What is the best way to get a 5V regulated power supply from a 12V transformer?

[Firestrider]

Hi, this might not be electrical engineering, but I found no other forum to question about this.

I'm trying to get a 5V regulated power supply from a 12V, 300mA transformer. I can't seem to get it to work. I'm using a 5V regulator, two 470 microfarad capacitors and electronic wire in a solderless breadboard. From the 12V transformer I measured 12.30 volts once it was plugged in. Probably it became more because I snipped off the connector and some copper wire. I plugged the negative lead of the transformer into the bottom strip of the breadboard, and the positive lead into the F4 socket. I'm trying to get the regulated 5V power on the top strip of the breadboard. This is my current configuration:

• J4,5,6 = 5V regulator chip (input, ground, output)
• I6 to top strip = wire
• F1 to E3 = wire
• A3 to bottom strip = wire
• A7 to bottom strip = wire
• E5 to F5 = wire
• E7 to F6 = 470uF capacitor
• G1 to G4 = 470uF capacitor
• F4 = positive 12V lead from transformer
• Bottom strip = negative 12V lead from transformer

When I put test leads into the bottom strip and any socket on row 4 I get 12.30V, but when I put test leads into the bottom strip and the top strip I get 10V which climbs steadily by hundredths. I'm not sure what that means, but I want a 5V regulated voltage on the top strip. Can you please help me on how to do this?

 

[berkeman]

What do you mean by the positive and negative leads of a transformer? A transformer by itself is an AC device. Does your transformer assembly include a full-wave bridge rectifier to give you pulsed DC?

Can you post a sketch of how you are hooking everything up?

 

[Cliff_J]

Put a 10k ohm resistor on the regulator from output to ground and measure the voltage across the resistor and see if it is indeed 5V.

 

[Firestrider]

Ya, the transformer takes the 110/120 VAC and regulates it down to 12VDC. I'll try to get a sketch. I'm trying to learn all the electronic stuff. Does negative and ground mean the same thing on a solderless breadboard?

http://img162.imageshack.us/img162/4706/circuit17ya.jpg

 

[berkeman]

Ya, the transformer takes the 110/120 VAC and regulates it down to 12VDC. I'll try to get a sketch. I'm trying to learn all the electronic stuff. Does negative and ground mean the same thing on a solderless breadboard?

What do you mean by 12Vdc out of the transformer? Does it say that? Does it contain an output diode bridge and smoothing capacitor? When you measure the DC and AC output components from the transformer, what do you measure for each?

As to the negative rail on the breadboard, the answer depends. If you have a single rail power supply, then yes, negative = ground. If you have split supplies, then you will have + something and - something, typically centered around the ground rail. Like, it's common to use +/-12Vdc for opamp circuits, with a center reference rail at ground.

 

[Firestrider]

This is exactly what it says on the transformer:
PLUG IN CLASS 2 TRANSFORMER
MODEL: HHD12-300
INPUT: AC120V 60Hz
OUTPUT: DC12V 300mA
POLARITY: - C +
DATE: 0506
MADE IN CHINA

Measured V DC: 12.29
Measured V AC: 35, 13, 5, infinity (alternating)

 

[berkeman]

Okay. The AC 35, 13, 5 numbers are probably in millivolts, so it looks like the wall transformer unit has a pretty steady DC output at no load.

So now all you need to do is connect your 5V regulator chip to the 12V input in order to make the 5V output. Are you using an LM340 (or LM7805 equivalent)? Which IC package is it in? Are you sure you are using the correct pinout for it? Where are you putting the 470uF cap?

 

[chroot]

He's using the word "transformer" improperly, using it to refer instead to an AC/DC adapter.

The real question, of course, is which specific 5V regulator are you using? There hundreds (if not thousands) of 5V regulators on the market.

- Warren

 

[Firestrider]

The 5V reg. chip is L7805CP, maybe I'm not using the correct "pinout"? Where would I find that? With the chip facing me I have the 12V input on the left, ground(neg) in the middle and 5V output on the right. I have one capacitor on the same 5-socket row as the 12V with the other end of the cap connected with ground, and the other cap connected to the same 5-socket row as the 5V with the other end of the cap connected with ground. The negative terminal on the caps are connected to ground.

 

[berkeman]

That's the correct pinout for the TO-220 package of the LM7805. But the pinout for the TO-92 version is opposite of that. You can find the datasheet for these regulators online many places. Even searching digikey.com can often get you to the part datasheet. The rest of your connections sound correct at this point. I agree with the earlier post about adding a little load to the 5V output. The 7805 regulators generally won't need it, but many voltage regulators have a minimum load current to keep their output in regulation.

 

[chroot]

I can't really follow your "connection list," either. From the looks of it, there's no path from, for example, J5, the regulator's ground, to the negative terminal of the AC/DC adapter.

