Active pre-charge/inrush control

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madin88   100 MW

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Re: Active pre-charge/inrush control

Post by madin88 » Jan 17 2017 12:40pm

fechter wrote:Nice work. I never tested without the switch.
fechter, it was mentioned here already 2 years before that your automatic circuit layout doesn't work!
Recently i reported here as well about that issue.
I think you can imagine that it was a bit of a peeve for me and others when building, measuring and checking everything twice and three times without getting it working properly in the end.
But i appreciate your work and don't want to blame you for it :wink:


@ Manny

BIG thanks for finding the solution. Great work!
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fechter   100 GW

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Re: Active pre-charge/inrush control

Post by fechter » Jan 17 2017 3:10pm

One test is worth a thousand guesses. All my testing was done with the version using a switch.

Time for a new schematic. Actually the one in the App Note looks about as simple as possible.
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Re: Active pre-charge/inrush control

Post by izeman » Jan 20 2017 7:18am

I can copy madin's post. I never had the switchless version working without sparking. :(


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fechter   100 GW

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Re: Active pre-charge/inrush control

Post by fechter » Jan 25 2017 10:34am

It's like the Einstein quote:
Things should be simplified as much as possible, but not more than that.
The earlier switchless circuit was oversimplified. I think Manny's updated schematic is about as simple as possible and still have the desired operation.
I wish I had that Application Note about 5 years ago. :oops:
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Re: Active pre-charge/inrush control

Post by Manny » Jan 29 2017 7:25am

I made a Excel calculator for the inrush control components.

hopefully it is useful. if you have problems or suggestions let me know.
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inrush limiter.xlsx
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Re: Active pre-charge/inrush control

Post by LewTwo » Jan 29 2017 9:12am

Manny wrote:I made a Excel calculator for the inrush control components.

hopefully it is useful. if you have problems or suggestions let me know.
Stupid question: Shouldn't there be a load somewhere ?
The more I learn the more obvious the immensity of my ignorance becomes.
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Re: Active pre-charge/inrush control

Post by Manny » Jan 29 2017 10:23am

LewTwo wrote: Stupid question: Shouldn't there be a load somewhere ?
eeuh yes. the circuit diagram is simplified, the Cfilter is the capacitor in the motor controller.

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Re: Active pre-charge/inrush control

Post by fechter » Jan 29 2017 11:36am

Manny wrote:I made a Excel calculator for the inrush control components.

hopefully it is useful. if you have problems or suggestions let me know.
That's nice!

I tried building one with a gate clamping transistor for startup. It seems the main FETs are able to switch on faster than the gate gets clamped so there is still a little blip at the start. It is very short, like 1uS, so probably won't make a noticeable spark. I built the circuit on a breadboard, so there could be issues caused by that too. I think this approach will be better as using a big capacitor across the zener makes it really slow to come fully on.
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Re: Active pre-charge/inrush control

Post by LewTwo » Jan 29 2017 12:04pm

Manny wrote:
LewTwo wrote: Stupid question: Shouldn't there be a load somewhere ?
eeuh yes. the circuit diagram is simplified, the Cfilter is the capacitor in the motor controller.
Thank thee
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Re: Active pre-charge/inrush control

Post by madin88 » Jan 29 2017 4:57pm

the automatic circuit works well now. Thanks for your PM help!

I measured that it takes about 2-3sec until i see 12V across the gate.
since this circuit is used for avoiding the spark on a removable downtube battery, it isnd't an issue (the CA anyway takes a few sec to boot). I would call it still idiot proof.
Without any load, the time until the gate voltage drops below 2V is quite long, and with "load" (CA + DCDC idle), it takes about 10sec.
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Re: Active pre-charge/inrush control

Post by markz » Jan 30 2017 12:30am

I used a large resistor high in wattage and touched the battery to the cable, then took it off for 1 to 1.5 seconds to reconnect the wire to the battery and no spark. 72V pack. Doesnt take much for the pre-charge to work.

