# Precharge Resistor Selection



## Tony Bogs (Apr 12, 2014)

100 Ohm, 10W is a good starting point, but not a full spec.
Add the rest of the specs (overload rating in particular) and the rest of the precharge components (~96V is not a problematic rating for a PTC thermistor).
Value/ratings might need adjustment for higher voltages (140V is max for the Curtis?) when the overload rating is 5x nominal for 5 seconds.


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## GoElectric (Nov 15, 2015)

Hi Tony,

The HV doesn't touch a lot more than that, unless you include the contactors themselves, lol. Yeah, there will probably be some heating elements, but as you say, no-worries there. 

What is the 5X rule (for overload protection?) 

I came up with 100ohm/10W from the spec on another build, which needed it for the controller. That one was 25W and as big as my thumb. I think since the controller is already protected, that would have been the bulk of it. So, 10W sounds good and safe as long as it doesn't set off any alarm-bells for anyone here. 

The 100ohm value I think I would keep the same - the resistor is still doing the same job, just not as much of it - again, unless someone says otherwise. 

Thanks.


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## Tony Bogs (Apr 12, 2014)

If the precharge resistor does not have an overload rating of at least 5X, 
the resistor may fail after a number of precharge sessions. 
Examples have been posted on this forum.
A PTC in series is needed as an overload protection if precharging takes longer than 
the overload spec of the resistor, for instance 5X nominal for 5 seconds at 25 deg C. 
It's basic electrical engineering. 
BTW, I am a BSEE: analog, digital and power electronics, datacommunication, computer sciences, systems design and schochastic analysis.


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## GoElectric (Nov 15, 2015)

Being an EE does give you some cred but it all depends upon experience. I know guys with engineering degrees who are smarter than me, but have no practical grasp of anything outside of their specialty. It helps if you have a great memory, which I don't.

Speaking of practical then, you are saying some resistors have a better overload rating than others? I'm guessing the little white ceramic ones are not the best then. 

When you say PTC, I think of positive temp coefficient resistor, is that right? I thought you meant something to do with the heating elements, but I think not. I see what you mean - that is a cool way to pre-charge, very safe.

As far as overload goes, worst-case I will have 150V (pack voltage) across the resistor, or 1.5A, which is a ton of watts, and maybe it will cause a failure. I guess I can wait and see how long the pre-charge takes, but I'm guessing less than a second. It will all be set up on the bench soon.

Thanks.

I will look and see what I can find on here about pre-charge failures. I expect all of them will be protecting the controller too.


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## GoElectric (Nov 15, 2015)

So far, nothing which applies to the question of the difference in pre-charge between cars which need it for the controller and not. Actually, I haven't seen an actual value for pre-charge yet (?).


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## boekel (Nov 10, 2010)

Tesla uses a 23 Ohm resistor:



















Don't know how many Watts....enough


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## Solarsail (Jul 22, 2017)

GoElectric said:


> I expect all of them will be protecting the controller too.


What else is there to pre-charge but the controller?

1.5A on a 100 ohm 10W resistor? Sounds like a firecracker to me. Even at momentary 5x overload, that is 50W vs. 225W supply. A PTC will help, but is it fast enough?

Try a common household GU10 halogen lightbulb, 20W 110V. It is about 45 ohm cold and 600 ohm glowing hot. This should work up to 96V. For 150V two in series will give you 90 ohm to about 700. If you desire more current, then use 35W (30 ohm) or 50W (18 ohm) lamps.

I believe Leaf pre-charge resistor is at 30 ohms.


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## GoElectric (Nov 15, 2015)

Hi SS,

Thanks. 

As I understand it, inrush current can damage the DC/DC and/or the Charger. I don't know how common this is, but if a single passive component can prevent it, then unless someone can convince me otherwise, I'm up for something. I just wonder why these components, which are built for such an event wouldn't be ummm, built to WITHSTAND such an event.

I like your idea of using a light so I can see what is going-on. I wouldn't use it permanently, but I should have thought of that. Should be able to scrounge-up a halogen light somewhere.

