Author Topic: How to use the same RELAY on 12V & 24V vehicles  (Read 12121 times)

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Offline JDWTopic starter

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How to use the same RELAY on 12V & 24V vehicles
« on: August 08, 2019, 01:19:37 am »
I designed a circuit that operates on both 12V or 24V vehicles (my 150mA-max output voltage regulator steps 12V or 24V down to 3.3V for an MCU and other digital circuitry).  I also need a relay on the same PCB, and the MCU will use a transistor to switch the relay ON/OFF via the relay coil's GND side.  Normally, this is very easy when dealing with a single voltage like 12V only or 24V only.  I would just choose a 12V or 24V relay and pull the coil's high voltage from Vin before my voltage regulator.  But in this application, I need to use the same device on both 12V cars and 24V trucks.  I cannot use a 24V relay coil on 12V because it just won't switch.  And using a 12V coil on a 24V circuit would fry the coil because I need to keep the relay power for long periods of time (during the entire duration the vehicle is being driven).

I could use jumpers to add a resistor in line with the relay coil, but that relies on the human element.  If this device is ever mass produced and professionally installed, installers would without a doubt forget to set the relay coil jumper, and such would result in blow coils and complaints.  So I want to avoid jumpers as a solution to this problem.  I am wondering if there is a solution that is fully automatic and doesn't relay on the human element to adjust the relay coil voltage.

Thank you.
« Last Edit: August 19, 2019, 11:54:49 am by EEVblog »
 

Offline Richard Crowley

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #1 on: August 08, 2019, 01:45:49 am »
Why not drive the relay coil with an appropriate CURRENT to pull it in?
So that a constant-current circuit would automatically adjust for the supply voltage.
 
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Offline Dundarave

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #2 on: August 08, 2019, 01:47:20 am »
How about using a 5V relay powered by a little 12/24V-to-5V buck circuit board? 

Then no matter what the vehicle supply is, it would still drive the relay at 5V.

Just a thought.
 
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Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #3 on: August 08, 2019, 02:07:40 am »
Why not drive the relay coil with an appropriate CURRENT to pull it in?
So that a constant-current circuit would automatically adjust for the supply voltage.

Thank you very much for the idea. Would you be able to provide a link to a circuit similar to what you are pondering?  I'm curious about the implementation of a Current Source in light of the fact my Vin will be from a vehicle that uses either 12V or 24V, and there is noise and even Load Dump voltage spikes to consider.

How about using a 5V relay powered by a little 12/24V-to-5V buck circuit board? 
Then no matter what the vehicle supply is, it would still drive the relay at 5V.

Thank you for the idea.  But for reasons of cost, board space and keeping noise to a minimum (my current power supply uses an LDO), I'd like to avoid adding a small switcher to power only the relay.
 

Offline NiHaoMike

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #4 on: August 08, 2019, 02:11:22 am »
Use 12V relays with a 12V LDO.
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Offline floobydust

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #5 on: August 08, 2019, 02:28:01 am »
It is common to use PWM to lower relay coil power.
Larger relays can have a coil that uses say 2W, so to run cooler, first you give 100% on for 100msec so the relay pulls in, and then switch to 50% PWM at say 20kHz. Fujitsu has application note on doing this if you contact them. 50% PWM gives 1W holding power so a big relay will run cooler.

For your problem, you could use 50% PWM on 24V system and 100% PWM for 12V system.

After all, a relay is just an inductor anyway, so pulsing it fast just keeps it in continuous-mode.
« Last Edit: August 08, 2019, 02:30:33 am by floobydust »
 
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Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #6 on: August 08, 2019, 02:40:34 am »
It is common to use PWM to lower relay coil power.
Larger relays can have a coil that uses say 2W, so to run cooler, first you give 100% on for 100msec so the relay pulls in, and then switch to 50% PWM at say 20kHz. Fujitsu has application note on doing this if you contact them. 50% PWM gives 1W holding power so a big relay will run cooler.

For your problem, you could use 50% PWM on 24V system and 100% PWM for 12V system.

After all, a relay is just an inductor anyway, so pulsing it fast just keeps it in continuous-mode.

Absolutely wonderful solution.  That's the ticket.  THANK YOU!
 