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Re: Active pre-charge/inrush control

Post by cycborg » Jan 30 2017 12:06pm

I'm thinking about using something like this for a much less demanding application - a small DC motor that's rated for 12 V and draws 6 A unloaded. I want to run it from a wall-wart supply with similar ratings.

Problem is at startup, when it draws much more than 6 A until the BEMF builds up. The supply doesn't current-limit; it just faults, then resets, then faults, then resets, while the motor starts, then stops, etc. without ever getting fast enough to build BEMF.

So I need to ramp up the voltage to the motor over, say, 5 s to keep the current under the supply's limit, then run at 6 A for a few minutes until the motor accomplishes its mission, which is to run a fan that inflates an air bed.

Questions: (1) How would this circuit react to a load like this, which is more complicated and dynamic than a big capacitor? (2) Any thoughts on component selection? Zener voltage, FET package, etc.

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Re: Active pre-charge/inrush control

Post by Steu851 » May 15 2017 8:57am

I have built the active inrush schematics 3a without switch https://endless-sphere.com/forums/viewt ... &start=550 but I have a spark every time I connect the battery.
I read Manny's post but it's not so clear for me.
I understand that if I put a 10 uF capacitors in parallel to zener it can work, but it's very slow. Can I use a 4,7 uF?
In his more complex schematics TP1 is - battery, TP2 is - Load and TP3 is + ?
What is the value of C2? 10 uF?

My battery is 36V and max current will be 16A,
I put a 2200uF capacitor on output to reduce inductance of my long battery cables and i am testing using some 12V bulbs as load.

Thank you and sorry for my English

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Re: Active pre-charge/inrush control

Post by fechter » May 15 2017 2:55pm

Steu851 wrote:I have built the active inrush schematics 3a without switch https://endless-sphere.com/forums/viewt ... &start=550 but I have a spark every time I connect the battery.
I read Manny's post but it's not so clear for me.
I understand that if I put a 10 uF capacitors in parallel to zener it can work, but it's very slow. Can I use a 4,7 uF?
In his more complex schematics TP1 is - battery, TP2 is - Load and TP3 is + ?
What is the value of C2? 10 uF?

My battery is 36V and max current will be 16A,
I put a 2200uF capacitor on output to reduce inductance of my long battery cables and i am testing using some 12V bulbs as load.

Thank you and sorry for my English
You could try 4.7uF and see if you still get a spark.

In Manny's schematic, I think TP2 and TP3 would be connected to each other.
I think C2 could be quite small or maybe left out completely. 0.1uF would be my guess.
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Re: Active pre-charge/inrush control

Post by Manny » May 15 2017 2:57pm

Steu851 wrote:In his more complex schematics TP1 is - battery, TP2 is - Load and TP3 is + ?
yes correct.
Steu851 wrote:I put a 2200uF capacitor on output to reduce inductance of my long battery cables and i am testing using some 12V bulbs as load.
what is the total capacitance controller and extra cap's?
do you connect the lamps after startup ?

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Re: Active pre-charge/inrush control

Post by Steu851 » May 15 2017 4:19pm

I don't know capacity of controller, it's a Yamaha PW motor, and I am talking of an auxiliary battery connected i parallel with the main one using 2 Schottky diodes.
Bulbs are connected to the circuit before connecting battery

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Re: Active pre-charge/inrush control

Post by markz » May 16 2017 7:08pm

I was thinking when I got the spark on my sunwin controller, a simple switch say a SPDT with a suitable resistor is all thats needed.

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Re: Active pre-charge/inrush control

Post by jmasta » Jun 29 2017 9:58pm

fechter wrote:
The attachment Automatic Precharge 3 schematic.jpg is no longer available
Hi Richard,

I am using this schematic and am looking into adding a secondary killswitch. I recently experienced a failure in which the motor was continuously fed full power. The SPDT switch already installed is too large and not accessible while riding. Therefore I modified the schematic to add a simple SDST switch (either momentary or latching). Would the proposed modification below be advisable, or do you have a better recommendation?

