- Variac question - 12 Updates
- Tek 465 B Part Needed - 1 Update
| three_jeeps <jjhudak@gmail.com>: Nov 07 09:41AM -0800 On Wednesday, November 7, 2018 at 3:39:07 AM UTC-5, GS wrote: > designed to be 1:1 under full load. Depends on current either way you can > end up say 160 volts no load. > Greg I can't advise on the modification to connect the variac to earth ground. One would have to know the design of the variac to accomplish this safely, if indeed it is a good idea at all. IMHO, I would not modify the autotransformer. One need to be aware of the 'upstream' power distribution network. In the states, residential customers are fed by three wire split phase distribution transformer that sits on a power pole or power utility box. The 120 VAC power is between ground and one half of the phase winding. I would also place the isolation xfmr after the variac. Variac (short for variable autotransformer) is an interesting device in that there is no isolated secondary winding. A portion of the primary winding serves as a secondary. There are taps along the primary winding that allow for stepping up or down the output voltage. One can use a variac for: reforming power supply capacitors in equipment that has been sitting idle for a number of years, testing the tolerance of electronic equipment/designs to under/over voltage conditions, gradually increasing the voltage to equipment to avoid 'shock' of the components, dimming incandescent light bulbs, and testing dropout of electronic devices under low voltage conditions. We used variacs to lower the supply voltage to computers to prove that the machines we designed would function down to at least 80% of line voltage, and when a threshold was hit, the machine would gracefully shutdown in x milliseconds. It was a requirement of the system. Isolation transformer are used a lot in servicing certain type of electronic equipment (e.g. tube TVs, amplifier, radio transmitters, etc.). It removes the direct path to ground of the electrical power, significantly reducing the chance of death. Ask any service tech or engineer who has seen their scope probe or DMM probe almost vaporize when the accidentally probe a circuit that effectively causes a path to ground through the probe. good luck J |
| "pfjw@aol.com" <peterwieck33@gmail.com>: Nov 07 10:20AM -0800 RANT WARNING: Isolation transformers are critical for any workbench, and after good hand-tools the first thing any radio person should get... perhaps even before said tools. Variacs, on the other hand, are either very handy diagnostic tools or worse-than-useless anchors fit only to let the magic smoke out of an otherwise salvageable radio. I am not really sure if there is an in-between other than their original function as light dimmers. Without both current and voltage metering, they are worse-than- useless. With the proper metering, they can be very handy tools. Myths: A variac will help reform caps (with specific reference to tube/ valve radios). Fact: Only on those vanishingly few radios with solid-state rectifiers that pass B+ current at any voltage, and only if done over more time than one cares to consider. Bench DC supplies are best for this typically futile purpose in any case. In the case of a tube rectifier, most of them do not start to pass DC until the filament voltage reaches between 65% and 75% of nameplate rating. If the B+ on the particular radio is say.... 350V, that means that the first voltage the caps-to-be-reformed would see would be something between 227V and 263V... not exactly a soft-start. Myth: A variac will allow one to apply voltage gently to a radio, so as to discover problems before they become fatal. Fact: No, not at all. True, a variac does allow a radio to see the minimum amount of current to trigger its functions... an AA5 with bad filter caps may begin to hum at 80V rather than 120V, similarly with a transformer radio. But if the caps are not so bad as to show audible hum, the variac ceases to be useful. And a variac will *not* tell you whether a repaired radio is OK or not. However, if the proper meters are put on the Isovariac, then some real diagnosis can happen: An AA5 which should, by calculation draw say... 35 watts or so (0.3A), and draws 42 watts is dissipating 7 watts of heat somewhere... perhaps the output transformer. And, unless one has calibrated eyeballs, the difference of 7 watts will not show up on a dim-bulb tester. Comes down to a current meter with fine enough increments to give meaningful information. Such a tool is useful right down the line from initial diagnosis to testing the completed results. End Rant. And it could have been much longer. As to order-of-parts: I keep the Heathkit IP 5220 (this one: http://www.byan-roper.org/steve/steve-at-play/antique-electronics-and-2/heathkit-ip-5220-variable.html ) From what you are seeing, it is pretty clear that the Iso goes after the Variable in this design. Peter Wieck Melrose Park, PA |
