Power supply - Led Display

[Kane2oo2]

Hello
i have a VERY OLD meccano transformer/power supply from a hornby train set. It works and has a control knob to adjust the voltage. Has both AC/DC pins on.
This is the information it has printed on it:
220/250 volts 50/60 cycles 12 Watts

Alternative continuos ratings
A-12 volts D.C 8 watts
B-15 volts A.C 9 V.A

my problem is ... i have no way of keeping the voltage constant. So i was thinking of creating another circuit which i could connect to this one that had a regulator, which would need to be adjustable so i could have full control over the voltage and would need to range from 0 - 15v (DC only)

Another feature i would like to implement would be an LCD display or one of those LED displays to display the current voltage to a certain degree of accuray (poss 0.00) .

Will this be plausable if so what sort of circuits should i be searching for and also if someone could point me in the direction of some sites etc.

Thanks Alot - Kane

 

[Johnson777717]

Heya Kane,

For the variable regulation of the power supply, I would suggest using a LM317 IC. I just built one of these circuits and is working very well for me. The circuit is also very simple.

Check this site for the datasheet on the LM317. There are some suggested circuits within the datasheet that you can use.
**broken link removed**

Also, check out this link for some of the LM317 circuits, which are derived from the example circuits in the LM317 datasheet.
http://electronics-lab.com/index.html
go to "Projects" then to "Power" then select one of the LM317 power supply circuits. There's also included, a tutorial on working with the LM317 and such.

As far as displaying the output voltage on a LCD or LED, this circuit may get into the digital realm, which I am not very experienced in yet so hopefully someone can help you out with this. I do know of a meter which will display the voltage in a graphical type manner (LED bar graph display) which uses the LM3915 IC. This circuit wouldn't give you an actual number read-out, but would give you a bar-graph type readout.

 

[Kane2oo2]

right...

thanks for this info

i have a question about the attached circuit though, as i already have a power supply and just need something to "regulate/vary" the voltage what part of the circuit would i use?
would it just be from the part which i have marked with a dotted line?

or would i have to use the whole circuit and purchase the transformer etc?

Kane

 

[Johnson777717]

Heya Kane,

The two capacitors directly to the right of the dotted line are placed there due to the transformer power supply. The 2200uF capacitor is for ripple filtering and so that the voltage is supplied from the capacitor, rather than the bridge. This is setup to allow smoother voltage to the remaining regulator circuitry, as well as protect the bridge from overheating from "constant" use. In other terms, the 2200 uF capacitor is the the middle-man who takes the raw material from the transformer and refines it for use by the circuit. The 100nF capacitor is installed for a high frequency response, again for the supply voltage and additional smoothing of the voltage to the regulator circuit. Thus, the 100nF capacitor is the retail store which puts the finishing touches on the product and supplies to the customer, which is the LM317. A final note, these two capacitors are there to filter the input voltage from the transformer.

You may need to do a little investigation. If the power supply that you already have (The train power supply) is only a transformer and a bridge, you'll need to include the circuit as you have labeled with the dotted line.

If the train power supply already has some sort of filtering capacitors (If you see a large capacitor in the range of a couple thousand uF) then you should be okay with eliminating the 2200uF capacitor. In this case, I would leave the 100nF capacitor in the circuit (which is what the datasheet specifies for input voltage filtering) Thus, you're just eliminating the 2200uF cap.

 

[Johnson777717]

Another point Kane:
I used the exact same circuit diagram for my regulator circuit. Be advised that the numbers on the schematic don't correspond with the pin numbers for the devices.

For instance, in the schematic, the numbers surrounding the LM317 aren't defining the actual pin numbers. I think the numbers are there from some CAD/drawing program which automatically places the numbers at the connection points for the devices.

So...be sure to check for the proper pin outs for the LM317. By the way, the "IN", "ADJ" and "OUT" are correct. I just didn't want you to get confused with the numbers. I almost did.

 

[Johnson777717]

Final point, I swear

Also, you should know that the LM317 regulates the positive voltage only. If you want to regulate the negative voltage, you'll need to add a LM337 IC. See the below link for how to do this.
https://www.aaroncake.net/circuits/supply3.htm

Last, but not least, be advised that the LM317 will only handle up to 2 amps of current, so if you're planning to draw more than 2 amps, you'll need to go with something else. Also, the LM317 (and the LM337) will produce heat, for the amount of voltage and current that it is regulating.

