# [EVDL] contactor control circuit



## EVDL List (Jul 27, 2007)

I'm working on a simple "electronic control module" for my car. I want it
to take care of turning on/off the contactor and other accessories based
on several inputs (such as the pot low switch, key switch, etc). I'm
working out the input and output circuits and have a question about a
couple of my ideas.

first, it has been said that a diode across the contactor coil is not the
best because it slows down the turn off time (which I understand). What it
I had a 50V MOSFET to pull down the contactor to turn it on with a 40V (or
somewhere just a bit lower than the FET max voltage) anti-parallel with
the FET? Like this:

+12V
|
)))))) (contactor coil)
|
+---+
| |
|-- _/
| / \ (~40V zener)
---|-| ---
|-+ |
| |
(NMOS) +---+
|
ground

I'm thinking this will help the contactor turn off faster while protecting
the FET. I was also wondering if it would be a benefit to add a capacitor
across the coil, to help smooth out spikes. Basically the cap is to try to
make the reactive load look more resistive in theory.

Second, regarding high voltage inputs. I want to detect if the controller
is shorted on. I intend on using opto-isolators for all HV inputs, so to
drive them I came up with a simple resistor network like this:

(voltage divider)
(+HV)--/\/\/---+---/\/\/-----+--(HV-)
R1 | R2 |
+-/\/\/--|>|--+
R3 (opto diode)
(current limit resistor for diode)

The voltage divider will drop the high voltage to a more reasonable
voltage for the opto-isolator (ie 150V in => 4.4V out is R1=33k, R2=1k.
I=4mA. R3=330). This should simply indicate if there is high voltage
present or not. The big question I have is about the two isolated voltages
(HV and logic supply). If they are really isolated, how much potential
could possibly be between them? How much isolation does the opto-isolator
need to provide?

Thanks for any advise!

-Jon Glauser
http://www.evalbum.com/555



_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

The customary circuit of late is to put a regular diode in series with a
zener diode. The zener is rated at the voltage of the FET or transistor you
are using to turn on the contactor. I usually use a zener rated at 2/3 the
transistor/FET value. When the voltage of the diode zener is reached the
diode turns on and does it's work.

Mark Grasser



-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Jon Glauser
Sent: Thursday, March 13, 2008 11:08 AM
To: [email protected]
Subject: [EVDL] contactor control circuit


I'm working on a simple "electronic control module" for my car. I want it
to take care of turning on/off the contactor and other accessories based
on several inputs (such as the pot low switch, key switch, etc). I'm
working out the input and output circuits and have a question about a
couple of my ideas.

first, it has been said that a diode across the contactor coil is not the
best because it slows down the turn off time (which I understand). What it
I had a 50V MOSFET to pull down the contactor to turn it on with a 40V (or
somewhere just a bit lower than the FET max voltage) anti-parallel with
the FET? Like this:

+12V
|
)))))) (contactor coil)
|
+---+
| |
|-- _/
| / \ (~40V zener)
---|-| ---
|-+ |
| |
(NMOS) +---+
|
ground

I'm thinking this will help the contactor turn off faster while protecting
the FET. I was also wondering if it would be a benefit to add a capacitor
across the coil, to help smooth out spikes. Basically the cap is to try to
make the reactive load look more resistive in theory.

Second, regarding high voltage inputs. I want to detect if the controller
is shorted on. I intend on using opto-isolators for all HV inputs, so to
drive them I came up with a simple resistor network like this:

(voltage divider)
(+HV)--/\/\/---+---/\/\/-----+--(HV-)
R1 | R2 |
+-/\/\/--|>|--+
R3 (opto diode)
(current limit resistor for diode)

The voltage divider will drop the high voltage to a more reasonable
voltage for the opto-isolator (ie 150V in => 4.4V out is R1=33k, R2=1k.
I=4mA. R3=330). This should simply indicate if there is high voltage
present or not. The big question I have is about the two isolated voltages
(HV and logic supply). If they are really isolated, how much potential
could possibly be between them? How much isolation does the opto-isolator
need to provide?

Thanks for any advise!

