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50 ohms to 0.02 ohm transformer

Started by amdx November 3, 2016
On 3.11.16 19:55, John Larkin wrote:
>> This is regardless which way you build your transformer. > > Leakage inductance can matter. > > It would be interesting to make a toroidal transformer where the > 1-turn winding wraps around the entire core. Visualize that! It can be > approximated by paralleling multiple 1t windings. > > http://www.spaceneedle.com/wp-content/uploads/2013/08/view-from-heli-250x211.jpg
It has been for years a common practice in transistor RF amplifiers to make the single-turn windings of copper/brass tubing and threading the multiple-turn winding through the tube inside. This probably is like your paralleled windings. In recent ARRL Handbooks there are fine examples (in 2014 edition on page 17.32). -- -TV
  On 3.11.16 19:55, John Larkin wrote:
> > Leakage inductance can matter. > > It would be interesting to make a toroidal transformer where the > 1-turn winding wraps around the entire core. Visualize that! It can be > approximated by paralleling multiple 1t windings.
I build 'wall current monitors' to measure the beam current in particle accelerators. Those are basically single-turn transformers very much like that: The secondary winding is a toroidal shell completely enveloping the core, with a small gap across which I pick-up the output signal. Accessorily, the secondary winding is also the vacuum chamber. I also make hybrid transformers that basically create the sum and difference of two input signals on two separate outputs. One of the issues limiting performance at HF is the capacitance between the two input windings and the difference winding. I tried putting the difference winding on its own separate core, and then coupling the two cores with a single-turn winding threading both. This was in practice a piece of air core coax shorted at both ends, with the two cores inside. It worked, but it wasn't enough of an improvement to be worth the bother. Jeroen Belleman
On 11/3/2016 2:39 PM, Tim Wescott wrote:
> On Thu, 03 Nov 2016 19:37:44 +0200, Tauno Voipio wrote: > >> On 3.11.16 19:15, amdx wrote: >> >>> I don't know, the schematic on the Last page of this pdf. It looks to >>> me like the osc tube drives the 0.02 ohms directly. >>> http://www.hparchive.com/Boonton/Boonton-Manual-260A.pdf >> >> >> You are not reading the schematic correctly. There is a transformer >> formed by the tuned circuit in the coil turret and its link coil. >> >> Besides, there is the impedance of the thermo-cross RF current meter on >> the way, as well. > > Oh lordy, I saw that days ago and didn't catch it. > > Andy: the DC resistance of that thermocouple unit is going to be pretty > close to the RF resistance, and I'll bet it's around 50 ohms -- or at > least a hell of a lot closer than 20m-ohms. >
I did a quick crude experiment this morning before work. I set my 50 ohm sig gen to 400mvpp, I then connected it to the input of the 260A (thermocouple input). The the sig gen voltage dropped to 20mvpp, from this I get an input impedance of 2.5 ohms for the thermocouple input. Maybe time for a second transformer. Mikek
On Friday, November 4, 2016 at 4:55:39 AM UTC+11, John Larkin wrote:
> On Thu, 3 Nov 2016 15:12:53 +0200, Tauno Voipio > <tauno.voipio@notused.fi.invalid> wrote: > > >On 3.11.16 14:55, amdx wrote: > >> I want to build a 50 ohm to 0.02 ohm transformer. > >> It needs a 50 to 1 turns ratio. > >> It takes a low A sub L core to make such a thing. > >> I have a toroid with an A sub L of 75. That only needs > >> 29 turns for my frequency of interest 500KHz to 1700kHz. > >> I also have some potcores that I could gap to lower the A sub L. > >> > >> What would make a better transformer a toroid or a gapped potcore? > >> > >> I plan 50 turns with a 1 turn secondary. > >> > >> What problems can I expect with a 1 turn secondary? > >> > >> Thanks, Mikek > > > >For a toroid, a turn is simple: A wire through the toroid > >hole is a turn, a wire outside is nothing. > > > >IMHO, your real problem is on the 20 milliohm side to make > >the circuit connections so that you're not going to lose all > >of your signal in the connection resistances and inductances. > >This is regardless which way you build your transformer. > > Leakage inductance can matter. > > It would be interesting to make a toroidal transformer where the > 1-turn winding wraps around the entire core. Visualize that! It can be > approximated by paralleling multiple 1t windings.
