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Voltage reducer for 6v trafficators
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Bob Kellock



Joined: 31 Jul 2010
Posts: 5
Location: S.W. Wiltshire

PostPosted: Sat Aug 07, 2010 9:08 pm    Post subject: Reply with quote

Coincidentally I've just had a similar problem. I bought a pair of boxed trafficators (not for the Nash!) advertised on eBay as being 12 Volt but on opening them up I found a label on the windings "Trico 6 volt" but, as they were in excellent condition I decided to keep them.

Firstly I measured the resistance with a DVM which was about 0.85 ohms. With that figure, running them at 6.5 volts results in a current of 6.5/0.85 = 7.6 Amps precluding the use of an LM388 and it appears that the 10amp LM396 has been discontinued so I decided to go for a series resistor.

Low resistance readings from a DVM are notoriously unreliable so I connected the trafficator to a battery by a long length (about 30m) of thin (0.5 sq. mm) cable which acted as a resistor. That gave a voltage across the trafficator of 4.9V and a current of 6.2A so the trafficator resistance is 4.9/6.2 = 0.79 ohms.

I wanted to drive it at 6.5V on which case the the current is 6.5/0.79 = 8.2A. Assuming a battery voltage of 12.6 the resistor needs to drop 6.1V at 8.2A so it should be 6.1/8.2 = 0.75 ohms and able to dissipate 8.2*8.2*0.75 = 50.4 watts.

The nearest value available from RS with a 50 W rating is 0.68 ohms (RS stock number 160-893 costing £1.68 ) so, if that is used, the current is 12.6/(0.79 + 0.68 ) = 8.6 A and the trafficator voltage is 8.6 * 0.79 = 6.8 V which I reckon is OK. The resistor dissipation is 8.6 * 8.6 * 0.68 = 50.2 W which seems to be a bit marginal but, provided that it is fixed to a decent lump of metal with some heatsink compound and it isn't left switched on for more than a couple of minutes, there won't be any problem - it will get a bit hot.

I've wired them up in that manner and they work well.

Bob
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Nic Jarman



Joined: 05 Oct 2008
Posts: 1031
Location: Stoke by Clare, Suffolk

PostPosted: Tue Aug 10, 2010 10:34 am    Post subject: Reply with quote

I minor point is a coil (trafficator) is an inductive load and is not resistive. I has impedance so putting a DVM across it to read its resistance is meaningless. When the switch is thrown the impedance is almost zero so there is a large surge in current. You are asking the resistor to handle 230 watts momentarily. This is not built in reliability.
Good luck and I hope it works, keep us posted.
PS Fluke DVMs are very good.
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Bob Kellock



Joined: 31 Jul 2010
Posts: 5
Location: S.W. Wiltshire

PostPosted: Tue Aug 10, 2010 3:48 pm    Post subject: Reply with quote

Nic, I'm sorry to have to say so, but almost all of what you wrote is completely wrong.

A solenoid coil is both inductive AND resistive with the latter being due to the resistance of the wire. The impedance due to the inductive element is an exponential function of the rate of change of current (di/dt) through the solenoid and is zero when di/dt is zero.

When set to measure resistance, a DVM applies a calibrated constant current through the DUT (Device under test), measures the voltage across it and calculates its resistance using Ohm's law. Because the current is constant, i.e. dv/dt = 0, the impedance of the inductive element is 0 and the displayed result is that of the resistive element only.

At the instant the switch is thrown the di/dt is extremely high; consequently the impedance due to the inductive element is very high so there is no surge - quite the opposite - it builds up from 0.

If you still think that I've got it wrong then take a look at
http://www.play-hookey.com/dc_theory/rl_circuits.html
where it says "The current through an inductor cannot change instantaneously, so at this instant [switching on] there is still zero current through R, ...".

You're right about Fluke DVMs being very good Smile
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Nic Jarman



Joined: 05 Oct 2008
Posts: 1031
Location: Stoke by Clare, Suffolk

PostPosted: Wed Aug 11, 2010 12:06 pm    Post subject: Reply with quote

If only it was so simple to use a single formula. As I have had extensive experience of switching inductive loads, in particular solenoids on a Strohm auto and in this link you can see that the solenoids have been replaced with pneumatics as the spikes that are a result of switching on and especialy off can cause electronics to become unreliable.
http://www.german-traders.com/products/metal_industry/Lathes/706132-turning-automatic-lathe-longitudinal-strohm.html
The equiptment I was installing, testing and commissioning was a detect and sequential shut down device. However the switching spikes would constantly give false detects and loading the voltage rail with capacitors did not work. Perhaps you can tell me how this problem was solved?
Your method leaves a lot to be desired. You should have connected the trafficator to its correct supply and measured the current, this would give you all the info you need. Instead of pushing the restistor to the max a series/parallel solution would be far superior.
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MVPeters



Joined: 28 Aug 2008
Posts: 822
Location: Northern MA, USA

PostPosted: Thu Aug 12, 2010 2:50 am    Post subject: Reply with quote

I'm just curious:
(it seems to me that the simplest solutions would be to either change the trafficator units/solenoids or fit some sort of darn great resistor to drop the 12V to 6V)

But what's the effect of using 6V trafficators on a 12V circuit? They'd operate quite 'smartly', but would they actually burn out quickly?
I do know that 6V starter motors last well on 12V - given the short duration of trafficator use, would they last as well?
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Nic Jarman



Joined: 05 Oct 2008
Posts: 1031
Location: Stoke by Clare, Suffolk

PostPosted: Thu Aug 12, 2010 10:16 am    Post subject: Reply with quote

A large resistor is what Bob proposed and there is nothing wrong with that. The problem is pissipating 50 watts of energy. Even if the resistor is rated at 50 watts if there is not adequate air flow around it or the heatsink it is mounted on it can overheat and fail. If one uses a set of 4 resistors in a series/parallel arrangement this will spread the heat. I still think the 2 6v batteries in series with a centre tap is the best answer to having 12v and 6v in the same vehicle. There is a simple elegance to it.
Also your question regarding putting 12v across 6v equipment is about duty cycle. If you hold your starter motor on the solenoid will eventualy burn out - I have seen this happen. The trafficators will take 12 volt for a brief time but not one I would try. Starter motors seem quite tollerant but just don't hold them on for too long.
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MVPeters



Joined: 28 Aug 2008
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Location: Northern MA, USA

PostPosted: Thu Aug 12, 2010 11:57 am    Post subject: Reply with quote

Nic said: "The problem is pissipating 50 watts of energy..........."

& Mike's response was: "I'd never thought of a water-cooled resistor" ! !
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Bob Kellock



Joined: 31 Jul 2010
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Location: S.W. Wiltshire

PostPosted: Thu Aug 12, 2010 1:58 pm    Post subject: Reply with quote

MVPeters wrote:
Nic said: "The problem is pissipating 50 watts of energy..........."

& Mike's response was: "I'd never thought of a water-cooled resistor" ! !


But other people have Smile

See http://www.micro-ohm.com/power/pwrhs700wc.html
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