Pre charge resistor Wattage

I need to make a pre charge for my new 24 S Turnigy.

All cells arrived nicely balanced at 3.8-3.10 volts.

I'm going with 100 ohm, is a ceramic type overkill ?, say 17 watts because I calculate that would be over 100 watts, even though for a second might burn a small resistor ? The ceramic can take a lot of power for a short time.

It's no big deal only I don't really like the size of the 17W caramic ones, but I could live with it. But if the smaller one say a 2w would not burn then I'd use that.

why do u have to use 100R? why not 10k?

On my 24s lipo system I use a 1/2 W 300 Ohm resister with no problems. If you use a 1k ohm one you will weld closed the contacts of your switch.

The power rating of the pre-charge resistor is low, as it only carries a high current for a fraction of a second. I used a 330 ohm 1/2W when I used a pre-charge system, and it didn't even get vaguely warm.

The value depends on the time you're prepared to wait. For a 3000uF controller capacitance (pretty big) then a 330 ohm resistor will charge it in around a second, which is usually fine. A 1k resistor will take around 3 seconds to charge, but because of the lower current and longer time won't dissipate as much heat.

In my view, a resistor of around 100 to 330 ohm, 1/2W, is usually fine.

Thanks for the replies.

It will be nice to not,have that big spark any more.

this might sound daft.... but why not use a bulb ?

pick the right bulb, and it should light/glow as you engage it... then once it's job is done (controller capacitors are charged) it will go out....

works as a nice save pre-charger and a warning light too

knighty said:

this might sound daft.... but why not use a bulb ?

pick the right bulb, and it should light/glow as you engage it... then once it's job is done (controller capacitors are charged) it will go out....

works as a nice save pre-charger and a warning light too

Click to expand...

Worth a try.

Would it work with a 220V bulb ?

o00scorpion00o said:

knighty said:

this might sound daft.... but why not use a bulb ?
pick the right bulb, and it should light/glow as you engage it... then once it's job is done (controller capacitors are charged) it will go out....
works as a nice save pre-charger and a warning light too

Click to expand...

Worth a try.
Would it work with a 220V bulb ?

Click to expand...

any bulb will work... just need to size it right, too little and it'll pop, too big and it won't light up...

try a 240v and see how it goes ?
you'll be running just short of 90v hot of the charger ?

knighty said:

o00scorpion00o said:

knighty said:

this might sound daft.... but why not use a bulb ?
pick the right bulb, and it should light/glow as you engage it... then once it's job is done (controller capacitors are charged) it will go out....
works as a nice save pre-charger and a warning light too

Click to expand...

Worth a try.
Would it work with a 220V bulb ?

Click to expand...

any bulb will work... just need to size it right, too little and it'll pop, too big and it won't light up...

try a 240v and see how it goes ?
you'll be running just short of 90v hot of the charger ?

Click to expand...

99.6V charging to 4.15 V per cell.

cool

a 60watt bulb at 220v is 806 ohms resistance
806 ohms resistance at 100v (same bulb) is 12watts

it'll work fine, and it might just glow for you if you're lucky

test it with an old bulb first before you buy one... that way if it sucks you can always forget about it

the higher wattage the bulb, the higher the resistance, and the better it is for what you want because more resistance = less spark

you could always try something like this....
http://www.ebay.co.uk/itm/LIG0230-LED-Pilot-Light-Indicating-Warning-Lamp-22MM-230V-Panel-Mounting-2PK-/190690684219?pt=UK_Home_Garden_Wall_Lights&hash=item2c660ced3b
not sure how many watts it is... but I think it should be enough

Cool, worth a try if I've space left in the Topeak bag after the 8x Turnigy.

As Jeremy said...they only take the current for second or so. so any low wattage can be used..the smaller it is though the hotter it will get. I have used quarter or eigth watt in the past on my 100 volt system..and it does get hot..you will burn your self ...I know

So half watt or so just fine..otherwise whatever you have to hand

I got it all up and running, and used a 100 ohm 2W resistor I found in an old satellite receiver power supply. It gets warm very warm but not roasting.

Works a treat, no spark what so ever on 24S .Why didn't I do this a long time ago ? :lol:

Now I wish it would stop raining so I could test this high voltage less current theory, well to me it's a theory until I prove it lol

o00scorpion00o said:

Now I wish it would stop raining so I could test this high voltage less current theory, well to me it's a theory until I prove it lol

Click to expand...

Stop raining? In Ireland? A vain hope, I would have thought...................

Glad you got it sorted, BTW.

Cheers Jeremy,

Yeah no hope of a dry spell for a while I'd say.

I hope I don't forget to turn the controller amps down from 85, on 24 S and 120 % throttle it would be a bad thing if I hit the throttle I. The 120 mode!

nicobie said:

On my 24s lipo system I use a 1/2 W 300 Ohm resister with no problems. If you use a 1k ohm one you will weld closed the contacts of your switch.

Click to expand...

This makes no sense. A 1k ohm resister will not hurt contacts any more than a 300 ohm would. If anything, there would be less danger of damage to contacts.

