# 13.5 volt current limited power supply

Discussion in 'The Projects Forum' started by BrainFog, Feb 7, 2011.

1. ### BrainFog Thread Starter Member

Jan 24, 2011
122
4
I have recently used building a desulfator as an excuse to really get into building circuits and electronics. I have learned quite a bit from my desulfator thread but there is still much to learn. It is mostly filled with my countless questions: http://forum.allaboutcircuits.com/showthread.php?t=49043

In order to prevent the batteries from going flat and essentially killing them as they are being desulfated I will need to constantly charge them at a low current.

The plan to to build what I am sure most people on this forum have a solid understanding of. A power supply. As the batteries state that the optimal float charge is 13.5 volts that is what I am going for, preferable regulated. Seeing as desulfators work slowly and take little current themselves and that I do not want to damage the batteries it would also need to be current limited. I think that as little as 100ma would be enough but maybe I should go with 150ma or 200ma, any more and I suspect that all I would be doing is heating my batteries. Seeing as the battery desulfator gives off potentally damaging pulses I am also considering doing something to prevent them from flowing into my power supply, maybe some kind of inductor.

I have never designed a circuit before and am unsure how to go about it but I will give it a go. I will use those immortal words "what is the worst that can happen" as I play with electricity.

To start with I will need a transformer to lower the voltage from 220-240v. I know the regulating process reduces the voltage and other factors will need to be taken into account. What transformer do I need to buy/make to get the voltage that will eventually give me 13.5 volts?

I know that a simple rectifier will turn 50Hz AC into 100Hz DC which is then smoothed slightly by a capacitor, then smoothed again by other componants. This is where my knowledge gets sketchy. I have found basic regulated circuit schematics but I don't know what values the components require for 13.5 volts they all seem to be for 12 volts. What would you advise?

How would I current limit the circuit without compromising voltage?

Would inductors provide the necessary protection from the desulfator and if so where would it/they be best placed and what values?

I hope I have made sense and have explained everything clearly, if not give me a poke.

Thank you

2. ### wayneh Expert

Sep 9, 2010
12,361
3,220
Don't bother making one. You'll need one that outputs at least about 12v or so, up to ~16v, and rated for maybe 2X the most current you'll use. You can use a lower current rating, but it may reduce the lifespan. When fully rectified (to the peak AC voltage), the voltage output will be higher than the (typically RMS) rating. If you're buying, consider buying a modern transformer-less charger. They're much more efficient.
There are MANY strategies for battery charging, but for simple, low current as you've described, it's tough to beat the LM317 adjustable regulator. I believe there are circuits for it available here, all ready to go.

3. ### BrainFog Thread Starter Member

Jan 24, 2011
122
4
Being quite new to electronics I am not yet familiar with many of the terms what does 2X and RMS refer to?

The LM317 looks promising, I will have to look into it a bit more. same applies to zener diodes. I will get back to you on it.

As for transformers, they seem to be quite expensive for what they are unless I am looking in the wrong places. What I considered was making one like you make a toroidal inductor, they are almost identical is design. Seeing as I would need a low amps would this be at all possible?

Seeing as I wish to make a power supply rather than buy one and my voltage it not too common I guess I could try making a transformerless one. I was initally put off as when I first found out about them there were quite a few warning on the page about the risk of shocks and beginners should stay away. Were they exaggerating?

4. ### Adjuster Well-Known Member

Dec 26, 2010
2,147
300
2X = two times. RMS = Root Mean Square, denoting AC voltages or currents having the same heating power as equal DC values.

http://en.wikipedia.org/wiki/Root_mean_square

For sine-wave AC as used for the mains, the peak value is √2 times the RMS value.

I agree with the last poster: making your own mains transformers is not a good idea. Apart from the safety aspect, winding any mains transformer by hand is going to be pretty hard work. Toroidal types are actually more difficult to make.

