- Type of wire to use for shielded test lead.... - 7 Updates
- How do you test a Selenium Rectifier? - 8 Updates
| tubeguy@myshop.com: Dec 05 11:49PM -0600 I am restoring a Eico VTVM model 221. The DC requires a test lead with shielded wire and a 15M resistor in the probe. It did not come with the leads. I have all the connectors and the resistor, but need shielded cord. Can I just cut the ends off a guitar cable to make one? I dont want to buy a whole roll of cable, which willnever get used after this project. I also considered the cable for CB radio antennas, but thats darn stiff. And tv ant coax has that funky aluminum foil for shielding, and is also stiff. What else can I use? I will use this meter for tube radios in the RF and IF stages, so I do need the shield. |
| Phil Allison <pallison49@gmail.com>: Dec 05 11:04PM -0800 > shielded wire and a 15M resistor in the probe. It did not come with the > leads. I have all the connectors and the resistor, but need shielded > cord. Can I just cut the ends off a guitar cable to make one? ** Probably be fine ,just cut one end off though - the 1/4 inch plug is needed !!! > I will use this meter for tube radios in the RF and IF stages, so I do > need the shield. ** DMMs don't come with shielded leads, only scopes do. .... Phil |
| Ralph Mowery <rmowery28146@earthlink.net>: Dec 06 09:58AM -0500 In article <dndh0e1emhi75knhs17hlbhtt5l2ebscln@4ax.com>, tubeguy@myshop.com says... > What else can I use? > I will use this meter for tube radios in the RF and IF stages, so I do > need the shield. Any shielded wire will work. Just be sure it is rated for the voltage you are going to measure. Not being familiar with the Eico, but the 15 meg resistor seems very high. Maybe it is a 1.5 meg. The VTVM I have used a 1 meg resistor in the end of the probe. |
| Ralph Mowery <rmowery28146@earthlink.net>: Dec 06 10:02AM -0500 In article <5ac31ee4-e414-4976-9ad7-4e548c68f139@googlegroups.com>, pallison49@gmail.com says... > > I will use this meter for tube radios in the RF and IF stages, so I do > > need the shield. > ** DMMs don't come with shielded leads, only scopes do. You missed the part where it is a VTVM, not a DMM. The old VTVMs I have seen often have a shielded probe. |
| Fox's Mercantile <jdangus@att.net>: Dec 06 09:11AM -0600 On 12/6/18 8:58 AM, Ralph Mowery wrote: > Not being familiar with the Eico, but the 15 meg resistor seems very > high. Maybe it is a 1.5 meg. The VTVM I have used a 1 meg resistor in > the end of the probe. It's 15 Meg. <https://stevenjohnson.com/manuals/mfiles/eico221.pdf> -- "I am a river to my people." Jeff-1.0 WA6FWi http:foxsmercantile.com |
| Ralph Mowery <rmowery28146@earthlink.net>: Dec 06 10:49AM -0500 In article <tY-dnSlwVL6moZTBnZ2dnUU7-IWdnZ2d@giganews.com>, jdangus@att.net says... > > the end of the probe. > It's 15 Meg. > <https://stevenjohnson.com/manuals/mfiles/eico221.pdf> Ok, I see that the input impedance is 25 meg ohms where most are around 10 meg. That explains the difference. |
| Fox's Mercantile <jdangus@att.net>: Dec 06 10:16AM -0600 On 12/6/18 9:49 AM, Ralph Mowery wrote: > Ok, I see that the input impedance is 25 meg ohms where most > are around 10 meg. That explains the difference. Comparatively: That's 25,000 ohms/volt on the 1000 range. With 11 meg, it's only 11,000 ohms/volt. Typical multi-meters like the Simpson are 20,000/volt, but at the 29 volt range, that's only 400 Kohms, which will load a high impedance source considerably more than the VTVM at either 11 or 25 Meg. However, if you're using the 20 volt range for measuring AVC voltages, it's 1.25 meg ohms/volt. -- "I am a river to my people." Jeff-1.0 WA6FWi http:foxsmercantile.com |
| makolber@yahoo.com: Dec 05 09:18AM -0800 > Each plate is only good for about > 25 volts , wow I always thought those were just heat sinks I seriously would not have guessed that I would learn something new about selenium rectifiers today. :-) mark |
| "Dave M" <dgminala@mediacombb.net>: Dec 05 12:42PM -0600 Actually, they ARE heat sinks. Selenium rectifiers were much less efficient than silicon, or even germanuim rectifiers, with a forward voltage drop of about 1V per cell (plate). That means that if your rectifier has 16 cells, then the rectifier, as a whole, will have AT LEAST 16 volts of forward voltage drop. Multiply that by the current through the rectifier, and you have the number of watts that has to be dissipated, thus, the need for the heat sink plates. I remember selenuim rectifiers being used in car battery chargers, capable of charging at 50 amps. The plates, as I remember, were about 8" square, and had to have a fan inside to cool the rectifier since there was so much heat to be removed. Cheers, Dave M |