| tabbypurr@gmail.com: Nov 07 11:02AM -0800 > purpose in any case. > In the case of a tube rectifier, most of them do not start to pass DC > until the filament voltage reaches between 65% and 75% of nameplate rating. If the B+ on the particular radio is say.... 350V, that means that the first voltage the caps-to-be-reformed would see would be something between 227V and 263V... not exactly a soft-start. Not so. The trick is to meter the B+ and wind the variac up uberslowly around the point when the rectifier begins to conduct. That way the rectifier passes only tiny current and you can indeed wind the B+ up from close to nothing. BTDT. Of course there may be easier ways now. It's true that their uses are very few nowadays. But marginally more than zero. NT |
| "pfjw@aol.com" <peterwieck33@gmail.com>: Nov 07 11:13AM -0800 > Not so. The trick is to meter the B+ and wind the variac up uberslowly around the point when the rectifier begins to conduct. That way the rectifier passes only tiny current and you can indeed wind the B+ up from close to nothing. BTDT. Of course there may be easier ways now. > It's true that their uses are very few nowadays. But marginally more than zero. Rectifiers are go/no-go devices. Variacs are not current-limiting devices. The voltage a (tube) rectifier passes will be a function of the minimum trigger voltage on the filament as a percentage of the "correct" filament voltage x the "correct" B+ voltage. So, if the rectifier filament trigger voltage is 4 V vs. the nominal 5 V (80%), then the passed voltage will be nominal B+ x 0.80. There is no 'winding' up. This is the same argument as "just a little bit pregnant". The logic is just as faulty. Peter Wieck Melrose Park, PA |
| Ralph Mowery <rmowery28146@earthlink.net>: Nov 07 02:35PM -0500 In article <34d141f2-f78f-4309-83a1-055dc1eb6e69@googlegroups.com>, peterwieck33@gmail.com says... > purpose in any case. > In the case of a tube rectifier, most of them do not start to pass DC > until the filament voltage reaches between 65% and 75% of nameplate rating. If the B+ on the particular radio is say.... 350V, that means that the first voltage the caps-to-be-reformed would see would be something between 227V and 263V... not exactly a soft-start. There is a 'more correct way" to do the reforming if one wants to try. If a tube rectifier you pull that tube or tubes and install solid state rectifiers. Then pull the other tubes to prevent cathode stripping. Most electrolytic capacitors are formed at the factory at about 10 % or so over the rated voltage. As they sit on the shelf they will degrade somewhat. It sometimes makes sense to reform even new ones if they have been on the shelf for many years. That is where a bench supply comes in handy. I wish I could find the magazine I had around 1965 give or take a few years. In it was an automatic restorer. I think it used a bunch of neon lights, but could be wrong. It may have been Electronics Illistrated. Don't think it was Popular Electronics. Maybe Radio Electronics. |
| tabbypurr@gmail.com: Nov 07 12:31PM -0800 > > Not so. The trick is to meter the B+ and wind the variac up uberslowly around the point when the rectifier begins to conduct. That way the rectifier passes only tiny current and you can indeed wind the B+ up from close to nothing. BTDT. Of course there may be easier ways now. > > It's true that their uses are very few nowadays. But marginally more than zero. > Rectifiers are go/no-go devices. valve rectifiers are not. The current they pass depends on filament temperature/voltage. > Variacs are not current-limiting devices. they are voltage controllers. The rectifier is a current controller when run undervolted. > The voltage a (tube) rectifier passes will be a function of the minimum trigger voltage on the filament as a percentage of the "correct" filament voltage x the "correct" B+ voltage. So, if the rectifier filament trigger voltage is 4 V vs. the nominal 5 V (80%), then the passed voltage will be nominal B+ x 0.80. There is no 'winding' up. I'm not at all clear about your argument, but they certainly do wind up on a variac. I've done it. > This is the same argument as "just a little bit pregnant". The logic is just as faulty. > Peter Wieck > Melrose Park, PA Try it some time. You'll find out what you got wrong. NT |
| Fox's Mercantile <jdangus@att.net>: Nov 07 04:57PM -0600 I have a large (20 amp) Sola harmonically balanced constant voltage transformer. I cut the neutral common and use a SPDT center off switch, so I can select Utility Neutral, floating (i.e. isolation) and Local Neutral (tied to the ground at the transformer.) Following that I have a 20 amp Variac, that I can wind up from 0-132 VAC. With a 2-5-10 selectable amp meter. Finally, a custom 4-terminal dynamic watt meter. 0-1500 watts. What do I use it for? Very rarely. The time I used it was to test the applied voltages on some 1947 to 1959 boat anchors to see "just how much over voltage" they were at 125 vac line vs 110 vac line voltages. Solution? Using a 5-10 amp filament transformer in the buck mode to drop the line voltage back down to 115 vac. Do I waste my time reforming capacitors? No. And certainly NOT in customer equipment. Saving $5-20 on a repair is NOT worth a warranty repair. Sure, I'll fix it for free, but my reputation takes a hit. "Why didn't you fix it right to begin with?" Apparently, "because I was lazy" is not an appropriate answer. -- "I am a river to my people." Jeff-1.0 WA6FWi http:foxsmercantile.com |
