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
C1 is 1,000uF. This is a very common value. You could use a pair of 470uF, or three 330uF, or a larger capacitor, like 2,200uF.
C2 is 0.1uF, frequently written as 100nF (nanofarads), probably the most common value there is. A capacitor of that size is required on virtually ANY IC.

The 317 regulator model in Circuitmaker is limited to around 800mA output. A real 317 regulator is rated for up to 1.5A output, but this depends on the difference between the input and output voltage. If the difference is large, the output current will be very significantly reduced to protect the regulator from being destroyed due to overheating.

Not at the lower voltage range. You might get 800mA out at 17 volts. Power dissipation will be far too high when the output voltage is low.

It's not going to work well like that. The regulators will have to dissipate far too much power.

Let's just take the 5v output for example.
5v x 1A = 5 Watts. Simple, right? Well, how much power does the regulator have to dissipate?
24v in - 5v out = 19v across the regulator at 1A current, so 19v x 1A = 19 Watts! That regulator will make a good room heater. You'll be dissipating nearly four times as much power in the regulator as you would in the load; just under 21% efficiency. It's even worse with the 2v supply.

3. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
As far as C1 and C2 are concerned try to find at least a 0.1 uF at the very minimum, a 0.33 uF or better is preferred and most any electrolytic can serve as C1 but considering we don't know what's coming in you may want a 2,200 uF or 4,700 uF rated for at least 35V.

For constant output voltages nothing beats a fixed regulator, preferably of the LDO type. It's just far easier to rectify and filter your main incoming voltage then branch off to the individual regulators. You're starting out with a pretty high voltage input so a large heat sink is going to be a necessity.

4. ### InnocentOfTheWorld Thread Starter Member

Apr 15, 2010
47
1
Thanks sgtwookie and marshallf3...
SGTWOOKIE you are right that power dissipation will be very high when output voltage will be low.....so what should i do???? I have to design it to vary from 1.2 to 17 volt capable of supplying 800 mA and at constant voltages of 2,5 and 9 volts to be capable of supplying 1A...Should i use a greater heat sink or there is some alternative design????
i am using a transformer supplying 25-0-25 V and +12V...
from 25-0-25V i will use it to regulate from 1.2 to 17V and -1.2 to -17V..for sure in negative case almost same circuit like lm317t but in that place lm337t...
What you think if i use 12V(rms) from transformer and then design a circuit for constant voltage of +5,+7 and +9 V?????If you agree can you tell me the circuit diagram that can work also capable of supplying 1A....

Last edited: Dec 26, 2010
5. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
Don't try to overcomplicate what should be a simple design. Large heat sinks can be expensive unless you find one on eBay or a surplus place. Of course at times I've just bought a large piece of aluminum related to door parts from Home Depot and used them if space isn't at a premium.

I would highly recommend using the 317 as the variable output (get the one he recommended) then using individual regulators for the fixed outputs. The 2V (sure you don't mean 2.2V?) one will be the hardest to come up with but there are tons of buck/boost converter ICs that can easily handle that. Alternately, if you do some research, you'll find plenty of ICs already designed to do what you want to do and not eat up all the wattage involved.

Try going through National's online Webench design stuff, Maxim may also be another source but there are plenty of others.
http://www.national.com/analog/webench/power_architect

A quick check showed they have 54 devices just for the 2V part alone. They'll give you complete schematics and will often offer samples if you ask. Just take your time with their online design software and don't settle on the first one you come up with, look at several of the possibilities as you may only need 4 of the same IC to achieve all the outputs you're wanting including the variable one.

Last edited: Dec 25, 2010
6. ### InnocentOfTheWorld Thread Starter Member

Apr 15, 2010
47
1
yap marshallf3 i am using the same circuit with lm317t using for regulating from 1.2 to 17 volt....
but for constant output voltages of +5,+7 and +9 volts
if i use lm7805,lm7807 and lm7809 and what is the maximum input of voltage i can supply to these????
i am rectifying an 25V(rms) voltage....

for constant voltage of +5,+7 and +9 volt with 1A current... How i will modify my circuit????
please tell me marshallf3 i am waiting

Last edited: Dec 26, 2010
7. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
The LM78xx series all take up to around 35V input so obviously there's a great amount of heat created to reduce even 25V down to 5V if you're drawing any current from them. Most people that use them try to supply only a few volts above the desired output to them.

http://www.fairchildsemi.com/ds/LM/LM7805A.pdf

Did you try that design feature in National's website? The ICs aren't really any more expensive, just a few extra outboard parts and you won't have anywhere near the heat sink requirements..

