Variable Power Supply

[shellcode]

power supply l200

I'm looking to build a variable power supply for my second electronics project (the first being the transistor tester I inquired about here a few weeks ago). I'd like a few recommendations on what I should build.

So far I'm thinking that a 3-30 volt design (of which there seem to be many) would work whether I am doing something that involves smaller electronics (ie. microcontrollers) or something more electromechanical (ie. relays and motors). Is this a good idea?

I also think that a power supply should be able to vary current output as well as voltage (again, so that it can be used in various types of projects). Again, correct me if this is wrong.

It would be nice if the power supply could have multiple outputs, but I am a beginner and I would rather the design not be too complex.

Finally, there is the question of regulated or unregulated. Is a regulated power supply really necessary with most projects? If so, how much more complicated is it to build? Again, I am just starting with electronics.

Thanks in advance for the tips.

 

[IanP]

difference between lm117 and lm317

If you want to "play" with electronics then a regulated power supply is a "must".
If you can adjust voltage (3-30V is a handy range) and adjust the current (0-2A is also a handy range) then in many cases you will be safe trying new circuits or testing components.
The simplest adjustable regulator is based on very popular voltage regulator, the LM317, and you can find circuits in it's data sheet. Because it is really very simple circuit you can start with this one and then build a laboratory power supply like for example this one:
http://home.planet.nl/~heuve345/electronics/course/lesson10.html

Other alternative to the 317 device is to employ the L200 voltage/current regulator:
http://www.st.com/stonline/books/pdf/docs/1678.pdf , but steel keep in mind the laboratory option.

Also very handy is to have fixed voltages such as +5V and +12V and -5 and -12, and you can do this by adding 7805 and 7812 for (+) voltages and 7905 and 7912 for (-) voltages voltage regulators.

And to make you confused even more, have a look at some switch mode voltage regulators. Their advantage is efficiency: just imagine a linear regulator fed of 35Vdc and the output is at the lowest end, say, 3Vdc, and you draw the maximum current of 2A. A lot of power is lost and converted into heat, so one has to ensure that the components are mounted on a sufficient heatsinks..
Here is the link where you can find the switching regulators:
**broken link removed**
(see in particularl LM2576ADJ)

So, if you build adjustable laboratory voltage regulator with additional 4 fixed voltages (+/-5V and +/-12V) yopu will be coverer for power needs in hobby electronics for years to come..
Good luck and regards,
IanP

 

[shellcode]

l200 power supply

Thanks for the reply.

That first link is far too complicated. There's no way I could build that (now). However, I did look up the LM317 regulator. A lot of feasible schematics exist that utilize the LM317 (ie. **broken link removed**). That one says that the voltage is 1.2-30V and the "maximum" current is 1.5 amps. Does this mean that I can adjust the current from 0-1.5 amps?

 

[IanP]

l200 voltage regulator

In 317 voltage regulator current is internally limited to 1.5A, and you can't chenge it. But (check out this application note: **broken link removed** ) you will find voltage/current regulator based on 2 x 317 regulators. You can try this way, or, what I also indicated, have a look at L200 voltage/current regulator..
Regards,
IanP

 

[shellcode]

psu l200

In 317 voltage regulator current is internally limited to 1.5A, and you can't chenge it.

But couldn't I just put a potentiometer on the output of the power supply to vary the current between 0 and 1.5 amps?

But (check out this application note: h**p://www.national.com/an/AN/AN-178.pdf ) you will find voltage/current regulator based on 2 x 317 regulators.

This document only mentions the LM117 not the LM317. Is that what you meant? What is the difference between the two?

You can try this way, or, what I also indicated, have a look at L200 voltage/current regulator..

I didn't find any good circuits for the L200. Do you have a link to some?

Thanks.

 

[IanP]

lm317 current boost

Using a potentiometer to adjust the current will also affect the voltage and you want the voltage to be constant till the current riches its limit value..

117 and 317 are in fact the same devices. The difference is in temperature range, and 117 is rated for -55°C to +150°C and 317 from 0°C to +125°C..

For L200 try thes links:
**broken link removed**
**broken link removed**
and some more here:
**broken link removed**

Regards,
IanP

 

[shellcode]

lm117 lm317 difference

Do you recommend that I go with the 2xLM317 solution or that I use an L200 regulator?

Using a potentiometer to adjust the current will also affect the voltage and you want the voltage to be constant till the current riches its limit value..

Of course. That should have been obvious.

For L200 try thes links:
h**p://www.zen22142.zen.co.uk/Circuits/Power/l200.html
h**p://www.st.com/stonline/books/pdf/docs/1678.pdf

For that first link, will the circuit need to be modified to be used with US 120V/60Hz mains power? Obviously I would use the appropriate transformer and bridge rectifier but will any of the values of the components need to be modified? Also, where would I add an ammeter and voltmeter to that circuit?

Finally, a general question. What is the purpose of a negative voltage power supply. My understanding was that the polarity of a voltage is dependant on the connection between the power source and the circuit. For example, a battery connected to a diode would only allow electrons to flow with one polarity and not the other. What, then, does a negative voltage power supply accomplish that could not be accomplished simply by inverting the connection to the positive and negative terminals of a battery or power supply?