I squared R means 1.5 amp across a 100 ohm resistor is 225 watts, not 25 watts. I can imagine that might cause a cheap resistor to fail eventually, even if it is only on for a second.


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## piotrsko (Dec 9, 2007)

With a capacitor current flow is logarithmic in decrease. Initial current is huge but rapidly decreases to eventual zero as the caps accept charge. AKA r-c time constant.


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## GoElectric (Nov 15, 2015)

Sorry, I suppose there are people who don't know that. I should have said "for a fraction of a second." 

Is there nobody out there who has a precharge?? What size (and why)?


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## piotrsko (Dec 9, 2007)

Last time I looked inside my SOL1 precharge was a 150. 10 watt ceramic ( I think). At 300 volts that would be something like 20 amps but only for 3 seconds until the relay drops out. Probably gets pretty warm but that is generally the only use per trip.

Use a 100 watt lightbulb. Down side is it is fragile so you should carry spares


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## GoElectric (Nov 15, 2015)

Hi - Mr. P. and that is good to know. I assume you are talking about a Soluton(?) controller? 

Anyway, the max current you are getting is roughly the same as what I will get with 150V and 100ohms, so that is good. And 10 watts sounds reasonable. 

Awesome, I think I'll go watch some porn (just kidding).


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## Tony Bogs (Apr 12, 2014)

piotrsko said:


> Last time I looked inside my SOL1 precharge was a 150. 10 watt ceramic ( I think).


Many high power ceramic wire wound resistors have the required overload spec, usually 5X nominal for 5 seconds. 

I am using a 47 Ohm wire wound ceramic resistor with the 5 seconds overload rating in the input stage of my charger design. 
The PTC (auto reset fuse) in series has prevented the resistor from melting down in all cases where precharging took longer than than a few seconds. 

It looks like the Tesla resistor is rated for continuous pre-charging. 
Would also be my choice when designing a US$100.000+ car.


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## EVfun (Mar 14, 2010)

The Curtis 1238 has a built in precharge function provided you have it enabled in software. You don't need 2 precharge systems, just one and I don't see any reason to not use the one Curtis has built into the controller.

The older Curtis 1221 and 1231 controllers did not have a built in precharge function and such a system should be provided. When I was running a Curtis 1221B controller I used a standard E26 household light socket with a low wattage build, in my case a 7.5 watt red group bulb. That kept the controller charged within a few volts of pack voltage. Now I'm running a Zilla controller which has the precharge function built into it, the previous precharge system has been completely removed.


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## GoElectric (Nov 15, 2015)

Hi. Yes, I know the 1238 has precharge, but am not sure about the charger, or DC/DC. I am thinking 50-100 ohms, and like the idea of the PTC, but don't know if it is necessary - I guess it could be considered a safety feature in case of a failure? Otherwise, if designed correctly, I'm not sure why the precharge would ever go on too long.

Thanks, 

Jim


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## Tony Bogs (Apr 12, 2014)

The PTC is indeed a safety feature in case of a failure. 
Just like any other fuse. If it is left out, the ceramic resistor often acts as a fuse.
That's also fine ( If carefully designed ); in the 80's there were ceramic resistors with a solder joint for controlled blowout (leaf spring actuated).

The PTC has the advantage of auto-reset.


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## GoElectric (Nov 15, 2015)

I guess what I was thinking was I might prefer to have a fuse which just blows, so I know where the problem is. I can see the use of a resettable fuse for a motor or something which might draw too much current. When it re-sets, one can adjust the motor controls or load (not thinking of an EV here), but if my charger or DC/DC draws too much current, I don't want it to turn on again until I fix it (?).


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## Tony Bogs (Apr 12, 2014)

A resistor-LED circuit in parallel shows the tripped PTC. 
But in many cases not needed: the smell of a tripped (very hot) PTC is very distinctive.

In some cases a non-resettable fuse is the better option. 
I'm using a "standard" fuse for the auxiliary power supply in my charger design. It is even soldered in, no holder. 
The power supply has about all imaginable safety features built-in. When that fuse blows, there must be a non-recoverable cause.


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## GoElectric (Nov 15, 2015)

Thanks Tony - I appreciate it!

Jim


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