Formula to calculate bulb resistance, based on power. Who knows?

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DreamV4

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Resistance of bulb is changing after it is on, so I cannot use tester, I need to calculate, based on power and voltage of running engine (13.5V?). To make matter more complicated, bulb as resistor of COP will get voltage possibly only about 5%, so actual resistance will be somewhere between cold bulb (tester) resistance and calculated one based on power specs.
There is no store around, I cannot just buy resistors.
If COP has 1 Ohm primary, I need to attach bulb with resistance 2 Ohm
 

02GF74

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The formula is pretty striaght forward R = V/I but you cannot use that since you don't have a constant current. If I had that type of bulb - which I don't - I could produce the data. From memory, the bulbs I have a BA15 bulbs 23 W and 15 W (indicator and brake light) and H4 headlamp.

You should try to get the resistors I posted about here: (59) Igniter tester, 10 sec. to make. 2.5 Ohm | Yamaha Star V-Max VMAX Motorcycle Discussion Forum (vmaxforum.net) They will work far better than bulbs and be more reliable too. Unless there is a specialist electrical store near you, you would need to order them via the post - 2 ohm I could get to you for £ 2.26 , 1.5 ohm for £ 2.00 1,8 ohm for 1.87 (not including tax) but postage would be expensive plus it woud take 1-2 weeks (my guess).

Mouser: 279-HSA52R0J or 279-THS102R0J $ 3.00

There are ceramic and wire wound types but I posted that one since you can mount them onto a metal plate, and then mount the plate to the frame so they are not vibrating in free space.
 

DreamV4

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Resistance=Voltage (square)/power=180/34=5.3 Ohm (if constant current)
Voltage is 13.4V (running engine)
34 is power of 3157 bulb with twisted wires(27W +7W)
20210828_100515.jpg
 

Kincheloe

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May be able to help, meaqsure the voltage across the lamp (meter in parallel with lamp) then the current through lamp ( meter in current mode) test leeds in series with lamp . Now with this info, voltage times current equal Power ( P=IE ). Bigest problem being unless you have two meters you can't read both at the same time.. Be sure you switch your meter to volts before reading voltage again. Hope this helps
 

DreamV4

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Resistance is too high (post #3), I am decreasing current and spark twice. It is stupid! That is why it starts hard probably.
Bulb is 5.3 Ohm +Ford coil is 0.5-0.7 Ohm= 6 Ohm!!!
OEM coil is 3 ohm
I need to buy 2 ohm resistors or attach extra 4 bulbs!!!
 
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Resistance is too high (post #3), I am decreasing current and spark twice. It is stupid! That is why it starts hard probably.
Bulb is 5.3 Ohm +Ford coil is 0.5-0.7 Ohm= 6 Ohm!!!
OEM coil is 3 ohm
I need to buy 2 ohm resistors or attach extra 4 bulbs!!!


I think you are measuring that wrong. When I used to bypass the old GM PATS system(the one with the chip bulb in the key), there was a different calculation I had to do to come up with the correct resistance value when I couldnt get the exact resistor to fake out the system.

Have you considered spark plugs with resistors already built in? Maybe that would help with also keeping with a cheap path.
 

DreamV4

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I think you are measuring that wrong. When I used to bypass the old GM PATS system(the one with the chip bulb in the key), there was a different calculation I had to do to come up with the correct resistance value when I couldnt get the exact resistor to fake out the system.

Have you considered spark plugs with resistors already built in? Maybe that would help with also keeping with a cheap path.
spark plugs are in high voltage circuit. I will attach extra bulbs, I have a box of old bulbs.
Found 2.2 ohm resistors!!! Perfect!!!
 

02GF74

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Or use headlight bulbs.
Resistive spark plugs won't help as it is the LT circuit that needs the resistor.
That link doesn't work ; what wattage are those resistors youve ordered?
 

DreamV4

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Or use headlight bulbs.
Resistive spark plugs won't help as it is the LT circuit that needs the resistor.
That link doesn't work ; what wattage are those resistors youve ordered?

eBay item number:
401261266178


Lot of 5 2.2 ohm 2R2ΩJ 10 watt Axial Ceramic Cement Power Resistor 10W New

100% Brand New
Size:L=42mm W=10mm H=9mm
Power:10 Watt
Tolerance:±5%(J)
Quantity:5pcs
 

DreamV4

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Resistance of bulb is 180/Power...(volts(square)/power)......
if current is constant.
Ignition coils get % of current time depending on RPM. It is close to impossible to calculate resistance at 1000 rpm and 10k rpm. unless clamp ampermeter is used...!!!!!
I guess the smart way would be to install cement resistors.
BTW, these resistors models/specs should be saved somewhere on forum as best ones for cheapest COP. $6 coil+$2 resistor.
Coils seat pretty tight, but I still want some holder.
 
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