Questions about my AC-DC power supply project - Page 1
Forum section: electronics / beginners

author: DaMaDo
author_rank: Contributor
timestamp: January 20, 2012, 12:22:01 am
content: I've been following the instructions from Electricity and Electronics by Randy Slone (2000 print) and was curious about my O-scope reading from the AC section....especially from the rectifier. Does that look right?I wanted to see the scope's readings before and after connecting the capacitors (and the effects of the bridge rectifier) so I can better understand the current each step of the way.The scope's ground plugs were attached to "circuit common" which is the topmost orange wire between the top two transformer secondaries. Primaries are in parallel, secondaries are in series.Here's a picture of the AC part (still considering how to solder/connect the caps)Here's the scope's take on the AC secondary output (seems right to me, although not sure why it's a bit jaggedy):Here's what it says about the output from the rectifier (this looks weird to me, was expecting a camel hump looking thing):This is going to be the regulating part (which has a pot for positive and a pot for negative which I don't understand why yet...I'm used to just one for "volts"):

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 12:31:57 am
content: Try attaching the scope ground to the black wire and the probe to the red wire coming from the rectifier in your picture. You should see a clean rectified sine wave (a series of humps).

author: DaMaDo
author_rank: Contributor
timestamp: January 20, 2012, 12:41:35 am
content: Hmm, connecting the probe's ground to the rectifier negative and the probe to the rectifier positive I get:

author: Psi
author_rank: Super Contributor
timestamp: January 20, 2012, 12:52:52 am
content: Can we see a pic of the rectifier output with the scope set to DC, so we can see the noise and the DC offset together.Unrelated safety question - have you checked if your ground terminal is actually connecting to the case? The paint maybe blocking the connection.

author: DaMaDo
author_rank: Contributor
timestamp: January 20, 2012, 01:16:15 am
content: Sure, all set to DC gives this:Earth ground registers continuity with all the bolts so it is going through the case

author: amspire
author_rank: Super Contributor
timestamp: January 20, 2012, 01:17:53 am
content: You need some kind of load on the rectifier output. Attach a 1K or 10K resistor from the red to the black lead and you should get the proper wavefoms.The rectifier junctions have a capacitance of something like 300pF. Your scope is set to AC coupling, and so it may be you are connecting to the wires to the wires with no DC load for the diodes at all. So you may be seeing the rectifier act more like a bunch of capacitors then a rectifier.

author: DaMaDo
author_rank: Contributor
timestamp: January 20, 2012, 01:51:16 am
content: Thanks amspire, I think this is more like it. Although I dunno what I did when I first tried to connect it but i learned my fuse worksTransformer buzzing is apparently a bad sign.Anyway, once I fixed that here's the result grounded to common:and grounded to the negative wire:

author: amspire
author_rank: Super Contributor
timestamp: January 20, 2012, 02:06:26 am
content: That looks much better. The distortion at the top of the waveform could be distortions in your mains supply, or a combination of some slight saturation in the transformer core (because the manufacturers don't want to use any more steel then they can get away with) along with some leakage inductance in the transformer.Now just a warning, be careful where you connect the scope ground lead to, especially when you start to construct the supply. If something in the power supply gets connected to ground, that is the only ground point you can clip the scope leads to.It is not a bad idea to learn how to use the two scope inputs in differential mode (Ch1 minus Ch2) so you can see waveform across components that you cannot clip the earth leads to.Don't ever clip the two ground leads of the scope to two different parts of the circuit at the same time, as they will just act as a dead short. You might get a lot more then a buzzing transformer.

author: DaMaDo
author_rank: Contributor
timestamp: January 20, 2012, 02:14:01 am
content: Thanks again, I did not know any of that about the scope grounds.What do you mean by "If something in the power supply gets connected to ground, that is the only ground point you can clip the scope leads to."?The "floating circuit common" according to the book taps out of the top secondaries in the picture...so that's the only point I can connect the probe ground to? Is it bad to connect it to the rectifier negative?I'll look up differential mode. That sounds interesting.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 02:24:27 am
content: Quote from: DaMaDo on January 20, 2012, 02:14:01 amGround is not floating. It is anchored to the earth as if by a big heavy weight. Richard doesn't mean any common "ground" point in the circuit, he means the real mains protective earth found on the third pin of the mains plug. The ground leads of the scope probes are both connected directly to that.So if any point of your circuit is connected to mains earth/ground, then that is the only safe place to connect the scope ground.(Note: in a lab power supply it is not usual to connect any part of the circuit to mains earth, since the power supply is much more useful if the output is completely isolated from any other circuits.)

