OCXO Kit - Are These Ideas OTT? #ocxo #mods


Brian George
 

Just starting to build the OCXO kit for my own homebrew project. There is plenty of room in the project case so I wondering whether to incorporate two possible modifications :

a) use a front panel-mounted control for R21 (temp adjust) with a vernier slow-motion drive. Could this be useful or is the setting of R21 truly "set and forget" regardless of the ambient room temperature?

b) take the heater supply via a separate LM317 (with heat-sink) and use a front panel-mounted potentiometer to vary the regulated output voltage from "low" to +5v. It would probably be necessary to feed the LM317 with around +6v to compensate for the drop-out. This would allow control of the oven start-up current, manually cranking up to the full +5v as the oven approaches a stable operating temperature.

Any thoughts on whether or not all this is completely unnecessary / OTT - particularly from those who have built and used this kit?

Thanks
73

Brian
G3ZOH 


M0RON
 

Good morning Brian,
the ones that I have built have been set and forget, 15 minutes or so to warm up and drift is 0 from then on.
andy
--
The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.


Peter Horne
 

Hi Andy, Do you have any idea what your heater current is ?. I’m trying to set mine up at present and I seem to need a large heater current (about 170mA). This seems very large compared to the pictures on the build sheet.

73 Peter G3JRH

 

Sent from Mail for Windows 10

 

From: M0RON
Sent: 25 July 2019 11:42
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] OCXO Kit - Are These Ideas OTT? #ocxo #mods

 

Good morning Brian,
the ones that I have built have been set and forget, 15 minutes or so to warm up and drift is 0 from then on.
andy
--
The universe is made up of Protons, Neutrons, Electrons but contains only one M0RON.

 


Alan de G1FXB
 

Hi Brian,
just my thoughts you are free to do as you choose.

a/.??? I guess your comment on using multiturn & vernier control is a result of the headline "the adjustment point is critical" ?
It's the Xtal oven target temperature that is critical, in maintaining the final goal of minimising Xtal frequency variation.
Less so the actual heater current. It is whatever it is, to heat it up to temperature.

It's really a set & forget adjustment, not ambient dependant.
Once done you don't want the temptation to give it just a little tickle,
You may get lost, very lost and be glad of the vernier scale...........

Be careful not to induce noise on the remote front panel wiring.


b/.??? Current limiting just the heater feed is highly desirable on first test, and initial setting.
I would caution too much day to day manual intervention, at best it just delays the oven temperature reaching equilibrium.
At worst it may result in overshoots which were not envisaged, and result in the very thing you are looking to avoid.
99.9% of builders do nothing special and the design works just fine.

(For a none QRP-Labs application I thought about adding a simple constant current circuit (Not variable voltage) from a 78xx or LM317 data sheet.
Else a L200 device, not gone beyond the brainstorm what devices I'm most familiar with to see which, if any is most suited for a circa 300mA absolute max figure.
More than likely there are more modern devices available now.
Alternatively add a series current limiting resistor, even better use that inpart to contribute to the heater function or just over engineer the heater with a power FET, IRF510 anyone.? :-[
And none of which will fit into this kit.....)


If I was going to have the patience to play,
is to trial a suggestion from Dick W7WKR, and simply short C1 and see if it does all it promises with little or no compromise.
It "breaks" Hans choice of oven control operation, so will not be acceptable to all.
It would be interesting if it provides a little more positive control during the setup process, and then note if it affects the adjustment point, and real world short / long term control if only as an experiment.
(the main reason of lag will still be the heat needs to peculate from the heater through the xtal can to the temperature "sensor" and reach equilibrium,
If you don't wait long enough before adjustment for it all to stabilise it will overshoot.)
Gilding the lilly would be to have a LED to show oven heat in operation and allow several heat / cool cycles before tickling the adjustment each time.
If it was still touchy then maybe a pcb mounted multiturn pot is of value, it will likely not fit the existing footprint and in doing so interfere with the oven enclosures......

OTT is your choice, whatever you find works....
Just don't loose sight that Hans wrote the book
on the OCXO kit. (both the assembly manual & http://www.hanssummers.com/ocxosynth)
And perhaps Keep that vernier drive for another project, something nice like a ATU or VFO......


Alan

On 25/07/2019 10:45, Brian George via Groups.Io wrote:
Just starting to build the OCXO kit for my own homebrew project. There is plenty of room in the project case so I wondering whether to incorporate two possible modifications :

a) use a front panel-mounted control for R21 (temp adjust) with a vernier slow-motion drive. Could this be useful or is the setting of R21 truly "set and forget" regardless of the ambient room temperature?

b) take the heater supply via a separate LM317 (with heat-sink) and use a front panel-mounted potentiometer to vary the regulated output voltage from "low" to +5v. It would probably be necessary to feed the LM317 with around +6v to compensate for the drop-out. This would allow control of the oven start-up current, manually cranking up to the full +5v as the oven approaches a stable operating temperature.

