tag:blogger.com,1999:blog-7935874269355433120.post5109534190978583647..comments2022-11-30T03:54:00.412-05:00Comments on by: Brandon, the Random Man: Project Clean Up Part 1 Chapter 2GB Hoythttp://www.blogger.com/profile/01305484602935707261noreply@blogger.comBlogger7125tag:blogger.com,1999:blog-7935874269355433120.post-10059942040030640312012-09-25T21:32:08.911-04:002012-09-25T21:32:08.911-04:00Dave wanted me to post this, it's good to take...Dave wanted me to post this, it's good to take into consideration when building power supplies:<br />"Oh, it will work all right. But read my comment again and look at the Wikipedia article you referenced, specifically the second of the diagrams, where the upper AC wire is negative and the lower positive (happens for 1/120 of a second, 60 times a second, in North America). If your line voltage is 120 V rms, during that half cycle the blue wire will swing between 0 V and about -169.7 V relative to ground. And during that half cycle the blue diode will be conducting and will make the negative output terminal (which is connected to your rig's chassis) the same voltage as the blue wire (except for a fraction of a volt drop inside the conducting diode). That will kill you, good and dead, if you're grounded but touching the chassis during that half cycle. The fact that it's negative, not postive, doesn't help at all. (Well, it might, if you had a diode inside your body between your hand and your brain or your heart, connected with the proper polarity so as to conduct only when your hand was positive!) Remember those half cycles occur 60 times per second and last for 1/120 of a second, each.<br /><br />To be safe your chassis MUST be grounded to a good earth ground. And it can't be both grounded and connected to the negative output terminal of the circuit in the Wikipedia article, or your house fuses or circuit breakers are going to blow. But suppose you do ground the chassis, use the full wave bridge rectifier circuit, and then plug the thing in. If the first half cycle after you plug it in happens to have the upper AC wire go positive, then the fuse or circuit breaker won't blow until the next half cycle comes along, 1/120 of a cycle later.<br /><br />All this assumes that your AC wire is connected in such a way that the upper wire in the diagram is the hot side and the lower one is the cold side. But if it's the other way around then read my argument above again, but this time use the first diagram. Your chassis will still go negative varying between 0 V and -169.7 V relative to ground (less the drop in the diode of a fraction of a volt), but it will do so during those half cycles where the lower AC wire goes negative, not the upper one.<br /><br />So this circuit is guaranteed dangerous, whichever way you plug it in. The original way is NOT dangerous, provided your chassis is really grounded. But the original circuit simply won't work 50% of the time as you found out (when the plug is in the wrong way around). The full wave circuit will work 100% of the time whichever way the plug is plugged in, and will also be dangerous 100% of the time (ignoring the opposite half cycles from the ones I'm worried about, so I guess in that sense it's only dangerous for 50% of the time, 1/120 of a second at a time and as I said it won't kill you for that first 1/120 of a second).<br />" <br />From Dave, ex-W8EZEGB Hoythttps://www.blogger.com/profile/01305484602935707261noreply@blogger.comtag:blogger.com,1999:blog-7935874269355433120.post-50705317183349705212012-09-20T08:24:30.952-04:002012-09-20T08:24:30.952-04:00Wait, what? This is the circuit I'm talking ab...Wait, what? This is the circuit I'm talking about: http://en.wikipedia.org/wiki/Diode_bridge<br />I see this circuit as keeping the positive side positive, and the negative side negative no matter which side of the AC side is 'hot'. <br />although this brings up something I've always wondered about, if the return line and the ground are connected at some point in time (Like at the supply box), shouldn't there always be a potential difference between ground and hot line?GB Hoythttps://www.blogger.com/profile/01305484602935707261noreply@blogger.comtag:blogger.com,1999:blog-7935874269355433120.post-49384908125398399062012-09-20T03:11:28.241-04:002012-09-20T03:11:28.241-04:00You wrote: "Another way to solve this particu...You wrote: "Another way to solve this particular problem is to use a full wave bridge rectifier. Then polarity becomes moot as both cycles transmit power."<br /><br />Draw the circuit diagram, and consider the waveform on the hot side of the AC line (relative to ground). On negative half cycles it will be connected (through a conducting diode) to the chassis. So you'll only electrocute yourself on the negative half cycles. On the positive half cycles the chassis will be a diode's voltage drop above ground potential so you'll live through those half cycles.<br /><br />Don't do it!!!David, ex-W8EZEnoreply@blogger.comtag:blogger.com,1999:blog-7935874269355433120.post-11349931887555856332012-09-18T14:53:24.025-04:002012-09-18T14:53:24.025-04:00To be fair, Mike originally stated that if you wer...To be fair, Mike originally stated that if you weren't going to use a three pronged plug, you should use an isolation transformer. Another way to solve this particular problem is to use a full wave bridge rectifier. Then polarity becomes moot as both cycles transmit power. Thanks for the safety reminder. I will be getting another post of this soon, showing the transmitter in action. Then it will be on to an accompanying receiver, I'm thinking something tubed and regen although the Desert Ratt is tempting. Am taking suggestions.GB Hoythttps://www.blogger.com/profile/01305484602935707261noreply@blogger.comtag:blogger.com,1999:blog-7935874269355433120.post-32911501912161550322012-09-18T13:54:24.965-04:002012-09-18T13:54:24.965-04:00I never realized that the Twin-Tube 80 used a tran...I never realized that the Twin-Tube 80 used a transformer-less, AKA "line operated" power supply for the B+. Not a problem provided you take precautions. -Never- assume that the A.C. line polarity is correct. -Always- check between the transmitter chassis and earth ground to ensure the chassis is not 'hot'. Best of luck with the project and 73.......Steve Smith WB6TNL "Snort Rosin"Steve "Snort Rosin" Smith WB6TNLhttps://www.blogger.com/profile/01570621886789949035noreply@blogger.comtag:blogger.com,1999:blog-7935874269355433120.post-6312908147056342352012-09-18T06:40:57.191-04:002012-09-18T06:40:57.191-04:00Heh, don't think that didn't cross my mind...Heh, don't think that didn't cross my mind! ;-)GB Hoythttps://www.blogger.com/profile/01305484602935707261noreply@blogger.comtag:blogger.com,1999:blog-7935874269355433120.post-78941201077084407982012-09-18T01:49:10.835-04:002012-09-18T01:49:10.835-04:00So paint the white wire black, and the black wire ...So paint the white wire black, and the black wire white ;-) .David, ex-W8EZEnoreply@blogger.com