Need a circuit designed to limit charge to XX amps max

I need a circuit designed that will limit the maximum EVSE pilot signal "duty cycle / pulse wave modulation" to the following:

1) EVSE pilot signal > than XX duty cycle, output XX pulse wave modulation (PWM) only
2) EVSE pilot signal <= to XX duty cycle, let the EVSE's PWM pass through unchanged

This device would be placed between the EVSE plug pilot signal pin and the cars pilot signal pin. That signal is a 1kHz 12/-12 volt square wave with the following duty cycle:

10% - 96% = 6 amps - 80 amps
>96% = Error

One simple option might be:

Duty cycle from EVSE is > XX, then PWM = 96%. This wouldn't work with duty cycles greater than XX% to continue operating at XX or lower, but would merely not work over XX.

I want to be able to limit to XX%, not stop above XX%.


Link to Tesla forum thread of the same topic:
http://www.teslamotorsclub.com/showthread.php/21714-How-to-make-a-30-amp-J1772-extension-cord-for-public-charging?p=445524&viewfull=1#post445524" onclick="window.open(this.href);return false;

It might depend on how accurate you want to be.

If I understand your requirement, then perhaps you might consider a 555 based monostable that triggers on the trailing edge of the last pulse, and which then enables a triac that carries the through-signal [only] after a set time. If the set time is shorter than the interval between pulses then it will have no effect on the through signal [e.g., a set time of <40us would have no effect on a duty cycle <96%]. If the set time was, say, 300us then it would limit the duty cycle to 70%, yet have no effect on, say, a 50% duty cycle signal (and would also carry + and - signals, irrespectively).

donald said:

It might depend on how accurate you want to be.

If I understand your requirement, then perhaps you might consider a 555 based monostable that triggers on the trailing edge of the last pulse, and which then enables a triac that carries the through-signal [only] after a set time. If the set time is shorter than the interval between pulses then it will have no effect on the through signal [e.g., a set time of <40us would have no effect on a duty cycle <96%]. If the set time was, say, 300us then it would limit the duty cycle to 70%, yet have no effect on, say, a 50% duty cycle signal (and would also carry + and - signals, irrespectively).

Click to expand...

All good points... thanks.

Tony

how are we powering this?

simple is best, but we might be back to having to deal with a 120 to 240v capable powersupply - once we have all that, it's probably just back to the Open EVSE guts with some extra logic.

If we can do it on just the power in the pilot then a super simple analog would be OK. -- not to overthink this -we're just trying to keep a tesla-inside car from killing a 30A max J1772 extension by mistake, right?

essaunders said:

how are we powering this?

simple is best, but we might be back to having to deal with a 120 to 240v capable powersupply - once we have all that, it's probably just back to the Open EVSE guts with some extra logic.

If we can do it on just the power in the pilot then a super simple analog would be OK. -- not to overthink this -we're just trying to keep a tesla-inside car from killing a 30A max J1772 extension by mistake, right?

Click to expand...

Yep, that's the goal. I want a nice light, simple, and "cheap" J1772 extension cord, and it's sorta dumb to make it 80 amp capable when almost all the public charge stations are 30 amps.

But, since some EVSEs are indeed 70 or 80 amps, the 30 amp cord needs smart protection from a Tesla driver who can suck down 80 amps.

The power has to come from the 100-250 volt power going through the cable, since I have no idea how much "power" is actually in the pilot signal. So, for at least a millisecond, there will be a pilot signal over 30 amps until it can be changed to 30 amps or less.

We absolutely don't need all the stuff in the OpenEVSE; no relays, no GCFI, no diode check, no stuck relay logic, et al.

Hi Tony,

You should check out "Hydra" on the OpenEVSE site. It was designed by nslayer to split a J1772 into 2 the and share the source pilot. It sounds like you want Hydra with a single output.

Hydra is losely based on OpenEVSE. I bet it would be perfect for your need with a few code changes.

chris1howell said:

Hi Tony,

You should check out "Hydra" on the OpenEVSE site. It was designed by nslayer to split a J1772 into 2 the and share the source pilot. It sounds like you want Hydra with a single output.

Hydra is losely based on OpenEVSE. I bet it would be perfect for your need with a few code changes.

Click to expand...

Yes, I was thinking about that. I was hoping for something far, far simpler for this one job.

Thanks,

Tony

Just match the cable to the vehicle.

pchilds said:

Just match the cable to the vehicle.

Click to expand...

Of course, except... in the mass market, somebody would eventually have a 15 or 30 amp cord and try and pull 80 amps through it.

How about a 30 amp fuse?

johnrhansen said:

How about a 30 amp fuse?

Click to expand...

Certainly, the simple, safe answer.

TonyWilliams said:

... The power has to come from the 100-250 volt power going through the cable, since I have no idea how much "power" is actually in the pilot signal. So, for at least a millisecond, there will be a pilot signal over 30 amps until it can be changed to 30 amps or less.

We absolutely don't need all the stuff in the OpenEVSE; no relays, no GCFI, no diode check, no stuck relay logic, et al.

Click to expand...

You can't run it off the mains voltage AND have no relays in the extension. You are either letting the EVSE control the mains, which means that there's no 120/240v available to run your electronics, or you're turning on the EVSE immediately, which means your circuit needs to be a full EVSE with relays in order to properly work with the car. I suppose you could use a small battery to run it until the mains come on...

On top of that, if you're letting the EVSE control the mains, then you'll have to plan on relaying the signalling the car is doing when it pulls the pilot down to start charging.

davewill said:

You can't run it off the mains voltage AND have no relays in the extension. You are either letting the EVSE control the mains, which means that there's no 120/240v available to run your electronics, or you're turning on the EVSE immediately, which means your circuit needs to be a full EVSE with relays in order to properly work with the car. I suppose you could use a small battery to run it until the mains come on...

