knob for variable capacitor

[medical-nerd]

Hiya

I've just purchased a few variable capacitors - see pic
150pF / 210pF

They have a slow motion drive.
 
What type of knob is used for these  - it would need an inner and outer mechanism.
The other option I am considering is using the outer coarse movement to hold a pointer and use just the inner slow motion as the tuning knob.

Suggestions please???

Cheers

[Rerouter]

If it works like i am thinking, you put a dial on the outer, and a small knob on the middle, allowing for a very fine tuning range if it suits you.

[vk3yedotcom]

Yes, I think  your last option is best.   

The gear reduction ratio in these is normally only about 3:1 so it's no impost that you are only turning the thinner shaft. 

Also, as less torque is required due to the gearing, it will be easier to turn - a great consideration when making fine adjustments eg tuning in a station.

[cdev]

maybe "vernier" or "vernier drive" like they use to have in old radios.

Also, planetary gear (reduction) coupling might be another helpful search term.

They attach to the shaft and bring about a substantial reduction in rotation.

Very useful for magnetic loop antennas and similar.

[vk3yedotcom]

maybe "vernier" or "vernier drive" like they use to have in old radios.

Also, planetary gear (reduction) coupling might be another helpful search term.

They attach to the shaft and bring about a substantial reduction in rotation.

Very useful for magnetic loop antennas and similar.

This is true but I wouldn't use these on the OP's capacitor which already contains a reduction drive.

Add-on reduction drives are normally based on the capacitor they're connected to having 1/2 turn to go from min to max capacitance.   1/2 turn is also 0 to 100 on their dial (if there is a dial)

Some drives even have stoppers to prevent them from going more than half a turn (so if you were to use them with a potentiometer which has about 0.8 turns then you need to remove it).

If you were to attach the reduction drive to a capacitor that already has an inbuilt drive you will get very fine tuning due to the greater mechanical reduction.  This may be useful for something like
a magnetic loop or receiver which requires very fine adjustment.  But the pointer will go around more than 360 degrees or the 0 - 100 scale will go off the end as there may be 3 instead of 0.5 turns. 

Not so much a problem if you can determine the frequency another way (eg using a frequency counter in a VFO) but its not ideal if you're just relying on a scale or pointer (unless you can memorise the number of times it's gone around).