-Jon Glauser
http://www.evalbum.com/555



_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> Mark Grasser wrote:
> 
> > The customary circuit of late is to put a regular diode in
> > series with a zener diode. The zener is rated at the voltage
> ...


----------



## EVDL List (Jul 27, 2007)

Use a NTE4933 Zener Overvoltage Transient Suppressor which is Bidirectional 
for this type of circuit. This type number is design for 12 volt coils.

Roland


----- Original Message ----- 
From: "Mark Grasser" <[email protected]>
To: <[email protected]>; "'Electric Vehicle Discussion List'" 
<[email protected]>
Sent: Thursday, March 13, 2008 3:06 PM
Subject: Re: [EVDL] contactor control circuit


> The customary circuit of late is to put a regular diode in series with a
> zener diode. The zener is rated at the voltage of the FET or transistor 
> you
> are using to turn on the contactor. I usually use a zener rated at 2/3 the
> transistor/FET value. When the voltage of the diode zener is reached the
> diode turns on and does it's work.
>
> Mark Grasser
>
>
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On 
> Behalf
> Of Jon Glauser
> Sent: Thursday, March 13, 2008 11:08 AM
> To: [email protected]
> Subject: [EVDL] contactor control circuit
>
>
> I'm working on a simple "electronic control module" for my car. I want it
> to take care of turning on/off the contactor and other accessories based
> on several inputs (such as the pot low switch, key switch, etc). I'm
> working out the input and output circuits and have a question about a
> couple of my ideas.
>
> first, it has been said that a diode across the contactor coil is not the
> best because it slows down the turn off time (which I understand). What it
> I had a 50V MOSFET to pull down the contactor to turn it on with a 40V (or
> somewhere just a bit lower than the FET max voltage) anti-parallel with
> the FET? Like this:
>
> +12V
> |
> )))))) (contactor coil)
> |
> +---+
> | |
> |-- _/
> | / \ (~40V zener)
> ---|-| ---
> |-+ |
> | |
> (NMOS) +---+
> |
> ground
>
> I'm thinking this will help the contactor turn off faster while protecting
> the FET. I was also wondering if it would be a benefit to add a capacitor
> across the coil, to help smooth out spikes. Basically the cap is to try to
> make the reactive load look more resistive in theory.
>
> Second, regarding high voltage inputs. I want to detect if the controller
> is shorted on. I intend on using opto-isolators for all HV inputs, so to
> drive them I came up with a simple resistor network like this:
>
> (voltage divider)
> (+HV)--/\/\/---+---/\/\/-----+--(HV-)
> R1 | R2 |
> +-/\/\/--|>|--+
> R3 (opto diode)
> (current limit resistor for diode)
>
> The voltage divider will drop the high voltage to a more reasonable
> voltage for the opto-isolator (ie 150V in => 4.4V out is R1=33k, R2=1k.
> I=4mA. R3=330). This should simply indicate if there is high voltage
> present or not. The big question I have is about the two isolated voltages
> (HV and logic supply). If they are really isolated, how much potential
> could possibly be between them? How much isolation does the opto-isolator
> need to provide?
>
> Thanks for any advise!
>
> -Jon Glauser
> http://www.evalbum.com/555
>
>
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
> 

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> > Mark Grasser wrote:
> >
> > The customary circuit of late is to put a regular diode in
> >
> ...


----------



## EVDL List (Jul 27, 2007)

> Jon Glauser wrote:
> > What it I had a 50V MOSFET to pull down the contactor to turn it on
> > with a 40V [zener diode] anti-parallel with the FET?
> 
> ...


----------



## EVDL List (Jul 27, 2007)

Can I ask a basic question that this thread already
assumes/understands? (...and I don't)

Most high level schematics of EVs I see show simple,
direct connectivity of the contactor coil to power and
a switch.

Is adding circuitry more the norm now?

And, is the circuitry just to aid in the SPEED that a
contactor will open or close by controlling the energy
into the coil? If so, why is this critical? I mean,
how much faster can it be?

Or is it a protective measure to the coil, or
contactor switch, or the power source.

