# Precharging



## Tomdb (Jan 28, 2013)

I am thinking more and more about building the parts together for my test bench and EV.

For the test bench a manual switching precharge resistor is okay. However for a vehicle I would like to automate it, full or partial. So pre-charge and indicate when its done or just one signal for drive activation. This would mean that one 12volt signal drives the complete circuit that takes care of precharge and main contactor. 

I was wondering why not use an mosfet (of sufficient voltage ofcourse) since pre-charging at a fast rate does not require large amounts of current (dependent on your capacitance size). So it would require a mosfet, optocouples and dc-dc converter plus a few passives.

Then the automation part, you need to monitor the voltage over the resistor. However no need to get specific you just need to know when its roughly zero. So again an opto-coupler which pulls the signal low if the voltage difference is too great. When the voltage difference becomes small enough optocoupler stops conducting and the signal goes high.

Anyone know of a company or other diy-ers using mosfets for precharge? My main reason I would investigate this approach is mainly cost for the pcb+parts would be roughly 8 euros vs 50+ for an contactor

Designed an pcb and simulated it in lt-spice.


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## jhuebner (Apr 30, 2010)

Worth a try. I didn't study your implementation in depth though.

Just a word on resistors: you won't believe how many resistors I've cooked in precharge. They work well for quite a while and then fail.

Currently I'm using an 11W wire wound resistor for the inverter (860µF) and a 50W wire wound for the charger (~10000µF). The same 11W resistor worked in the charger for half a year and then became infinite resistance.

I cooked another one because I operated the heater over the precharge resistor. Make sure to interlock


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## dcb (Dec 5, 2009)

anyone considered a current mode buck converter for a pre-charger? Or even a simple linear current (or power) regulator, i.e. a small brick transistor biased to dissipate a fixed amount of power? less amps at higher voltage drops.


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## Tony Bogs (Apr 12, 2014)

Yes, I am considering it for my build. 
Certainly for the charger: buck is already there. 
Controller: isolated boost (from auxiliary LV battery). Low power, no big challenges there.
Get the cap voltages at the same level as the main battery (info exchange via f.i. CAN) and then connect.


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## dcb (Dec 5, 2009)

in terms of a controlled precharge, while this is the "surplus transistor brick" mentality, this sort of approach could be useful, though if there is an obvious simple way to get a more constant 150w dissipation from q2, rather than peaking at 500w, I'd like to hear it.

inductors can be tricky to source cost effectively.

(part numbers are not real world examples)
edit: not sure how it bootstraps q1, but the idea is the low cap voltage equates to low current through q1, which lowers the current in q2. as the voltage drop across q2 comes down, it allows more current.


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## Tony Bogs (Apr 12, 2014)

This inductor (L1/R1) is easy to source and the circuit is a lazy man's diy cheap precharger .

Don't think this one will blow out.


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## Duncan (Dec 8, 2008)

Re - Pre-charge resistors 
I'm using a kettle element - I don't think I'm going to make that fail


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## Tony Bogs (Apr 12, 2014)

That's even better. Lower dynamic thermal resistance.


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## Tony Bogs (Apr 12, 2014)

About dynamic thermal resistance (impedance Zth)), wire wound resistors and semiconductors (to replace resistors). 
Wire wound resistors have a much higher maximum allowable internal temperature than semiconductors. Usually 5 times the rated power for 5 seconds is allowable for wire wound.

Thermal impedance.
If the load is a single pulse of say about 50 msec, a good estimate for the dynamic correction factor (based on surface area of a 11W) is about 2. Above 0.2 sec the factor is close to 1. At 1 msec the factor is ~15.

This would mean a power limit somewhere in the 1 to 5 kW range for a 11W rated wire wound resistor for a 50 to 100 msec single pulse.


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## Tomdb (Jan 28, 2013)

This is what my setup looks like, I cant seem to get it to work in ltspice with an cap between output and ground.

This is really wierd I think, or am i missing something fundamental here?

Ordered my boards, If all else fails I got a nice isolated HV output control.


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## Tomdb (Jan 28, 2013)

Okay, the mosfet precharge works a charm. Now got to test what kind of amps it can do.

One small cock up, if you measure between the battery positive and the output (system postive) using the setup show above, guess what you will pre-charging through that. Noob electronics error. Means I have to use an transistor to also switch on and off the measurement resistor that is connected to the opto, easily fixed since I got isolated 12v available.


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