For instance, if the input voltage is 30 volts at 1.5 amps and you've adjusted the output voltage to be 5 volts at 1.5 amps, the IC is going to generate a lot of heat due to the voltage that you are dropping. Thus, a proper heat sink is required for the LM317 and the LM337. Finally, I think the LM317T is designed with a thermal shutoff, which shuts itself off if it becomes too hot, or if it is supplied with excess voltage / current.


I think I'm done now.

 

[Kane2oo2]

almost there

right .... i think im almost their now

about the 2200uf capacitor ... the transformer is enclosed in a metal case type thing so i a unable to see the circuitry, so what should i do?
would it harm anything if i did put a 2200 in just incase?

also a question about the output voltage. In the schematic it shows an input voltage of 28V and and output of 1.2 - 25v
my input voltage will only be around 12-15V (measured straight from the terminals - even though on the casing it says only 12v)
so what could i expect for the output voltage?

and for the LED display i am going to try to build this circuit
https://www.electronics-lab.com/projects/test/014/index.html

**edit** - oh and one last thing if my input voltage is not going to be more than 20v is there another regulator that would be better suited (ie which would enable me to go from 0 to my max output?)

Thanks alot for your help
Kane

 

[Kane2oo2]

i think im jus about to answer my own question (lol)

right from what ive read the input voltage needs to be around 3v greater than max output voltage (so on mine if this is 15V my max output would be 12v?)

and i looked at other regulators on nation.com and the lowest value of Vout is 1.2 so i guess i will just use this one

Kane

 

[Kane2oo2]

one last question

is the 4k7 a variable resistor or potentiometer?
and how can you tell?

Kane

 

[Nigel Goodwin]

one last question

is the 4k7 a variable resistor or potentiometer?
and how can you tell?

Kane

It's a potentiometer wired as a variable resistor. Generally variable resistors aren't available, there's not much point when a potentiometer does the job by ignoring one pin.

 

[Johnson777717]

Thanks for the link on the LED display. I think I'll build this one too!

 

[Johnson777717]

FYI Regarding the LED display driver:

I'm having trouble finding the ICL7107 IC at a reasonable price.
Mouser wants $20 for one, Allied wants $17 for one. Digikey wont sell them unless you buy 30 at a time at $6 :shock:
Maxim sells them for around $4, but there's an 8 week lead time.

Some replacements are:
NJU9202 - which has been discontinued. Digikey will sell these singularly at around $5 but I can't find a datasheet to match the pins with the ICL7107 :? The NJU website doesn't have this IC datasheet because they've discontinued.

TSC7107- I can't find anyone who sells these.
NTE2050- Circuit Specialists doesn't carry these, and Mouser wants $20 for one.

I'd like to order from Digikey because I already have some other stuff that I want to order, but again, I can't find the datasheet on the NJU9202.

 

[Kane2oo2]

yeh thats a pain for me as well ...cant seem to find any

have tried looking for other digital voltmeter circuits but unable to find any

 

[Johnson777717]

Kane, I found a datasheet for the NJU9202 which is a listed replacement for the ICL7107. If you're interested, go to the "Datasheets / Parts Request" portion of this forum. You should be able to find the thread on the NJU9022.

Or, go to this link:
**broken link removed**

Thanks to ChrisP

 

[Kane2oo2]

thanks alot Johnson777717

however im not too hot on electronics. i can build circuits etc
but what i need to know is, does this NJU9202 jus go straight into the other circuit or are there some pins that need switching round?

ive searched google to try and find some already built circuits with this in but no luck

Kane

 

[Kane2oo2]

right ive had a look at the datasheet for each (im learning )

it seems the first 20 for each are the same?
but the NJ9202(BM) has 2 extra pins ...are these extra pins just ignored? and then the remaining pins need to be matched to that of the 7107?

Kane

 

[Johnson777717]

Good question. I'll research this and post the results.

 

[Johnson777717]

Heya Kane,

Looking at the datasheet for the NJU9202, the BM suffix (NJU9202"BM") seems to denote that a DMP42 package is used. The DMP-42 package seems to be a surface mount package, which is pretty difficult to solder. The two additional pins labeled NC may stand for "No Connection". I reason this because in the datasheet for the NJU9202, these "NC" pins aren't used in the suggested schematics. I'm assuming that the manufacturing process was somehow cheaper to build the BM suffix IC's this way? They probably set it up so that they wouldn't have to change a particular die, or something, from manufacturing one of their other IC's.

Because I won't be using the DMP-42 package (It's too hard to solder), but rather the DIP 40 package(NJU9202 BD), these NC pins aren't included and thus won't be an issue with creating the circuit.

Otherwise, the pins match exactly to that of the ICL7107. I'm going to check the datasheet against the components used in the circuit for the ICL7107 and see if there are any differences.