470 Ohm resistor purchase

[KLA]

I stand corrected too
According to this schematic:
http://evdl.org/docs/curtis_1221b_schematic.pdf
The total capacitance that needs to be charged is 7700uF (35 x 220uF) in this case.

Because allowable resistor short time overload (STOL rating) is typically 2.5 times the rated power for 5 seconds, a 2W 470 Ohm resistor should easily be able to handle the precharge duty without burning up. With that capacitance and 470 Ohm resistor it takes 18 seconds (5 tau) to fully charge the capacitors. However, the majority of power dissipation on the resistor happens in the first tau period (3.619 seconds), and the average power dissipated on the resistor during that period is 3W, below the 5W over 5 seconds STOL rating. For the next tau period the average power dissipated is < 0.4W and so on.

[JohnnieB]

Interesting schematic.

I've never seen a schematic of a controller before, but I have seen block diagrams and those were enough to satisfy my curiosity since my basic interests are more along the line of helping others troubleshoot existing cart drive systems rather than repairing what is hidden inside the black boxes; And helping others upgrade their cart with off-the-shelf black boxes rather than designing or redesigning the black boxes. However, I do get carried away with the technical details every now and then.

The Curtis 1221B that the schematic was reverse engineered from, is a fairly high amp controller (500A to 600A versions) and appears to have upwards of three dozen MOSFETs in parallel to pass that many amps as PWM-DC, and a similar number of filter capacitors. Lower amp rated controllers have fewer MOSFETs and capacitors, so the current flow through the by-pass resistor will be lower.

There may be, and very likely is, better ways to protect the contactor (aka as solenoid in golf cart parlance) contacts from the inrush of current caused by discharged filter capacitors in the controller than a fixed ohmic value resistor, but 270 ohm for a 36V systems and a 470 ohm for 48V systems has worked well enough over the years that the cart manufacturers continue to use them.

Ironically, it appears that the bean counters at the cart manufacturing companies may be asleep at the wheel because the unit cost of a PPTCD is less than that of a 10 Watt resistor. Of course, there could be another explanation why they haven't changed to something else.

[KLA]

Originally Posted by JohnnieB

Interesting schematic.

...However, I do get carried away with the technical details every now and then.

Yeah, I hear you. As an electrical engineer (in automotive), I can't help looking at things with an eye on how to make it cheaper and more effective.

Originally Posted by JohnnieB

...Lower amp rated controllers have fewer MOSFETs and capacitors, so the current flow through the by-pass resistor will be lower.

Or more precisely, the current flow will be the same, but the time during which it flows will be shorter. And therefore the energy dissipated will be less.

Originally Posted by JohnnieB

Ironically, it appears that the bean counters at the cart manufacturing companies may be asleep at the wheel because the unit cost of a PPTCD is less than that of a 10 Watt resistor. Of course, there could be another explanation why they haven't changed to something else.

The most likely explanation is that is has worked so far and nobody has given a lot of thought to it. After all, the price pressure in golf cart manufacturing is probably far less than in automotive.

OT: Speaking of automotive. I noticed in your sig that you use silver soldered cables. I use crimped cables for reliability reasons.
The reason stranded wires in automotive applications are always crimped, never soldered, is that during the soldering process the flux wicks back between the strands all the way under the insulation and cannot be removed. But in the presence of moisture the flux corrodes copper even at room temperature (agressively during soldering as that is its purpose) and over time continues to reduce the cross section of the strands up to 2" back from the solder joint under the insulation.
The second reason is that the solder forms with the copper a fairly brittle alloy on the copper strand surface when it wicks between strands right at the back end of the terminal. But that's where the highest mechanical stress occurs during vibration. That's why soldered stranded cables like to break right at the terminal.

[JohnnieB]

Originally Posted by KLA

1. Yeah, I hear you. As an electrical engineer (in automotive), I can't help looking at things with an eye on how to make it cheaper and more effective.

2. Or more precisely, the current flow will be the same, but the time during which it flows will be shorter. And therefore the energy dissipated will be less.

3. The most likely explanation is that is has worked so far and nobody has given a lot of thought to it. After all, the price pressure in golf cart manufacturing is probably far less than in automotive.