I was considering using a momentary SPST switch to act as a reset button. Would the circuit would turn off quickly enough to be effective? I am running 12S with a hard battery cutoff at 40.8V. I could program the minimum ESC voltage to shutoff at something close to this value (say 38V, instead of the default 8V). Would it be better to use a latching switch instead?

Thanks!
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Re: Active pre-charge/inrush control

Post by izeman » Jun 30 2017 5:19am

i found out that this circuit does NOT like to be shut off under load.
if you draw high current, and during that short the gate to GND, it will blow your FETs. at least this is what i found out.

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Re: Active pre-charge/inrush control

Post by madin88 » Jun 30 2017 10:47am

izeman wrote:i found out that this circuit does NOT like to be shut off under load.
if you draw high current, and during that short the gate to GND, it will blow your FETs. at least this is what i found out.
do you remember the battery voltage and the voltage rating of the FET's?

I think if the FET's may blow or not depends on how much room is left from actual battery voltage to the maximum voltage rating of the used FET's.
If high current flow gets interrupted, there will always occur a voltage spike. How high it gets depends on the inductance of the supply line.
The spike can be lowerd by using fat and short wires together with capacitors in those main leads (the closer to the circuit the better).

Otherwise i see no reason why the "kill switch mode" should not work, as the turn off process should be instant.
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Re: Active pre-charge/inrush control

Post by fechter » Jun 30 2017 11:28am

Grounding the FET gates with a switch will make the FETs turn off as fast as possible. Normally this is what you want as a kill switch. There will be some inductive ringing that might create some voltage spikes, but normally I would expect the main caps in the controller to minimize those. A TVS diode across the drain-source might be a way to minimize spikes.

I can't explain why sudden turn off should blow the FETs, unless as madin88 suggest the voltage rating is too close to the pack voltage.

Maybe we can get Manny to do a scope test of a high current turn off.
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Re: Active pre-charge/inrush control

Post by madin88 » Jun 30 2017 12:14pm

fechter wrote:There will be some inductive ringing that might create some voltage spikes, but normally I would expect the main caps in the controller to minimize those.
the problem is the caps become also disconnected instant. than there is NOHTING more in the line that would iron out the voltage spikes.
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Re: Active pre-charge/inrush control

Post by jmasta » Jun 30 2017 1:23pm

What if you used a momentary switch to temporarily ground the FET gate and reset the controller? Hold it down until the motor shuts off and then release

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Re: Active pre-charge/inrush control

Post by fechter » Jun 30 2017 6:40pm

madin88 wrote: the problem is the caps become also disconnected instant. than there is NOHTING more in the line that would iron out the voltage spikes.
You are correct. The controller caps will only snub anything downstream of them. The wiring from the battery to the controller will still have inductance. If those wires are relatively long or the loop area is large, you can get a pretty good spike. If the FETs are rated for quite a bit more than the pack voltage, they shouldn't be blowing.

If you drop the power even for a millisecond, you will still have this spike, so a momentary to trip the controller won't help. Plus you want to completely cut the power if the controller is malfunctioning.
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Re: Active pre-charge/inrush control

Post by Arlo1 » Jun 30 2017 11:45pm

fechter wrote:
madin88 wrote: the problem is the caps become also disconnected instant. than there is NOHTING more in the line that would iron out the voltage spikes.
You are correct. The controller caps will only snub anything downstream of them. The wiring from the battery to the controller will still have inductance. If those wires are relatively long or the loop area is large, you can get a pretty good spike. If the FETs are rated for quite a bit more than the pack voltage, they shouldn't be blowing. .
A proper cap will still absorb this although you might see something at the fets it will be minimal compared to what you would have without.

But yes the FETs need to be rated for much more then the fully charged battery.
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