The problem with making a 1-turn winding on a toroid would be making it non-progressive. As long as you have a turn in the plane of the toroid, you've got an external field and leakage inductance. The tricks for making non-progressive winding do seem to involve a second turn going back around the toroid in the opposite sense (viewed from above the plane of toroid). -- Bill Sloman, Sydney
On 2016-11-04 14:00, bill.sloman@ieee.org wrote:
> On Friday, November 4, 2016 at 4:55:39 AM UTC+11, John Larkin wrote: >> On Thu, 3 Nov 2016 15:12:53 +0200, Tauno Voipio >> <tauno.voipio@notused.fi.invalid> wrote: >> >>> On 3.11.16 14:55, amdx wrote: >>>> I want to build a 50 ohm to 0.02 ohm transformer. >>>> It needs a 50 to 1 turns ratio. >>>> It takes a low A sub L core to make such a thing. >>>> I have a toroid with an A sub L of 75. That only needs >>>> 29 turns for my frequency of interest 500KHz to 1700kHz. >>>> I also have some potcores that I could gap to lower the A sub L. >>>> >>>> What would make a better transformer a toroid or a gapped potcore? >>>> >>>> I plan 50 turns with a 1 turn secondary. >>>> >>>> What problems can I expect with a 1 turn secondary? >>>> >>>> Thanks, Mikek >>> >>> For a toroid, a turn is simple: A wire through the toroid >>> hole is a turn, a wire outside is nothing. >>> >>> IMHO, your real problem is on the 20 milliohm side to make >>> the circuit connections so that you're not going to lose all >>> of your signal in the connection resistances and inductances. >>> This is regardless which way you build your transformer. >> >> Leakage inductance can matter. >> >> It would be interesting to make a toroidal transformer where the >> 1-turn winding wraps around the entire core. Visualize that! It can be >> approximated by paralleling multiple 1t windings. > > The problem with making a 1-turn winding on a toroid would be > making it non-progressive.
I fail to see how a one-turn winding that wraps around the entire core can be progressive. A multi-turn progressive winding, yes, but a single-turn winding has no current in the plane of the toroid. Making proper connections to such a winding may be challenging. Jeroen Belleman
On 4.11.16 13:26, amdx wrote:
> On 11/3/2016 2:39 PM, Tim Wescott wrote: >> On Thu, 03 Nov 2016 19:37:44 +0200, Tauno Voipio wrote: >> >>> On 3.11.16 19:15, amdx wrote: >>> >>>> I don't know, the schematic on the Last page of this pdf. It looks to >>>> me like the osc tube drives the 0.02 ohms directly. >>>> http://www.hparchive.com/Boonton/Boonton-Manual-260A.pdf >>> >>> >>> You are not reading the schematic correctly. There is a transformer >>> formed by the tuned circuit in the coil turret and its link coil. >>> >>> Besides, there is the impedance of the thermo-cross RF current meter on >>> the way, as well. >> >> Oh lordy, I saw that days ago and didn't catch it. >> >> Andy: the DC resistance of that thermocouple unit is going to be pretty >> close to the RF resistance, and I'll bet it's around 50 ohms -- or at >> least a hell of a lot closer than 20m-ohms. >> > > I did a quick crude experiment this morning before work. > I set my 50 ohm sig gen to 400mvpp, I then connected it to the > input of the 260A (thermocouple input). > The the sig gen voltage dropped to 20mvpp, from this I get an input > impedance of 2.5 ohms for the thermocouple input. > Maybe time for a second transformer. > > Mikek
Do not do that - you risk of burning the RF current meter and the terminator resistor. The 5763 will put out around 10 W of RF when properly matched. -- -TV
On 11/4/2016 8:20 AM, Tauno Voipio wrote:
> On 4.11.16 13:26, amdx wrote: >> On 11/3/2016 2:39 PM, Tim Wescott wrote: >>> On Thu, 03 Nov 2016 19:37:44 +0200, Tauno Voipio wrote: >>> >>>> On 3.11.16 19:15, amdx wrote: >>>> >>>>> I don't know, the schematic on the Last page of this pdf. It looks to >>>>> me like the osc tube drives the 0.02 ohms directly. >>>>> http://www.hparchive.com/Boonton/Boonton-Manual-260A.pdf >>>> >>>> >>>> You are not reading the schematic correctly. There is a transformer >>>> formed by the tuned circuit in the coil turret and its link coil. >>>> >>>> Besides, there is the impedance of the thermo-cross RF current meter on >>>> the way, as well. >>> >>> Oh lordy, I saw that days ago and didn't catch it. >>> >>> Andy: the DC resistance of that thermocouple unit is going to be pretty >>> close to the RF resistance, and I'll bet it's around 50 ohms -- or at >>> least a hell of a lot closer than 20m-ohms. >>> >> >> I did a quick crude experiment this morning before work. >> I set my 50 ohm sig gen to 400mvpp, I then connected it to the >> input of the 260A (thermocouple input). >> The the sig gen voltage dropped to 20mvpp, from this I get an input.
