Additional Alternator Warning Light

[alanrichey]

Hi all, more electrical advice needed :-)

 

After driving my Superspec a few times I realised the warning lights are very difficult to see, a combination of positioning, angle of the cluster mounting and reflection off the plastic screen. So I decided to put additional warning lights high on the dash on the right. The indicator and oil pressure lights were simple, I just took a tapping off the back of the cluster and used that to put warniing lights in parallel. But the alternator light worries me a bit, and reading all the historical posts on the subject gives conflicting advice.

 

The problem is that, unlike the Oil and Indicator warning lights, which are directly fed from the instrument cluster connector, the alternator light goes through a diode and has a 100 Ohm resistor across the connections, both on the printed circuit on the back of the cluster. So I didn't want to risk just taking a tapping from the connector.

 

So:

 

1. If I do take a tapping off the instrument cluster connector do I need to have an additional 100 Ohm resistor across the connections ? Or is this going to screw something up ? (As I recall 2 x 100 OHms in parallel = 50 Ohms ?)

 

2. What exactly is the purpose of the resistor across the connections ?

 

3. I saw on one post that current needs to flow both ways, so you need a bulb, not an LED, but in that case why has the existing one got a diode in circuit ?

 

4. Would I better off taking a tapping direct off the alternator ? if so would I need a diode and the resistor as well ?

 

Thanks

 

Al

[fry61]

Hopefully others will confirm; the 100 ohm resistor in parallel with the LED(which should have its own resistor in series to drop the volts down) is used instead of a filament lamp; SO ---- remove the 100 ohm resistor & connect your new warning lamp (must be old fashioned filament type) instead; 5 watt seems to be the norm this is enough to excite the alternator.(Which the LED on its own can't -- too little current)

[alanrichey]

OK, makes sense. But I wasn't intending on removing the original one in the cluster, just augmenting it. So what if I simply put my new lamp in parallel with the original, so that they both light up and the 100 Ohm resistor keeps the current high enough to excite the alternator. Will that do any harm ?

[brumster]

Agreed completely with Florin. Depends where you're tapping off - if it's after the resistor you might find the voltage drop across the bulb means the LED doesn't even light. So effectively we're saying wire it 'direct' to the alternator. I think without seeing a wiring diagram!

[fry61]

Put your LED (with built-in resistor in series) in parallel with the new warning lamp ( must be a filament lamp) no need for any other electrics. The new 5 watt lamp gives enough current flow to excite the alternator & the LED just adds a little bit. If memory still works the 5 watt lamp has a resistance of 48 ohms.

[alanrichey]

OK, my apologies, I don't seem to have explained myself very well as none of the answers relate to my problem and I didn't actually mention LED.

 

Let me try again.

 

The present system appears to be (The Robin Hood Wiring Diagram only gives colours of wires, not functionality so it's a bit of a guess),:

 

1. A wire from the alternator arrives at the Blue Connector plug to the back of the instrument cluster. I can access this wire under the dash.

 

2. When I examine the back of the cluster (not the one on the car, I have a spare one), I can see that printed circuit connection from the appropriate connector goes through a diode to one side of the the Alternator Warning Light (I almost certain it's a filament bulb. Were LEDs actually in use when the SuperSpec was sold ?) and the other side of the warning light is tied to 12V (ignition live). In addition, there is a 100 Ohm resistor on the circuit board in parallel with the lamp. Now I do NOT intend to take the cluster off and try and solder direct to the circuit board, that comes under the 'too much hassle' heading.

 

Now, these lamps are tiny, so I can understand the logic of them adding an extra resistor to ensure a large enough current flow to excite the alternator.

 

So all I am really asking is can I now put another lamp, probably filament, between the alternator output and 12V (ignition switched), without any extra components and without disturbing the existing setup ? or will this somehow screw up the charging/warniing system. ?

 

Hope that makes more sense.

 

Al

[sj-bradley]

Yes, As Florin above states, you should be able to do just that.

 

That 100 ohm resistor in parallel with the filament bulb would be fine.

 

(Note - measuring the resistance of an ordinary lamp when cold will give you a much lower figure than when the lamp is lit as resistance goes up with the high temperature)

 

Let's see 5 watt lamp over 12 volts = 0.42 amps

12 volts over 0.42 amps = 29 ohms when the lamp is lit - but much lower when cold.

 

So the 100 ohm resistor was only just enough to excite the alternator.

 

You would have no problem in paralleling the lamp in this case.

 

Simon.

[fry61]

Point 3 of your original post led me a-stray ???

[sj-bradley]

Which bit? That hot lamps are a higher resistance?

 

Simon.

[sj-bradley]

Ohm's law and the Power law.

 

Volts = Amps x Resistance

Power = Volts x Amps

 

If you measure the resistance of a lamp when cold, it will be only 1 - 2 ohms or so.

Go by the wattage instead. So for a 5 Watt bulb over 12 volts would give around 1/2 amp through it.

Then 12 volts / 0.5 amps would give you around 24 ohms, which would be the resistance when hot.

 

Simon.

[alanrichey]

Point 3 of your original post led me a-stray ???

Ah, I missed that as well :-) So I did mention an LED. Sorry.

 

That 100 ohm resistor in parallel with the filament bulb would be fine.

 

Sorry, are you saying I need another 100 Ohm resistor in parallel with my additional filament bulb ? Surely adding another bulb is going to increase the current anyway making the alternator even more excited :-)

[sj-bradley]

No. You would be fine putting just one 5 Watt lamp where you thought it should go.

 

Most of the current will then go through this lamp instead and you should still get enough through the parallel LED in the instrument cluster to light up both.

 

Simon.

[alanrichey]

Thanks, sounds good. I'll post the result

[fry61]

Point 3 of your original post led me a-stray ???

Your post crossed my post, Simon.(cause I can't type quickly) The alanrichie first post was my point.

[brumster]

Putting the diode in I suspect just turns it into an "alternator" failure warning lamp, rather than the dual function of an "alternator or battery" warning lamp. So it will work when the potential difference flows in the appropriate direction of the diode - in this case I assume it's when the alternator output drops below that of the battery, and current flows from battery back to alternator. But with the diode, it won't light if the alternator voltage is greater than the battery voltage (ie. stuffed regulators).

 

The other thing is that typically LED's need just over 2v PD before they even consider lighting - and their intensity doesn't vary quite so linearly above that, and then they pop when over-volted.... whereas with a 12v bulb, you at least get some analogue indication (via brightness) of how bad the potential difference is

 

So not to worry about having the diode in there; just turns the lamp into a one-way warning, if you like.