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LTspice and pulse voltage

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lk:
Greetings,
I was playing around with LT Spice, and i wanted to simulate a relay switch between two power sources, i planned to do it with a negative pulse on the voltage source, i don't know if this is the right way to do this. According to the datasheet for the relay the switch time is 1ms.

but what i do know is that the massive spikes can not be good at all, but i guess that is normal behavior when you cut the current path with an inductor in there.


I guess my question is, does this setup properly simulate the relay action, eg the 1 ms switch time where the power is "disconnected". Maybe there is a problem with my raise and fall times, could it be that my load is infact without power for 3 ms?

this is 24 volts and i peak load is 6 A.

-l


PA0PBZ:
Congratulations, you just invented the boost converter.  :)
I'm not sure what you are trying to do, but yes, these spikes are to be expected in this setup.
If you are trying to switch a relay, shouldn't the relay coil be parallel to the supply?

lk:
Doh :)
No I'm not trying to switch the relay, I'm trying to make the "output" of my relay drop voltage as little as possible when the "relay" switches, that's why I'm talking about the 1ms switching time. So I'm trying to figure out if i need to do something to my output to keep it high, or just hope that the 24v PC power supply in this case can survive. That's why I'm messing with the pulses.

So i have 2 independent 24 volt inputs that i need the relay to switch between to the same output, that switch takes 1ms, so i was trying this to see what i could do so that i see as little drop on the output as possible. 

i hope this makes some sense

-lk

TerminalJack505:
I'm not sure I understand what you're trying to do but if you're trying to keep the relay's coil energized while you switch between two power sources then you might try the attached schematic. 

It will, in a sense, allow you to power the relay from two power sources simultaneously.  In fact, the power supply with the higher voltage will power the coil.  If one power source falls off then the other will pick-up automatically. 

You can use the circuit for switching between two power sources as well.  You can bring one power source online while the other power source is still online thus preventing the relay from dropping out.

The freewheeling diode across the coil will keep the coil energized for a short while in the case where both power supplies fall off.  (You need it anyway to avoid the voltage spikes.)

jahonen:
If you want to simulate a relay contact changeover, use the voltage controlled switch component, "SW". You just need to parametrize it.

Regards,
Janne

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