Offline digsys

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #7 on: August 08, 2019, 03:21:02 am »
If you already know the voltage is going to be 12/24V, then switch in / out a 12V Zener? or use a 3V3 relay. A lot less messy?
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Offline Psi

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #8 on: August 08, 2019, 03:23:57 am »
Another solution is a solid state relay with wide input voltage range.
But the PWM solution will be cheaper
« Last Edit: August 08, 2019, 01:38:00 pm by Psi »
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Offline MosherIV

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #9 on: August 08, 2019, 06:57:06 am »
Hi.
The automotive subsystem board I work on has an on board buck converter to convert 24V to 12V with current upto 12A.
From talking to the electronics team, this is a common solution when the subsystem has to be multipurpose.
 
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Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #10 on: August 08, 2019, 01:37:49 pm »
Look at circuit #4 at http://www.4qdtec.com/csm.html
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Offline Psi

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #11 on: August 08, 2019, 01:47:10 pm »
It's been a while,
but from memory automotive electronics are supposed to handle double the battery voltage for 60 minutes without damage. I don't think it has to actually be functional at double the voltage though.
So for a relay you might monitor input voltage and have MCU choose not to energize the relay if the input voltage is way to high. You'd probably be monitoring input voltage anyway to choose which PWM duty to select.
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Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #12 on: August 08, 2019, 03:51:46 pm »
A quick and dirty solution that should work but the main transistor will dissipate some heat at 24 V. Emitter resistor should be calculated for the specific current desired.



« Last Edit: August 08, 2019, 06:31:48 pm by soldar »
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Offline David Hess

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #13 on: August 08, 2019, 10:02:48 pm »
I would either use a current mirror to multiply the current from a logic output or include a separate 12 volt regulator for the relay.  Both solutions have to dissipate 12 volts time the coil current however.

Pulse width modulation avoids having to dissipate power but is more complex.  This suggests using a simple switching drive circuit like a blocking oscillator which applies power to the relay and then modulates its switching to maintain a constant current.
 

Offline james_s

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #14 on: August 08, 2019, 10:17:48 pm »
How about use an efficient switching regulator to step 12/24V down to 5V and use a 5V relay, then use a small linear regulator to drop 5V down to 3.3V for the micro?
 

Offline Nusa

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #15 on: August 08, 2019, 10:56:46 pm »
Remembering, of course, that when we say 12V and 24V in automotive systems, we really mean 14.4V and 28.4V when the engine and charging system is running. And whatever you make has to tolerate large spikes, especially during startup, but expect some noise all the time. Also temperature extremes may come into play in some climates. This is understood when you buy automotive relays, but may not be if you source something generic.
 

Offline floobydust

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #16 on: August 09, 2019, 12:26:35 am »
A linear drop of 24V to 12V is not practical. It's converting the excess voltage into HEAT.

OP did not give details about his relay, but high current contacts ~30A need 1.5-2W of coil power.
Automotive 40A cube relays i.e. Panasonic CB 12V 117mA 103R coil or 1.4W so you have 1.4W to dissipate (both in the relay and 24-12V dropper) in automotive high ambient temperatures, and a heatsink plus vibration is not a good mix. That's 2.8W of heat on your board for my example.

PWM needs no hardware change, as long as the transistor switches good at 20kHz. A software change to go from on/off to PWM on an MCU output pin. I've done it in products and it works great, just as the relay manufacturer suggests. I use a SOT-223 part, BCP56 or BC817 for smaller relays. They are not rated for raw vehicle +ve transients at only ~40V VCE.
 

Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #17 on: August 09, 2019, 12:49:56 am »
A quick and dirty solution that should work but the main transistor will dissipate some heat at 24 V. Emitter resistor should be calculated for the specific current desired.



That circuit would face more than 24V because when the engine is running, you could get up to 28V on a truck, just like you can see up to 14.4V on a passenger car.  Furthermore, I must account for a Load Dump.  I will use a TVS with a Vc (voltage cut-off) somewhere between Vc=50v to54v.  That Vc must be lower than the maximum voltage all of my regulators can handle in order for the TVS to protect the regulators.

So in your schematic, that 12-24V nominal incoming voltage would be AFTER my TVS, which means it would see a maximum voltage of 50 to 54V, depending on which TVS I use.  And I am not sure your design is best suited to that.
 

Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #18 on: August 09, 2019, 01:13:36 am »
A linear drop of 24V to 12V is not practical. It's converting the excess voltage into HEAT.

