Requesting Assistance with using Relays in Car Circuitry (Preventing Counter-EMFs)

Hello, this is my first post. I'm going to post two diagrams that involve relays. These are planned headlight modifications on my car that I will implement soon. I understand logic gates, but I need to ask for advice in terms of preventing damages from counter electromotive forces (associated with relay coils). This is the first time I'm going to add a relay to my car.

The solution to CEMFs I read about is to place a diode (or resistor) in parallel with the relay coil that is oriented in a way such that it does not conduct during normal operation (oriented toward the +). This way, it somehow diverts the CEMF current.

1) This is a DRL module that replicates the operation of the DRL on the OEM Canadian model of my car (I'm in the US). The high beam bulbs are switched over to a series (from parallel originally) to reduce the brightness. This occurs at the intersection of ignition-ON and shifting to any gear other than park (AND-gate). Then the circuit is broken upon activating the headlights, passing lights, or fog light switch (OR-gate).
**broken link removed**
^^note that the high beam bulbs are in parallel, so connecting the ground wire of only one bulb to a + will make both bulbs into a series (the other bulb is not shown in the diagram, sorry).

2) Here is a fog light modification. On my car, the fog lights will not turn on without low beams, so this mod allows me to bypass that so the fog lights turn on when I wish.
**broken link removed**

So, my question: what do you guys think about the CEMF's? If I implement these mods, will I be safe from CEMF's? Also, what specific diode should I use? I currently have the 1N914 type handy.

Thanks.

You did not specify the relay coil's resistance.

The 1N914 can withstand a non repetative current of 0.5 Amp for 1 second. Auto relays are usually about 100 Ohm so that means 120 mA. Therefore the 1N914 will be adequate.

The CEMF (usually called "back EMF") will be limited by the diode to about 0.8 Volt and thus will not do any damage.

I don't see why you need another relay in your "modified" diagram. It could be done with diodes.

I don't know what is in the "control" box, but I doubt if you can do any harm by bypassing it.

You have the diodes in the correct direction in your first diagram. But I would have thought that F should be gnd, not +12 Volt.

Thanks for the reply ljcox,

For the high beam DRL module, this below is the diagram I should have posted. Sorry for the ambiguity with the first one. F gets switched to a + to put the high beam bulbs in series for DRL mode (reduced brightness)
**broken link removed**

As long as the stock relay is open whenever my relay (the one placed inline in the bulb's ground wire) is closed, then it should be fine. This was my task to handle in the logical design and is one reason I have the other relay (circuit breaker).

As for the coil resistance, I don't know because I didn't buy the relays yet. That was my next question. I can get the $3 SPDT ones off oznium, but maybe I should stay away from the cheap ones. I heard Bosch is a good one. What price bracket should I look for? And what specific brand or name would you recommend for this job?

Lastly, should I just conservatively expect a lower resistance with the coil (more current), and get a stronger diode?

Thanks again

You're welcome.
Thanks, now understand the circuit & what you're trying to do.

I'm not an expert on auto relays. The one in the Altronics catalogue that I have has a coil resistance of 90 Ohm. So the current will be in the 130 ~ 160 mA range depending on the battery voltage.

Therefore, the 1N914 would be adequate to suppress the back EMF as it can withstand 0.5A for 1 second. But it would not be suitable for the diodes at B, C or D.

For these diodes, the 1N4000 series, ie. 1N4001, 1N4004, etc. would be necessary as they can pass 1 Amp continuously.

So I recommend that you use one of the 1N4000 series for the coils also.

Another point just occurred to me.

Are the driver side and passenger side lamps of equal wattage?

If not, the lower wattage lamp will glow at virtually full brightness whereas the higher wattage one will be very dim.

Thanks for the most recent post... I have no questions there. I'll get some new 1N4000 series diodes though.

I did overlook this quote last time though:

ljcox said:

I don't see why you need another relay in your "modified" diagram. It could be done with diodes.

Click to expand...

That relay breaks the bypass when the low beams (or passing lights) turn on so that the control block resumes control over the fogs. So, I'm conditionally bypassing the block (when the low beams are off).

Here's why. Its hard to explain, but its really simple logic.

1) the fog lights must turn off with the high beams (on my car) to remain legal

2) the high beams do not turn on unless the low beams are already on (even if you press the high beam switch). However, the passing lights (pulling the high beam switch backwards) still work anytime.

3) the fog lights also won't turn on unless the low beams are already on (which is what I'm bypassing)

All 3 of these restrictions are handled by the same control block. So bypassing the block removes #1 and #3. I only want to remove #3 and keep #1. So bypassing the block only when the lows are off allows the high beams to still shut the fogs off because the block resumes control. When the low beams are on, I don't need the bypass because the fogs work anyway. So I only need to allow the fogs to work when the low beams are off. When the low beams are off, the high beams won't work anyway, so the fog circuit doesn't have to be broken by the highs. If the highs did work without the lows, then this design wouldn't be sufficient. See what I mean?

Then the passing lights need their own input because they work anytime (even without the low beams).

Are you saying there is a way to accomplish this without the relay? I've never heard of a way to implement a NOT-gate without a relay.

Again, thanks much for your help- much appreciated

ljcox said:

Another point just occurred to me.

Are the driver side and passenger side lamps of equal wattage?

If not, the lower wattage lamp will glow at virtually full brightness whereas the higher wattage one will be very dim.

Click to expand...

They're both the same bulbs- 55W incand H1 bulbs

ArcWindsor said:

Are you saying there is a way to accomplish this without the relay? I've never heard of a way to implement a NOT-gate without a relay.

Click to expand...

No, you're right. I had missed the fact that you need to short the box when the relay is released.