4. OT: Speaking of automotive. I noticed in your sig that you use silver soldered cables. I use crimped cables for reliability reasons.
The reason stranded wires in automotive applications are always crimped, never soldered, is that during the soldering process the flux wicks back between the strands all the way under the insulation and cannot be removed. But in the presence of moisture the flux corrodes copper even at room temperature (agressively during soldering as that is its purpose) and over time continues to reduce the cross section of the strands up to 2" back from the solder joint under the insulation.
The second reason is that the solder forms with the copper a fairly brittle alloy on the copper strand surface when it wicks between strands right at the back end of the terminal. But that's where the highest mechanical stress occurs during vibration. That's why soldered stranded cables like to break right at the terminal.

1. Those thoughts invade my thought processes also, but I tend to look for better efficiency first and then cost.
I'm a retired Clinical Engineer and worked mostly for the end users (Hospitals) and it nearly takes an act of God to make a change in an FDA approved medical device, but I still tried occasionally.

2. I was thinking continuous flow after the initial inrush. Few caps and devices in parallel, therefore less leakage, therefore less current flow.

3. Also, the golf cart industry didn't re-invent the wheel every year like the automobile industry used to.

4. I'm old school and was thoroughly brainwashed by the instructors at a NASA soldering school back in the 70's.
A high strand count cable with a good soldered on lug has less ohmic resistance than the same cable with a crimped on lug, but that is a minor consideration. The trick is to seal the cable termination air-tight. If that is properly done, I don't think there is very much difference in the performance or life expectancy between a properly crimped lug, and a properly soldered lug. (There are non-corrosive solder fluxes), for golf cart usage.

The first picture is a DIY cable with an open to the acid laden air type crimp lug attached to a low strand count cable, like those used in automobiles. Current flow through the high cueent cables in carts with electronic speed controllers is pulsed DC. The frequency of the PWM is in the 15-18 kHz range, so high stand count conductor is needed to reduce the skin effect.

The second and third pictures are IR photographs of Crimped and Soldered cable lugs with high current passing through them.
Note the temperature color scales are different on the two pictures. (The cable in the background is about the same temperature in both photos)
The IR pictures were lifted form a Trojan Battery White Paper.

[radioman]

If the solenoid contacts ever fail to make a 2.5 watt 470 ohm resistor would get very hot if you keep trying to make the thing go. A ten watt resistor would be a better choice to be on the safe side. It needs to be able to handle the cart system voltage B+ to B-. As long as everything is in working order the 2.5 watt resistor is ok but we all know things with golf carts don't always work as designed.

[scottyb]

Even the 10w resistor gets hot enough to burn your finger when the solenoid fails to close and throttle is applied.
Also I don't believe the vibrations in a 15mph golf cart don't begin to match an automobile over the road operation.

[KLA]

Originally Posted by radioman

If the solenoid contacts ever fail to make a 2.5 watt 470 ohm resistor would get very hot if you keep trying to make the thing go. A ten watt resistor would be a better choice to be on the safe side. It needs to be able to handle the cart system voltage B+ to B-. As long as everything is in working order the 2.5 watt resistor is ok but we all know things with golf carts don't always work as designed.

That's why I suggested the self-resetting fuse resistors (PPTCD). They would basically limit the current to their hold current or less.

[radioman]

No. If you use a self resetting resistor and it opens, if the solenoid contacts does close before it resets you will get a big splat inside the solenoid and weld the solenoid contacts together. Stay with the recommended resistor.

[KLA]

Originally Posted by radioman

No. If you use a self resetting resistor and it opens, if the solenoid contacts does close before it resets you will get a big splat inside the solenoid and weld the solenoid contacts together. Stay with the recommended resistor.

The PTTCD device has to supply a non-trivial current only under 2 conditions:

Case 1.
Controller connecting to the batteries the VERY FIRST TIME when the capacitors are discharged. In this case the controller CANNOT operate the solenoid because it does not have enough operating voltage (discharged caps). Once the capacitors are charged, only the quiescent current of the controller (typically a few 10s of mA) flow through the resistor.
Think of the PTCCD not as a fuse (it isn't), but a self regulating resistor that allows only a certain fixed max power to be dissipated on it.

Case 2.
A controller short (B+ to B- short in the controller). In which case the PTTCD opens (has a large resistance) and no large current can flow. If you have a B+ to B- short while the solenoid is closed, no resistor can help welding the contacts.

[radioman]

I am sticking with the 470 ohm ten watt resistor.