Searching for engineer experienced in 24V vehicle electrical design.

Hello,

My company decided to port our 12V automotive product design to 24V for sale in the European truck market. I am experiencing a problem with the transistion and was wondering if you, or someone you may know, would be willing to help. I am attempting to either prevent, or quickly extinguish, the arcing effect across a set of relay contacts.

Reply to this post if you think you can help and I'll supply more details.

Gerb

Reply to
gerbermultit00l
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I'm far from qualified to comment on the issue, but could it be something as simple as the problem (and solution) described here:

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Chris

Reply to
kmaryan

Because this is an area that I'm interested in, I googled this a bit further and came up with instances of people using a MOV for preventing this sort of thing. In general, the aim is to prevent the current surge associated with switching inductive and to a certain extent capacitative loads. The MOV, diode idea and transient voltage suppressors are all candidates. Google any of the above, or "relay arc" for more info.

I'd be curious to know what you come up with.

Chris

Reply to
kmaryan

preventing

surge

arc"

I am switching a wire wound resistor using a 24V high current relay. The relay acts as a high side driver between the battery and grounded load. Wire wound resistors have some inductance but it is very low (I believe below 1uH). I do not think this is a contributing factor to my problem. I may be wrong.

SAE says 24V automotive systems typically sit at 28V while charging. They can drift up to 32V under normal operating conditions. I am driving a 1 ohm load. My product seems to have no problem passing 28A at 28V. A pretty good spark is generated when the relay contacts separate, but this is manageable. My problem occurs when system voltage rises to 32A at 32V. When the relay contacts separate a sustained arc of approx 2-3 seconds is created. This arc is severely damaging to the relay contacts. It melts the contact acting as anode. One or two of these arcs kills the relay. The anode contact melts away and the relay can no longer conduct.

I have tried all recommended approaches. RC network across relay contacts, RC network across load, diode across load, MOV. No suppression network seems to work. The only thing I found that works is to increase the gap between relay contacts. I have not had a chance to do extensive testing on modified relays yet to see if it is the cure.

My stock relays have fine grain silver contacts and are gapped 0.4mm apart. The relays I modified are gapped to approx 0.85mm. They seem to start arcing at about 36V which is outside the normal operation range of the vehicle and should be good enough for me.

I'm looking for an expert opinion from someone who has been down this road before. Am I missing something? What does it take to extinguish an arc? Am I on the right path, etc....

Thanks for your input Chris. I welcome you, and others, to join in for more trouble shooting.

Gerb

Reply to
gerbermultit00l

What are your "stock relays" rated at? 0.4mm seems like a really small gap for even a 12V power relay.

range of the vehicle

Way too close for me! Especially in an environment with extremely broad temperature and humidity requirements...

Tim.

Reply to
Tim Shoppa

Yes, some most certainly do. It can cause problems with current sense resistors, for example.

Of course in this case, you've got the inductance of the resistor plus the inductance related to the loop area of the wiring. And high-current DC circuits just love to arc away anyhow.

One way around it would be to use a hybrid switch.

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

--
I think the most important things to do when trying to quench an arc
are to get the contacts as far apart as possible as quickly as
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Reply to
John Fields

Or hire a sub-contactor.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

Hi, your problem is an old one when switching high currents and the practical solution to this is a relay with blowout magnets at the contacts.These extinguish the arc quickly by forming a magnetic field thst repels the plasma field .hope this helps

Reply to
Don Murray

Completely off the subject, but I suddenly wondered whether wire wound resistors introduce significant inductance. I've always thought of them as just resistors that were manufactured a different way.

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our ancestors."-- Heironymus Fracastorius, 1546
Reply to
Gregory L. Hansen

[...]

One solution, that I've used sometimes, is to simply put a (FET) transistor across the contacts. Turn the transistor on shortly before the contacts open, and turn it off when they are fully open. It's a simple solution -- and often cheaper than the alternatives.

