Author Topic: How to use the same RELAY on 12V & 24V vehicles  (Read 12131 times)

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Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #25 on: August 09, 2019, 06:18:15 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.
...If you want something more efficient you need to go to switching solutions.

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Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #26 on: August 09, 2019, 06:32:35 am »
You can also probably find relays with two coil windings for different voltages. The control circuit could select which one to activate or it could just activate both and disconnect the low voltage one when the voltage surpasses a certain point. For optimal results you might need a custom design though. Also a very simple and reliable solution.
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Offline Nusa

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #27 on: August 09, 2019, 06:33:05 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.
Or you could take that same concept, but instead of two relays, make it one 12V relay and one resistor in series for the 24V scenario. With two activation points, you can choose if vehicle voltage is applied before or after the resistor.
 

Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #28 on: August 09, 2019, 06:37:51 am »
Or you could take that same concept, but instead of two relays, make it one 12V relay and one resistor in series for the 24V scenario. With two activation points, you can choose if vehicle voltage is applied before or after the resistor.

But a single resistor doesn't offer any special protection for the relay coil.  Remember one must account for a Load Dump, and on a 24V vehicle with no factory protection, that could hit as high as 200V!  This is why a TVS diode is necessary.  (On modern passenger cars, you usual have factory protection that caps the Load Dump spike to about 60V, but even that 60V is higher than many power supplies account for, which again is why a TVS is needed that has a Vc lower than the Vin-max of whatever regulator is used.)
 

Offline soldar

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Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #30 on: August 09, 2019, 07:03:03 am »
Dual voltage relays: https://www.newark.com/space-age-electronics/ssu-pam-2/relays-control-spdt-24v-7a-panel/dp/08N1908

Too large and much too expensive even in high volume.  Again, I am considering Hongfa relays for size and price reasons.  I've used them in the past so I know the quality is good.  In other words, the relay cost is restricted to less than US$2.
 

Offline MosherIV

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #31 on: August 09, 2019, 07:04:19 am »
In automotive electronics, the circuit has to be able to withstand voltage surges and drops BUT not necessarily work at the extremes. It depends what the circuit is being used for (sorry I forgot what the original post said).
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Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #32 on: August 09, 2019, 07:22:50 am »
In case it is of use for others reading:
Quote
The SSU-PAM-4 Relay provides 10.0 Amp Form C contacts.
The relay may be energized across a wide voltage range from 9VDC to 40VDC.
The 15mA operating current is constant across the operating range.
The input DC voltages are polarity sensitive and diode protected.

The SSU-PAM-2 Relay provides 7.0 Amp Form C contacts.
The relay may be energized by one of two (2) input voltages: 12VDC or 24VDC.
The input voltages are polarity sensitive and diode protected.
A red LED is provided which, when illuminated, indicates the relay coil is energized.

https://www.newark.com/space-age-electronics/ssu-pam-2/relays-control-spdt-24v-7a-panel/dp/08N1908
Probably integrates the electronic current control.
« Last Edit: August 09, 2019, 07:30:54 am by soldar »
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Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #33 on: August 09, 2019, 03:27:21 pm »


By the way and just in case anybody is interested, this was the circuit normally used to maintain constant motor speed in battery powered cassette players. If the battery set was nominally 6V, the player would use a 3 or 4 volt motor and use a similar circuit to limit current and motor speed would remain constant as the battery's voltage declined.
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Offline floobydust

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #34 on: August 09, 2019, 07:23:31 pm »
OT I don't think so, constant current does not give constant speed. Low cost cassette player motors have centrifugal switches for constant speed or a tachogenerator.

That circuit you post is a disaster - from 24-12V with a 0.5A coil?! 12W of heat total? It's a coffee warmer.
 

Offline floobydust

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #35 on: August 09, 2019, 07:42:58 pm »
... 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....

Have you considered that during cranking (12V) drops quite low and your relay might cut out or chatter. I will have to dig and get the exact info about the push-to-start, I thought it's just a digital input on an ECU.

A reminder 24V vehicle electrical systems, especially trucks, are brutal.  For 24V, the standard test is -600V, +200V spikes, so no off-the shelf Vreg can take it.
I no longer design for load dump as it's one specific failure mode (loose battery connection) and modern alternators use zener diodes for the rectifiers. It's also expensive additional circuitry, so I tell sales/marketing the product will not cover that beyond blowing a fuse.

I did use one of the National Semi (TI) SMPS IC's and they died and blew up in automotive use despite being designed for it. The problem was Vin dV/dt would make the buck converter latch up and short. Very few SMPS IC's now have this spec on their datasheet. It is a trap to avoid.
« Last Edit: August 09, 2019, 07:44:46 pm by floobydust »
 

Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #36 on: August 09, 2019, 10:16:20 pm »
OT I don't think so, constant current does not give constant speed. Low cost cassette player motors have centrifugal switches for constant speed or a tachogenerator.

I have never seen centrifugal switches and I don't even know how they could work because flutter would be terrible. The old, cheap cassette players that I ever saw just had a simple circuit like that.