Just a few sentence summary of what zener diodes (or
potentially FETs, as proposed in this thread) are
accomplishing here would be GREAT! Also, if you have
one handy, a web URL that points to a typical
schematic would be also appreciated.

Thanks for your patience,

Scott


____________________________________________________________________________________
Be a better friend, newshound, and 
know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ 

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> S Collins wrote:
> > Most high level schematics of EVs I see show simple,
> > direct connectivity of the contactor coil to power and
> > a switch.
> ...


----------



## EVDL List (Jul 27, 2007)

Most EVs really use the simple switch method of turning on the contactors.
Unless you have a Zilla controller, then you have a little computer (kinda
like the ECU in gas cars) running everything (the Hairball).

Usually there is some kind of diode (one way valve), zener diode(a diode
with a specific reverse turn-on voltage), or other surge suppressor across
the coil to reduce the voltage spikes coils make when they turn off. These
voltage spikes can damage other things connected to the 12V system.

A MOSFET is just an electronic switch (a transistor).

In my case, I'm trying to build an ECU (Electronic Control Unit) for my
car similar to the Hairball. My circuit is simply something that would
allow a low power 5V microcontroller to turn on such a large load as the
contactor. The microcontroller (mini computer) puts out a small signal to
the FET, which turns on the "high" power contactor. Here something is
required to keep the voltage from getting too high or it will destroy the
MOSFET and possibly my ECU.

The reason I bring up the opening speed of the contactor is mostly for the
emergency situations. The faster it opens with current flowing through it,
the less likely it arcs itself to death (AFAIK).
If there is much voltage present across an open contactor, then when it
closes it will arc some. If there is enough voltage across it, it could
weld itself shut. But it's all a mater of energy, which has a relationship
to time. So since you cant lower the voltage of your battery pack, you can
reduce the time, thereby reducing the energy put into the arc. The best
way to reduce arcing is to avoid opening/closing them with any voltage
across them.

I hope I answered your questions in my rambling 

-Jon Glauser
http://www.evalbum.com/555

<quote who="S Collins">
> Can I ask a basic question that this thread already
> assumes/understands? (...and I don't)


_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> S Collins wrote:
> 
> > Most high level schematics of EVs I see show simple,
> > direct connectivity of the contactor coil to power and
> ...


----------



## EVDL List (Jul 27, 2007)

<quote who="Lee Hart">
> The only drawback to putting the zener across the MOSFET instead of the
> coil is that the power dissipation in the zener is higher.

So is there any advantage then to putting the zener across the MOSFET
instead of something across the coil?

Is there any real benefit gained by trying to make it open/close faster if
there are other systems in place to make sure it only opens/closes with no
voltage across the contactor? Or am I chasing a diminishing return?

When people say they put a zener back to back with a regular diode, which
directions are they facing?

How does the surge suppressor compare to the zener+diode or does it really
matter?

> There's no need for R2 or R3; a single resistor will limit LED current
> to the desired value. But it is common to split R1 into two resistors,
> one in the + and one in the - lead to the opto's LED.

Thanks for pointing out something that should have been obvious  Silly me!

> Finally, you can find optocouplers that work adequately with much lower
> currents than 4ma. The 6N139 for example is fully specified at 0.4ma of
> LED current; 1/10th of what you assumed.

Again, thanks. I dont know very much about opto-isolators (yet). This
example gives me something to start with.

-Jon Glauser
http://www.evalbum.com/555

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

I built a precharge controller that does a lot of
useful things. 
When the car's "run" turns on:
1. it first waits to make sure the throttle-off switch
is on, so it never turns on anything until the
throttle is off.
2. Then it turns on a relay that precharges the
controller through a resistor and measures the current
flowing. If it never stops flowing this is a fault,
and it reports it.
3. Once the precharge current stops flowing, it will
turn on the main contactor using a FET.
4. After a delay, then it PWM's the FET to reduce the
current draw by the coil from 1amp to 600ma.
5. It then monitors the battery current and reports
it.
6. If the throttle-off switch goes on and battery
current is still flowing, this is a fault, and it
shuts off the main contactor.
7. It monitors the motor RPM and reports it.
8. If the RPM exceeds its limit, it turns off the main
contactor.