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Reply to
Bjarne Bäckström

Right- at these current levels, any passive type suppression network will be huge. The problem is that contact voltage vs current curve exceeds the sustained arc threshold for your relay. You can use a smaller auxiliary relay to keep this in check with a voltage divider. At

32V the main relay opens on a 22V open circuit voltage, and the smaller aux relay then drops out after delay and breaks the circuit completely at a much reduced current 11 amps. K2 would be a 12V relay coil. View in a fixed-width font such as Courier.
Reply to
Fred Bloggs

The current rating of the contacts is a strong function of the force with which they are held closed. He most certainly has reduced this rating significantly by doubling the spacing- maybe halved it. Fiddling with the armature travel is a dumb thing to do.

Reply to
Fred Bloggs

Hmm.... my solution was going to involve a timed MOSFET, but I don't think I'll bother now.

Maybe add a diode from the 1-ohm to Ground though. This clamps any negative HV spike from the cabling inductance.

--
Tony Williams.
Reply to
Tony Williams

Nah. Just plop an Infineon PROFET Smart SIPMOS switch in there. Get raw performance, short-circuit and thermal protection, and an error indication, all at a low cost. For example, use a BTS443, BTS432, BTS442, etc.

At first they called these a "Sense Highside Switch," and later, protected-POWER "Smart High Side Switches," to help your searching.

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Poke around a bit... BTW, DigiKey has a nice in-stock selection.
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--
 Thanks,
    - Win
Reply to
Winfield Hill

First of all let me say thank you for the sudden overwhelming burst of support. I'll try to address all responses if it makes sense to do so.

1.) Rich Grise Wrote

Unfortunately this is not an option. I am confined to the space allocated to me in our product packaging. I have an area of approximately 26mm wide x 26mm long x 30mm high to fit a relay solution. This device also needs to be PCB mounted. And finally, it must be able to function in the temperature range from -40C to 72C. It must survive without damage (non operational) from -50C to 125C.

I am using the chosen device near its rated limit but still within a reasonable safety margin of 15% below published limit (which the manufacture says is very conservative). I would like to see a larger safety margin and am working on that.

My problems start to occur when system voltage increases out of the specified limits for sustained durations of time (as will occur in any vehicle from time to time). I tried implementing a new over voltage protection scheme that will shut the relay off and protect the contacts long before system voltage rises to a level that will be damaging to the relay.

2.) Tim Sh>What are your "stock relays" rated at? 0.4mm seems like a really

Take a look at the relay specs for yourself here:

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Tim also wrote: "

True, I'm playing it close. However, these relays have been used in rougher environments than what I am attempting and seem to survive. I'm confident that with a little tweaking I can get this to work.

3.) Spehro Pefhany wrote:

resistors, for example.

high-current >DC circuits just love to arc away anyhow.

I do not believe I have a big problem with inductance. I say this because if the load did have a large amount of inductance I should see a fairly large negative voltage spike (at least a couple hundred volt) when turning off the load. I see a negative going voltage spike of about 8V, not even enough to go below ground from the 28V starting point.

Loop inductance due to wiring harness may be an issue with my test setup. My product is inside an environmenal chamber and the power supply is outside the chamber. The power lead going to the product is approximately 3m and so is the ground return line. Wire gage is 8AWG. I do see a small bump in voltage across the main power terminals of my product (about 10V for 1us) when it makes or breaks the load. This bump decreases as I lower load current so it must be inductance.

The in-vehicle application might be different. The power lead will be restricted to below 2m and the ground return will be tied to the vehicle chasis local to the product.

Now Spehro, how about this Hybrid Switch you were talking about?

4.) John Fields wrote:

The manuacture offers a number of c>Opening up the gap between the contacts on relays you already have

quickly as they did before, which will give the arc a chance to live

mechanicals it doesn't usually end up serendipetously.

I don't know if I agree with what you say in this instance. I am increasing contact gap by filing down the armature stop of the relay so the entire armature mechanism can swing away from the opposing contact a little more. I am not affecting the mechanism the contact is mounted to or its spring tension. You would have to see the internals of the relay to understand what I say. I wish I could post pictures. The manufactures images of this relay only show it with a cover, none without.

Think of this relay design being like a door with a door stop behind it preventing the knob from hitting the wall. A spring pulls the door open and towards the door stop. I'm just grinding the door stop down a little. The spring is under a certain amount of tension when the armature is held 0.4mm from the opposing contact (stock relay). The spring wants to pull the contact back to rest but it can't because of the doorstop. By grinding the doorstop down I allow the door to swing further back towards rest. The spring is still under tension, just a little less since it is closer to rest position.