Simple transformer-less power supplies are dangerous and should be avoided, because they do not isolate the output from the mains. On the other hand, modern switched-mode types often do achieve mains isolation. They don't use big heavy mains transformers, but often do use small ferrite-cored types working at higher frequencies. Again, these are not ideal home-build projects. The designs are fairly involved, and there is a fair amount of scope for going wrong with potentially serious results. They are reasonably cheap these days, so why not just buy one?

5. ### wayneh Expert

Sep 9, 2010
12,361
3,220
Sorry. 2X means two times and RMS means root-mean-square, which in turn means time-weighted-average, if that helps. It's just a measure of the average voltage of a rippling signal, such as alternating current coming out of the wall at 60 cycles per second. It's LESS than the peak voltage. It's what a good AC voltmeter will read.
Definitely worth having this in your mental toolbox.
Probably not. I haven't made one and I don't plan to. I wouldn't embark on this unless/until you're more advanced.

You can certainly make your own transformer, but you'll need a core and some copper wire. These aren't cheap either. Plus, you'll ultimately have to connect your device to mains power and I believe such projects aren't supported here for safety reasons. My advice is to keep looking for the right transformer or, better yet, switch-mode power supply (SMPS). They're not rare, and maybe someone here can guide you to the right source.

6. ### BrainFog Thread Starter Member

Jan 24, 2011
122
4
As much as I hate to admit it it looks like making one will not be as easy as I had hoped, a compromise is needed until I am more avanced. Nothing is as easy as we hope though. Still I would like a go at efficiently modifying voltage and creating a current limiting circuit.

A while ago during a project I asked around to see if anyone wouldn't mind giving me some unwanted power supplies. One that just came to mind was a smallish one that had clearly never been used and was fairly new. It claimed to produce 12v and 150ma, turned out the voltage was 14-15 volts. I used it for a while and seemed to work perfectly. Would this be the sort of power supply I could use as a base?

7. ### wayneh Expert

Sep 9, 2010
12,361
3,220
Maybe, but just barely. Was that an AC or DC output, and if it was AC, how was it measured? It makes sense that the peak voltage (the 14-15) would be higher than the voltage when measured at the rated load, which might have been as low as 12v at 150mA. If you want to charge a battery at 100mA and 13.5v, though, it might not cut it. Plus it leaves no room for a regulator such as the LM317, which will drop at least about 1.2v.

Something like 16v AC at 0.5-1A would be ideal, if a bit of overkill.

8. ### BrainFog Thread Starter Member

Jan 24, 2011
122
4
I will have a look around. That power supply is a bit puny isn't it. Still I have just found a way to modify a pc power supply to produce 13.5v, a slight overkill.

I was considering zener diodes as it would be a good intro to using them.

What would be the best way to current limit the thing at a set milliamp? It is not really as simple as adding a resistor is it?

Wow I am drained, I will return to this project after sleep.

Apr 27, 2007
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10. ### wayneh Expert

Sep 9, 2010
12,361
3,220
Not quite. You need active circuitry (resistors are passive) to limit voltage and/or current properly. A simple, constant voltage supply based on the LM317 will deliver increasingly smaller amounts of current as the set point voltage is approached. It may even be left connected, since the current vanishes at the set point. However it will try to deliver max current trying to charge a discharged battery, and you need to protect against that. That could be as simple as never using it that way. And there are circuit designs for it, including using other regulators. But the LM317 is widely available and very well known.

Careful with the PSU. They're great, but can behave oddly at low current. For instance, mine delivers ~16v until the load current comes up over ~100mA, and it then jumps into nice 12v regulation. And the 5V and 12V legs are not entirely independent - a load on the 5v side will affect regulation of the 12v.

11. ### BrainFog Thread Starter Member

Jan 24, 2011
122
4
I see, so basically the L200 and LM317 contain all the active circuitry designed to lower voltage and current in a small easy to program package.