That circuit you post is a disaster - from 24-12V with a 0.5A coil?! 12W of heat total? It's a coffee warmer.
It "burns" 6 W which is the con side. On the plus side it is extremely simple. I already said as much when I posted it so I don't understand your negative attitude. You are repeating what I said.
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Offline floobydust

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #37 on: August 10, 2019, 12:47:12 am »
OT Cassette-player brushed DC motors use either internal centrifugal switches, voltage regulator, or a tachogenerator+transistor/IC for speed control. I worked in a repair shop and have seen 100's of them. The capstan flywheel deal's with flutter. The point is constant current control is not used in them.

6W is an awful lot of heat, just pick out a heatsink and you will see it needs to be huge. I haven't seen a relay with 24R coil and 0.5A coil current- for another 6W, it's pretty big too. So I thought it's a circuit that could work but the example's details are coffee warmer kind of hot and made me cringe. You could do it with one transistor as the MCU's output voltage is stable at 3.3V for CC control.

The car's pushbutton start somehow is tied into the keyfob RFID, so I still think it's a low current 12V digital signal. You would have to guess the switch's current rating and match that with a relay. Some people use a 4N35.

weird video cassette motor internals:
 

Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #38 on: August 10, 2019, 02:56:58 am »
Have you considered that during cranking (12V) drops quite low and your relay might cut out or chatter.
Yes, I have.  There is a very quick digital message that is sent across one or both of those wires  when you have the break held down and then you push that Push Start button, such that it would only take milliseconds to transfer. That data enables starting. In other words, that data will be passed completely before cranking even begins.  The purpose of the relay is to sever those lines (at times I specified with my MCU) to ensure that the starter enabler data is never passed and therefore no one could crank the engine.

 In other words, by the time the data is passed and you start cranking, it’s OK if the relay cuts out because the data has already been passed to enable cranking in the first place.
 

Offline NiHaoMike

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #39 on: August 10, 2019, 05:00:51 am »
Since it's a small signal, have you considered an opto MOS solid state relay?
A reminder 24V vehicle electrical systems, especially trucks, are brutal.  For 24V, the standard test is -600V, +200V spikes, so no off-the shelf Vreg can take it.
It's not that expensive to clamp those just like how kV level spikes are clamped on AC mains - use a MOV.
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Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #40 on: August 10, 2019, 07:53:26 am »
Since it's a small signal, have you considered an opto MOS solid state relay?
A reminder 24V vehicle electrical systems, especially trucks, are brutal.  For 24V, the standard test is -600V, +200V spikes, so no off-the shelf Vreg can take it.
It's not that expensive to clamp those just like how kV level spikes are clamped on AC mains - use a MOV.

If you read through my previous relies, you will see I mention a TVS diode, which is a far better choice than MOV.  I designed a switching power supply based on a Linear Technology (now Analog Devices) LTC3637 and a 1500W TVS diode with Vc=49.9V to cap spikes to less than the max Vin of 76V.  It is designed for exclusive use on 24V trucks, which is why my design is based on their example 12.5V-76V design presented in the datasheet.  My design actually works on 12V too, but the average current consumption is too high to make it a good choice on passenger cars.  The LTC3637 is also an expensive chip too, which is why I am considered TI chips for this new design I am working on now.
 

Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #41 on: August 10, 2019, 10:10:19 am »
OT Cassette-player brushed DC motors use either internal centrifugal switches, voltage regulator, or a tachogenerator+transistor/IC for speed control. I worked in a repair shop and have seen 100's of them. The capstan flywheel deal's with flutter. The point is constant current control is not used in them.
I appreciate your experience and knowledge but don't say something was never done just because you haven't seen it done. It may be that American/Japanese industry favored different solutions than European industry. I have no doubt that as time advanced better solutions were developed with ICs, tachos, etc. but I was thinking of units I repaired which dated to the late 1960's and probably did not have even silicon transistors, let alone ICs. Everything was as basic as you could get. They were Spanish made but might have been knock offs of better brands like Philips. I distinctly remember encountering that circuit and thinking it was ingenious. I can't see why it would not work. Varying only the input voltage and maintaining the load constant the current and voltage should remain constant.

At any rate, I do not want to hijack this thread any further with this topic so that's my last word on this topic.

6W is an awful lot of heat, just pick out a heatsink and you will see it needs to be huge. I haven't seen a relay with 24R coil and 0.5A coil current- for another 6W, it's pretty big too. So I thought it's a circuit that could work but the example's details are coffee warmer kind of hot and made me cringe. You could do it with one transistor as the MCU's output voltage is stable at 3.3V for CC control.

It seems to me you are looking to argue for the sake of arguing. The circuit I showed will allow a 12 V relay to work at 12 and at 24 V and when working at 24 V will dissipate the same heat as the relay, whatever that is. If you want to argue a vehicle can only activate one relay at a time because the heat of two relays would overheat the vehicle ... well, that's just silly.

Again, as I said when I showed it, it is a simple circuit, quick and dirty, and it works. Criticizing it for being simple, quick and dirty is silly and criticizing it for dissipating as much as the relay it is controlling is double silly. A vehicle has many relays and the heat they produce is insignificant compared to other sources of heat in the vehicle. Any vehicle, at any one time, is dissipating in heat produced by electrical items hundreds of watts and the heat produced by a relay or two is just insignificant. Even more so if the relay is activated for a very short time.

