Compatibility of Modern DPDT switch for a 1967 Project

Thread Starter

Ray Bryant

Joined Jan 31, 2019
8
Hello,

I'm working on a project whose schematics were published in 1967, the blueprints call for a DPDT switch. The switches in the diagrams appear to be knife switches, which I have found online for sale, however they are about 4 dollars more expensive than alternative modern style DPDT switches. Because I need to buy around 50 switches I want to get the modern ones and I want to make sure this is a safe move. I've attached pictures of the old and new switch, as far as I know their properties are the same.
 

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Yaakov

Joined Jan 27, 2019
1,612
As long as the ratings are the same, they are equivalent with the proviso that the toggle switch is specified as "break before make", meaning it will never have the contacts on at the same time.

Also, make sure the switch operates, mechanically, in the direction you expect so the lever makes sense in the application.
 

Reloadron

Joined Jan 15, 2015
5,604
Also, make sure the switch operates, mechanically, in the direction you expect so the lever makes sense in the application.
That's a good point. On a DPDT switch like pictured when the switch is to the left as in the picture the center (common) contacts are generally made to the contacts on the right. Also, consider if you need a switch with a center off which is what you get with a knife switch when the switch is not closed on either side. Finally as mentioned, the current ratings and that's about it.

Ron
 

Yaakov

Joined Jan 27, 2019
1,612
Also, consider if you need a switch with a center off
Yes, this is a very good point, and ON-OFF-ON configuration is probably the best choice to reproduce all the possibilities of the knife. Unless the center off function of the knife switch was an unwanted side effect in the original design.
 

djsfantasi

Joined Apr 11, 2010
6,691
Yes, this is a very good point, and ON-OFF-ON configuration is probably the best choice to reproduce all the possibilities of the knife. Unless the center off function of the knife switch was an unwanted side effect in the original design.
Might be nice to see the schematic. Perhaps we could discern the actual requirement.
 

Yaakov

Joined Jan 27, 2019
1,612

Well, this project doesn't have critical electrical requirements for the switches, it is the state of each switch that needs to be reproduced. In the context of logic gates, I am guessing there is no valid case for OFF in the ON-OFF-ON geometry of the knife switch but not having read the entire text I'd encourage you to make sure they never specify the center position for testing or troubleshooting, or experimentation.

It would seem that no OFF position is actually a benefit.
 

Thread Starter

Ray Bryant

Joined Jan 31, 2019
8
I just watched a youtube video of someone demonstrating a project and it does not appear to need the central 0 position. I'm going to read through the text one more time and do some further research. Thanks for the help everyone.
 

rphare

Joined Nov 20, 2015
9
Of course, we are all curious what 1967 project used 50 knife switches!
The "center off" feature that Reloadron mentions is usually described as "ON-OFF-ON" in the switch literature. Base on the legend plate in your switch graphic, that particular switch may be "ON-NONE-ON", i.e. it toggles from one side or the other but has no stable center position.
 

Reloadron

Joined Jan 15, 2015
5,604
That is pretty cool and brings back memories. The book is, as mentioned, an E book and can be simply viewed online. Yes, just about any DPDT switch will work as you are not switching any current. Personally I would go with the mini switches you linked to. Again, really cool project.

Ron
 

shortbus

Joined Sep 30, 2009
7,840
You could do this with a micro controller. Couldn't help myself.:) To the TS, this is a common thing said here quite often, but my saying it is meant as a joke.
 
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