I have managed to do a bit of rummaging and everything I have follows the trend of low voltage high current. Even my TV is 12 volts. The only candidate I have found is my 20 volt 2 amp laptop charger which is still used quite often, it is 2-3 years old. What is the technical name for the circular connector they use? I will need the female version of the connector as I want the charger to remain intact and in perfect working order. I am assuming that the laptop itself chooses how much current to drain from the charger and the charger will not choose to adjust the power in any way itself.

Do not worry I have no intention of using a computer psu, I know that they are designed for high current and are not to be messed with unless I know what I am doing.

Just a random thought, people often fear high voltage as it is what is dangerous; but is it not true that low voltage and high amps are far more likely to kill than high voltage and low amps?

For now I have a good amount of information to get through on L200s and LM317s as well as checking availability and price.

Thank you everyone for your help.

12. ### wayneh Expert

Sep 9, 2010
12,361
3,220
They vary, but power plug adapters are round. Radio Shack carries a number of them, and I'm sure you can find more. BUt anyway those round connectors are pretty much only used for power. Note that you need to specify both diameter and length.
Depends what you mean by "high" and "low", and chicken-or-egg problem. High voltage is normally required to get much current to flow through a human. Static is very high voltage (easily over 10,000v) but very low current and not terribly dangerous. A car battery is "low" voltage but capable of very high current. But you can grab both poles with little risk, though, because your skin is not very conductive and little current will flow. A 9v battery on your tongue is unpleasant, but not near lethal. These low voltages COULD be lethal if applied to a large enough portion of your body, to get a significant current to flow.

13. ### BrainFog Thread Starter Member

Jan 24, 2011
122
4
Hey

I have gotten lots of books out of the library on electronics and begun reading them. I wish that some would not assume certain knowledge; but I have learned a lot even from the confusing parts.

I have also looked into lm317 and lm200. lm317 seems to be more of a voltage regulator but can regulate current but in order to regulate them both I would require two. Still i can think of no end of uses for them so they are going on my "to buy" list as they are under £1 each. The lm200 seems to be exactly what I want; regulates both current and voltage. I have found them in a shop near college for £4 and an online parts supplier that I am about to buy quite a bit from sells them for under £2 each so no breaking the bank. http://uk.farnell.com/stmicroelectronics/l200cv/v-reg-adj-2-8-36v-pentawatt5-200/dp/1094204

My only worry is getting my calculations right for the resistors as I have not mastered the kind of maths needed yet.

Just to confirm the 0.22uf and 0.1uf capacitors must be ceramic?

On the schematic why does R2 have a line through it?

Thanks

14. ### Adjuster Well-Known Member

Dec 26, 2010
2,147
300
If you mean R2 as shown in Fig. 1 in the data-sheet, R2 is a variable resistor, and the line is actually an arrow indicating that it is adjustable. A similar symbol for a pre-set adjustable resistor shows a line more like a letter T drawn slanting across the symbol.

You would do well to study such symbols until you are thoroughly familiar with them. This link shows a few, er, variants on the variable resistor. http://www.learnabout-electronics.org/resistors_09.php

Last edited: Feb 12, 2011
15. ### BrainFog Thread Starter Member

Jan 24, 2011
122
4
Thanks, I admit I need greater familiarity with the symbols but there are so many important things to learn. I am spending a few hours every day learning a bit more about electronics to build up my knowledge.

All my searches for answers keep bringing up things like cars and mobile phone so just a few questions:

Turned out I was looking at the wrong schematic which is a bit embarrassing. I think I need to use Figure 27 on this PDF: http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000053.pdf Sadly my knowledge although growing is still quite weak when it comes to schematics. What are the best values for R1 and RL I read somewhere that sometimes they must be within a certain range? I assume the value of R2 depends on the value of R1 and standard diodes are needed of at least 13.5v. What kind of capacitor and how many farads should I be using, in this case is it simply the more the better?

Does anyone know any good webpages with calulators for working out resistors needed?