I started out with single 550 ohm 10w resistor and got a pretty good sharp using 24s lipo charged to 100V. Paralleling 2 of them for 275 ohm made the spark weaker, but still more than I wanted. 3 of them for ~170 ohm did the trick. Still get a tiny tick sometimes if make the main connection fast, but hardly noticeable. Go with a 150 ohm resistor and you can't go wrong. I just ordered 10 of them 10W for $6 shipped. 150 ohm should work for any voltage to about 150V. FWIW my controller has 100V 1000mF caps.

My harnesses use a 5w 1k ohm. It works on 24-100v. Its over kill but its not like i want anything coming back to me wither. Jeremy is right it only sees that rush for a fraction of a second but Id rather not risk it. Ive had my share of overpowered failed resistors.

When I used a 1k ohm resistor on my 24s system, with the momentary switched closed it dropped the voltage to around 60v, which left a voltage difference of 40v. This 40v was too much for any of the main system switches/relays that I tried, and I tried a lot of them, some quite expensive. They kept welding their contacts together and wouldn't release. After asking on E-S what to do, Big Moose suggested using a lower ohm resistor. That did the trick and have had no further trouble, just so I remember to use the pre-charge switch first.

The energy lost in the resistance charging a capacitor is 1/2*C*V*V, same as the energy finally stored in the capacitor. It does not vary with the resistor value. It just takes longer with a larger resistor.

http://www.physicsforums.com/showthread.php?t=251092

The precharge resistor must be small compared to the initial load, and large enough to control the peak current. For example in a 100 volt system a 10 ohm resistor would limit the peak current to 10 amps. A 100 ohm resistor would limit it to 1 amp. The problem with the 1000 ohm resistor cited earlier was the voltage did not charge enough due to the loads in the controller. Usually the controller has a 10K bleed resistor but in that example there was even more load. In any case choosing a value between 10 and 100 ohms should work for most systems up to 100 volts.

One quick thing to add...

We are talking about a momentary switched parallel circuit, that never should make a 1/2W resistor to get hot.

Don't just stick these in line with your battery feed. If it gets hot... u've screwed up.

The amount of energy required to charge the capacitors depends on the capacitance and the voltage. It can be sufficient to heat a 1/2 watt resistor, depending on these factors and the thermal capacity of the resistor.

Im trying to figure out whats the issue with 1k. Even when i ran 24s I plugged the precharge in first then the main and unplug the precharge. I got no spark. Im not beind defensive but what is the issue here the wattage or the ohm. I used one of the online calculator an 1k 5w for 5s was the only one that i saw that stayed within the specs of the resistor but from what i read it doesnt even matter that i can go much lower in resistance.

icecube57 said:

Im trying to figure out whats the issue with 1k. Even when i ran 24s I plugged the precharge in first then the main and unplug the precharge. I got no spark.

Click to expand...

Probably something to do with the controller, size and how many caps have to charge. I know I got a big spark using a 550 ohm 10W resistor with my 72V 1500W controller until I went to 170 ohm. After that I never tried anything higher than that on any of the others I did. 170 ohm worked fine on 3 different controllers from 50V to 100V so I'd say 150 ohm would work good on about any controller.

The problem here is that people may be fitting the pre-charge resistor in the wrong place. If you're going to use this method of reducing the connection spark, then it's very important that you DO NOT connect the pre-charge resistor so that it feeds the controller "ignition" wire. You MUST only use the pre-charge resistor in-line with the main power feed to the capacitors and FETs.

The reasons are two fold:

- Firstly there is a risk of controller damage if you feed the ignition wire through another dropper resistor, as it may mean that the FET driver power regulator drops out of regulation, meaning that the 5V rail may also drop and you could possibly get some odd conditions, with FETs partially turned on in the output stage. This probably wouldn't cause damage, but it might, so it is better not to take the risk.

- Secondly, and perhaps more importantly, the controller will try and draw the "ignition" current through the pre-charge resistor if it's wired in the wrong place. This means that the pre-charge resistor cannot ever charge the capacitors to the full supply voltage, so you will still get a spark when you make the connection, unless you reduce the pre-charge resistor to a low value. This ignition current flowing through the pre-charge resistor will also make it get hot. Normally the pre-charge resistor won't see anything like enough dissipation to get hot.

Below are two possible ways to wire up pre-charge without accidentally powering the "ignition" feed via the pre-charge resistor. The first uses a separate pre-charge switch (or connector) and "ignition" switch, the second combines two functions and uses the "ignition" switch as the pre-charge switch. For the second option you switch the "ignition" on, then a second or so later switch the main power on. The diode stops the possibility of the main power feeding the "ignition" via the pre-charge resistor if they are accidentally switched in the wrong order.





In my view, it's actually better to get rid of the whole pre-charge thing and just use a slow electronic switch, as detailed elsewhere. That way you can have a simple, low current, on/off switch anywhere on the bike and use it to turn on and off the power with no worries about sparks etc.