| "pfjw@aol.com" <peterwieck33@gmail.com>: Dec 05 11:37AM -0800 I remember selenuim rectifiers being used in car battery chargers, capable of charging at 50 amps. Did not most battery chargers use copper-oxide rectifiers? As I understand it, they are both more rugged and more weather resistant than selenium. On the other hand, they are very similar in appearance. Peter Wieck Melrose Park, PA |
| Ralph Mowery <rmowery28146@earthlink.net>: Dec 05 03:08PM -0500 In article <G8ydnbcn5sPXgZXBnZ2dnUU7-d_NnZ2d@giganews.com>, dgminala@mediacombb.net says... > voltage drop. Multiply that by the current through the rectifier, and you > have the number of watts that has to be dissipated, thus, the need for the > heat sink plates. Yes, the actual plates are heat sinks, but I was loose with my words and just using that as a refference as to how many sections there are. That is each plate coresponds to a diode type junction in series. So if you count 5 plates it is similar to having 5 dioides in series. Each one good for about 25 volts of inverse voltage and dropping about a volt. That would make around 5 volts forward drop and good for about 125 piv. Using the term plate was to me just a quick way of estimating how much voltage drop and PIV the stack could handle. |
| Allodoxaphobia <knock_yourself_out@example.net>: Dec 06 01:34AM > Did not most battery chargers use copper-oxide rectifiers? As I > understand it, they are both more rugged and more weather resistant > than selenium. On the other hand, they are very similar in appearance. I still have, and use, an "antique", Big, Honkin' 50 amp 12/6V battery charger -- and it uses selenium rectifiers. It's out in the bitterly cold, dark, detached garage ATM -- or I would quote from it's metal name tag. A hand-me-down from my father ... and maybe even, his father. In the past I was able to start my old, dilapidated, hardly-any- compression Jeep with no onboard battery! Still have the charger -- not the Jeep. :-) Jonesy -- Marvin L Jones | Marvin | W3DHJ.net | linux 38.238N 104.547W | @ jonz.net | Jonesy | FreeBSD * Killfiling google & XXXXbanter.com: jonz.net/ng.htm |
| Phil Allison <pallison49@gmail.com>: Dec 05 08:10PM -0800 Dave M wrote: > Actually, they ARE heat sinks. Selenium rectifiers were much less efficient > than silicon, or even germanuim rectifiers, with a forward voltage drop of > about 1V per cell (plate). ** But silicon diodes like 1N4004s or 1N4007s you see everywhere have a similar voltage drop - about 1V at 1 amp. https://www.onsemi.com/pub/Collateral/1N4001-D.PDF > voltage drop. Multiply that by the current through the rectifier, and you > have the number of watts that has to be dissipated, thus, the need for the > heat sink plates. ** But the *REAL* reason is the low reverse voltage capability - 25V per diode rather than 400V or even 1000 volts. FYI: Silicon diodes are remarkable devices - a finger nail size, 4 diode bridge is adequate for a 1kW DC supply using only the PCB foil as a heatsink. https://au.element14.com/vishay/w10g-e4-51/no-of-phases-single-phase/dp/1497580?st=1.5 amp bridge Think there is another reason too in that Selenium diodes cannot be allowed to run as hot as Silicon, or their life span is drastically shortened. ... Phil |
| tabbypurr@gmail.com: Dec 05 08:33PM -0800 > I seriously would not have guessed that I would learn something new about > selenium rectifiers today. :-) > mark The big plates are heatsinks. Further in are the smaller selenium plate rectifiers. NT |
| tubeguy@myshop.com: Dec 05 11:42PM -0600 On Sun, 2 Dec 2018 10:57:07 -0000 (UTC), Cursitor Doom >> Just replace it with a 1N4007 diode and about 100 ohms series resistor. >Not kosher if you want to keep the internals original! Many collectors >are extremely fussy about that. I plan to leave it on the chassis for looks, but replace it. My voltage is real low, so it seems it's weak or worse.... |
| 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 - 15 updates in 2 topics"
Post a Comment