>> impedance of 2.5 ohms for the thermocouple input.
>> Maybe time for a second transformer. >> >> Mikek > > Do not do that - you risk of burning the RF current meter and the > terminator resistor. The 5763 will put out around 10 W of RF when > properly matched. >
Yes, I'm aware of all the cautions regarding the thermocouple. Almost every website I read has a thermocouple warning, the only person that says "relax, if the thermocouple is still good your unlikely to burn it out," is a guy that worked at Boonton and has his inspection # on many units. My sig Gen with a 50 ohm output impedance would not drive enough current to harm the thermocouple. When the Multiplier is set to 1, there is 20mv across the 0.02 ohm resistor, that is one amp of current! About all I can get out of my sig gen is 200ma. But yes, the part is unobtainable, so I use care. Easy to monitor the voltage across the LOW connector and GRD, that is the voltage across the 0.02 ohm resistor. Thank, Mikek
On Fri, 04 Nov 2016 14:10:16 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 2016-11-04 14:00, bill.sloman@ieee.org wrote: >> On Friday, November 4, 2016 at 4:55:39 AM UTC+11, John Larkin wrote: >>> On Thu, 3 Nov 2016 15:12:53 +0200, Tauno Voipio >>> <tauno.voipio@notused.fi.invalid> wrote: >>> >>>> On 3.11.16 14:55, amdx wrote: >>>>> I want to build a 50 ohm to 0.02 ohm transformer. >>>>> It needs a 50 to 1 turns ratio. >>>>> It takes a low A sub L core to make such a thing. >>>>> I have a toroid with an A sub L of 75. That only needs >>>>> 29 turns for my frequency of interest 500KHz to 1700kHz. >>>>> I also have some potcores that I could gap to lower the A sub L. >>>>> >>>>> What would make a better transformer a toroid or a gapped potcore? >>>>> >>>>> I plan 50 turns with a 1 turn secondary. >>>>> >>>>> What problems can I expect with a 1 turn secondary? >>>>> >>>>> Thanks, Mikek >>>> >>>> For a toroid, a turn is simple: A wire through the toroid >>>> hole is a turn, a wire outside is nothing. >>>> >>>> IMHO, your real problem is on the 20 milliohm side to make >>>> the circuit connections so that you're not going to lose all >>>> of your signal in the connection resistances and inductances. >>>> This is regardless which way you build your transformer. >>> >>> Leakage inductance can matter. >>> >>> It would be interesting to make a toroidal transformer where the >>> 1-turn winding wraps around the entire core. Visualize that! It can be >>> approximated by paralleling multiple 1t windings. >> >> The problem with making a 1-turn winding on a toroid would be >> making it non-progressive. > >I fail to see how a one-turn winding that wraps around the entire >core can be progressive. A multi-turn progressive winding, yes, >but a single-turn winding has no current in the plane of the toroid. >Making proper connections to such a winding may be challenging. > >Jeroen Belleman
Make a bunch of 1-turn secondaries, each driving a twisted pair. Connect the pairs outside the plane of the toroid. Like the Space Needle. I had dinner in the Space Needle once. The tables rotate around on a track to slowly to scan the view. If you lean your elbow on the window sill, it slowly creeps out from under you. If you go the the bathroom and then go again later, it's in a different direction. I guess the Space Needle has low leakage inductance. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Thu, 3 Nov 2016 07:55:14 -0500, amdx <nojunk@knology.net> wrote:

>I want to build a 50 ohm to 0.02 ohm transformer. >It needs a 50 to 1 turns ratio. >It takes a low A sub L core to make such a thing. >I have a toroid with an A sub L of 75. That only needs >29 turns for my frequency of interest 500KHz to 1700kHz. >I also have some potcores that I could gap to lower the A sub L. >What would make a better transformer a toroid or a gapped potcore? >I plan 50 turns with a 1 turn secondary. >What problems can I expect with a 1 turn secondary? > Thanks, Mikek
