I posted this question on the EVDL and i got a pretty good answer from Lee
Hart. Below is his response.
Steve Powers wrote:
> I need a simple circuit that drives a relay... turn on if current goes
> over 50 Amps. If current drops below 25 Amps, the relay will open...
> Side note, I may want a feature to "delay" the relay engagement for some
> time (say 500 ms or so).
You're in luck. There *is* a simple circuit to do it. One component. Two
components if you want that 500 msec delay. In fact, it's so simple that
nobody will think of it. Everyone will hunt for complicated ways to do it.
Buy a reed switch. It needs a contact that can switch the desired load.
For example, let's say you want to switch 12vdc at 1 amp. Go to
www.digikey.com and search for "reed", then "magnetic", then select a
current of "1a". It will come up with 21 candidates. I'll arbitrarily
pick the first one, Digikey #374-1076-ND, a Meder Electronics
KSD-1A35-1015, for $1.23 each. It's rated at 1a max, 200vdc max, 20w max
so it can easily do the job.
Its "must operate" spec is 10-15at. This means it operates (closes) with
a magnetic field strength of 10 to 15 ampere-turns. If you make a 1-turn
coil in a wire whose current you want to sense, and put this reed switch
in the hole in center, it will operate when 10-15 amps flows in the wire.
You need *less* sensitivity, so you don't need even a 1-turn coil. Just
place the reed on the wire, at right angles to it (like a cross). This
forms a fraction-of-a-turn coil, and that is all you'll need. Adjust the
spacing between the reed and the wire to get the desired 50 amp pull-in.
The easiest setup is a block of some non-magnetic material that clamps
around the wire, and has a hole at right angles that the reed slides
into. Slide the reed in/out of the hold to adjust the spacing, and thus
the operate current.
It doesn't mention the drop-out ampere-turns, but it's generally about
half of the operate ampere-turns. If you need to adjust it separately
from the operate ampere-turns, then wind a coil of wire (suitable for
the current you're switching) around the reed, and connect it in series
with the reed. When the reed closes, the current through its closed
contact creates a second magnetic field, which can add or subtract from
the field produced by the big wire. Experimentally determine how many
turns and in which direction to wind this coil.
To get a delayed turn-on and turn-off, make the block that holds the
reed to the wire out of copper or aluminum. It acts like a coil with a
shorted turn, which slows the magnetic field from reaching the reed. For
a start, a 1/2" diameter x 1" long piece of aluminum with a 3/32"
diameter hole for the reed will slow it down about 0.5 seconds.
If you like this idea, a donation to our Sunrise EV2 project would be a
good way to say "thanks". :-)
Lee A. Hart
The way I would implement this is to add a small resistor in parallel with
the 10k resistor used in the selector switch for the power mode. Just by
adding that resistor, the controller power is limited even when you select
power. Use the basic formula for resistors is parallel (product divided by
sum). With the formula and some trial and error, you can figure out what
resistor value to use. Then, use Lee's idea for a reed relay. Use the
reed relay to break the circuit where where you added the extra resistor in
parallel. When the relay engages the power is restored to full. By that
time, any backlash or slop in the drivetrain has been taken up and you can
proceed as normal. Sounds like about a <$10 fix to a very annoying
problem. If anyone tries it, let me know the results.
On Mon, Oct 31, 2011 at 8:06 PM, Steve Powers <stevepowers007@...>wrote:
> I'm sure some smart people can figure this out. Depending on the real
> cause, it may not be that difficult. I am not an Electrical Engineer, but
> I do have an idea. The controller power is controlled by a resistor. I
> don't recall all the details, but I believe it is possible to force the car
> into econ mode for the first x.xx seconds of power on (for each
> acceleration). The circuit could reset each time the power drops below a
> certain level.