Now say someone wants to close the door. It takes a larger magnetic force to get it started on its way back to close because of the increase in distance (equivalent to a higher pull-in voltage requirement). Once the door passes the 0.4mm mark on its way towards being closed (the opposing contact) it is just like a stock unit. The hold in force is the same whether the door was originally opened 0.4mm,

0.8mm, or 1.2mm

Drawing the armature in from a further distance will requires more pull-in force. But, I have more than adequate pull in force now. A stock relay pulls in at about 14.4V at 25C. I do not need to activate the relay unless system voltage is above 25V. So, if the pull in voltage of the relay increased to 16V because of my modification it still does not affect me.

This discussion about weak contact pressure brings up an important point. I might have found something that contributed to my recent failures. I was driving the relay coil with a specific high voltage NPN small signal transistor. I found that the relay coils resistance was about 100 ohm lower than what was published. So, the required drive current for this coil went up. The transistor did not have enough DC current gain at higher temperatures to keep up. Therefore the transistor was not turning on all the way. There was only 14V across the coil! This was barely enough to pull it in, and in some instances it would not. Since then I replaced this transistor with one that has more than adequate current gain. The armature now snaps into place at any temperature.

I agree. I chose this relay for one reason. It is the only one I could find that fits my packaging requirements that can operate at the capacity I need it to. It just so happens that the stock unit cannot handle those rare instances where system voltage ramps up out of spec. However, lucky for me this relay is also highly customizable. I can have it built for what I need. I just need help in determining what is good enough.

What do my competitors use? I could not tell you. This is a new product to the market and we do not have competitors. That, and I was asked to design a device for a market I am not familiar with. Everything I learned about 24V vehicle systems so far either from a lacking SAE specification on the subject or just hear say. So, I'm venturing into a new frontier. Not the best situation to be in, but I am there nevertheless.

5.) D>Hi,

Yep, I looked into this. Not practical for the small outline of this relay. I asked the manufacture to try and find a solution.

6.) Bjarne Bäckström wrote:

I also considered this design approach. It may still be feasible and I intend on using it as a last ditch effort. However, there are a few obstacles to overcome.

Since I am switching my load with a high side drive I would need a P-Channel FET across the contacts. You just can't find high voltage, high current P-CH FETS with a low enough on resistance to keep internal power dissipation down while passing up to 30A. That and I have no room to heat sink the device. My total allocated space for relay and FET is 26mm x 26mm x 30mm. Not that heat sinking would do any good since my electronics are sealed inside a small glass-filled nylon enclosure. I would need to hold the device on for 50ms to 100ms at a time. It would get hot after a few activations. This might be managable if ambient temperatures could be held to a reasonable level. However, my product will be mounted under the hood of heavy trucks and utility vehicles. Temperatures could easily approach 50C.

Then there is the issue of time. My management wants to release this product later this spring. I need time to come to a solution and then do adequate testing before I let it hit the street. I've passed the point of no return with my current design approach.

7.) Fred Bloggs proposed a clever design idea. If I don't get anywhere with my current design I'll surely try this. Heck, I might just do it anyway for piece of mind. It performs a similar function to the FET solution I was contemplating but will probably be cheaper to implement.

I would have to reduce the size of my main relay in order to fit a small secondary relay in my packaging space. I know of a few smaller main relays that can easily carry the current levels I am working with. What was killing me is making and breaking the contacts at high voltage. Fred's solution would correct that with a modest increase in component count.

Questi> The current rating of the contacts is a strong function of the force

Fiddling

John Fields also brought this up. I tried to voice an arguement against this above. Maybe this was just my ignorance. Mind to ellaborate on why you say this? Just trying to learn. I went into this project taking relay technology for granted. I never intended to get so far in depth with it but I am very quickly finding out otherwise.

8.) T >Hmm.... my solution was going to involve a timed >MOSFET, but I don't think I'll bother now.

I tried the diode trick but found it had no effect. I believe this is because I am not experiencing problems due to inductance. I think I'm just over stressing the relay contacts at high voltage extremes.