I don't use a zener or diode on the coil, I do have a
pulldown resistor on it, so it makes sure it turns off
the main contactor when power is off to the precharge
controller. Should I put a 15v zener on it also to
shunt overvoltage? I has worked without any problem.

Jack




> --- Doug Weathers <[email protected]> wrote:
> 
> >
> >
> ...


----------



## EVDL List (Jul 27, 2007)

By putting the zener across the FET you save the FET but you haven't done
anything to quench the energy being generated by the coil. This energy would
be pushed out the positive rail by pushing against the FET and then
Negative. I personally wouldn't do it. I am perfectly happy with the zener
butted against a diode and then across the relay but my situation is taking
place on a PCB. There are some web sites out there that show the circuit and
theory. Here is the one I found.

http://www.jaxxeninc.com/images/tech_info.pdf

Mark Grasser
Balyntec
Marine Products, LLC
828-581-4601
[email protected]

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Jon Glauser
Sent: Friday, March 14, 2008 12:43 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] contactor control circuit

<quote who="Lee Hart">
> The only drawback to putting the zener across the MOSFET instead of the
> coil is that the power dissipation in the zener is higher.

So is there any advantage then to putting the zener across the MOSFET
instead of something across the coil?

Is there any real benefit gained by trying to make it open/close faster if
there are other systems in place to make sure it only opens/closes with no
voltage across the contactor? Or am I chasing a diminishing return?

When people say they put a zener back to back with a regular diode, which
directions are they facing?

How does the surge suppressor compare to the zener+diode or does it really
matter?

> There's no need for R2 or R3; a single resistor will limit LED current
> to the desired value. But it is common to split R1 into two resistors,
> one in the + and one in the - lead to the opto's LED.

Thanks for pointing out something that should have been obvious  Silly me!

> Finally, you can find optocouplers that work adequately with much lower
> currents than 4ma. The 6N139 for example is fully specified at 0.4ma of
> LED current; 1/10th of what you assumed.

Again, thanks. I dont know very much about opto-isolators (yet). This
example gives me something to start with.

-Jon Glauser
http://www.evalbum.com/555

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> Jon Glauser wrote:
> 
> > why a regular diode in series with the zener
> 
> ...


----------



## EVDL List (Jul 27, 2007)

Lee, I think you explain this in a recent post, but I want to make sure 
I understand:
For the most part, the contactor's load contacts will be opening with no 
current across them, because the controller will already have ramped 
down when you let your foot off the throttle. So the arc you're talking 
about happens even without current, because of the voltage across the 
contacts coupled with the permissivity of air. Do contacts arc on 
closing due to the voltage, or not because they're starting out with 0 
volts across them?

Thanks.

Bill Dennis

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> Mark Grasser wrote:
> 
> > There are some web sites out there
> > that show the circuit and theory. Here is the one I found.
> ...


----------



## EVDL List (Jul 27, 2007)

Thanks for all who replied...

So if I understand correctly, straight on/off
switching allows for high-voltage spikes in the coil
due to inductive kickback inherent to coils, thus
putting the coil wire itself at risk of early failure
and exposing nearby electronics to EMF pulses(?).

Then, if we use a basic (but properly sized) diode to
prevent kickback, we run the risk of a too-slow
"break" of the contact connection, thus allowing for
more arc than would normally be the case for straight
on/off control. This risks early failure of the
contactor "points", including a potential for them to
weld together (not good if you REALLY need to turn off
your EV .

So, then a move to some sort of Zener/FET design is
what is being discussed here to deal with both issues
- avoid/minimize kickback problems and keep the speed
up on the contactors opening up.

I hope I got it right...

Thanks,

Scott



____________________________________________________________________________________
Looking for last minute shopping deals? 
Find them fast with Yahoo! Search. http://tools.search.yahoo.com/newsearch/category.php?category=shopping

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

Sounds right to me.

Mark Grasser



-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of S Collins
Sent: Friday, March 14, 2008 2:50 PM
To: [email protected]
Subject: Re: [EVDL] contactor control circuit

Thanks for all who replied...

