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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#41
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using mosfets as rectifiers?
Ken Smith wrote...
I have someone else checking their records. I didn't turn up a copy of the schematic. I think I, remember where the charge current came from. Remote preamp Long cable Main unit ................. +15 ..................... +15V .--------------------------. ! . GND . / .--------------------------. \ . . / . . \ . Signal in question . !----------------------------------------- Reciever !- . . !- Q1 . . !- . -15 . ! .--------------------------. -15V . . ................. ..................... With Q1 on, the signal is at -15V. With Q1 off the signal is at +15V This works fine until we add some more info. There is some big currents switching in the pre-amp section. Q1 turns on just as the other thing switches off and remains on for a few mS. The other circuit switching causes the preamp's GND to jump vs the ground in the main unit. The combination of the cable capacitance and the receiver circuit caused Q1's current to flow backwards. Either the bias on Q1 wasn't enough to keep the voltage below 0.7V or the current was backwards just when Q1 was supposed to go off. The cable could be as much as a mile long. The large signalling voltage swings were needed to deal with the ground differences. OK, got that. Now describe the rest of what happens, the symptoms. Thanks, - Win (email: use hill_at_rowland-dot-org for now) |
#42
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using mosfets as rectifiers?
In article ,
Winfield Hill wrote: Ken Smith wrote... [.. schematic snipped ..] With Q1 on, the signal is at -15V. With Q1 off the signal is at +15V This works fine until we add some more info. There is some big currents switching in the pre-amp section. Q1 turns on just as the other thing switches off and remains on for a few mS. The other circuit switching causes the preamp's GND to jump vs the ground in the main unit. The combination of the cable capacitance and the receiver circuit caused Q1's current to flow backwards. Either the bias on Q1 wasn't enough to keep the voltage below 0.7V or the current was backwards just when Q1 was supposed to go off. The cable could be as much as a mile long. The large signalling voltage swings were needed to deal with the ground differences. OK, got that. Now describe the rest of what happens, the symptoms. It looked something like this. This was over 10 years ago so I'm not certain on the details: ***.................................**********. ....*...............................*.......... ....*.............................**........... ....*............................*............. ....*...........................*.............. ....*..........................*............... ....*.........................*................ ....*.*..*............********................. ....*.*.**......*..***......................... ....**.*.**********............................ ......*.........*.............................. 1 2 3 4 5 6 7 At 1, the big stuff starts to turn off At 2, there was some crashing around as coil voltage reverses At 3, there was a blip as we go from fixed voltage to resistive damping At 4, the coil current has stopped and the gate drive is removed. At 5, the MOSFET's drain voltage has risen to some part way voltage At 6, the voltage begins a normal shaped exp. rise to +15V At 7, the voltage is most of the way up. The 4-6 time was several mS long (as in maybe 3) -- -- forging knowledge |
#43
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using mosfets as rectifiers?
"Jim Adney" bravely wrote to "All" (09 Jun 04 20:30:53)
--- on the heady topic of " using mosfets as rectifiers?" JA From: Jim Adney JA In my limited experience, Schottkys have a lower forward drop than JA even Ge. Ge diodes are about 0.3 volts Vf while I think Schottky diodes have about 0.5 volt but a microwave type diode can be less than 0.1 volts. A*s*i*m*o*v .... A couple of volts below threshold. |
#44
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using mosfets as rectifiers?
On a sunny day (Wed, 09 Jun 2004 20:30:53 -0500) it happened Jim Adney
wrote in : My shottky diodes are already dropping just under 4 tenths of a volt, so switching to a ger diode gets me very little and might not help at all as some of them have very high leakage currents. In my limited experience, Schottkys have a lower forward drop than even Ge. Perhaps depends, I looked up AA119 Ge diodde, and 250mV at a few mA. Schottky depends a lot on the current? Now I look at BAT85 datasheet, at 25C it is 300mV at 10 mA, a lot better at higher temp and lower current. Hey, maybe we could make a xtal radio with a Schottky! JP |
#45
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using mosfets as rectifiers?
