# Dennis' Open source precharge controller



## Jaesin (Mar 6, 2011)

I am no EE but I find this circuit very interesting. I just have one questions.

Why have two diodes in series(D3 and D4)?


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## Guest (Oct 16, 2011)

Why do you think no one is interested. It has been left as an open thread. Free to copy and use. You never asked anyone to say that they are going to use it. I think its great for those who need a precharge circuit. Not all controllers need one. Mine has a precharge circuit built in. Good for a Curtis setup.


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## Jaesin (Mar 6, 2011)

I am currently installing a Curtis setup and I was looking for exactly this so I did a search and thank goodness it showed up.

I have another question about it though. I have a couple of these relays laying around but I was wondering about solid state relays. 

Would there be any drawbacks to using solid state relays?


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## Dennis (Feb 25, 2008)

> I am currently installing a Curtis setup and I was looking for exactly this so I did a search and thank goodness it showed up.
> 
> I have another question about it though. I have a couple of these relays laying around but I was wondering about solid state relays.
> 
> Would there be any drawbacks to using solid state relays?


I strongly advise against using solid state relays because they are very susceptible to failure from voltage spikes and if you look at RLY-2, it must hold off pack voltage which I do not trust a solid state relay to do since their off state is just electrons being prohibited in a N or P channel of a MOSFET transistor or the C and E junction of a bipolar transistor where as relays have an actual air gap between the load side and supply side when in the off state. Also RLY-1 is being powered with a current source and therefore I do not know how a solid state relay will behave when hooked up to this current source.




> I am no EE but I find this circuit very interesting. I just have one questions.
> 
> Why have two diodes in series(D3 and D4)?


To reduce sensitivity along with R3 and so the circuit will switch on at about 95% instead of waiting longer. If you do build this circuit then be sure to select a small value precharge resistor or you will be highly annoyed at the wait time because my circuit absolutely will not allow the contactor to work until the capacitors are almost completely charged up and a large resistor value for precharge will take a long time to charge them up.


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## EVfun (Mar 14, 2010)

I thank you for your contribution. I am not currently using a controller that needs an external precharge circuit but this looks pretty good. I am a little concerned with RLY-1 and what happens in the event something has gone wrong and the user is shutting down with full pack voltage across the precharge resistor (and so across the relay contacts when they open.) The relay you selected can break up to 300 volts DC and low currents. This might be a good place for a 250 vac relay that was double pole so you can get a 500 vac interrupt rating. Roughly speaking, a relay can break a DC voltage about 1/4 of the AC voltage at roughly 1/4 the AC current rating. AC has an arc that extinguishes 100 or 120 times a second (Europe or USA.) Another option is a DC rated relay, I like the KUEP-11D15-12.


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## Dennis (Feb 25, 2008)

> I thank you for your contribution. I am not currently using a controller that needs an external precharge circuit but this looks pretty good. *I am a little concerned with RLY-1 and what happens in the event something has gone wrong and the user is shutting down with full pack voltage across the precharge resistor (and so across the relay contacts when they open.) *The relay you selected can break up to 300 volts DC and low currents. This might be a good place for a 250 vac relay that was double pole so you can get a 500 vac interrupt rating. Roughly speaking, a relay can break a DC voltage about 1/4 of the AC voltage at roughly 1/4 the AC current rating. AC has an arc that extinguishes 100 or 120 times a second (Europe or USA.) Another option is a DC rated relay, I like the KUEP-11D15-12.



RLY-1 is for connecting the 12 volt power to a contactor when precharge has completed. Look closely at the schematic. You are referring to RLY-2 which provides power for the precharge circuit and precharge resistor when the key ignition power is on. The current is very low such that RLY-2 will not have a plasma arc over that is sustaining when it opens when the user has the key in the off position. Also the maximum voltage I specify my design for is 144 volt nominal pack that is fully charged which will be at 177 volts. I have also thoroughly tested my design and it works perfectly.


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## gravelydude (Sep 6, 2008)

Hi Dennis,

I think that this is a great idea (especially since I will be connecting my Curtis 1231 soon!) Did you make up a PC board for your application? Do you have any pictures of your implementation? I for one very much appreciate your sharing with the forum on your circuit. Thanks again.

JACK


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## Jaesin (Mar 6, 2011)

I am designing a mostly surface mount board for this circuit. 

I want to have a two color LED on the dash that changes color when the contactor is engaged. So when you turn the key on, it's red. When it is engaged, it turns green. 

So I need a 5 pin relay for RL2. Does anyone have any suggestions? 
I've currently designed for PCN-124D3MHZ,000 as I am only running 80v at the moment. 