Yes, "ground" usually refers to the negative terminal of an arbitrary power supply. All other voltages in the circuit are thus measured with respect to the negative terminal of the power supply.

You'll need to connect the supply across the input (input and ground terminals) of the regulator, in parallel with one capacitor. The output (output and ground terminals) of the regulator should also have a capacitor placed across them. The output of the regulator is measured at its output pin.

- Warren

 

[Firestrider]

Yes, there is a connection from the reg. to ground, I forgot to include that wire in my sketch. I knew I ordered 10 330-ohm resistors with 2 prongs, instead I got 10 100K-ohm resistors with 10 prongs, are they the same thing?

 

[berkeman]

By a 2-prong resistor, I assume you mean a traditional axial package? And by a 10-prong resistor, I assume you mean a SIP resistor array? Those arrays can either be 5 separate 2-connection resistors, or 9 resistors with one common connection. Look up the part number to figure out which resistor array configuration your SIPs are.

Looking at your sketch earlier, and adding a ground connection on the voltage regulator seems like it should work. At least if the LM7805 is in the TO-220 package. Does it still not work? If not, maybe clean up the sketch a bit, and add annotations for the voltage that you read at each point in the circuit with respect to the - (ground) line. That will help us to debug the problem.

 

[berkeman]

BTW, one "gotcha" that messes up every EE student at least once is the break in continuity in the outside bus bar nets in many prototype plug boards like the one you sketched. Typically, the continuity is only carried through half of the bus bar length down the edges of the proto plug board, and you have to bridge the break if you want your bussing to continue for the full length of the proto board. Maybe beep out the continuity of your proto plug board to understand what sets of holes are actually connected to each other...

 

[chroot]

I have no idea what you're talking about with the resistors. Do you mean you have a "resistor pack," which has 5 resistors in one package?

- Warren

 

[mugsby]

a sip resistor package on a busbar breadboard?

 

[berkeman]

a sip resistor package on a busbar breadboard?

Well he called it a "solderless breadboard", which makes me think of the small 0.1" array plug boards. And SIP resistors will fit fine on those proto boards. But come to think of it, a TO-220 package LM7805 will not plug into that kind of small plugboard, at least not without damaging the proto board for future use. We need pictures!

 

[mugsby]

for some reason i was thinking of a smd resistor array :tongue2:, to-220 will fit, to-92 needs the leads bent.

 

[berkeman]

Yeah, but the TO-220 leads are pretty broad, so if you stick them in the wrong way, they bend the spring contacts out enough that the contacts won't be tight for normal-size leads from then on. It might be possible to jam in the TO-220 leads the other way, so that the flat part of the leads lines up with the slot in the spring fingers, but that might also glob up the plastic of the proto board (I'm not sure). Worst case, the OP can solder short wire leads on the TO-220 leads to help him plug the voltage regulator into the plug board.

 

[Firestrider]

Well I got it to work, it reads 5V now, but I think I burned out an LED in the process :). My meter read 10m-ohm resistance for the LED, and 0 for the resistance between the LED and resistor. The part number for the resistor is 30867 at jameco, but I get 100K-ohm SIP arrays instead of those single 330-ohm ones, and I cannot find the pinout. My temp. probe read 40degC for both the regulator and the LED, is that normal?

Also, how hard is it to make an ALU with TTL chips?

EDIT: I measured the resistance for one of the 5 resistors on the SIP and it measured 100.5K-ohm. This disappoints me because that is not what I ordered, and they put the part number of the 330-ohm on the bag of the SIPs

The breadboard I have is P/N 20722 at jameco, and the 7805 fits fine.

 

[berkeman]

Remember that the LED is directional. The cathode (the flatter side) goes toward the negative, and the anode (the longer of the two leads) goes to the positive. You should be able to use the diode-check setting of your DVM to get about 2V forward voltage in the anode-to-cathode direction, and open circuit in the opposite direction.

40C is reasonable for the regulator, depending on the current going through it. The LED is plastic, and will generally not get warm unless it is on real bright for a while.

As for making an ALU with TTL chips, I did that way back in undergrad. It was about a month-long project, with the first half drawing the schematic, and the 2nd half building the ALU in the lab on breadboards. Pretty fun project!

 

[Firestrider]

Ok, I think I burned out one LED because the anode-to-cathode voltage read 5V. My other LED read 1.6V from anode-to-cathode, and 5V from cathode-to-anode, or reversed leads, is that normal? The one that read 1.6V didn't light up. And my diode test seems to not work since I get infinity readings every time.

Did you use any programs to draw the schematic using breadboards back when you were an undergrad?

If I may ask, what degree and job do you have? I want to major in electrical engineering that designs mostly microprocessors and circuits on motherboards for computers, but I like the math part just as much.

 

[chroot]

You always need to use a current-limiting resistor with an LED, because it's forward-bias resistance is almost zero. An LED, like any other diode, will act almost a short-circuit in the forward-bias direction. It'll burn itself up unless you limit its current.