 

[IanP]

lm317 an178

I think L200 is more universal option.
Try circuit 2 from this link: **broken link removed**

These circuit don't need to be modified for 120/230V 50/60Hz. If you ensure that the input voltage after rectifier bridge and smoothing capacitor will not be higher than the max input voltage of the L200, which is 40Vdc, you will be just fine.

Negative power supply is often used as +/- simetrical supply by opamps and in general in ac circuits where you want to keep ac signal in both directions without using capacitors to "cut off" the dc component.
Good example will be here an audio power amplifier supplied of +/-XX volts and the speaker is directly connected between the output and 0V..without need for big capacitor to remove the dc component..if the same amplifier were supplied only of single voltage..

Regards,
IanP

 

[shellcode]

l200 current adjust

I think L200 is more universal option.
Try circuit 2 from this link: h**p://eu.st.com/stonline/books/pdf/docs/1318.pdf

By "circuit 2" do you mean figure 2 in that PDF, the programmable current regulator? How exactly is it programmable? Also, I'm not sure how exactly these circuits will work. What I envisioned is a power supply with two knobs. One to control voltage and the other to control current. That way I can get any combination of (approximately) 0-30V and 0-2 amps. In other words, voltage and current can be controlled independently. In Figure 2 there are no potentiometers used at all. In the schematic at **broken link removed** there is only one potentiometer. How would the voltage and current be controlled if these schematics were built?

 

[IanP]

how to make a variable power supply

It looks like that was not the correct link ..
So below is the drawing .. with a diagram where you have 2 potentiometers: one to adjust voltage and one to adjust current.
Both adjustments are independent .. as you wished ..
Regards,
IanP

 

[bamdad]

power supply l200

If you need to boost the current limit of, say, LM317, or any other regulator, you can use the current boost architecture to pass the higher current levels from some bypass transistors. things like 7A@5V are easily realizable with two BD680s.

 

[shellcode]

variable power supply 0-30v diagram

Is Vi the input positive voltage and Vo the output positive voltage from the bridge rectifier. What happens with the negative?

Also, what is the thick black line that the capacitor and pin 4 of the LM741 connected to? Is that ground?

 

[IanP]

regulated power supply with l200

Attached you will find a circuit that incorporates L200+741. Tis circuit will show you connections to transformer+bridge. As the smoothing capacitor use at least 2200µF electrolytic capacitor, rated for 40V (or more), and add these 2 capacitors to the output: 10µV/35V tantalum and 100nF ceramic.
These thick lines can be connected together and treated as GND(0V), or as stated before, point A can be connected to small negative voltage if you wish to have the output from 0V, and not from ≈3V..
Regards,
IanP

 

[Ante]

power supply for l200

Hi shellcode,

If you follow this thread you will learn a lot: https://www.electronics-lab.com/forum/index.php?topic=196.0

I hope it helps!

 

[shellcode]

difference between lm117 & lm317

That seems like a nice thread. However, since I don't really need it to output 5A or more I could just go with the original design. I'm guessing that the original is https://www.electronics-lab.com/projects/power/003/index.html.

There are two problems however.

First, they say: "...is the possibility to preset the maximum output current which can be drawn from the p.s.u., effectively converting it from a constant voltage source to a constant current one...". I'm not sure what that means. Does that mean I can independently adjust current and voltage? It sure doesn't sound like it. I don't want current being constant and voltage going crazy or vice versa. I'd like to make my voltage/current adjustment and have the power supply work at those levels.

Also, that looks really complicated. This is only my second circuit ever.

 

[IanP]

current boosting circuit for 7905

Let's assume, for the sake of argument, that the voltage is adjusted to 15V and current limit is set to 1A. With small loads the voltage remains at 15V level. When you reach the current limit value by connecting >15Ω resistor (in this case) the voltage starts to drop as the current regulator takes control over voltage regulator. So if from that point you go to short circuit the current still stays at the 1A level but voltage (at short circuit) will be 0V.
It looks like as the load increases you first "travel" on costant voltage line and when you reach the current limit you travel on constant current line.
Normally, you will adjust certain current limit but your circuit under test will never reach this level so the power supply will look like a voltage regulator supplying voltage at la evel you set, and you will be not even thinking abot the current limit..
On the other hand, current limit is handy if you don't want to much "fireworks" on you banch incase you make a mistake in a circuit..

If you think a circuit is to complicted build somthing that you are familiar with..

Regards,
IanP

PS. For some reasons I could't open that link from your last post..it timed out..

 

[mannyphil]

lm741 based regulated variable power supply

i'm mostly interested in the current regulation. for the voltage I can handle. can someone give me more tips or sites on current regulations?

 

[rauol]

how does a variable power supply work

You can use the Standard 723 Ic to build a linear regulator , the current limit also can be changed by using one external opamp.

 

[Ante]

what is difference between lm317 and lm117

Hi shellcode,

Yes, you can independently adjust current and voltage on this PSU! You can preset whichever value for voltage and current separately and they will not go crazy on you!