One for the electrical/electronic gurus
 

Looking for some technical advice please from those in the know! if i put four 6 ohm resistors in parallel what is the total resistance of them all? less than 6ohms or more? What im looking to do is put resistors in the earth from my injectors to the ecu, but there is only one wire (as they are batched together) The resistors need to have a total cuircuit resistance of 48 ish ohms, how can this be achieved, thanks in advance.

Resistor total value in parallel is a bit more complicated to work out than series, where it's all just added together to find the total.
To find the resistance of parallel resistors it is Rt= R1XR2XR3/R1+R2+R3.
To do it this way would be a over complicated way, you could instead use a 10W wire wound resistor of say 47ohm, which is a VERY common value, in practice there won't be a problem between 48ohm and 47ohm, a 10W wire wound will carry roughly 3/4 amps, is this enough ?
If not these can be bunched if needed ?
tabetha

it needs to carry 6 amps, thats 1 amp per injector. But if your equation is correct they would have a total resistance of 16 ohms in parralel? four resistor packs at 6 ohms each. 4x4x4x4 (256) divided by 4+4+4+4 (16)= 16 ohms total, not enough. :sad: if i put them in series they may over heat?

I'm getting the feeling that something is going terribly wrong here, so could you please elaborate a litte what you are trying to acchieve.
Putting resistor in series; you just add the values. 4pcs 6Ohm resistor make 24Ohm.
Putting identical resistors in parallel; divide by quantity. 4pcs 6Ohm resistor make 1.5Ohm.

Since U=RI and P=UI=U^2/R=RI^2

If you have a 48Ohm resistor and put 1A through it, you have also 48 Volts across it. Something that your ecu can not supply.
Also, if you use 48Ohm and an injector in series, the injector will surely not open oen bit, when 12V is applied.

If you apply 1A through a 48Ohm resistor, you will generate 48W of heat in the resistor, again something that of course will never happen in a 12V system.

injectors won't need an amp to open, probably only a few milliamps.
you don't need a total resistance of 48 ohms, you only need to make sure each injector circuit has resistance of around 12ohms.
so in a batch configuration you'll need about 24ohms resistance per batch of two injectors.
parallel is quite easy if you know the formula as stated many a time.

thanks for your concern lol, im just going to buy some wire wound ceramic resistors off of ebay and put one in each individual earth from the injectors (the ecu does not supply a voltage, only an earth, but more than 6 amps total will cook it) Im just running low impedance injectors with my ecu thats all.

will do mate, the injectors are 4 ohms each (LPG gas injectors) as said ill buy six 8 ohm wire wound 25 watt resistors and put one in each earth, then join all the earths from the resistors together as i only have/want one wire going to the ecu/changeover reylay (from petrol to gas)

So you are trying to make low impedance injectors to work with a high impedance ecu? So, if you have a 4 Ohm injector and want it to look like 12Ohm for the ecu, then just add 8 Ohm in series to each injector. The current will be 12V/12Ohm=1A. So you will get P=4Ohm *1A*1A= 4W of power and U=4Ohm*1A=4V of voltage to the injector. So just make sure the injector is opening in the first place with the 4V (around 3V when cranking in the cold). If the injectors are in a batch configutation (2+2), you will need half the resistance not double, so 6 Ohm resistor. The power converter to heat in the 6Ohm rsistor will be P=R*I^2=6Ohm*2A*2A=24W. If you don't have a batch install, the power will be P=12OHM*1A*1A=12W, so just use a single resistor 25W is ok if you can actually mount it to something in the engine bay that dissipates heat. Remember also that the injectors are driven by PWM which almost never will reach 100% duty, so in an actual case, much less heat is generated.

An example of a good resistor is:
http://uk.rs-online.com/web/search/s...duct&R=1073891
Notice how the 25W rating drops to 12W only when it is not mounted to any metal for heat dissipation (convection only).

They are being batched mate (all six together, one earth to ECU) does that mean i still need one resistor after every injector? They will never be used when cranking as they are lpg, and they do open when in series with an 8ohm resistor as i tried them on the bench. Thanks

So, you are driving six injectors (for a V6?) from one single ecu channel? And the channel has the capacity to drive 6 high impedance (12Ohm) injectors, so a total of 2 Ohm load and 6A of current?

If so, then batch the 6 4Ohm injectors (this makes 0.67Ohm) and put one 1.3Ohm 50W resistor in series of the batch.
Just make sure your ecu is up to this.

yep, six 12 ohm injectors on one channel (it may split into two nearer the ecu but that doessnt matter i think) so one 1.3 ohm 50w it is then lol