On Wed, 09 Jun 2004 20:30:53 -0500, Jim Adney
wrote: On Tue, 08 Jun 2004 12:06:05 -0400 Albert wrote: I've already found (and ordered) a 2.2 volt turn on mosfet for my switching transistor. This should give me the the head room I need to drive the gate with a lower voltage. I don't follow this stuff much, so I didn't know that such things existed. I still have to ask: Is 2.2V where the turn-on starts, or where it is really fully on? It's where it starts to turn on. However, even if the resistance was only 100 ohms between the source and drain, it would probably be enough to key the transmitter (keying line on a modern ham transceiver). At best, I need to control a milliamp of output current and worst case would be 50 ma of output current needed. So I can use a partially conducting mosfet. Thinking about this, I gather that you intend to run these MOSFETs backwards, so that the "rectifier" is on when it is parallel to the body diode and off when in the usual forward direction. Does that work? I never considered the possibility.... No..... I asked about mosfets as diodes. I was hoping I could bias the gate high and the mosfet would only conduct with positive input voltage...but it appears they are somewhat unidirectional. Since I need to control the input to the gate when positive input voltage is detected, I probably can't use this method as I don't have a steady supply voltage as the unit is self powered (no external dc input voltage). I am however going to use a mosfet to switch my load, which is the keying line on the radio transceiver. So, mosfets as rectifiers is out. Mosfet as a switch to pull the keying line low when I get enough audio voltage WILL be used. My shottky diodes are already dropping just under 4 tenths of a volt, so switching to a ger diode gets me very little and might not help at all as some of them have very high leakage currents. In my limited experience, Schottkys have a lower forward drop than even Ge. Even is the Ge drops a slightly lower voltage, the leakeage current is much higher, which is one reason why we use silicon. There just isn't anything to gain by switching to Ge. I don't know your application, but I wonder if you could transformer couple your signal and boost it a bit, just enough to give you the extra voltage you need to keep this job simple. With a bipolar transistor acting as a switch, I am probably limited severely by the transformer step up ratio. But, the mosfet needs very little real power applied to the gate to make it switch, so looking for a higher turns ratio audio type transformer might be the answer as well. I have to say that I've been reading all the responses and I've learned alot. But, you (Jim) seem to have the clearest understanding of my original question and my reason for seeking help here. I wonder what the practical limits are for turns ratio for 8 ohm input impedance? If I could get a 50 to 1 step up ratio, I would have no problem generating enough voltage to turn my mosfet switch on! I don't think transformers can be built with that much of a step up ratio (and still be a relatively wide range audio frequency device). I thank all who have commented. Regards, A -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#46
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using mosfets as rectifiers?
On Sun, 06 Jun 2004 22:28:43 -0400, Albert wrote:
I have an extremely low power application (recovering a small amount of dc power from the audio output of a PC soundcard). I am trying to key a transmitter when the soundcard puts out audio. It's actually a self powered VOX, deriving all it's needed power from the audio output of the soundcard. Right now, I'm almost succeeding, but need another half volt or so to make the keying reliable. I am using my laptop computer, which has only speaker out jacks. I am feeding the speaker output into the low impedance side of an audio transformer and taking the stepped up voltage from the secondary (8 ohm to 1K ohm impedance transformer). Laptop headphone outputs are usually more than 8 ohms. The 8 ohm winding transformer may be needlessly pulling down levels even with an unloaded secondary.. snip |
#47
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using mosfets as rectifiers?
In article ,
Albert wrote: It's powering the gate of a small signal mosfet. The mosfet needs about 3 volts to turn on completely. The mosfet is hooked to ground and the positive keying terminal on the microphone jack. It (the keying line) is already pulled up by the electronics inside the radio, so it needs to be pulled down to ground in order to key the transmitter. The mosfet pulls the keying terminal to ground when it conducts. Can't you steal a tiny bit of current from that keying line without activating the key input to power some low-power Opamp? That would be enough power for the turned-off state. When you activate the key, you would need to get power from the audio input (or provide a large enoug buffer capacitor for the longest on-interval). cu Michael |
#48
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using mosfets as rectifiers?
Albert wrote:
With a bipolar transistor acting as a switch, I am probably limited severely by the transformer step up ratio. But, the mosfet needs very little real power applied to the gate to make it switch, so looking for a higher turns ratio audio type transformer might be the answer as well. You can use the same transformer that you already have, and use two schottky diodes and two capacitors to make a voltage duplicator, then apply that voltage to the gate of the mosfet: | ||-+ ||- o-----o-||-+ - | | ^ | | || | | | -||-----o | | || - --- | ^ --- | From transformer | | | --------o-----o----o created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de --- El Meda. http://ingemeda.tripod.com/ |
#49
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using mosfets as rectifiers?
Albert wrote...
I wonder what the practical limits are for turns ratio for 8 ohm input impedance? If I could get a 50 to 1 step up ratio, I would have no problem generating enough voltage to turn my mosfet switch on! I don't think transformers can be built with that much of a step up ratio (and still be a relatively wide range audio frequency device). Step-up and step-down transformers are basically the same, but connected backwards. Up to 10,000-ohm CT to speaker (4-ohm) used to be made for driving speakers from vacuum tubes. The impedance ratio is the square of the turns ratio, so that'd be a 1:50 xfmr. Radio Shack used to sell transformers similar to this. Now they only offer the # 273-1380, rated at 1000:8 ohms or an 11:1 ratio. Ocean State Electronics has many types of small audio transformers. http://www.oselectronics.com/ose_p119.htm You could use two of these, say a 1000:8 (p/n 45-714) plus a 100000 to 1000 (p/n 45-702) to achieve an impressive overall 112:1 ratio. Thanks, - Win (email: use hill_at_rowland-dot-org for now) |
#50
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using mosfets as rectifiers?
