Duff flasher unit

[Pete Lewis]

 there a whole lot of variables in these   with 2x     and 4 x  and 2 + 2 + 5   and so on 

Pete

[Chris A]

1 minute ago, cliff.b said:

If you upgrade your brain to LEDs then you can only think one way 🤔

One way is better than the current (!) no way . . 

[cliff.b]

Just now, Chris A said:

One way is better than the current (!) no way . . 

Lol💡

[johny]

15 minutes ago, cliff.b said:

Well I assumed that the current through the heater filament is limited by the resistance of the bulbs that are in series with it and 4 bulbs in parallel would allow twice the current to pass compared to 2. This would mean the heater gets hot much quicker and speed up the flash rate, so guessing the heater resistance itself is lower in the 4 bulb version to compensate.

If the 4 bulb version is used on a 2 bulb circuit then I would expect the "heavier duty" heater to heat up slower than intended due to the reduced current, resulting in a slower than expected flash.

Surely though if the bulbs were limiting the current they would light up? I think the heater is a higher resistance than the bulbs so it limits the current and it must be low enough not to cause the bulbs to glow (easier the more bulbs in parallel there are).... 

[cliff.b]

6 minutes ago, Pete Lewis said:

 there a whole lot of variables in these   with 2x     and 4 x  and 2 + 2 + 5   and so on 

Pete

I suspect that they would all work, but the flash rate will vary and in some cases be unacceptable.

[cliff.b]

13 minutes ago, johny said:

Surely though if the bulbs were limiting the current they would light up? I think the heater is a higher resistance than the bulbs so it limits the current and it must be low enough not to cause the bulbs to glow (easier the more bulbs in parallel there are).... 

Yes, I agree. I suppose the heater and the bulbs are effectively 2 resistors in series so the total current flow during the heating stage will be a function of both (V=IR) but as you say, the heater resistance must be higher so the current flowing through the bulbs isn't high enough to make them glow.

So if the resistance of the bulbs is lower because there are more in parallel, I am now thinking the resistance of the heater would go up to some extent to compensate and limit the current increase.

Does that make more sense?

[cliff.b]

Also, while on this subject, during my fault finding I saw from the wiring diagram that the +12v to the indicator flasher unit comes from the same connection as the brake light but via the hazard warning switch. 

But the hazards have their own permanently live supply from the cigar lighter connection, so wondering why the indicator supply goes through the switch and can't find any internal diagram for it.

Is this just to remove the power to the indicator circuit when the hazards are operational?

[johny]

13 minutes ago, cliff.b said:

Yes, I agree. I suppose the heater and the bulbs are effectively 2 resistors in series so the total current flow during the heating stage will be a function of both (V=IR) but as you say, the heater resistance must be higher so the current flowing through the bulbs isn't high enough to make them glow.

So if the resistance of the bulbs is lower because there are more in parallel, I am now thinking the resistance of the heater would go up to some extent to compensate and limit the current increase.

Does that make more sense?

yes that sounds right and of course then the heater will produce more heat (W=I*R) but they must either increase its resistance a bit more to reach a balance point or use a different bimetallic strip. Either way your flasher should flash a bit slower and I wonder if the longer time the heater is energised could have caused its early failure?

[cliff.b]

11 minutes ago, johny said:

yes that sounds right and of course then the heater will produce more heat (W=I*R) but they must either increase its resistance a bit more to reach a balance point or use a different bimetallic strip. Either way your flasher should flash a bit slower and I wonder if the longer time the heater is energised could have caused its early failure?

Who knows. Based on the construction I assume they only have a certain life anyway. I will see how long this one lasts 

[johny]

Think theyre intended to last indefinitely but of course the contacts will eventually burn....

[Pete Lewis]

copied from vehicle lighting regulations  circa 1066!!

cant remeber the whole story but on warranty testing manufactureres  units returned there was also a time period of how long on and how long off 

so you do get a  well defined flash and not a load of fast winking  we made a rig to time this but it all went in the skip when they closed us down 2002

 

Every indicator (other than a semaphore arm, that is an indicator in the form of an illuminated sign which when in operation temporarily alters the outline of the vehicle to the extent of at least 150 mm measured horizontally and is visible from both the front and rear of the vehicle) shall when in operation show a light which flashes constantly at the rate of not less than 60 nor more than 120 flashes per minute. However, in the event of a failure, other than a short-circuit of an indicator, any other indicator on the same side of the vehicle or combination of vehicles may continue to flash, but the rate may be less than 60 or more than 120 flashes per minute. Every indicator shall when in operation perform efficiently regardless of the speed of the vehicle.

pete

[NonMember]

6 hours ago, cliff.b said:

Well I assumed that the current through the heater filament is limited by the resistance of the bulbs that are in series with it and 4 bulbs in parallel would allow twice the current to pass compared to 2.

If the heater is in series with the bulbs and (crucially) if it's in parallel with the contacts, so that it heats up when the bulbs are NOT lit (as was stated earlier in the thread) then the current through the heater must be lower than the operating current of the bulbs and therefore must be dominated by the heater's resistance, not the bulbs'. The current is therefore not in any way limited by the bulbs and increases by only a small fraction when there are four of them.

However, if the heater is very low resistance and wired in series with the contacts, so that current flows through it when the bulbs are lit, then you're right.

[NonMember]

5 hours ago, cliff.b said:

But the hazards have their own permanently live supply from the cigar lighter connection, so wondering why the indicator supply goes through the switch and can't find any internal diagram for it.

The indicators are powered from the ignition-switched (green) supply, as you noted.

The hazards need to operate with the engine off, so they're powered from the permanent live.

Since the output of the two is the same set of bulbs, the indicator supply needs to be disconnected when the hazards are in use, otherwise the ignition circuit would be powered along with the bulbs.

[cliff.b]

1 hour ago, NonMember said:

The indicators are powered from the ignition-switched (green) supply, as you noted.

The hazards need to operate with the engine off, so they're powered from the permanent live.

Since the output of the two is the same set of bulbs, the indicator supply needs to be disconnected when the hazards are in use, otherwise the ignition circuit would be powered along with the bulbs.

Yes, that makes sense and I have found a diagram which I think confirms that. So for indicator fault finding purposes, the hazard switch could be bypassed and a voltage applied directly to the L side of the flasher unit and the indicators should then work as normal.

If not, then I would expect applying 12v to the B terminal would make the indicator lights come on when selected, but not flash.

If still nothing then likely to be a poor connection/wiring/earthing 🤔

[cliff.b]

1 hour ago, NonMember said:

If the heater is in series with the bulbs and (crucially) if it's in parallel with the contacts, so that it heats up when the bulbs are NOT lit (as was stated earlier in the thread) then the current through the heater must be lower than the operating current of the bulbs and therefore must be dominated by the heater's resistance, not the bulbs'. The current is therefore not in any way limited by the bulbs and increases by only a small fraction when there are four of them.

However, if the heater is very low resistance and wired in series with the contacts, so that current flows through it when the bulbs are lit, then you're right.

Yes, in this flasher unit the heater is wired across the contacts so heats when the contacts are open, with a circuit to ground through the bulbs.

2 X 21w bulbs will have a resistance in the region of 3.5 ohms and as discussed previously, the heater will be somewhat higher than that. I tried measuring the resistance of what was left of the heater in my old flasher and got 15ohms, but part of it had broken off. Probably 20+ when intact.