Any thoughts on whether or not all this is completely unnecessary / OTT - particularly from those who have built and used this kit?

Thanks
73

Brian
G3ZOH?


Alan G4ZFQ
 

It would probably be necessary to feed the LM317 with around +6v to compensate for the drop-out.
Brian,

As I recall you would need more than 1V more like 2-3V dropout depending on current, look at the data sheet.

But as others have said it is not necessary and could upset the operation.

73 Alan G4ZFQ


Brian George
 

Alan (G4ZFQ), Alan (G1FXB) and Andy (M0RON) - thank you very much for all the useful information on my OCXO questions. Could not have hoped for more - particularly in this heat!!

Decision made - I will build it exactly as per the manual but may be tempted to tinker with the value of R5. 

Thanks again to everyone who replied on and off list.

73

Brian
G3ZOH


geoff M0ORE
 

If you use a LM78L05, it will current limit at about 100mA itself. When the heater has warmed up and the current drops, it will voltage regulate to 5 volts. It must be the "L" version. Another way is to use two LM317's, one to control the voltage to the required amount and another to current limit. I've just made a battery charger to charge at 300mA up to a maximum voltage of 4.2 Volts. Just happen to have a bunch of 317's lying around.

Geoff

On 25/07/2019 10:45, Brian George via Groups.Io wrote:
Just starting to build the OCXO kit for my own homebrew project. There is plenty of room in the project case so I wondering whether to incorporate two possible modifications :

a) use a front panel-mounted control for R21 (temp adjust) with a vernier slow-motion drive. Could this be useful or is the setting of R21 truly "set and forget" regardless of the ambient room temperature?

b) take the heater supply via a separate LM317 (with heat-sink) and use a front panel-mounted potentiometer to vary the regulated output voltage from "low" to +5v. It would probably be necessary to feed the LM317 with around +6v to compensate for the drop-out. This would allow control of the oven start-up current, manually cranking up to the full +5v as the oven approaches a stable operating temperature.

Any thoughts on whether or not all this is completely unnecessary / OTT - particularly from those who have built and used this kit?

Thanks
73

Brian
G3ZOH 


m0bmn
 

Hi Brian

Just be clear for you.

If the LM7805 is the standard T220 style its rated at 1Amp, if its the small version , the size of the BS170, its rated at the 100mA that small version is the LM78L05

Hope that helps, the LM317 is adjustable and is also rated at 1 amp but unless you have a bunch of them already (Like Geoff) I would go for the  T220 style LM7805 (you still may need the heatsink)

Well that’s my thoughts anyway , (may not be the thoughts of the management hi)

73 Paul

Sent from Mail for Windows 10

 

From: geoff M0ORE via Groups.Io
Sent: 25 July 2019 19:44
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] OCXO Kit - Are These Ideas OTT? #ocxo #mods

 

If you use a LM78L05, it will current limit at about 100mA itself. When the heater has warmed up and the current drops, it will voltage regulate to 5 volts. It must be the "L" version. Another way is to use two LM317's, one to control the voltage to the required amount and another to current limit. I've just made a battery charger to charge at 300mA up to a maximum voltage of 4.2 Volts. Just happen to have a bunch of 317's lying around.

Geoff

On 25/07/2019 10:45, Brian George via Groups.Io wrote:

Just starting to build the OCXO kit for my own homebrew project. There is plenty of room in the project case so I wondering whether to incorporate two possible modifications :

a) use a front panel-mounted control for R21 (temp adjust) with a vernier slow-motion drive. Could this be useful or is the setting of R21 truly "set and forget" regardless of the ambient room temperature?

b) take the heater supply via a separate LM317 (with heat-sink) and use a front panel-mounted potentiometer to vary the regulated output voltage from "low" to +5v. It would probably be necessary to feed the LM317 with around +6v to compensate for the drop-out. This would allow control of the oven start-up current, manually cranking up to the full +5v as the oven approaches a stable operating temperature.

Any thoughts on whether or not all this is completely unnecessary / OTT - particularly from those who have built and used this kit?

Thanks
73

Brian
G3ZOH 

 


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Brian George
 

Hi Paul and Geoff, All understood and thanks for the clarification. I have learnt quite a bit from our exchanges today. Thank you !

73

Brian
G3ZOH