Need an automatic off switch

[DeafBrad]

Hi Everyone,

I have a 2011 Think City Electric car that has a bug. It is a known problem with these cars where if you turn the car on with the heater controls set to on, you can fry the pre-charge resistor in the battery pack. The result is a bricked car with the possibility of it never running again.

The solution to the problem is to always make sure the heater is off before starting the car. As long as the heater is off before you start the car you are fine. But all it takes is forgetting one time!

I would like to wire in a switch to the wire running to the heater controls to ensure there is no power to the heater when the car is started. I have been looking online for a momentary kind of switch that resets itself to off when the car is switched off and stays off until the switch is turned on.

I found an scr switch that looks like will work. (also called a thyristor switch). When turned on, it stays on until the power is cut. When the power is cut, the switch reverts back to off and stays off until it is turned on again.

I need someone knowledgeable to tell me if this will work, or give me other ideas of what I can do. The heater fan circuit has a 12v, 20 amp fuse so the switch would need to be able to handle that. If there are other ideas, it would need to be automatic to ensure the heater is always off when the car is turned on.

Thanks for any help you can provide!

Brad

 

[ctechbob]

A latching relay setup would work as well. You could build it out of a couple(or 3) of standard 12v relays and a momentary switch/button. Push the button and the relay latches on until power is cut to it, whereupon it would stay in an off state.

Standard 12v Bosch relays are good for 20-30 amps so you'd have plenty of current capability, and they're rated for continuous use.

 

[Danno]

Unless you value the heated seats more than the car, an option is to permanently disable the heated seats.

 

[earlyre]

who said anything about heated seats danno?

 

[SeaJay]

Originally Posted by earlyre
who said anything about heated seats danno?


No heated seats? What kind of a barbaric car is that.

 

[ragtoplvr]

Thyristors are used for AC not DC. Will probably not work unless you can verify something else turns off ALL power when the car is off. How much current does the heater draw. This determines the size of the relay. If for example the heater draws 50 amps, this takes a much higher power and more expensive relay

 

[EdwardC]

Would a time delay relay work? Something like the below could be set to delay the relay power-on for some amount of time, giving you time to boot your car. If you need to switch more current, you could throw another relay in between.

https://www.amazon.com/Timer-Relay-...mp;psc=1&refRID=CYXQ50Z123XZDV2Q1JS2

 

[SpitFire6]

Originally Posted by ragtoplvr
Thyristors are used for AC not DC. Will probably not work unless you can verify something else turns off ALL power when the car is off. How much current does the heater draw. This determines the size of the relay. If for example the heater draws 50 amps, this takes a much higher power and more expensive relay



Sorry but the thyristor can be used for DC as OP suggested.
In fact, its a rather poor choice for AC when better semiconductors are out there. Triac for example.

 

[DeafBrad]

Originally Posted by ctechbob
A latching relay setup would work as well. You could build it out of a couple(or 3) of standard 12v relays and a momentary switch/button. Push the button and the relay latches on until power is cut to it, whereupon it would stay in an off state.

Standard 12v Bosch relays are good for 20-30 amps so you'd have plenty of current capability, and they're rated for continuous use.


Great information! So would a dpdt relay like this work? https://www.ebay.com/itm/US-Stock-JQX-12F-2Z-DC-12V-30A-DPDT-General-Purpose-Power-Relay-8-Pin/372001097799?hash=item569cfe4847:g:2mQAAOSwbopZWgDb ??

 

[DeafBrad]

Originally Posted by EdwardC
Would a time delay relay work? Something like the below could be set to delay the relay power-on for some amount of time, giving you time to boot your car. If you need to switch more current, you could throw another relay in between.

https://www.amazon.com/Timer-Relay-...mp;psc=1&refRID=CYXQ50Z123XZDV2Q1JS2


It is a possibility. One Think owner did that. His solution I believe eliminates most of the problem, but not all of it. I believe the resistor could still blow if the car is switched to the on position but the drive motor is not started. The first on position boots up the car, and the second on position is what actually activates the drive motor. I suppose by finding a circuit that is only active when the drive motor is on would work.

