12 volt relay in an automotive environment

LoopyByteloose

I am setting up a little relay board that uses a BS-24 to drive a ULN2803 that will power a bank of 12volt relays.

The problem is that the power source will be automotive 12 volt.

Upon reading about automotive voltage regulation, the supply varies as much at 14.5VDC on the high side to under 12 volts on the low side. I've got a serial LCD that can tolerate 14VDC for the backlight, so I've inserted a diode to drop the supply by 0.7vdc under peak conditions.

But now I am wondering it I will have occasional low voltage drop outs on the relays.

Does anyone have practical experience with 12vdc industrial relays in an automotive setting? Or is it best to use automotive relays to avoid trouble?

Alternatively, are there 12 volt low dropout regulators that are specifically designed for such situations? While the project is pretty much built and I am just testing it, I fear that I may have to start over with different relays and more voltage regulation.

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Ain't gadetry a wonderful thing?

aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan

sam_sam_sam

To me the best way to do this is to use a switching regulator to power the Basic Stamp and·LCD Display and use 5 volt Relays

If you use a 12 volt relay then use a 12 volt··regulator and if the Back Light can work good at 12 volts then use a 12 volt··regulator on it as well

Does anyone have practical experience with 12vdc industrial relays in an automotive setting? Or is it best to use automotive relays to avoid trouble?

That depend on how many amps you are wanting to switch ON and OFF

The coil amps (on the 40 amp contact relays)·tend a little higher than most relays· just use more than one input/output pin set·on the ULN2803 chip

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·Now wanting to learn Spin· Thanks for any··that you may have and all of your time finding them

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Sam

Post Edited (sam_sam_sam) : 4/19/2010 12:01:34 PM GMT

Dr_Acula

12V relays should be fine in an automotive situation. I wouldn't worry about voltage drop with a diode - the relay coil will just get a little warmer. Many relays rated at 12V are meant for automotive situations anyway - eg 'horn' relays which are a good robust generic automotive relay that is available at a good price as they make lots of them. If you already have the diode soldered in then leave it there. 12V relays have a large operating range - larger than the range your battery would ever experience.

I have been using a 12V relay to switch my radiator fan on and off and it has been working fine for five years now.

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www.smarthome.viviti.com/propeller

Chet

12 V relays tend to do OK in automotive applications. The voltage can drop to 10V when starting, depending on the ambient temperature, batteray condition and the type of starter. The drop out voltage has not been a problem however for my applications. Having said that, the ones I use tend to be on only for short periods. I do this for hobby projects and not industrial applications. I would be very careful with electronics however. Voltage spikes in the electrical systems can easily exceed the 15V level. I ALLWAYS use regulators for the solid state stuff.

Chet

bill190

Don't buy any "new" automotive relays! All you get is a part number and no data sheet nor any technical information to be found *anywhere* (on the internet) about the specifications of the relay (like we are used to in the electronics field).

But if you go to an automotive·wrecking yard, you can get a pretty good idea of the ratings for a relay by seeing what it was used for. And plenty of relays and other electronics there.

Relays which are designed to be "on" all the time are called "continuous duty rated". This would be like for ignition on, headlights, A/C compressor, etc.

Other relays are designed to only be on momentarily. Like for a horn.

(And as I pointed out above, good luck going to an auto parts store and asking for a specific rating of relay! All they know is part numbers...)

Note that these relays can get to be quite hot if on for a long period of time. So best to mount in the engine compartment where they will get good air circulation.

Note that a relay is a *coil*. Sort of like that which is used to make sparks! And when power is removed from a relay coil, thousands of volts can be released from the coil! Thus it is a good idea to install a "backwards diode" or "flyback diode" as discussed on the second page here...
http://www.physics.unlv.edu/~bill/PHYS483/relay.pdf

And here is something which is *really* interesting to read about. Called "Relay Contact Life" (A 10 on the geekyness scale!)...
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3236.pdf
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LoopyByteloose

Thanks all.
As I've already completed building this with 12volt DPDT industrial relays that draw about 55ma, I am just going to press it into use in a real environment and see what happens.

The relays (contacts rated at 10amps 250VAC) will have to perform between about 13.8 volts and 11.00 volts for the most part. I am not too sure about drop during an engine start. But it that is a problem, a separate gel cell may have to be inserted with a blocking diode to hold a higher voltage.

The problems with 5 volt relays are that for similar functionality, they may have to pull more milliamps (I am near the 500ma total limit for the UNL2803 when all 8 lines are powered) and I cannot get 5 volt relays that fit on a modular rail with sockets (the idea is to easily service wiring configuration and relay replacement -- no 'stupid' relay board with relays soldered in.)

It appears that the UNL2803 has 'backwards' diodes included internally. I generally call these fly back diodes.

Relay Contact Life is always an greater issue with DC. It seems that AC shuttles contact metal back and forth while the contacts don't suffer much wear. But with DC, all the material moves in one direction and a much lower rating will quickly erode the useful contact. Automotive relays are designed to work with the more hostile DC environment, but I've begun to wonder about the actual operating voltage range. It seems to be about 5volts or +/- 2.5 volts.

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Ain't gadetry a wonderful thing?

aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan

erco

A relay's holding voltage is much lower than it's pull-in voltage. That is, a 12 volt relay may need 10-12 volts to activate, but once it's on, it won't release until the voltage drops below a threshold, as low as 4-8 volts depending on the relay.

So if the relays are already on before the starter cranks, you may not have any problems at all.

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·"If you build it, they will come."

stamptrol

A good source for heavy duty automotive relays is "Parts for Trucks" or any other supplier to the long haul truck or heavy equipment business.

The standard relay is available in 12 or 24 volt coils and 20, 30, 40 amp contacts. Often, they also have a normally closed contact available with a slightly lower current capability.

Or, checkout DigiKey part #Z2248-ND ( OMRON G8JN ) which is a nominal 12 v relay and will pull in at 8 volts and stay in until 1 volt, gives 35 amp contacts with a coil current in the 50 mA range. Its in a package about 1 inch (25 mm) on each side. The normal voltage swing on an automotive system is not a concern for the relay.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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