Not necessarily.  Consider the LM9076 datasheet.  This 150mA LDO can handle a nominal 40V input (no limit on duration) and can handle 55V for longer period and even 70V for transients of 10ms or less (1% duty).  It comes in an SOIC housing version that dissipates heat through the circuit board traces.  And because of its currently handling, and the fact it is simpler, less costly, and less noisy than a switcher would be, I am considering the LM9076 to power my 3.3V MCU.  Normally, the MCU and all my other circuitry will draw no more than 2mA -- and that's without even sleeping the MCU at all.  With all my other circuitry (but NOT including a relay) would draw about 90mA.  A 150mA regulator will give me much needed headroom.  So when installed in a 24V truck, you are basically dropping 28V (with the engine running) down to 3.3V.  So it is possible without resulting in a fireball of heat.

What I am considering now is using this Hongfa HFD27/003-H 3V relay (High Sensitivity version with coil resistance of 60Ω).  The relay coil can be triggered by as little as 2.4V or as much as 7.0V.  I would trigger the GND side of the relay coil using an output pin on my PIC16F1508 MCU via NPN transistor (and yes, I will have a 1N4004 across the relay coil to suppress spikes), and I can use the PWM module of the PIC to send 20kHz or even 40kHz at a 50% or 60% duty cycle to reduce the current to the relay after waiting 0.5s or so for the relay to switch.  At a PWM 100% duty, it's Ohm's Law:

I = E/R
I = 3.3V/60Ω = 55mA (without PWM)

And yes, it will indeed draw 55mA because don't forget the voltage is 3.3V.  (Again, it's Ohm's Law.)

Now with PWM, I could drop that current draw and therefore the heat dissipation down.  And technically I do have enough headroom on my LM9076 to accommodate the 3V relay's 55mA even without PWM.  But I think PWM is important because this will be a normally open relay used to cut two signal wires coming from the PUSH start switch in a vehicle and the relay will need to be powered the entire time the vehicle is being driven.  For trucks, that could be 10 hours straight.

I started this thread to see if there were alternatives to using my 3.3V rail to power the relay though.  The reason is because even with the 1N4004 diode across the relay coil, I'm concerned about having it on the same 3.3V rail that powers my PIC.  Maybe it would be fine.  Thoughts?

Of course, powering the positive side of the relay coil would eliminate any noise concerns for my MCU.  And I could use the 12V of that same relay in such a case.  But as you can see from the relay's datasheet, the 12V relay cannot handle anything above 27.7V, and a 24V truck with the engine running will see 28V.  So I could not use that for both 12V and 24V.  And if I used the 15V relay instead, note that it's lowest voltage is 12.0V, which means anything lower than 12V simply would not switch the relay.

Anyway, the current handling of that Hongfa relay is 2A which is really more than I need.  I think a max of 500mA or perhaps even 300mA would suffice.  But there is no economical solid state relay that would suit my application.  They are all pretty outrageously priced.

Again, the reason I even need a dual-switch relay is to break 2 signal wires coming from the PUSH Start button (switch) of a vehicle, which will result in starter immobilization.  Cutting something else is not optional.  I need to cut those two wires.  But again, they are signal wires not seeing more than 12V, and not passing more than a couple hundred milliamps.

Further thoughts in light of this would be appreciated.

THANK YOU!

 

Offline james_s

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #19 on: August 09, 2019, 03:42:34 am »
If you're dropping 28V down to 3.3V and trying to draw 90mA then the regulator is going to be dissipating more than 2W. I don't know what sort of thermal resistance you can get from a SOIC package offhand but I'd be shocked if it doesn't get searing hot under those conditions. Something a lot of people don't seem to understand about linear regulators is that no matter what type it is, the amount of heat generated depends only on the voltage drop across it and the current being drawn through it and there is no way around this. Just because a regulator is rated to handle 40V on the input and a load of 150mA doesn't mean it can handle both of those limits at the same time, or continuously. 3.3V from a 24V input is a very large drop for a linear regulator, you'll be burning up more than 7 times as much power in the regulator into waste heat as ends up in the load, there is no way around that fact.
 