> So, build a circuit that actuates a switch if the power goes above a
> threshold. For this example, lets say 50 Amps. That number could be
> anything. Read on to understand better. The circuit would reset below a
> certain level (10 Amps or so). So, here is how it works. You power up the
> car and the contactor is unpowered. In that unpowered state, an extra
> resistor is either in series or parallel with the one to the controller.
> That extra resistor forces the controller into low power, but it is enough
> to allow the gears to properly mesh (take out the play). Then, after a
> chosen time (250 ms, 500 ms or so) the contactor would turn on. The time
> is actually the time it takes the mechanical contactor to engage. It's
> really that simple. The contactor has normally open and normally closed,
> so you can wire the extra resistor either in series or parallel (whatever
> is required to put it in slow mode). After the fixed time, the contactor
> would pull in and change the power mode automatically. That would allow
> full power only after the gears are meshed. It causes a very slight lag in
> acceleration (probably unnoticeable). Then, when you let off the power and
> the Amps drop below the threshold the ckt resets. It does this over and
> over always ensuring that the gears have time to mesh before going over a
> certain Amp draw. Its a "mechanical" solution proposed by a Mechanical
> Engineer. A EE or controls expert might try to intercept the signal to the
> pot box and somehow modify it with some "logic." I also considered that,
> but have no idea how to do something like that.
> This is the same as putting the car in econ, slamming on the accelerator,
> and shifting it into power as soon as the gears mesh.
> I think it would work, but unfortunately, I'm not smart enough to design a
> ckt to drive a contactor based on the Amp draw from the battery pack.
> Probably very easy to do this, so I hope someone smarter than me can figure
> it out and provide the simple circuit. The engage may even need a time
> If this doesn't work, probably lowering the battery trip point may help.
> Then, you would have to be more careful not to over discharge the battery
> pack, but this also may help.
> If it is right on the threshold of the controller being too wimpy, you may
> try to raise the 200 Amp limit on the controller, but that is also risky
> and possibly cause other damage.
> I'm open to other ideas if anyone has some.
> On Mon, Oct 31, 2011 at 7:41 PM, Reed Bement <reedb@...> wrote:
>> From reading the forum for the last five years and my personal
>> observations the three most frequent problems with late model (no belt
>> drive) version of the Force are:
>> 1. Leaking drive shaft seals. Leads to premature failure of gear box
>> bearings (growling at speed). Easy to detect just park over cardboard.
>> 2. Short CV joint life. Most likely due to the car being so heavy because
>> of the battery pack. If this is the problem, the clunking or click will be
>> worse or better depending on how tightly the steering wheel is turned.
>> 3. Loose retaining bolt on motor pinion gear. This is a tricky one. If
>> the retaining bolt is loose, the pinion gear can move out on the spline
>> leading to large backlash in the gear box and possible destruction if it
>> comes completely off while the car is in motion. The pinion gear moves away
>> from the motor and against the retaining bolt in reverse. If you notice
>> that you only seam to get backlash after or during the use of reverse, esp.
>> if the steering wheel is turned tight (increases torque required to start),
>> suspect this and deal with it promptly.
>> With regard to starting on a hill, my experience is that this is all
>> about the controller low voltage drop out setting. I'm currently running a
>> 144V controller with a 156V pack. I can climb a 15 degree hill from a dead
>> stop (I have the 11:1 gearbox). I couldn't come close to doing this with
>> the controller setup for the 156 volt pack. I would recommend setting up a
>> laptop on the controller serial port and trying to see where you are pack
>> voltage wise with respect too the high and low limits. When these issues
>> are taken care of the cars seem fine for every day use. My wife drives ours
>> every on a very hilly island.
