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cheap 12V Delay on Break Relay for fog-light "turn assist" lights.

TbirdMan

Apr 17, 2011
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Hello, I have an issue perhaps someone could help me with. My car has Fog-Lights that I ordered from Japan(they look nice and are a considerable upgrade in quality). As OEM in Japan there are "Turn Assist" lights that project in the direction the turn signal is activated but don't flash. My car has no such provision and since It has a single flasher unit wired in before the turn signal switch, I am left with few options to get the constant directional voltage I need. As i see it, I can eliminate the single flasher before the switch and go with a two single flashers or try to find a dual flasher to put after, or I can wire in a Delay on Break relay at the light that will turn the flashed signal into a steady one. My problem is I cannot seem to find a simple, basic 12V DoB relay for a reasonable price. Is there something out there i missed, or am I going to have to build my own?
 

TbirdMan

Apr 17, 2011
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Could I make something with a transistor and capacitor? controlling a relay?
 

TbirdMan

Apr 17, 2011
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Ok, after searching, does anyone think this:
cache.php

would do what I am wanting to do?
 

(*steve*)

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Yeah, that may work. You may need to choose a capacitor by trial and error. Start with around 1000uF (a guess) and go from there. Make sure the capacitors are rated for 16V or higher.

A low current relay will allow you to use a smaller capacitor (or have a given capacitor hold in longer). Ensure the relay can switch the load, preferably with plenty of capacity in reserve -- contacts rated at least twice the current is probably a good idea.
 

TbirdMan

Apr 17, 2011
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Sounds good, picked up a 1000uF and a "grab bag" of smaller ones from rat shack today. The relay I am using is an automotive relay and i cannot really find the specs on it, you think this will handle the load?
http://cgi.ebay.com/ebaymotors/Buic...fCarQ5fTruckQ5fPartsQ5fAccessoriesQQxZ27QQyZ1
the only issue is it has an internal resistor, so I don't know how that will affect the circuit. If necessary, I can open the relay and remove the resistor.
 

Resqueline

Jul 31, 2009
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Common coil resistances of auto relays is around 270 ohms. With that and 1000uF you get approx 0.3s hold time. Double the resistance and the hold time doubles.
Alternatively double the capacitance. Measure the coil resistance of your relay. The internal resistor may lower the hold time somewhat (10-30%?).
Relays for electronics may have coil resistances as high as 400-700 ohms and still be able to handle currrents of 5-10A.
If you can't get enough hold time with a reasonable sized capacitor then you might add a transistor and a couple of resistors.
In that case the resistor in the relay would be useful to have in place, to protect the transistor.
 

TbirdMan

Apr 17, 2011
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Common coil resistances of auto relays is around 270 ohms. With that and 1000uF you get approx 0.3s hold time. Double the resistance and the hold time doubles.
Alternatively double the capacitance. Measure the coil resistance of your relay. The internal resistor may lower the hold time somewhat (10-30%?).
Relays for electronics may have coil resistances as high as 400-700 ohms and still be able to handle currrents of 5-10A.
If you can't get enough hold time with a reasonable sized capacitor then you might add a transistor and a couple of resistors.
In that case the resistor in the relay would be useful to have in place, to protect the transistor.

Thank you, this is very helpful, I assume 1KOhm Pot allow me to adjust the hold time? I plan to experiment with this a bit before i settle on a final configuration.

one other possible issue occurred to me. How much effect will the added load of this have on my flasher? will I need to switch to a solid state flasher unit?
 

Resqueline

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I think a pot is only feasible for use with a transistor circuit, otherwise it'll dissipate a bit much power, although it can be done directly with a wire-wound pot.
I don't think the added load will affect ordinary flashers to any degree, but if this should prove to happen then the transistor circuit is the way to go.
 

TbirdMan

Apr 17, 2011
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Where in the circuit would i put the resistor to hold it longer?

Ok, after testing my relays, i am getting 83Ohm on the base and 95Ohm when i remove the resistor (which is actually in parallel with the coil) I imagine this changes the size of the capacitor i need considerably? (wild guess it will be more on the order of 0.1sec or less?)
 