9.) Tim Sh>My god, the mind boggles. 28V * 28A = 784Watts. Into a resistor. >

On a truck. What the heck is this guy doing? Icemelting? Space

Something similar to that :).

10.) W>Nah. Just plop an Infineon PROFET Smart SIPMOS switch in there.

I would love to use a solid state solution. I wanted to do this in a bad way. Problem though. I can't keep a FET cool enough. My product must operate up to 70C ambient. The FET would be passing up to 30A for

2.5 minutes. Even if I could use a ultra low RDSON part of say 4 mohm it would still be dissipating 3.6W. Stack a few in parallel and I still won't be able to remove the heat. My electronics are embedded in a small sealed glass filled Nylon box (sealed from the elements that is). Even if I could fit a heat sink inside my packaging space I still don't have a way to get the hot air out and cool air in. We cannot re-tool the housing to accomodate a heat sink at this point.

That and then there is the issue of reverse battery connection and the internal body diode of the FET(s). I would need to add a rather large diode in series with the load to prevent damaging reverse current flow. This diode would take up even more space.

To put it bluntly, I'm try to cram 20 pounds of sh__ in a 5 pound bag.

Well, that's all for now. Hopefully you all won't fall asleep reading through all this. Sorry for dragging this on forever but I wanted to spell out more details to clue you in on my specific situation.

Thanks a bunch

George "Gerb" Marutz II

Reply to
GerberMultit00l

Well, what a crazy set of requirements; you might try spelling them out for us at the get-go, so efforts aren't wasted on non-starter solutions. Your "port our 12V automotive product design to 24V" description doesn't convey much of the reality.

But back to Infineon's PROFETs, there's their 800-pound gorilla, the BTS555. This little 5-lead TO-218 baby has an Ron of only 2.5 milliohms max at 25C and 2.7 milliohms typ at 85C. Therefore it should dissipate under 2.5W at 30A at 70C. That's a trivial amount of heat to get rid of with a small bit of modestly-heat-conductive material.

The BTS555 shouldn't be damaged by reverse wiring, because the "Reverse battery protection by self turn on of power MOSFET" feature means that while current will flow (just as in the forward direction), the FET will turn on, shorting out the intrinsic diode with its high voltage drop, and thereby preventing any BTS555 switch-overheating damage.

--
 Thanks,
    - Win
Reply to
Winfield Hill

May be worth asking the relay mfrs if they can supply a contactor with a main set of normal contacts and a separate contact pair which is early-make_late-break. Fred's solution could then be implemented with just one extra resistor.

That also solves a minor problem on Fred's circuit, K2 failing to de-energise if there is no load.

--
Tony Williams.
Reply to
Tony Williams

My intentions were not to waste anyones time. If I did I sincerely appologize. My original thoughts were that I would meet up with someone and discuss this outside the forum. However, responses came in quick and became overwhelming. So, I decided to carry on the conversation here. Unfortunately I wasn't able to fill in the details fast enough. That and I need to watch what I say. My company has a new product coming to market and I do not want to give any of their trade secrets away.

Back to the matter at hand. The BTS555 is truely a beast!!! I can't believe they crammed such a device in that small of a package.

About reverse battery connection and this FET. Reverse current may not damage this part. However, while reverse current flows, my 1 ohm load will heat up considerably. It may be a few minutes before whomever installed the product wrong to realize their mistake. By this time the load could approach 150C. I hate to be so vague, but need to watch what I say. I have protection mechanisms in place that will prevent further heating. However, this is destructive. The device will not recover and would become a warranty return. I would rather prevent reverse current flow all together.

The good thing is that the BTS555 is packaged in such a small format that I might be able to place a large body reverse current blocking diode in series with the load and have some room to spare.

Anyone know of a high current, low forward voltage drop, Schottky diode in a TO-220 or equivalent package?

George "Gerb" Marutz

Reply to
Ge0rge Marutz

If he says that relay stays on for many minutes ( he wrote "2=2e5") then replacing that bulk ceramic with an Epcos "switching" PTC varistor, removing 12V zener, making K2 a subminiature 24V coil type with almost a small signal current rating, would make for a long lasting configuration with self-turn-off and absolutely *no* arcing on that big relay. The drawback is can he tolerate possibly a few seconds latency at turn-off.

Reply to
Fred Bloggs

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