>> On 2011-10-31, at 1:56 PM, Steve Powers wrote:
>> > I have to chime in here because you must be reading my mind. I got a 99
>> > Force earlier this year (the one with the gearbox) and I really like the
>> > way the car drives except for that occasional jerky start. On my car, it
>> > is only occasional, but it is very annoying when it happens. I asked the
>> > previous owner and he said it has always been that way and was that way
>> > another Force he had as well. Mine is a little different because it
>> > worse when stopping on a hill. The car will roll back a little. You want
>> > to use a lot of power, so you press the accelerator to the floor -
>> there it
>> > goes - a nice big crunch as you accelerate. The first time you hear it
>> > think you left the transmission back there. So, then you become a lot
>> > careful when driving. What I think is: (1) there is excessive play in
>> > these gearboxs in the Forces that have gearboxes. Basically, the whole
>> > transmission is poorly designed and poorly manufactured with poor
>> > tolerances. (2) The controller doesn't like starting from a dead stop
>> > full throttle. There isn't much I can see that can be done. There
>> > definitely is play in the gearbox. I think they all have that. I would
>> > think though that the controller program could me modified to improve
>> > rampup when going from a dead stop. I don't know what parameters are
>> > there, but I suspect something could be done. If it is ramping up too
>> > fast, and there is a setting for that, just lengthen it. If it is
>> > out, that is a whole different situation. Not much can be done about
>> > if the controller at 156 V and saggy lead batteries is just plain
>> > and can't handle that start from a dead stop on a hill. On conversions,
>> > people sometimes have this issue and they change the ramp speed on the
>> > controller. If anyone knows how to fix this, I'd sure like to know
>> > like I said it is extremely annoying and seems to be on every Force out
>> > there. With careful driving it can be minimized, but I should be able to
>> > slam down the accelerator from a dead stop, even on a hill and not have
>> > car jerk into motion. Very interesting on my car, I have tried to create
>> > this jerk motion on demand to figure out the cause. Sometimes, no matter
>> > what I do, it is smooth as can be. I can slam my foot to the floor and
>> > perfect acceleration. Other times, the controller cuts out for a very
>> > brief moment and then you get the jerk motion. Unlike other cars with
>> > speed sensors, I don't have any other controller issues. So, I don't
>> > it is a speed sensor issue. I have to say that when it happens, it is so
>> > bad (apparently on all these cars) that it convinced me not to pursue a
>> > Ion batt pack. The jerky start is annoying enough (even though it is
>> > occasional) that I can't see investing in an expensive battery pack for
>> > of these cars. That says a lot about how annoying this "feature" on all
>> > these cars really is.
>> > Steve
>> > On Mon, Oct 31, 2011 at 1:11 PM, Stephen Taylor <sparrow262@...
>> >> **
>> >> Have you checked the CV joints?
>> >> Stephen Taylor
>> >> From: scndbsn <dianne.stengel@...>
>> >> To: email@example.com
>> >> Sent: Monday, October 31, 2011 10:02 AM
>> >> Subject: [solectria_ev] Jerking at beginning of accelleration and radio
>> >> interference
>> >> All -
>> >> Our '99 Force has been running like a top except for two things:
>> >> 1. When we start accelerating from a dead stop, the car jerks and
>> >> for about 1-2 seconds then starts going smoothly. Often, the same in
>> >> reverse. If the car is rolling even slightly, we don't have the
>> problem. We
>> >> believe it's related to the speed sensor, perhaps in programming
>> >> Do others experience the same thing or, if this is unusual, does anyone
>> >> have any suggestions on how to fix?
>> >> 2. When we are in motion, there is a lot of static and interference
>> >> AM radio stations (not with FM). When we come to a dead stop, the
>> >> interference is gone. Feels like the antenna is picking up some
>> >> from the motor or controllers but would like to hear anyone's
>> >> and suggestions for how to get AM stations clearly. Shielding radio or
>> >> motor? Other?
>> >> Thanks to all in advance!!
>> >> Jack and Dianne
>> >> [Non-text portions of this message have been removed]
>> > [Non-text portions of this message have been removed]
>> > ------------------------------------
>> > Yahoo! Groups Links
[Non-text portions of this message have been removed]