. I think you could make a few mild changes to this to turn a contacter on.fechter wrote:OK, here we go...
Theory of operation:
When the switch is turned on, R1 feeds voltage to the FET gates, which will be clamped by the zener diode.
Initally, the controller (-) will be at the same voltage as controller (+).
As the FETs turn on, the controller (-) line starts to pull down, causing current flow through the capacitor.
The dV/dt of the controller voltage will be constant to match the zener current. This will cause the current flowing into the capacitors of the controller to be constant during the precharge. Once the controller is fully on, current ceases to flow through C1 and the gate voltage climbs to 12v. Turning the switch off shorts the gate, immediately turning off the FETs and slowly charging C1.
C1 needs to be non-polar and rated for the full pack voltage.
Some approximate math: Lets say the pack voltage is 62V, so the voltage across R1 is 50V. This makes the zener current 50uA. With 1uF for C1, this will allow a dV/dt of 50V/second on the output.
If the controller capacitors total 1,000uF, the current would be a constant 50mA during precharge, for about 1 second.
If the conroller capacitors total 10,000uF, the current would be 500mA.
fechter wrote:I don't think the capacitance of the board will be a problem. If you do the math on it, it is actually quite small compared to the gate capacitance and other things in the circuit. I do think having the negative leads coming off in opposite directions would be good to help even out the current sharing, which could explain why the one on the left blew first.
I think the problem was overdissipation. The datasheet for a IRFB4310 is here: http://www.mouser.com/ds/2/200/irfb4310zgpbf-72754.pdf
This one is just as an example but others are similar.
The maximum continuous drain current is over 100A.
The maximum pulsed drain current is 560A.
This seems like it should be plenty to charge a capacitor. But if you go down the page to Fig. 8, Maximum Safe Operating area, I think is where we find the answer. Just for example, let's take our 60V pack. If the charge rate is too fast, we end up outside the safe operating area on the right side of the graph. Maximum power dissipation is 250W. At 60V and 10A, we'd blow that baby for sure even though the FET can handle 560A.
In the circuit I posted above, the current will be constant, and in a fairly extreme case is limited to 500mA, which would be a dissipation of 30W, well within the safe area.
I forgot to ask; what voltage and controller you are using?
mvly wrote:I am using Lyen's 18FET 4110FETs version 1 if that matters. I have a version 2, but I never got around to send it back to him to mod it for my uses with my current bike. I am not sure if the version 2 is any better at this.
I am running 74V Lipo (20s).
I am using 47K and 10K as the bottom resistor.
fechter wrote:With my circuit, you could also have the controller 'key' wire permanently attached to the positive wire and use the circuit switch to turn everything on and off.
Arlo1 wrote: I think you could make a few mild changes to this to turn a contacter on.
fechter wrote:Arlo1 wrote: I think you could make a few mild changes to this to turn a contacter on.
OK, here you go!
This arrangement has the contactor on the negative side so we can use N channel FETs. The FETs don't need to be super high current, but it wouldn't hurt. It would be possible to use P-channel FETs and design the circuit to switch on the positive side, but I don't see any real advantage to this.
If the contactor coil is rated for the pack voltage, it can look like the drawing. In most cases, the contactor coil is rated for a lower voltage, so either a big resistor in series with the coil or a DC-DC converter can be used between the circuit and the coil. The main DC-DC coverter for a vehicle could be driven off Q2. If using a DC-DC, make sure Q2 is rated for the current it takes. Both Q1 and Q2 should have some kind of heat sink, but they should only need it during the two seconds during precharge.
Q1 does the precharging. Once the controller gets up to full pack voltage, the dV/dt drops to zero and allows Q2 to turn on. One advantage of this arrangement over the typical resistor precharge is any additional drain from the controller circuitry or a DC-DC converter won't prevent it from reaching full voltage.
If the controller is shorted, the fuse will blow, preventing the contactor from kicking in. The fuse should be around 2A plus whatever the DC-DC or coil takes.
In practice, turning on the switch starts precharge, which will take about 2 seconds. After that, the contactor automatically engages.
Arlo1 wrote:I planed to do this and post a schematic but hey its likely you would have had to help me edit it so it would work. I will make sure I build this asap! Thank you very much!
fechter wrote:Arlo1 wrote:I planed to do this and post a schematic but hey its likely you would have had to help me edit it so it would work. I will make sure I build this asap! Thank you very much!
What's the coil voltage on your contractor and your pack voltage?
fechter wrote:Sounds like a small dc-dc converter (wall wart) might be good for driving the coil. A big resistor will get pretty hot.
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