My 2nd/3rd questions are:

I couldn't find a 3watt 27K surface mount resistor in digikey. I'm trying to source everything from there to save on shipping. 

Can I use 4ct 6.8K .75 watt thick film resistors in series?
Most of the resistors I have sourced are thick film, is this OK?


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## Dennis (Feb 25, 2008)

> I am designing a mostly surface mount board for this circuit.
> 
> I want to have a two color LED on the dash that changes color when the contactor is engaged. So when you turn the key on, it's red. When it is engaged, it turns green.
> 
> ...


I would use the 12 volt power that energizes RLY-2 coil so you know key power is working and that the coil should be getting power. For the green LED that you wish to use for full charge indicator, I would have it hooked up to the RLY-1's output contacts that close the 12 volt circuit to provide power for the main contactor coil. That way you know that the relay is working and the contactor should engage.




> My 2nd/3rd questions are:
> 
> I couldn't find a 3watt 27K surface mount resistor in digikey. I'm trying to source everything from there to save on shipping.
> 
> ...


It does not have to be all surface mount. Even boards today use a combination of surface mount and through hole topology. The 27K ohm, 3 watt resistor is Digi-Key part number: P27KW-3BK-ND that is made by Panasonic. 

Also please be sure to epoxy dip the entire assembled board so that moisture from cool temperatures does not come in contact with any exposed areas of the board as these transistors are fairly sensitive and non-pure water can form a conduction path which can cause serious problems.


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## Jaesin (Mar 6, 2011)

Epoxy dip is great advice, thanks. I definitely would not of thought of that.

About the resistor (R4), I suppose my question is sort of academic. As long as I understand it correctly. 27K * 144V would if fully grounded would create 0.768 watts of heat. 144*144/27K. And if the load was separated by 4x 6.8K resistors, the formula for each resistor would be (144/4) = 36*36/6.8k = 0.1906 watts per resistor. 

Like I said earlier, I am not an electronics engineer. I just want to be sure I understand this concept. I will probably go with whatever is cheaper. But to be sure, do I understand this correctly?


About the relay, I was thinking of using a bicolor LED. Like digikey: 67-1327-ND or 492-1175-ND (much brighter). I don't want it to change from red to orange, I want it to change from red to green. I suppose, I could use some sort of transistor based switching scheme but a 5 pin relay seems much simpler.

Are small board mount 5 pin relays uncommon?


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## Dennis (Feb 25, 2008)

> About the resistor (R4), I suppose my question is sort of academic. As long as I understand it correctly. 27K * 144V would if fully grounded would create 0.768 watts of heat. 144*144/27K. And if the load was separated by 4x 6.8K resistors, the formula for each resistor would be (144/4) = 36*36/6.8k = 0.1906 watts per resistor.


You MUST use a 3 watt rated resistor because at 100+ degrees summer time the resistor's effective wattage rating is reduced because it cannot shed as much heat into the the ambient environment. You must remember that my circuit is going to be exposed to temperature extremes and that a 144 volt pack fully charged will be at 177 volts.

If you really do want to stick with surface mount resistors then you should use four, one watt, 6.8 Kohm resistors that you have wired in series.







> About the relay, I was thinking of using a bicolor LED. Like digikey: 67-1327-ND or 492-1175-ND (much brighter). I don't want it to change from red to orange, I want it to change from red to green. I suppose, I could use some sort of transistor based switching scheme but a 5 pin relay seems much simpler.
> 
> Are small board mount 5 pin relays uncommon?


The current source that powers RLY-1 is designed to work very well with the relay I chose or the other one I chose for the other voltage ranges. If you change it, then you risk the circuit not functioning properly.


I added a revision to my schematic to include an LED status using a Bi-color LED. A pull down general purpose transistor (2n2222) has been added to the red LED side to shut off the red LED when the green LED side receives power so it will not be the ugly orange color from both of them being on.











In my second revision I plan to do away with RLY-2 and instead have the user use a contactor for the positive side and one for the negative, but have the negative side come on first from key power which I will use as the pre-charge begin phase since it will complete the circuit for the pre-charge resistor. I also plan to have one design that works at 36 volts to 480 volts DC (600 volts DC fully charged battery bank). This new design will not use a transistor to detect deferential voltage by the way. I have thought of an ingenious way to be build a very reliable pre charge. I am keeping this a secret on how it will be done for now though.


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## Jaesin (Mar 6, 2011)

Revision 2 Sounds great, I can't wait to see it!


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