So if I understand correctly, straight on/off
switching allows for high-voltage spikes in the coil
due to inductive kickback inherent to coils, thus
putting the coil wire itself at risk of early failure
and exposing nearby electronics to EMF pulses(?).

Then, if we use a basic (but properly sized) diode to
prevent kickback, we run the risk of a too-slow
"break" of the contact connection, thus allowing for
more arc than would normally be the case for straight
on/off control. This risks early failure of the
contactor "points", including a potential for them to
weld together (not good if you REALLY need to turn off
your EV .

So, then a move to some sort of Zener/FET design is
what is being discussed here to deal with both issues
- avoid/minimize kickback problems and keep the speed
up on the contactors opening up.

I hope I got it right...

Thanks,

Scott




____________________________________________________________________________
________
Looking for last minute shopping deals? 
Find them fast with Yahoo! Search.
http://tools.search.yahoo.com/newsearch/category.php?category=shopping

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

And just because you guys were giving me a hard time I second guessed myself
and went out to the lab to check reality against theory. Yes the diode
conducts at .7vdc or at its rated value if connected correctly.

Thought for a minute I was getting old again.

Mark Grasser



-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Roger Stockton
Sent: Friday, March 14, 2008 3:00 PM
To: 'Electric Vehicle Discussion List'
Subject: Re: [EVDL] contactor control circuit



> Mark Grasser wrote:
> 
> > There are some web sites out there
> > that show the circuit and theory. Here is the one I found.
> ...


----------



## EVDL List (Jul 27, 2007)

> Bill Dennis wrote:
> 
> > Lee, I think you explain this in a recent post, but I want to
> > make sure I understand:
> ...


----------



## EVDL List (Jul 27, 2007)

Just about. The MOSFET is only there because we want to control the
contactor with a low power signal such as from more circuitry. You really
only need the diode/transorb/zener and any normal switch.

And contact welding only happens on CLOSURE under load (I think I read
that on the list some time ago). Opening under load just burns the heck
out of the contacts.

-Jon Glauser
http://www.evalbum.com/555

<quote who="S Collins">
> This risks early failure of the
> contactor "points", including a potential for them to
> weld together (not good if you REALLY need to turn off
> your EV .
>
> So, then a move to some sort of Zener/FET design is
> what is being discussed here to deal with both issues
> - avoid/minimize kickback problems and keep the speed
> up on the contactors opening up.



_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

Yeah, I understand now that I see how they are arranged . Very nice
document giving an overview of several snubber circuits! Thanks!

-Jon Glauser
http://www.evalbum.com/555

<quote who="Roger Stockton">


> > Mark Grasser wrote:
> >
> >> There are some web sites out there
> >> that show the circuit and theory. Here is the one I found.
> ...


----------



## EVDL List (Jul 27, 2007)

> --- Roger Stockton <[email protected]> wrote:
> 
> >
> > There are two possible arcs that have been
> ...


----------



## EVDL List (Jul 27, 2007)

> Jeff Major wrote:
> 
> > One shouldn't forget the arcing of the load contacts when
> > they are closed with a capacitive load. Hence the reason for
> ...


----------



## EVDL List (Jul 27, 2007)

> Mark Grasser wrote:
> > By putting the zener across the FET you save the FET but you haven't
> > done anything to quench the energy being generated by the coil.
> 
> ...


----------



## EVDL List (Jul 27, 2007)

> Bill Dennis wrote:
> > Lee, I think you explain this in a recent post, but I want to make
> > sure I understand: For the most part, the contactor's load contacts
> > will be opening with no current across them, because the controller
> ...


----------



## EVDL List (Jul 27, 2007)

Sounds like what I'm trying to put together. Willing to share any of your
design? I had not considered Kilovac or other non-Albirght contactors,
thanks for the heads-up! I'll probably put in an option to enable/disable
my onboard economizer.

If I understand you, what you have is a half bridge. If you switch the FET
across the coil inverse of the other FET and remove the diode you're
there.

I'm looking into using a transorb instead of zeners and diodes.