| Phil Allison <pallison49@gmail.com>: Nov 07 06:20PM -0800 three_jeeps wrote: > Isolation transformer are used a lot in servicing certain type of > electronic equipment (e.g. tube TVs, amplifier, radio transmitters, etc.). ** But only those examples where the items have no AC supply isolation transformers - IOW they are *HOT CHASSIS* designs. All of them very old and very obsolete. Contemporary use of 1:1 isolation transformers would be for servicing devices like SMPSs. > It removes the direct path to ground of the electrical power, > significantly reducing the chance of death. ** That is wrong. Using an isolation transformer allows one to connect the common rail ( or any other point) of an off-line SMPS to safety ground. After which you can use a scope in the normal way to investigate various waveforms, maybe small ones like MOSFET drive signals. .... Phil |
| tabbypurr@gmail.com: Nov 07 07:42PM -0800 On Thursday, 8 November 2018 02:20:42 UTC, Phil Allison wrote: > > Isolation transformer are used a lot in servicing certain type of > > electronic equipment (e.g. tube TVs, amplifier, radio transmitters, etc.). > ** But only those examples where the items have no AC supply isolation transformers - IOW they are *HOT CHASSIS* designs. All of them very old and very obsolete. Well, they were standard & widespread in the 1990s. > > significantly reducing the chance of death. > ** That is wrong. > Using an isolation transformer allows one to connect the common rail ( or any other point) of an off-line SMPS to safety ground. After which you can use a scope in the normal way to investigate various waveforms, maybe small ones like MOSFET drive signals. One can, but they are also used to reduce shock risk. NT |
| Phil Allison <pallison49@gmail.com>: Nov 07 10:37PM -0800 > > > electronic equipment (e.g. tube TVs, amplifier, radio transmitters, > > ** But only those examples where the items have no AC supply isolation > > transformers - IOW they are *HOT CHASSIS* designs. All of them very old < and very obsolete. > Well, they were standard & widespread in the 1990s. ** Live chassis tube TVs and radios " standard & widespread " in the 1990s ???? FYI: The OP did NIT even mention "live chassis" so his post was wrong and so is yours. > > After which you can use a scope in the normal way to investigate > > various waveforms, maybe small ones like MOSFET drive signals. > One can, but they are also used to reduce shock risk. ** Biggest safety myth out. Using an isolation transformer unnecessarily INCREASES electric shock risk. .... Phil |
| tabbypurr@gmail.com: Nov 08 07:07AM -0800 On Thursday, 8 November 2018 06:37:49 UTC, Phil Allison wrote: > < and very obsolete. > > Well, they were standard & widespread in the 1990s. > ** Live chassis tube TVs and radios " standard & widespread " in the 1990s ???? Live chassis TVs were very common in the 90s. Live chassis radios disappeared in the 60s. > FYI: > The OP did NIT even mention "live chassis" so his post was wrong and so is yours. I was responding to your mention of 'hot chassis' not the OP's. What the OP mentioned is irrelevant. > > One can, but they are also used to reduce shock risk. > ** Biggest safety myth out. > Using an isolation transformer unnecessarily INCREASES electric shock risk. The UK government certainly disagrees with you. We have used isolation transformers for decades in bathrooms to reduce shock risk. Of course you're free to explain your pov. NT |
| "pfjw@aol.com" <peterwieck33@gmail.com>: Nov 08 08:31AM -0800 One of the things you will learn about Phil is that he suffers from a congenital fixation with the fallacy of illicit transference - meaning that what is true of the parts is true of the whole. My cat is grey, therefore all cats are grey (apologies to Ben Franklin). There are unique conditions in which an isolation transformer does not prevent the risk of electric shock. Therefore they increase the risk of electric shock. Idiots will find a way to solve for the most idiot-proof safety measures as conceived, because they were not conceived by idiots. And if one inserts one's self into an electrical circuit, one will be shocked. If you want to twist the dragon's tail, just get Phil started on newly-made multi-section can-caps. Peter Wieck Melrose Park, PA |
| jurb6006@gmail.com: Nov 07 04:55PM -0800 One of the "A3" attenuator boards. Tried to fix it but to no avail. |
| You received this digest because you're subscribed to updates for this group. You can change your settings on the group membership page. To unsubscribe from this group and stop receiving emails from it send an email to sci.electronics.repair+unsubscribe@googlegroups.com. |
No Response to "Digest for sci.electronics.repair@googlegroups.com - 13 updates in 2 topics"
Post a Comment