Yes, you'll learn how to use some simulation products like Spice as an undergrad in EE.

If you're asking just berkeman about his education, I apologize, but I'll answer too just so you have another data point. I have a BSEE from Virginia Tech, am working on an MSEE at Stanford. I'm a mixed-signal IC designer for a company called Intersil.

As I've said in another thread, your undergrad EE curriculum is going to be pretty much laid out for you in advance. Also, note that designing a microprocessor and designing a motherboard are worlds apart. One is microarchitecture, the other is system-level design.

- Warren

 

[berkeman]

Ok, I think I burned out one LED because the anode-to-cathode voltage read 5V. My other LED read 1.6V from anode-to-cathode, and 5V from cathode-to-anode, or reversed leads, is that normal? The one that read 1.6V didn't light up. And my diode test seems to not work since I get infinity readings every time.

Did you use any programs to draw the schematic using breadboards back when you were an undergrad?

If I may ask, what degree and job do you have? I want to major in electrical engineering that designs mostly microprocessors and circuits on motherboards for computers, but I like the math part just as much.

If the diode test setting on your DVM doesn't give you a different reading for the forward and reverse directions of an LED, it may just be a limitation of the meter. Not all meters use a check voltage large enough to turn on an LED. Be sure that that feature works with a regular diode, though, or your meter is busted.

Schematics back in my undergrad days were hand-drawn. What a pain, but it was ancient times back in the late 70's. I have a BSEE/CS and an MSEE, and I do both analog and digital circuit design. I've pasted in a bio reply below from an old thread in the career guidance forums FYI.

Hey chroot! You're just up the road from me. How do you like Stanford's MSEE program? I used to spend a lot of time in the Physics library back in my consulting days. I still think about pursuing an MS in Applied Physics at Stanford some day...

I work at www.echelon.com. We were a startup about 16 years ago when I joined, and thankfully now we are public and doing well. As you can tell from our website, we invented LonWorks technology, which is basically a networking technology that is optomized for multidrop monitoring and control applications, as opposed to data transfer applications like point-to-point Ethernet. LonWorks networking technology has become the standard for many applications like building automation, factory automation, and energy monitoring (like with our Networked Energy Services electric meters).

I worked at Bell Labs, HP and on my own as a design consultant before joining Echelon, and over the years I've done lots of different kinds of design work. As I mentioned earlier in the thread, I think that engineers that can mix several EE disciplines have extra value to a company, because they can really tie together multiple parts of a design. Whether it's helping out in the architecture planning, or even just helping out other engineers to get their work done on schedule during crunch time, having skills in multiple practical areas really pays off. When we interview candidates for positions in our R&D Lab, we look for a mix of analog, digital, software and IC design skills. You don't have to be a hotshot in all categories (just a couple), but being able to work in some depth with all the different specialties is a big help to the team.

I picked up a lot of my RF and analog background mostly after I got out of college, and I wish that I'd taken more of those classes in school, as opposed to the digital theory classes. I'd also recommend that you take at least software classes in compilers, OSs, data structures and C++ if you can. You'd be surprised how much software you write, even as a hardware EE. Also, I'd recommend picking up a little extra probability math, and then taking an in-depth communication theory class or two. There is just so much going on in the communication field right now -- understanding the math behind communications is a very important foot in the door at many companies right now, especially some of the ones on the leading edge...

Also try to build practical projects on your own if you can. Maybe make a habit of building a kit project over each semester break, and a couple of them over the summers. Build more complicated projects on your own using microcontrollers as soon as you can, and get comfortable designing and building practical stuff with them. When you build real-world projects, it really helps you to start to understand what is important in designing and building stuff, and helps you to "learn to ask the right questions" of yourself and your teachers. Good luck! -Mike-

 

[Firestrider]

Will I need new lower resistance resistors in order for the LEDs to light and not burn out?

 

[berkeman]

LEDs usually run about 5-20mA and about 2V. So if you want to put 10mA through an LED from a 5V supply, the series resistor will be about 3V/10mA = 300 Ohms.

 

[Firestrider]

I mean is there anyway I can use the resistors I have now (100K-ohm) to make 330 ohm resistors, because I ordered 330-ohm, but I got these.

 

[berkeman]

Not easily. 100K/330 is about 300, so that's a lot of parallel resistors! Where are you located physically? There has to be somewhere you can get a simple resistor quickly...

 

[Firestrider]

egh. I guess I can order another pack of 10 for $5.70, but I don't trust jameco.com. Is digikey more reliable?

Also I downloaded a demo of SPICE called TopSPICE, is there a breadboard template or layout?

 

[berkeman]

Yeah, Digikey is great, although I've never had problems with Jameco either. Keep in mind that they both will have a minimum order of like $20. If you're going to be doing a lot of prototyping, you might want to order an assortment if they have them.