On top of that, if you're letting the EVSE control the mains, then you'll have to plan on relaying the signalling the car is doing when it pulls the pilot down to start charging.

Click to expand...

The car isn't changing the EVSE pilot signal duty cycle when it ramps down the charge current.

Yes, the battery idea seems ok, except then you'd need a charger for it, etc. It seems that the teeny tiny amount of time that an 80 amp pilot signal is sent from the EVSE and the relays close to provide 100 - 250 volts that it could power and wake up our "device" to then send a XX signal duty cycle. Unfortunately, the standard allows up to 5 seconds for the car to respond.

Having a relay is not going to happen, or you just have another EVSE.

Perhaps I need to think of a way to turn on the battery powered device. Logically, it would be the 12 volts coming from the EVSE proximity signal.

TonyWilliams said:

... The car isn't changing the EVSE pilot signal duty cycle when it ramps down the charge current.

Yes, the battery idea seems ok, except then you'd need a charger for it, etc. ...

Click to expand...

No, but it does when it wants to end the charging or encounters a fault, so even if you let the pilot through until the charge starts, then substitute yours, you're going to have be able to monitor the substitute pilot signal for changes (and the real one for that matter).

I wasn't thinking so much of a rechargeable battery. I figured a couple of AAs would last quite a while if they were just being used between plugging the EVSE into the extension, and starting charging...But I just realized that either a charge timer or a completed charge cycle would mean the thing would be plugged in for hours with the EVSE off. What was Phil running his flashlight LED off of?

TonyWilliams said:

davewill said:

You can't run it off the mains voltage AND have no relays in the extension. You are either letting the EVSE control the mains, which means that there's no 120/240v available to run your electronics, or you're turning on the EVSE immediately, which means your circuit needs to be a full EVSE with relays in order to properly work with the car. I suppose you could use a small battery to run it until the mains come on...

On top of that, if you're letting the EVSE control the mains, then you'll have to plan on relaying the signalling the car is doing when it pulls the pilot down to start charging.

Click to expand...

The car isn't changing the EVSE pilot signal duty cycle when it ramps down the charge current.

Yes, the battery idea seems ok, except then you'd need a charger for it, etc. It seems that the teeny tiny amount of time that an 80 amp pilot signal is sent from the EVSE and the relays close to provide 100 - 250 volts that it could power and wake up our "device" to then send a XX signal duty cycle. Unfortunately, the standard allows up to 5 seconds for the car to respond.

Having a relay is not going to happen, or you just have another EVSE.

Perhaps I need to think of a way to turn on the battery powered device. Logically, it would be the 12 volts coming from the EVSE proximity signal.

Click to expand...

To keep it simple you may need an external power source. When connected the proximity circuit is only 1.5v.

At Minimum you will need a microprocessor, Opamp, DC/DC converter and passives. A li-poly and a simple charge controller/booster would power it for days.

The extender will need a 2.74k resistor to put the source into the connected state and a 1.3k switched by a transistor to activate the charge state at the right time.

You might be able to squeeze it all into an 8pin attiny85, you would need:
-analog input to read source pilot
-analog input to read extender pilot
-digital output to generate extender pilot
-digital output to switch 1.3k/transistor
-digital output for Error LED (extender will have to read the pilot for error states vent required, diode and unknown)

I know that it's really cool to make something elegant and complex, but I've been a technician for many years and have worked with engineers for a long time, and to be honest, over complication is always the biggest gripe my peers and I have had. I think technicians oversimplify, and engineers over complicate. So maybe my idea of a fuse and a placard is somewhat crude, but how often is someone going to try to charge at more than 7200 watts? I really think you should either use a 30 amp fuse, or build it with heavier wire so it can carry all of the possible current. Anything that limits the current is going to need to be passive...all this electronics I read about is a failure point especially if you need to work up some kind of power supply for it. How strong is this square wave that dictates the current? Can you just run it through a integrator of sorts that provides a voltage to an scr that just crowbars it to ground and limits the PWM to 50 percent, or whatever duty cycle makes 30 amps...? Would that burn anything up?

davewill said:

What was Phil running his flashlight LED off of?

Click to expand...

That is a Tesla-ism... 3.3 VDC for the remote charge port door on the car.

The transmitter is in the Tesla plug. I copied his idea and use it in my 40 amp Tesla UMC to J1772 conversions:

TonyWilliams said:

since some EVSEs are indeed 70 or 80 amps, the 30 amp cord needs smart protection from a Tesla driver who can suck down 80 amps.

Click to expand...

TonyWilliams said:

johnrhansen said:

How about a 30 amp fuse?

Click to expand...

Certainly, the simple, safe answer.

Click to expand...

If it is purely for current protection, and not to try to control a charge rate, then I agree a fuse is an obvious 'simplest' solution. You can get thermal resettable 30A fuses for a few bucks. You can reset them after they have tripped and cooled down again. As they can replace regular fuses, you could even supply with a single-time fuse, or a resettable fuse, depending on what the buyer wants and whether they feel they can resist the urge to try to pull 80A through it! If they reckon they have that level of discipline (!!) then they'd just go for the one-time fuse for a few cents instead, wouldn't they?

scr or triac? oh wait thats AC.

Link to Tesla forum thread of the same topic:
http://www.teslamotorsclub.com/showthread.php/21714-How-to-make-a-30-amp-J1772-extension-cord-for-public-charging?p=445524&viewfull=1#post445524" onclick="window.open(this.href);return false;