 

[ctechbob]

Originally Posted by DeafBrad
Originally Posted by ctechbob
A latching relay setup would work as well. You could build it out of a couple(or 3) of standard 12v relays and a momentary switch/button. Push the button and the relay latches on until power is cut to it, whereupon it would stay in an off state.

Standard 12v Bosch relays are good for 20-30 amps so you'd have plenty of current capability, and they're rated for continuous use.


Great information! So would a dpdt relay like this work? https://www.ebay.com/itm/US-Stock-JQX-12F-2Z-DC-12V-30A-DPDT-General-Purpose-Power-Relay-8-Pin/372001097799?hash=item569cfe4847:g:2mQAAOSwbopZWgDb ??


https://www.ebay.com/sch/i.html?_fr...S0&_nkw=bosch+12v+relay&_sacat=0

More along the lines of this. You'd need 3 total. The wiring could get a little confusing but its all logical.

 

[DeafBrad]

Originally Posted by ragtoplvr
Thyristors are used for AC not DC. Will probably not work unless you can verify something else turns off ALL power when the car is off. How much current does the heater draw. This determines the size of the relay. If for example the heater draws 50 amps, this takes a much higher power and more expensive relay



The heater is probably 400v and 3kw. The circuit I would control is actually the fan on/off and fan speed control. My assumption is there is a relay running off the fan circuit to activate the heater element. So controlling the fan circuit I should only need a relay to that can handle 12v and 20 amps.

 

[sloinker]

You could just build a start circuit with a push button and holding contact via a relay like a regular 3 wire control for motor starters. Series the appropriate sized contactor in the heater high leg. Control voltage will dictate the availability of the parts you can use.

 

[ctechbob]

Very quick, very dirty drawing on how to use 3 standard relays and a switch to get this done.

Things you will need from the car.

- Fused high current 12v source
- Switched 12v source, this will need to be energized when the car is in the normal 'run' position and no voltage when the car is off
- Ground

Parts needed

3 - 12v Bosch style relays. (I would seek out real Bosch branded ones. Local car audio store can supply these, very common there)
Momentary push button switch.

The first relay is simply a trigger for the 3rd relay.

The second relay energizes with key on to provide high current 12v to the last relay. When the key turns off, the last relay loses its power and unlatches.

The 3rd relay takes the input and latches itself on and doesn't turn off until power is cut to it.

There is a way you could use 2 relays and do it as well, but I prefer to switch grounds whenever possible instead of using switches with positive 12v and the 3 relays keep everything isolated a little better.

 

[sloinker]

You could also use a Time Delay On relay.

 

[ctechbob]

Now that I get to looking at it, does that car even have a 12v subsystem? If not, you can disregard anything I posted. I'm stuck in ICE world with 'normal' systems.

 

[DeafBrad]

Originally Posted by ctechbob
Now that I get to looking at it, does that car even have a 12v subsystem? If not, you can disregard anything I posted. I'm stuck in ICE world with 'normal' systems.


Thanks for your information and diagram. Yes, the car does have a 12v system. Electric cars have regular 12v batteries that runs lights, and stuff like that. Believe it or not, electric cars will not start with a flat 12v battery and need to be jumped. Electric cars have AC, and DC, so they have a DC to AC converter (for the drive motor), and a DC to DC converter (to step down from 400vdc to 12vdc). While simpler than gas cars, there is still a lot going on in electric cars.

 

[javacontour]

Looks like the big issue is if you turn the key off and then back on in a matter of say 5 seconds.

The heater can keep running for a while after the key is off.

So I think the solution is to still make sure you power off the heater before you shut off the vehicle, and leave it off until you get underway.

The three relays or the time delay relay solution should work should you want to employ them. Apparently, there are no more MLEC boards out there, so preserve what you have.