# What is a precharge resistor?



## McRat (Jul 10, 2012)

And do I need it with a DC series wound motor, Curtis 1204, and LiPo batteries? How about permanent magnet and LiFePO4?


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## Roy Von Rogers (Mar 21, 2009)

A controller has a large amount of capacitors that have to be filled (precharged) whenever you turn on a controller, and in doing so there will be a very large surge of electricity. Such a surge can do damage to components, so you put a resistor in line to slow that surge down to a save level, and once thos capacitors are close to full charge, you can turn on the full connection to the controller via the contactor and drive the vehicle.


Roy


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## Joey (Oct 12, 2007)

If you connect a voltage source (battery pack) to a bank of capacitors (controller), there is very little resistance, and a lot of current can flow into the capacitors until the voltage on the caps reaches the voltage of the battery. This is called in-rush and it is hard on componnets like your contactor/switch and the capacitors themselves. The solution is to put a resistor in the circuit to slow down the current going into the controller, until the capcitors are charged up. Then you remove the resisitor so that full power can be sent to the controller from the battery pack.

You will need a precharge circuit anytime there is high voltage connecting up to a capacitor, so it doesn't matter what motor you use. A few controllers have the precharge function built in, which is a proper design for a controller.


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## McRat (Jul 10, 2012)

THANKS!

If I read the schematic right, the Curtis 1204 does not have an internal one.
48v x 300amps (electric motorcycle project).

What size resistor we talking here? Something weird, or something I get at RadioShack?


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## frodus (Apr 12, 2008)

Read the manual, 250ohm, 5W
http://curtisinstruments.com/index.cfm?fuseaction=cProducts.DownloadPDF&file=1204%20%2811C%29%2Epdf

Page 7 explains why, page 6 indicates what size resistor.

And usually if the controller manual has it, it's a good idea to put one in there.


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## McRat (Jul 10, 2012)

frodus said:


> Read the manual, 250ohm, 5W
> http://curtisinstruments.com/index.cfm?fuseaction=cProducts.DownloadPDF&file=1204%20%2811C%29%2Epdf
> 
> Page 7 explains why, page 6 indicates what size resistor.
> ...


Thanks! All I found was a specification sheet, guess I need to learn to search better.

This is an Ebay item with no docs.


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## McRat (Jul 10, 2012)

OK, I'm ready to start hooking up the Curtis 1204.

But the circuit is for FWD/REV, and for a motorcycle, I'm not going backwards.

Per the schematic on PG 6 of the manual, I just put the M- directly to the motor negative, and put A2 right to the motor Positive for the direction I want the motor to spin? There are 3 posts on my motor. I connect 1-2 and #3 becomes the positive for CW. I connect 1-3 and #2 becomes the positive for CCW rotation.

Sorry for such a simple question, but I'm just getting started with this tech.


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## major (Apr 4, 2008)

McRat said:


> OK, I'm ready to start hooking up the Curtis 1204.
> 
> But the circuit is for FWD/REV, and for a motorcycle, I'm not going backwards.
> 
> ...


Page 33 shows the connection without the reversing contactors. Just put the motor where the light bulbs are.


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## McRat (Jul 10, 2012)

Thanks! Once again, RTFM bit me in the ass.


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## Elithion (Oct 6, 2009)

Precharge: what it is and how to do it.


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## miko81 (Apr 2, 2012)

hi guys,

I have also Curtis controller but 1221B 72-120V. 
I don't know did I get you right, do you all have some switch that when you sit in car turn on power from battery to contactor? (I have connect battery and will connect + to contactor and - to controller) 
and second, someone seed that when precharge is connected main power is turned off and when contactor let main power to controller precharge is off?


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## major (Apr 4, 2008)

miko81 said:


> hi guys,
> 
> I have also Curtis controller but 1221B 72-120V.
> I don't know did I get you right, do you all have some switch that when you sit in car turn on power from battery to contactor? (I have connect battery and will connect + to contactor and - to controller)
> and second, someone seed that when precharge is connected main power is turned off and when contactor let main power to controller precharge is off?





Elithion said:


> Precharge: what it is and how to do it.


____________________________

Also: http://www.diyelectriccar.com/forums/showthread.php?t=25419


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## Ziggythewiz (May 16, 2010)

I have your controller. I get in the car and flick a switch on the dash which connects a resistor of the size specified in the curtis manual across the contactor. I watch the voltage rise in the controller as I buckle up etc. When it's around 90V (of 120) I turn the key which closes the contactor. The precharge is now bypassed and can be turned off anytime from then till I go to get out of the car.


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## miko81 (Apr 2, 2012)

Ziggythewiz said:


> I have your controller. I get in the car and flick a switch on the dash which connects a resistor of the size specified in the curtis manual across the contactor. I watch the voltage rise in the controller as I buckle up etc. When it's around 90V (of 120) I turn the key which closes the contactor. The precharge is now bypassed and can be turned off anytime from then till I go to get out of the car.


You have put some small switch on/off to turn on precharge (small switch or need powerful switch) you dont lose strength on long cable to switch and back?? do you have some pictures off connecting it?? witch contactor do you use?


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## Ziggythewiz (May 16, 2010)

It's a small switch (~$5 from Walmart) running over 18? GA wire. I think it's rated for 10 or 15A at 24V, but I'm running it at ~1/6 A @ 120V. You can't lose power in the cable running to/from the switch as there is virtually no resistance compared to the big fat precharge resistor, and you wouldn't care if you did.

Contactor is an Albright 200somethinorother

This is not my pic but very similar. Instead of the resistor being direct connected to both leads, one is wired through the switch on my dash.