8. ### InnocentOfTheWorld Thread Starter Member

Apr 15, 2010
47
1
yap i tried that website but the circuit there were much complex... i can't understand them thoroughly as i am just a beginner....circuit will become complex even for just creating 5V and 1 A .... and i have to create 5,7 and 9 v....i am confused you tell me how to use that circuits mentioned in that website and which one there are many of them...

9. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
Even halfway experienced hobbyists always have to learn something new but for now I'd just stick to the adjustable + fixed regulators and face up to the fact you'll need a heat sink. Is your 24V supply already pre-made or can it be split such that you could get 12V out of it as well? That would help a heck of a lot for powering the fixed regulators.

10. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
You fill in the details as far as values and the rest of the connections go, don't forget that every one of the regulator ICs should have at least 0.1 uF bypass caps on their input and output pins and each should have an additional fairly decent sized electrolytic also on the output pins.

http://www.innoengr.com/images/rough_regulator.jpg

11. ### InnocentOfTheWorld Thread Starter Member

Apr 15, 2010
47
1
yes i have 25 v as well as 12 v out

12. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
Shouldn't be too hard then provided you make a + & - for your two variable regulators. If the transformer is 50VCT on one secondary just modify what I drew such that the CT goes to ground and a single bridge outputs the + & -

12V will be one heck of a lot better running to 78xx fixed regulators.

Another source of good heat sinks can be thrift shops or the few stereo repair shops that still exist. Find a blown up receiver that has a decent heat sink in it, you should be able to adapt that as well as use a lot of the filter caps that are in it. If you're not concerned about the looks you could even use the gutted box as your chassis.

I'm sure someone will come along in time and finish that crude illustration I posted.

13. ### InnocentOfTheWorld Thread Starter Member

Apr 15, 2010
47
1
thank you very much sir

14. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
Just remember that the mounting tabs of the LM317 & LM337 need to be isolated from the heat sink, on the LM78xx series they're at ground potential so they don't need any isolation.

15. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
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To heck with it, I didn't feel like working on anything else so I figured I'd go ahead and draw it up, besides it will make a good reference design for the future.

http://www.innoengr.com/images/general_projects/bench_regulator.jpg

You will notice a few things about the schematic that may appear a bit strange.

I'm showing grounds on both sides of the bypass caps because they should be connected with as short of traces as possible from the IC terminals to the ground.

C10 & 11 are in my opinion worth adding, they will greatly help to improve noise and filtering on those supplies. 10 uF is a standard used here but anything from 4.7 - 22 uF would be fine.

I also included a 12V fixed regulator, in my opinion you'll be unhappy if you don't have that output as well.
I did not include any reverse diodes across the ICs but you might consider adding them since simple 1N4000s are cheap. They're used to help protect the ICs should a filter cap prior to the IC shorts out, it's a rare occurence but there are so many caps involved I suppose it wouldn't hurt.

I prefer using the LM78xxACT versions for the fixed regulators as they're 2% and far better able to put up with accidental peak currents than most of the others, they're often even capable of 2A continuous output if you heat sink the heck out of them.

http://www.fairchildsemi.com/ds/LM/LM7805A.pdf

I showed the 7V one as you requested but 8V is the common value here. You can however find a few 7V regulators left if you look hard enough, (BA17807T for example) but it really was never used much at all.

If you need any help with component values don't hesitate to ask but I'd be sure to use good filtering on the initial supplies as well as decent sized electrolytics for C13, 15, 17, 19, 21 & 23. Pay the extra 5 or 10 cents to buy 105*C caps, it will pay off in the long run. Also be sure to use a bit higher voltage ones than you'd expect as in 50V ones for C1 & 2 then 35V ones for the rest of the adjustable sides, at least a 25V one for C3 & C22 and 16V ones will be fine for C17, 19 & 21.

As far as a fixed 2V output? You're going to have to use the variable again with a far lower voltage input or design something that comes off the 5V output that pulls 3V off of it such as a zener or a buck converter. Let me know how essential it is and if you mean 2.0V or 2.2V which is more common.

Last edited: Dec 27, 2010