On 11 Jun 2004 08:40:35 -0700, the renowned Winfield Hill
wrote: Albert wrote... I wonder what the practical limits are for turns ratio for 8 ohm input impedance? If I could get a 50 to 1 step up ratio, I would have no problem generating enough voltage to turn my mosfet switch on! I don't think transformers can be built with that much of a step up ratio (and still be a relatively wide range audio frequency device). Step-up and step-down transformers are basically the same, but connected backwards. Up to 10,000-ohm CT to speaker (4-ohm) used to be made for driving speakers from vacuum tubes. The impedance ratio is the square of the turns ratio, so that'd be a 1:50 xfmr. Radio Shack used to sell transformers similar to this. Now they only offer the # 273-1380, rated at 1000:8 ohms or an 11:1 ratio. Ocean State Electronics has many types of small audio transformers. http://www.oselectronics.com/ose_p119.htm You could use two of these, say a 1000:8 (p/n 45-714) plus a 100000 to 1000 (p/n 45-702) to achieve an impressive overall 112:1 ratio. Thanks, - Win (email: use hill_at_rowland-dot-org for now) Mouser also carries (carried?) a wide range of "import" type small audio transformers at very reasonable prices compared to Pico et al. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
#51
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using mosfets as rectifiers?
On Thursday, 10 Jun 2004 10:18:28 -500 "Asimov"
wrote: Ge diodes are about 0.3 volts Vf while I think Schottky diodes have about 0.5 volt but a microwave type diode can be less than 0.1 volts. Certainly the Vf depends on the current in all three types, but I think that I get more like 0.2V when I just check a Schottky on my Fluke. I've been using 1N5819s and 1N5820s to improve the efficiency of some low voltage switching supplies that I often repair, and the results seem consistent with this. Isn't a microwave diode still a silicon PN junction? If so, then it is only 0.1V by virtue of the low forward current. It's clear that we have to be careful here, because we all know that we can find a silicon diode that will measure 0.1V with 1A and another silicone diode that will measure 1V with the same current. It's just a matter of how close to the max I rating for the diode we are. I've got some nice 10,000 A silicon hockey pucks at work. I wonder what they show on a Fluke? ;-) - ----------------------------------------------- Jim Adney Madison, WI 53711 USA ----------------------------------------------- |
#52
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using mosfets as rectifiers?
"Jim Adney" bravely wrote to "All" (11 Jun 04 19:06:15)
--- on the heady topic of " using mosfets as rectifiers?" JA From: Jim Adney JA On Thursday, 10 Jun 2004 10:18:28 -500 "Asimov" JA wrote: Ge diodes are about 0.3 volts Vf while I think Schottky diodes have about 0.5 volt but a microwave type diode can be less than 0.1 volts. JA Certainly the Vf depends on the current in all three types, but I JA think that I get more like 0.2V when I just check a Schottky on my JA Fluke. I've been using 1N5819s and 1N5820s to improve the efficiency JA of some low voltage switching supplies that I often repair, and the JA results seem consistent with this. Of course it depends on Is how the Vf curve goes. For power rectification, say at about 3A, an Si diode will exhibit about 1V and a similar rated Schottky about 0.5V. Now for small signal applications this all goes out the window especially as it approaches the leakage currents. For example a 1N914 will show between about 500K and 1M ohm small signal resistance at Vf=0 (with signal input 10 times less than Vt of about 25mV) but I think this is due those leakage currents. Small signal transistor C-B junctions are often much better than that though the capacitance tends to be larger. JA Isn't a microwave diode still a silicon PN junction? If so, then it is JA only 0.1V by virtue of the low forward current. I think a microwave diode is designed to take advantage of quantum effects. It works with waves and fields as the operating principle for signals but probably like an ordinary diode for DC and low frequencies, as they seem to have a frequency range spec. For example odd things like the output starts to change before the input signal actually propagates to the output. Now if I had a diode that would have an output before I actually applied a signal, now you're talking! JA It's clear that we have to be careful here, because we all know that JA we can find a silicon diode that will measure 0.1V with 1A and another JA silicone diode that will measure 1V with the same current. It's just a JA matter of how close to the max I rating for the diode we are. Especially with pulse currents when a rectifier supplies current to a very large filter capacitor. For example Vf may be 1V at 3A but what does it become with a 100A current pulse? JA I've got some nice 10,000 A silicon hockey pucks at work. I wonder JA what they show on a Fluke? ;-) A negative reading? i.e. it supplies a measurable saturation current just from the temperature photons in your hand? ;-) A*s*i*m*o*v .... Over a hundred billion electrons were used in crafting this tagline. |
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