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## miko81 (Apr 2, 2012)

Tnx, that is something that I didnt know. 
Just one question, how long do you wait untill you precharger charges controller (you seed that you wait that power in controller reach 90V) how do you check power in controller, do you have some instrument (I dont have it) I will need to wait in seconds, do you have idea how long 10-15 or more seconds,?? and when you turn on contactor do you turn off precharge????

tnx very much!!!


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## dragonsgate (May 19, 2012)

I am running the Curtis 1231C-8601 controller 144 volts and an Albright SW-200b contactor. When I first did my EV build in 1999 a resistor was mentioned in the Curtis manual but they didn’t give a lot of info. I called the company I bought the motor and controller from and ask him about it. He didn’t seem too concerned but said he would send me a resistor. I stuck in on and ran it for a few months. When I reconfigured the wiring I didn’t bother putting the resistor back on. I have been driving with out a contact resistor all this time until a couple of weeks ago. The contacts still look pretty good. A bunch of you have been going on about it so I thought I would give it a try again. I am using an on /off push button switch that is activated by a solenoid. That way there is not a relay that is constantly on. I have liked the jump in push a button and go all these years so now I push the resistor button and then go back in the house and make the last bathroom call or whatever and when I come back down to the car it is warmed up so to speak. Push the go button and go. I usually leave the resistor on the time I am out running around. That way there is no wait to go and with the on/ off switch there is no draw on the aux. battery. I seems to be working.


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## major (Apr 4, 2008)

dragonsgate said:


> I am running the Curtis 1231C-8601 controller 144 volts and an Albright SW-200b contactor. When I first did my EV build in 1999 a resistor was mentioned in the Curtis manual but they didn’t give a lot of info. I called the company I bought the motor and controller from and ask him about it. He didn’t seem too concerned but said he would send me a resistor. I stuck in on and ran it for a few months. When I reconfigured the wiring I didn’t bother putting the resistor back on. I have been driving with out a contact resistor all this time until a couple of weeks ago. The contacts still look pretty good. A bunch of you have been going on about it so I thought I would give it a try again. I am using an on /off push button switch that is activated by a solenoid. That way there is not a relay that is constantly on. I have liked the jump in push a button and go all these years so now I push the resistor button and then go back in the house and make the last bathroom call or whatever and when I come back down to the car it is warmed up so to speak. Push the go button and go. I usually leave the resistor on the time I am out running around. That way there is no wait to go and with the on/ off switch there is no draw on the aux. battery. I seems to be working.


Precharge normally takes under 5 seconds. Like 2 or 3 seconds.


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## Ziggythewiz (May 16, 2010)

miko81 said:


> Tnx, that is something that I didnt know.
> Just one question, how long do you wait untill you precharger charges controller (you seed that you wait that power in controller reach 90V) how do you check power in controller, do you have some instrument (I dont have it) I will need to wait in seconds, do you have idea how long 10-15 or more seconds,?? and when you turn on contactor do you turn off precharge????
> 
> tnx very much!!!


I have a voltmeter hooked to the controller side of my contactor, so it will show the pack voltage that the controller is seeing. It takes about 8-10 seconds for the precharge. If you use a resistor with less resistance than the one specified in the manual it would go faster and you would get a higher precharge.

What I would really like, but haven't figured out yet, is to use a NTC thermistor as part of the circuit, so that it would start out around 500-750 ohm, but that resistance would decrease to near 0 over about 5 seconds as it warms up. That would give a faster, and complete precharge without increasing the current.


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## dragonsgate (May 19, 2012)

Precharge time is right at 15 seconds for me. About the amount of time it takes to get the seatbelt hooked and the tilt wheel in place. 15 seconds is not much except when one is waiting for it. I have seen it happen with computers over the years. This is not just me as I have heard others confess to this. When my pooter was on dial up it took for ever to load. When I got DSL it was OMG! That’s fast! Now after the new has worn off it’s still fast but you swear it is taking for ever to load. Same with ev. There is a good chance any one of you will catch yourselves tapping your foot waiting for the precharge some time in the future.


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## major (Apr 4, 2008)

dragonsgate said:


> Precharge time is right at 15 seconds for me. About the amount of time it takes to get the seatbelt hooked and the tilt wheel in place. 15 seconds is not much except when one is waiting for it. I have seen it happen with computers over the years. This is not just me as I have heard others confess to this. When my pooter was on dial up it took for ever to load. When I got DSL it was OMG! That’s fast! Now after the new has worn off it’s still fast but you swear it is taking for ever to load. Same with ev. There is a good chance any one of you will catch yourselves tapping your foot waiting for the precharge some time in the future.


Then decrease the Ohmic value of the precharge resistor. Just put a second resistor of the same value in parallel with your existing precharge resistor and it will cut the time in half. Or put a few more in parallel with it and reduce the time further. There is no reason to wait more than a few seconds, like 2 seconds.


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## dragonsgate (May 19, 2012)

I expected some advice like yours Major and was planning on doing something one of these days but Gees look how long it took me to get around to even putting the dang thing back on. One question though. If you lessen the resistance so it only takes a couple of seconds aren’t you increasing the inrush and defeating the purpose of the whole project?


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## major (Apr 4, 2008)

dragonsgate said:


> If you lessen the resistance so it only takes a couple of seconds aren’t you increasing the inrush and defeating the purpose of the whole project?


The main purpose is to mitigate the 1000A plus inrush arc which occurs with no precharge resistor across the contacts. Using a resistance value which limits that current to 10 or 20A is sufficient. Commercial drives with 330VDC bus use as low as 1Ω precharge. As the resistance decreases, the wattage needs to increase. Adding an additional resistor, or two, or three as I suggested will take care of that.


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