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TbirdMan

Apr 17, 2011
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alright, looks like i either need a different relay, a 2200uF cap, or i need to study up on transistors. Tested my circuit with another relay with 215Ohm coil and got much better performance, but still not quite what i need, but this is promising.
 

(*steve*)

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There are advantages and disadvantages in each approach.

A larger capacitor is simple. If you have several capacitors you can connect them in parallel (connect all their +ve leads together and all of their -ve leads together and these combined leads become the +ve and -ve of a larger capacitor).

A different relay means going out to get one. You will need to ensure that either the coil resistance is higher. But even that does not allow for the fact that the relay may drop out at a higher or lower voltage so the simple ratio of resistances, whilst indicative, is not predictive of the differences in hold-in time.

Using a transistor is a lot more wiring, resistors, and another diode more complex, but it will allow a much longer delay.

Here is a possible circuit:

attachment.php


The resistor needs to be about 1k (or higher) the transistor can be almost any NPN device capable of switching power to the relay. 2N2222 seems appropriate if you're in the US. Change the timing by changing the capacitor and/or the resistor. Don't reduce the resistor below about 470 ohms or above a few tens of k.
 

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TbirdMan

Apr 17, 2011
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Looks, like that should work, ill have to pick up some parts. Would a 10K pot work for the resistor, or should i just pick up a handful and trial and error it?
TDC.png

If i can't find the transistor you suggest, any guidelines for choosing another?
 
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(*steve*)

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You could try a 10k pot in series with a 470 ohm resistor -- the resistance should never fall to zero or your transistor will emit smoke.

You want an NPN transistor capable of switching between 500mA and 1A. It's not very critical. The current through the relay should not be more than 50mA I guess, but it's always better to be safe than sorry. The transistor will dissipate a little power as the capacitor discharges, but should not get too hot. It should dissipate under 1/4 watt at the most I expect (and even that, not for long).
 

Resqueline

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I've been debating myself if it would be better to separate the discharge path and the transistor switching point, like shown in the diagram below.
You might want to experiment and see what works best, but I'd want to put a 470 ohm resistor from base to ground anyway.
The most power the transistor will ever dissipate with your 83 Ohm relay is 600mW and the most current it will pass is 170mA.
The MPS2222A that the Shack carries is the same as a 2N2222 (for most practical purposes). It's rated for 625mW & 600mA.
You might also want to consider Shack pack # 276-1617 that contains 5 2N2222's (out of 15) transistors for the price of 3 MPS2222A's.
 

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TbirdMan

Apr 17, 2011
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Yeah, That sounds like the best deal. I am gonna eventually want at least two, and it is always good to have a couple spares. I try not to let all the smoke out of my electronics, they need that to work. ;)
 
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TbirdMan

Apr 17, 2011
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Well, your circuit seems to work wonderfully, thanks so much for all the help.
 

(*steve*)

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Well, your circuit seems to work wonderfully, thanks so much for all the help.

Great! But read below.

The most power the transistor will ever dissipate with your 83 Ohm relay is 600mW and the most current it will pass is 170mA.

That will be sailing very close to the wind for a 2222. I had done my back of the envelope calculations assuming a resistance about 4 times this value (and I really should have mentioned my assumptions).

Just check that the transistor doesn't get too hot if you leave the indicators operating for a while. (It may pass through the high dissipation stage quickly enough that the average dissipation is much lower.
 

TbirdMan

Apr 17, 2011
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One last question, If i wanted to use a different relay with a higher resistance coil, the load on the transistor should go down, right?
 

TbirdMan

Apr 17, 2011
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I am currently still bench testing the board, but i just left it on 100% for about over 60sec and the transistor didn't get much above room temp.
 

TbirdMan

Apr 17, 2011
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Ok, After testing i think i may swap the 10K pot for a simple 10K resistor. would is hold a bit longer if I replaced the 470Ohm that is in series with the pot with a larger one? I think it hold's enough as is, but I wouldn't mind it holding a tad longer.

also, any answer on the higher resistance relay question? I would like to replace the bulky auto relay with one that is easier to mount and more compact if possible.
 
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