Are you perhaps building an AM broadcast band to power line coupler? If so, how much RF power? If not, just ignore the following. Way back on college daze, I helped build such a contrivance for the campus AM radio station. The problem was how to keep 60Hz AC from going back through the transformer and AM modulating the final stages. The fix was to use a ferrite toroid, which coupled the RF quite nicely, but none of the 60Hz. As I barely recall from 46 years ago, it was an ordinary ferrite toroid core about 2" OD. However, I don't recall the ferrite material used or number turns involved. No need for a pot core since the transformer was inside an aluminum box. I'll see if I can find some details (time permitting). Google for "carrier current AM transmitter" Also, do you really need broadband for your unspecified application? Going from 0.5MHz to 1.7MHz without tuning is nice, but if your transmitter doesn't change frequency, an LC tuned arrangement (tapped coil or two capacitors) might be easier than a toroid. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
On 4.11.16 16:33, amdx wrote:
> On 11/4/2016 8:20 AM, Tauno Voipio wrote: >> On 4.11.16 13:26, amdx wrote: >>> On 11/3/2016 2:39 PM, Tim Wescott wrote: >>>> On Thu, 03 Nov 2016 19:37:44 +0200, Tauno Voipio wrote: >>>> >>>>> On 3.11.16 19:15, amdx wrote: >>>>> >>>>>> I don't know, the schematic on the Last page of this pdf. It looks to >>>>>> me like the osc tube drives the 0.02 ohms directly. >>>>>> http://www.hparchive.com/Boonton/Boonton-Manual-260A.pdf >>>>> >>>>> >>>>> You are not reading the schematic correctly. There is a transformer >>>>> formed by the tuned circuit in the coil turret and its link coil. >>>>> >>>>> Besides, there is the impedance of the thermo-cross RF current >>>>> meter on >>>>> the way, as well. >>>> >>>> Oh lordy, I saw that days ago and didn't catch it. >>>> >>>> Andy: the DC resistance of that thermocouple unit is going to be pretty >>>> close to the RF resistance, and I'll bet it's around 50 ohms -- or at >>>> least a hell of a lot closer than 20m-ohms. >>>> >>> >>> I did a quick crude experiment this morning before work. >>> I set my 50 ohm sig gen to 400mvpp, I then connected it to the >>> input of the 260A (thermocouple input). >>> The the sig gen voltage dropped to 20mvpp, from this I get an input. >>> impedance of 2.5 ohms for the thermocouple input. >>> Maybe time for a second transformer. >>> >>> Mikek >> >> Do not do that - you risk of burning the RF current meter and the >> terminator resistor. The 5763 will put out around 10 W of RF when >> properly matched. >> > Yes, I'm aware of all the cautions regarding the thermocouple. > Almost every website I read has a thermocouple warning, the only person > that says "relax, if the thermocouple is still good your unlikely to > burn it out," is a guy that worked at Boonton and has his inspection # > on many units. > My sig Gen with a 50 ohm output impedance would not drive enough current > to harm the thermocouple. > When the Multiplier is set to 1, there is 20mv across the 0.02 ohm > resistor, that is one amp of current! About all I can get out of my > sig gen is 200ma. > But yes, the part is unobtainable, so I use care. > Easy to monitor the voltage across the LOW connector and GRD, that > is the voltage across the 0.02 ohm resistor. > Thank, Mikek
The 5763 is a transmitting tube. It was a very popular driver tube in the tube era ham transmitters, driving usually a pair of 6146's for about 200 W output. -- -TV