Okay, now that that's cleared up, let me ask a question about diode OR-gates. I've seen them done with multiple ground sources (like the left picture), but I've never seen one done with two different positive sources (like the right picture). Would the right picture (Or-gate on the + side) work just as well as the left picture with, let's say, an LED or a relay coil as the load?

**broken link removed**

Yes, it makes no difference as to which side the diodes are connected.

Thanks ljcox,

Now its time for me to get some relays. Does anyone know if these ones I linked to will work for switching 55W incand headlight bulbs (half-strength as DRL)? I'm assuming it is fine since it is rated at 40A continuous duty and at 12V.

Here is the link:
relays continuous duty 12 volt & 24 volt DC power relays,starter relays

Here is the description:
40 Amp Cole Hersee Mini Relay
SPDT
Cap Material = Plastic
5 Terminals:
# 85 .250
# 86 .250
# 30 .250"
# 87 .250
# 87a .250
12 volts
Continuous Duty
Includes Mtg Bracket
Unsealed
Can Be wired to be used for small permanent magnet motors. Electric Door Lock Motors, Small Reversible Permanent Magnet Motor.
Resistor & Diode Models Available.
Order RN for Resistor,
Replacement for Bosch 0 332 204 159
Order RD for Diode Relay

Let me know what you think. If the relay is not sufficient, then can anyone suggest another relay off that link?

Also, what is the total current through the DRL bulbs? That is two 55W incand bulbs in series on 14V. I'm get 4.58A using 55/12, but how do I adjust for 14V with the alternator?

Thanks

It's complicated because the lamps are non linear resistances. Their resistance is low when cold and increases as they warm up.

Thus, the critical factor is the starting current.

I suggest that you measure the cold resistance of both lamps with a multimeter.

As I understand it, the lamps will be in series some times and in parallel at other times.

If so, then the worst case is one lamp across say 14 Volt.

So if the cold resistance was (and I'm guessing of course) say 0.7 Ohm, then the starting current would be 14/0.7 = 20 Amp.

If this is correct, then the 40 A contacts would suffice.

However, if the lamps are always in series, then using my guess of 0.7 Ohm, then the starting current would be 10 Amp.

With my DRL module, the lamps will always be in series.

The stock high beam relay (in the fusebox) handles the bulbs when they are in parallel. My module will never switch the full high beam load.

ArcWindsor said:

With my DRL module, the lamps will always be in series.

The stock high beam relay (in the fusebox) handles the bulbs when they are in parallel. My module will never switch the full high beam load.

Click to expand...

Did you measure the cold resistance?
I expect that a 40 Amp contact will be adequate anyway, but it may be worthwhile to check.

No, I didn't measure cold resistance yet. To do this, do I just stick the DMM probes onto each bulb contact with the selector switch set to measure resistance? And I do this when the bulbs haven't been run in a while? Is that all you meant?

Also, the members of the car forum (with the same car as I) have all done this with the high beams with good results. Although the module they used has different inputs than mine will, the general act of switching the bulbs in series was accomplished. So its been done before with a 10A fuse. Doesn't this show the current won't exceed 10A?

I'll measure the resistance soon anyway.

Thanks

ArcWindsor said:

No, I didn't measure cold resistance yet. To do this, do I just stick the DMM probes onto each bulb contact with the selector switch set to measure resistance? And I do this when the bulbs haven't been run in a while? Is that all you meant?

Click to expand...

Yes

ArcWindsor said:

Also, the members of the car forum (with the same car as I) have all done this with the high beams with good results. Although the module they used has different inputs than mine will, the general act of switching the bulbs in series was accomplished. So its been done before with a 10A fuse. Doesn't this show the current won't exceed 10A?

Thanks

Click to expand...

Not necessarily. Fuses take time to blow. Thus the current could be >10A briefly while it is warming.

However, I think the 40 A contacts will be adequate, but if you have the time to measure the resistances, then it would confirm.

I did measure the cold resistance of the PIAA H1 bulbs I have laying around (because I didn't have to pop them out of the headlight, so they were handy). I probably won't use them, but they likely have a lower resistance than the stock bulbs that are in my car now.

The DMM output for the resistance of the PIAA bulb fluctuated from 0.5 ohm to 2 ohm. It never stayed steady. I'm not sure how to interpret that. If the resistance is 0.5, then that is a crazy 28A for ONE bulb in parallel or for the series of both.

ArcWindsor said:

The DMM output for the resistance of the PIAA bulb fluctuated from 0.5 ohm to 2 ohm. It never stayed steady. I'm not sure how to interpret that. If the resistance is 0.5, then that is a crazy 28A for ONE bulb in parallel or for the series of both.

Click to expand...

The most likely reason is that you're not applying enough pressure to the lamp connectors.

Probes with sharp points are best. I often use my wife's dressmaking pins to get a good connection.

Sharp nails would be an alternative.

Is your DMM self ranging, or do you choose the range with a switch?

The self ranging ones take 2 ~ 3 seconds to give a stable reading.

If the lamps are about 0.5 Ohm cold, then 2 lamps in series will be about 1 Ohm. Thus the current will be about 14/1 = 14 A max.

This is well within the capability of 40 A contacts.

28A in parallel (for brights) would be too much for my stock circuitry (each headlight bulb is on its own 10A fuse), but I've seen on forums that people with the same car as I still used the same PIAA bulbs without blowing the fuse(s).

This contradiction likely indicates, like you said, an error with the DMM measurement. I'm going to have to measure again. I bet the cold-resistance is higher than 0.5 Ohms. I'm also going to have to measure the stock bulbs that I am using now.

Thanks

Remember that the max current will be brief since the resistance increases rapidly as the filament heats up.

Also, as I said in an earlier post, the fuse will be able to withstand a brief surge.