dc switch snubbing questions

Thread Starter

minkey01

Joined Jul 23, 2014
185
Hi.

I have some questions about this yellow highlighted DPDT switch in the attached circuit jpg and contact snubbing.

The top pole is switching 24 DC volt power on in the solenoid. Probably around 727 mA max. Should I be concerned with this switch? I've read that switching DC power can be a problem. Does it need a snubbing circuit in parallel across it?

It looks like the second lower pole of the switch already has one (C409).

Last question, will a new snubbing capacitor turn on the solenoid briefly while it charges up? It would seem like the switch in the off position would still be shorted while the capacitor is charging. I don't want the solenoid to flash activate when the AC power is first plugged in. I only want it to activate when I throw the switch.

Thanks for the help!

contact snubbing.jpg
 

MikeML

Joined Oct 2, 2009
5,444
Upper switch pole already properly snubbed; D18.

Lower switch pole snubber designer should be shot! Wiring capacitors directly across switch contacts is dumb! The peak capacitor discharge current as the contact closes is likely to do damage to the switch contact. The solenoid already has a RC snubber. Likely, they were trying to get rid of a "pop" in the audio.
 
Last edited:

Thread Starter

minkey01

Joined Jul 23, 2014
185
Upper switch pole already properly snubbed; D18.

Lower switch pole snubber designer should be shot! Wiring capacitors directly across switch contacts is dumb! The peak capacitor discharge current as the contact closes is likely to do damage to the switch contact. The solenoid already has a RC snubber. Likely, they were trying to get rid of a "pop" in the audio.
Now I'm more confused. That C409 design came out of a professional reel-to-reel that is over 30 years old and is still working. If it caused damage wouldn't it be dead by now?
 

MikeML

Joined Oct 2, 2009
5,444
How many times do you think that switch has been operated in 30 years?
What is the value of C408 and C409?
 

Thread Starter

minkey01

Joined Jul 23, 2014
185
How many times do you think that switch has been operated in 30 years?
What is the value of C408 and C409?
It operates when you start or stop the machine, so not too active. It activates the solenoid to pull in the pinch roller on the capstan. Maybe 4 or 5 times during a listening session. If they did that five days a week you would get 39,000? idk

C408 Mylar .033 mfd
C409 Mylar .1 mfd

The bottom pole of highlighted switch turns on the relay called "D9 solenoid start." It is for triggering that K9. This gives extra capstan solenoid power boost in the beginning.

The top pole of highlighted switch is the main power for the capstan solenoid.

Here is a picture of the old switch. The reason I am replacing it is because I need a manual one you can flip with your hand.

IMAG0154.jpg
 

Thread Starter

minkey01

Joined Jul 23, 2014
185
1) What if I add this snubber circuit to the top? Will the cap leak in the beginning and activate the solenoid?

2) Do you think that old switch picture is anything special? I would just like to replace it with a new DPDT manual rocker switch and put the C409 across the bottom pole.

Thanks for the help! Have a great day.

new snubber.jpg
 

MikeML

Joined Oct 2, 2009
5,444
1) What if I add this snubber circuit to the top? Will the cap leak in the beginning and activate the solenoid?
Not needed as long as D18 is there. Adding unnecessary stuff just adds risk of failure...

2) Do you think that old switch picture is anything special? I would just like to replace it with a new DPDT manual rocker switch and put the C409 across the bottom pole.

Both
C408 and C409 need to have a low value resistor (10Ω to 22Ω) in series with each. Just like you showed the 120Ω resistor, above. That will cure the contact erosion issue with having the bare-assed capacitors wired directly across the contacts...
 

Thread Starter

minkey01

Joined Jul 23, 2014
185
Not needed as long as D18 is there. Adding unnecessary stuff just adds risk of failure...



Both
C408 and C409 need to have a low value resistor (10Ω to 22Ω) in series with each. Just like you showed the 120Ω resistor, above. That will cure the contact erosion issue with having the bare-assed capacitors wired directly across the contacts...

Cool. Thanks, Mike!

Will these new resistors in series mess with the timing of that D9 relay area?
 
Top