DC rated switches.

Do you own or use a EV. Then this is a good place to discuss things.
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aminorjourney
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DC rated switches.

Postby aminorjourney » Sat Apr 05, 2008 12:19 pm

Hi folks.

The City El's brake switch and accelerator switch seem to be an Achilles heel. I've replaced so many switches in the past two years that I've lost track, but it seems to be that the switches just can't take the DC Voltage they need to.

Obviously, AC rated switches can't cope with anywhere near the voltage or current that a DC EV needs them to, and both the throttle switch (which switches the +43V low current to the controller) and the brake switch (which handles the +12V for the brake circuit) seem to fail with depressing regularity.

I could do several things here:

1) remove the throttle switch. It's only really a safety to make sure the contractors aren't engaged when the car is stationary but in gear. As long as the throttle pot doesn't jam or fail it should be okay. However, it's a bit risky not having one and perhaps a bit questionable on safety grounds. (Shorting out the switch on the way home to get you home doesn't count as it's an emergency fix).

2) Place a relay in the circuit for the accelerator and actually use a DC rated 12V switch to switch the relay to switch the larger 43V to the controller. But that adds another failure point....

3) Do something involving a solid state circuit. Much more robust, and perhaps more suited to life in an EV with very little suspension!


The only problem I have is that I must still have something which will turn the second heater coil on and off as I drive. In City Els the second stage of the heater turns off when you apply the throttle to prevent excess current drain on the batteries. With Lead Acid this is really important as the car only ever develops 120A, but I think the lithium with their increased range and ability to provide larger currents may negate this...

Help?


Nikki.
Nikki Gordon-Bloomfield

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ChrisB
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Postby ChrisB » Sat Apr 05, 2008 1:09 pm

Why are the switches failing Nikki ??

You say its due to voltage but whats the actual problem i.e what stops them working in the end, is it they fall apart, the contacts weld up, burn out etc etc

ChrisB
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Postby aminorjourney » Sat Apr 05, 2008 2:59 pm

Hi Chris,

The switches fail under load at 43V. They will not pass the current required. Probably due to contacts getting dirty due to arcing....
Nikki Gordon-Bloomfield

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ChrisB
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Postby ChrisB » Sat Apr 05, 2008 9:35 pm

Ah so its likely to be caused due to arcing and eventual destruction of the contacts.

Have you thought about installing snubber circuits across the switches to take out the arcing ??

Some interesting reading here
http://focus.ti.com/lit/an/slup100/slup100.pdf
Although it does get quite heavy and some of it isnt that relavant for your case but does give a good back ground into them :wink:

You can knock one up fairly easily with a basic capacitor and resistor across the switch

Image

Might help ??

ChrisB
I reject reality and substitute my own !!!!!!

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EVguru
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Postby EVguru » Sun Apr 06, 2008 2:02 pm

You have to remember that microswitches generally only have a rating for resistive loads on DC and that a lightbulb is not a pure resistive load. The inrush current when the bulb is cold is many times higher.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

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ChrisB
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Postby ChrisB » Sun Apr 06, 2008 2:25 pm

Most definately Paul , tungstans are renowned for very high inrush when cold.

I dont know what type of switches Nikki is using and or what rating they are and what current they are suppose to handle and what current they are now being expected to handle.

ChrisB
I reject reality and substitute my own !!!!!!

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Postby electricvehicles » Sun Apr 06, 2008 5:45 pm

My brake master cylinder has 3 oulets. One for the rear, one for the front and one unused spare. Should be the same as yours ( Fiat 500 ) Why not fit a brake pressure switch onto the spare outlet and do away with the pedal operated switch. Alternatively how about LED stop/tail bulb replacements ??
Regarding the micro switch to interlock with the forward/reverse contactors. I would strongly recommend NOT removing it. I know the controller has pot failure, broken wiper sensing, which should shut the controller down, but I have seen them not work as they should.
We use Cherry D49 switchs which are rated at 6A @ 30V Dc.
If you do want to try one let me know and I will pop one in the post, FOC I Hasten to Add :)

regards
Dave

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aminorjourney
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Postby aminorjourney » Mon Apr 07, 2008 6:32 am

Cheers Dave. Sadly the switches sound like the ones I have. They just can't survive the extra voltage I'm throwing at them :(

It's a common failure point, even at 36V. So it looks like a snubber then???

Nikki.


Oh, and the master cylinder - you mean on the El? Is it the same as Fiat 500? (The old fiat 500, I presume?)
Nikki Gordon-Bloomfield

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electricvehicles
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Postby electricvehicles » Mon Apr 07, 2008 9:49 am

Hi Nikki

Just a quick question, are you switching anything other that the SW182 forward/reverse contactor with the accelerator micro switch ?
Because according to my figures the:
SW182 ( intermitten duty ) is rated at 30-40W making a loading of only 1 amp at your battery voltage
SW182 ( continous duty ) is rated at 10-15W making a loading of only .38 amps at your battery voltage.
Even allowing for a very large inrush a 10A DC type V4 micro switch should easily handle the loading.

Regards
Dave

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Postby EVguru » Mon Apr 07, 2008 10:26 am

Contactors are an inductive load, so you don't get an inrush surge.

Do check the freewheel diodes though. If one of those is open circuit you'll soon have a switch failure.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named


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