SPDT Make Before Break Switch?

S

snowflake

Filing Flight Plan
Joined
May 22, 2012
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3
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Wells, ME
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Display name:
Michael
Can anyone (e.g., Jim Weir) help me locate a switch that has a "make-before-break" capability?

Problem: I am trying to connect a 12 -42 DC volt source to an electronic speed controller that has large capacitors across the input voltage leads - huge spark that is slowly pitting the electrical contacts.

I have kludged together a resistor in the positive lead of about an ohm, or thereabouts (1/2 watt appears to be sufficient) and that suppresses the spark. Next, I would like to cut the resistor out to handle DC currents ranging from 10 amps to 100 amps (but, not at the time of switching the resistor in or out of the circuit.)

My thought is that a SPDT, make-before-break, switch would allow me to have the resistor in the positive lead so that, when I connect the power source, the resistor allows the capacitors to charge without a huge spark. Then, I simply throw the switch (toggle, slide, whatever) to bypass the resistor.

Eventually, I unplug the DC power source, switch back to the "resistor" position, and am ready for the next use.

I've looked at Mauser and DigiKey, but cannot seem to find a switch that will do what I need. I strongly suspect the appropriate switch is staring me in the face, but there is a disconnect between my ears.

Any thoughts, recommendations?

Thank you.

Michael
 
Capt. Geoffrey Thorpe said:
http://www.westmarine.com/webapp/wc...toreNum=50523&subdeptNum=50543&classNum=50547

or

http://www.westmarine.com/webapp/wc...toreNum=50523&subdeptNum=50543&classNum=50547

Switch through the "both" position.
Click to expand...
Appears large, physically. I am looking for a DC switch that does not have to switch a lot of current, but needs to be able to pass it through once the contacts have been made...

I hadn't thought of marine sources, thanks - will broaden my search.
 
snowflake said:
Can anyone (e.g., Jim Weir) help me locate a switch that has a "make-before-break" capability?

Problem: I am trying to connect a 12 -42 DC volt source to an electronic speed controller that has large capacitors across the input voltage leads - huge spark that is slowly pitting the electrical contacts.

I have kludged together a resistor in the positive lead of about an ohm, or thereabouts (1/2 watt appears to be sufficient) and that suppresses the spark. Next, I would like to cut the resistor out to handle DC currents ranging from 10 amps to 100 amps (but, not at the time of switching the resistor in or out of the circuit.)

My thought is that a SPDT, make-before-break, switch would allow me to have the resistor in the positive lead so that, when I connect the power source, the resistor allows the capacitors to charge without a huge spark. Then, I simply throw the switch (toggle, slide, whatever) to bypass the resistor.

Eventually, I unplug the DC power source, switch back to the "resistor" position, and am ready for the next use.

I've looked at Mauser and DigiKey, but cannot seem to find a switch that will do what I need. I strongly suspect the appropriate switch is staring me in the face, but there is a disconnect between my ears.

Any thoughts, recommendations?

Thank you.

Michael
Click to expand...
I'm a little confused about your solution. Are you looking for a 3 position switch (off-on-on) with the first on position connecting power through a current limiting resistor and the second on position bypassing the resistor? If so there's no need for a make before break configuration because the caps won't discharge significantly between the first and second on position. And if your plan was to use something else to disconnect the power source completely all you need is a simple SPST switch that shorts out the resistor.

An alternative concept would be one or more IGBTs in parallel with a simple RC time delay to ramp the current up gradually.
 
You are correct about the three position switch. I tried a conventional SPDT switch, but the caps apparently discharged sufficiently in the short interval of "break" before "make" that there was some sparking that led to switch failure. I had theorized that the caps would retain their charge through the small resistor, but no joy; hence the desire to find a make-before-break switch. I have seen them, but back many years ago during my ham radio days and they were rotary switches; too large a form factor.

The IGBT approach could work, but then adds more complication. I was looking for a KISS solution. :)

Thanks, though, you've given me two things to explore (revisit why the caps discharged so rapidly and the use of IGBTs).

Michael
 
snowflake said:
Can anyone (e.g., Jim Weir) help me locate a switch that has a "make-before-break" capability?

Problem: I am trying to connect a 12 -42 DC volt source to an electronic speed controller that has large capacitors across the input voltage leads - huge spark that is slowly pitting the electrical contacts.

I have kludged together a resistor in the positive lead of about an ohm, or thereabouts (1/2 watt appears to be sufficient) and that suppresses the spark. Next, I would like to cut the resistor out to handle DC currents ranging from 10 amps to 100 amps (but, not at the time of switching the resistor in or out of the circuit.)

My thought is that a SPDT, make-before-break, switch would allow me to have the resistor in the positive lead so that, when I connect the power source, the resistor allows the capacitors to charge without a huge spark. Then, I simply throw the switch (toggle, slide, whatever) to bypass the resistor.

Eventually, I unplug the DC power source, switch back to the "resistor" position, and am ready for the next use.

I've looked at Mauser and DigiKey, but cannot seem to find a switch that will do what I need. I strongly suspect the appropriate switch is staring me in the face, but there is a disconnect between my ears.

Any thoughts, recommendations?

Thank you.

Michael
Click to expand...

I'd do it with a set of relays that way you just push the button and when one powers on it trips the other off.
 
A wafer switch may be the answer you seek.
 
What form factor are you looking for? Small toggle, 22mm hole, 30mm, etc....
 
I've seen this in the context of large rc helicopters (think 50V at 50A). The most common solution I've seen here is to avoid the switch entirely. It's just another moving part/point of failure. Hook up a small wire with an inline connector of some sort with your resistor. Plug it in to charge up the caps, connect the voltage source, unplug the resistor.
 
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