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


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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