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Offline BrianHG

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #20 on: August 09, 2019, 04:27:53 am »
Just use one of those TI simple switcher IC regulators which only need 1 inductor and 3 caps and 2 resistors to set the output voltage.  You will get a 4.5v to 42v input voltage range with a 350ma or 500ma output, almost no heat as they operate from 82-94% efficiency.

Here:
Integrated inductor module: (36v max)
500ma https://www.digikey.com/product-detail/en/texas-instruments/LMZM23600SILR/296-51378-1-ND/9745742

Place your own inductor: (42v max)
100ma https://www.digikey.com/product-detail/en/texas-instruments/LM2840XMK-ADJL-NOPB/LM2840XMK-ADJL-NOPBCT-ND/3527199
300ma https://www.digikey.com/product-detail/en/texas-instruments/LM2841YMK-ADJL-NOPB/LM2841YMK-ADJL-NOPBCT-ND/2056741
600ma https://www.digikey.com/product-detail/en/texas-instruments/LM2842XMK-ADJL-NOPB/LM2842XMK-ADJL-NOPBCT-ND/2056742
« Last Edit: August 09, 2019, 04:35:33 am by BrianHG »
 

Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #21 on: August 09, 2019, 04:55:41 am »
Just use one of those TI simple switcher IC regulators...

I've actually been pondering the LM5008A which has 6-95V Vin, 350mA output, and can produce a 3.3V output.

Alternatively, there is the LM5018, which has 7.5-100V Vin, 300mA output, and can produce a 3.3V output.

This would be more expensive than an LDO, but I cannot effectively dissipate 2W, so a switcher seems the only recourse.  And that would give me the current output to also drive a 3V relay as well, solving my primary problem.

Thanks to all who have contributed to this thread.
« Last Edit: August 09, 2019, 05:24:38 am by JDW »
 

Offline BrianHG

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #22 on: August 09, 2019, 05:12:28 am »
Just use one of those TI simple switcher IC regulators...

I've actually been pondering the LM5008A which has 8-95V Vin, 350mA output, and can produce a 3.3V output.

Alternatively, there is the LM5018, which has 7.5-100V Vin, 300mA output, and can produce a 3.3V output.

This would be more expensive than an LDO, but I cannot effectively dissipate 2W, so a switcher seems the only recourse.  And that would give me the current output to also drive a 3V relay as well, solving my primary problem.

Thanks to all who have contributed to this thread.
The SOT-23 devices I listed start at 4.5v.  The 7.5v devices may cut out during the starting of a 12v car in the winter with a weak battery.
 

Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #23 on: August 09, 2019, 05:22:24 am »
Just use one of those TI simple switcher IC regulators...

I've actually been pondering the LM5008A which has 8-95V Vin, 350mA output, and can produce a 3.3V output.

Alternatively, there is the LM5018, which has 7.5-100V Vin, 300mA output, and can produce a 3.3V output.

This would be more expensive than an LDO, but I cannot effectively dissipate 2W, so a switcher seems the only recourse.  And that would give me the current output to also drive a 3V relay as well, solving my primary problem.

Thanks to all who have contributed to this thread.
The SOT-23 devices I listed start at 4.5v.  The 7.5v devices may cut out during the starting of a 12v car in the winter with a weak battery.

Yes, I am aware.  But if I get that picky about low voltage, I need to consider that ridiculously small car batteries in today's cars, especially Japanese cars, can bottom out at 4.0V or slightly lower even in good weather.  I've measured this so I know.  As such, a SEPIC regulator would be best (buck & boost), but the cost is higher, which is something I need to avoid.

Another very important consideration is Iq (Quiescent current).  According to the LM2841 datasheet (Vin-min=4.5V), the typical Iq is 1.35mA with the device ON and no load.  In contrast, the LM5008A (Vin-min=6V) has a typical Iq = 485mA.
« Last Edit: August 09, 2019, 05:41:29 am by JDW »
 

Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #24 on: August 09, 2019, 06:14:27 am »
So in your schematic, that 12-24V nominal incoming voltage would be AFTER my TVS, which means it would see a maximum voltage of 50 to 54V, depending on which TVS I use.  And I am not sure your design is best suited to that.
That circuit can be designed for any voltage you want. The drawback is that it dissipates heat. The advantage is that it is simple.  If you want something more efficient you need to go to switching solutions.

Or you could go to a solution with two relays, one for 12 V, one for 24 V, and only activate the right one depending on the voltage. That would be quite easy to do.
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