-Jon Glauser
http://www.evalbum.com/555


<quote who="Jack Murray">
> I built a precharge controller that does a lot of
> useful things.
> When the car's "run" turns on:
> 1. it first waits to make sure the throttle-off switch
> is on, so it never turns on anything until the
> throttle is off.
> 2. Then it turns on a relay that precharges the
> controller through a resistor and measures the current
> flowing. If it never stops flowing this is a fault,
> and it reports it.
> 3. Once the precharge current stops flowing, it will
> turn on the main contactor using a FET.
> 4. After a delay, then it PWM's the FET to reduce the
> current draw by the coil from 1amp to 600ma.
> 5. It then monitors the battery current and reports
> it.
> 6. If the throttle-off switch goes on and battery
> current is still flowing, this is a fault, and it
> shuts off the main contactor.
> 7. It monitors the motor RPM and reports it.
> 8. If the RPM exceeds its limit, it turns off the main
> contactor.


_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> Jeff Major wrote:
> > --- Roger Stockton <[email protected]> wrote:
> >
> >> There are two possible arcs that have been
> ...


----------



## EVDL List (Jul 27, 2007)

> Jon Glauser wrote:
> > And contact welding only happens on CLOSURE under load (I think I read
> > that on the list some time ago). Opening under load just burns the heck
> > out of the contacts.
> ...


----------



## EVDL List (Jul 27, 2007)

> Josh Wyatt wrote:
> 
> > One question I've always wondered: What is the purpose of all
> > those large caps in the controller?
> ...


----------



## EVDL List (Jul 27, 2007)

Is this because discharged caps would allow a large
in-rush of current, thus allowing the arc? Do
precharged caps smooth/snub the current out with some
sort of damper action?

Scott


>One shouldn't forget the arcing of the load contacts
>when they are closed with a capacitive load. Hence
>the reason for precharging the caps in the
controller.




____________________________________________________________________________________
Be a better friend, newshound, and 
know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ 

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> Roger Stockton wrote:
> > Josh Wyatt wrote:
> >
> >> One question I've always wondered: What is the purpose of all
> ...


----------



## EVDL List (Jul 27, 2007)

> Josh Wyatt <[email protected]> wrote:
> > Perhaps there might be a resistor permanently in series between B+ and the + side of the cap bank?
> 
> That's exactly right; this "resistor" is actually the cable from the
> ...


----------



## EVDL List (Jul 27, 2007)

> S Collins wrote:
> > Is this because discharged caps would allow a large
> > in-rush of current, thus allowing the arc? Do
> > precharged caps smooth/snub the current out with some
> ...


----------



## EVDL List (Jul 27, 2007)

Morgan LaMoore wrote:
>


> Josh Wyatt <[email protected]> wrote:
> >> Perhaps there might be a resistor permanently in series between B+ and the + side of the cap bank?
> >
> > That's exactly right; this "resistor" is actually the cable from the
> ...


----------



## EVDL List (Jul 27, 2007)

Hi Scott,

When you close a switch connecting a source (battery)
to an uncharged capacitor, you will get an arc across
the contacts as they close. There is probably a
proper technical explanation, but this is it in a
nutshell. The voltage across the capacitor cannot
change instantly. So as it is connected to the
battery voltage, some current must flow before the
switch, or contacts, are closed. This charge current
flows in the form of an arc as the contacts close and
the voltage across the contact gap exceeds the
dielectric breakdown of the air or gas between the
contacts.

If the capacitor is charged to the battery voltage
before the contacts are closed, then there is no
voltage difference between the contacts and no arc. 
Hence the reason to precharge.

The amount of arc which occurs is dependent on the
current and time necessary to bring the cap voltage to
equal the battery voltage. The discharged caps in the
controller essentially appear as a short circuit to
the battery on contact closure. So you get a very
high current for a very short time. Adding a resistor
in series with the capacitor will decrease the charge
current and lengthen the charge time. So this
precharge circuit will still have an arc on its
contact closure, but it can be tolerable.

Hope that helps,

Jeff M




> --- S Collins <[email protected]> wrote:
> 
> > Is this because discharged caps would allow a large
> > in-rush of current, thus allowing the arc? Do
> ...


----------