author: DaMaDo
author_rank: Contributor
timestamp: January 20, 2012, 02:39:31 am
content: I guess this book is confusing me.It says in one page, "Do not connect circuit common to chassis ground."Then in another, "Remember that you have a "floating" circuit common that is the power supply ground, but not connected to the chassis ground."How do I interpret that?

author: amspire
author_rank: Super Contributor
timestamp: January 20, 2012, 02:50:42 am
content: Ian, thanks for the clearer explanation about the ground.The scope ground is directly connected to mains earth.Not knowing your circuit, I have no idea if it will have a direct connection to mains ground/earth or not, but Ian is absolutely correct - it is normal for a lab supply to be completely floating with respect to mains ground/earth and that means you can connect the oscilloscope ground lead to any part of the power supply circuit.It is easy to make a mistake - connect the scope earth to a point in the power supply circuit when the power supply output is connected to a load that is also connected to mains earth and you have a problem again.Mistakes are not good for the circuit you are testing, but most of the time, you blow parts on the bench and replace them. It is all part of the fun. You do though want to really look after your scope, and accidentally shorting a transformer output through the scope is just not a good thing to do. If you are not sure, connect the scope ground to the circuit via a 100 ohm resistor, and if it becomes a LER (light emitting resistor), then something is wrong.

author: BravoV
author_rank: Super Contributor
timestamp: January 20, 2012, 03:09:03 am
content: Quote from: amspire on January 20, 2012, 02:50:42 amLOL .. dunno why I found this "LER" sounds so funny.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 03:14:57 am
content: Quote from: DaMaDo on January 20, 2012, 02:39:31 amOK. In your picture showing the transformers, there is a green and yellow wire from the mains socket bolted to the case (the chassis). That is the chassis ground, also known as the mains protective earth. It is designed so that the circuit breaker will trip or a fuse will blow if the chassis ever gets a mains voltage on it from a fault, thus saving you from the possibility of electrocution.There is also a short orange wire connecting the secondaries of the two transformers. This is your floating "circuit common". It can also be called the "power supply ground", which is confusing. It is really the 0 V rail between the +ve and -ve outputs of the power supply. You must never connect this 0 V rail to the metal case of the power supply. It must always remain floating and isolated on the secondary side of the transformers.That is what the book is telling you.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 03:18:26 am
content: Quote from: BravoV on January 20, 2012, 03:09:03 amDoesn't seem to work for me. Usually I get SERs, being smoke emitting resistors, or smell emitting resistors. Usually it is the smell of burning that alerts me to the smoke

author: DaMaDo
author_rank: Contributor
timestamp: January 20, 2012, 03:32:08 am
content: Well back when my fuse blew, since it's a slow burning one, I got a nice light showThanks for explaining the difference between my circuit common and earth ground. I think I'm starting to get it, although my two questions below might disprove that.So is it correct to say that the circuit common coming from the secondaries is the "standard"(0V) by which anything less is considered negative and anything greater is considered positive? And it can't be connected to earth ground because earth ground is as 'low as it gets' so anything compared to earth ground is positive?

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 03:37:43 am
content: "So is it correct to say that the circuit common coming from the secondaries is the "standard"(0V) by which anything lesson the same circuitis considered negative and anything greateron the same circuitis considered positive?(Yes.)And it can't be connected to earth groundbecause earth ground is as 'low as it gets' so anything compared to earth ground is positive?because you don't want any connection at all between the power supply circuit and any other circuits including mains earth."The power supply output must be isolated so that current will only flow where you want it to flow and current will not flow where you don't want it to flow. Current flowing where it is not wanted is usually bad, and may cause bangs, pops, smoke, or other undesirable things.

author: amspire
author_rank: Super Contributor
timestamp: January 20, 2012, 03:51:15 am
content: Quote from: DaMaDo on January 20, 2012, 03:32:08 amNo, that is not correct. Nothing is "as low as it gets". The significance of mains earth is that every connection to mains earth in your house is shorted together. Your scope ground is connected to mains earth, and your power supply chassis and earth terminal will be connected to mains earth. If you manage to connect both sides of a transformer output to mains earth, by connecting one side to the chassis, and the other side to the ground clip from your scope, then you have just shorted your transformer out through the interconnected earth wiring of your house wiring.That is all.

author: DaMaDo
author_rank: Contributor
timestamp: January 20, 2012, 04:00:06 am
content: That's excellent guys, thank you!

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2012, 05:50:51 am
content: I was curious about the effect of an unloaded rectifier in my own power supply experiments, so here are the results I got for comparison.First, the unrectified AC:Then the DC output from a bridge rectifier with no load on it:The DC output after adding a load:And just for good measure, the loaded output with a smoothing capacitor added:The no load result from the rectifier is amusing. I wonder if it would be possible to come up with a physical explanation for that result? (I have not pondered it too much, but I might dwell on it before I go to sleep.)