And that's my last word on this issue.
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Online Zero999

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #42 on: August 10, 2019, 12:52:44 pm »
OT Cassette-player brushed DC motors use either internal centrifugal switches, voltage regulator, or a tachogenerator+transistor/IC for speed control. I worked in a repair shop and have seen 100's of them. The capstan flywheel deal's with flutter. The point is constant current control is not used in them.
I appreciate your experience and knowledge but don't say something was never done just because you haven't seen it done. It may be that American/Japanese industry favored different solutions than European industry. I have no doubt that as time advanced better solutions were developed with ICs, tachos, etc. but I was thinking of units I repaired which dated to the late 1960's and probably did not have even silicon transistors, let alone ICs. Everything was as basic as you could get. They were Spanish made but might have been knock offs of better brands like Philips. I distinctly remember encountering that circuit and thinking it was ingenious. I can't see why it would not work. Varying only the input voltage and maintaining the load constant the current and voltage should remain constant.

At any rate, I do not want to hijack this thread any further with this topic so that's my last word on this topic.

6W is an awful lot of heat, just pick out a heatsink and you will see it needs to be huge. I haven't seen a relay with 24R coil and 0.5A coil current- for another 6W, it's pretty big too. So I thought it's a circuit that could work but the example's details are coffee warmer kind of hot and made me cringe. You could do it with one transistor as the MCU's output voltage is stable at 3.3V for CC control.

It seems to me you are looking to argue for the sake of arguing. The circuit I showed will allow a 12 V relay to work at 12 and at 24 V and when working at 24 V will dissipate the same heat as the relay, whatever that is. If you want to argue a vehicle can only activate one relay at a time because the heat of two relays would overheat the vehicle ... well, that's just silly.

Again, as I said when I showed it, it is a simple circuit, quick and dirty, and it works. Criticizing it for being simple, quick and dirty is silly and criticizing it for dissipating as much as the relay it is controlling is double silly. A vehicle has many relays and the heat they produce is insignificant compared to other sources of heat in the vehicle. Any vehicle, at any one time, is dissipating in heat produced by electrical items hundreds of watts and the heat produced by a relay or two is just insignificant. Even more so if the relay is activated for a very short time.

And that's my last word on this issue.
He is right. Delivering a constant current to a permanent magnet DC motor will not result in a constant speed, but a constant torque.

Voltage regulation is required to achieve a constant speed.

Even if you're wrong about the original application of the circuit, you're right that it is suitable for enabling a 12V relay to work with either 12V or 24V.
 

Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #43 on: August 10, 2019, 01:09:31 pm »
He is right. Delivering a constant current to a permanent magnet DC motor will not result in a constant speed, but a constant torque.

Voltage regulation is required to achieve a constant speed.

Even if you're wrong about the original application of the circuit, you're right that it is suitable for enabling a 12V relay to work with either 12V or 24V.

Would constant torque with a constant load result in constant speed?

Most or all turntables I have had worked with shaded pole motors which will slow down with increasing load but the variability of the load was so insignificant that it was considered constant. I think it would be the same with the capstan of a cassette player. The load would be pretty much constant .

« Last Edit: August 10, 2019, 01:26:16 pm by soldar »
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Online Zero999

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #44 on: August 10, 2019, 06:11:38 pm »
He is right. Delivering a constant current to a permanent magnet DC motor will not result in a constant speed, but a constant torque.

Voltage regulation is required to achieve a constant speed.

Even if you're wrong about the original application of the circuit, you're right that it is suitable for enabling a 12V relay to work with either 12V or 24V.

Would constant torque with a constant load result in constant speed?

Most or all turntables I have had worked with shaded pole motors which will slow down with increasing load but the variability of the load was so insignificant that it was considered constant. I think it would be the same with the capstan of a cassette player. The load would be pretty much constant .
Interesting point. I don't know whether the load would be constant or not. I would have thought that a turntable would have more friction when the stylus is on the outside and less when it's in the middle.
 

Offline soldar

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #45 on: August 10, 2019, 06:46:04 pm »
Interesting point. I don't know whether the load would be constant or not. I would have thought that a turntable would have more friction when the stylus is on the outside and less when it's in the middle.
Yes, correct. And yet the load is so small and the force of the motor so multiplied by the speed reduction that the motor won't even notice it and the difference in speed will be negligible. And the same thing happens with the cassette spindle. The difference in motor load between the tape at the beginning and the tape at the end is negligible and the motor will not even notice it. You would need extremely precise instruments to measure it. The speed stabilization does not have to be 100% perfect, just good enough that a listener cannot hear it.  Even if between the beginning and the end of a tape the speed would slowly change by a very small factor nobody would even notice it. I bet there were greater differences in speed between players. So, I say that with constant load, as is the case, the circuit maintains the motor speed. I don't think I have any cassette players any more but if I did I would not mind doing some tests. (My apologies to the OP for the hijack.)
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Online Zero999

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #46 on: August 10, 2019, 08:20:56 pm »
The cassette players I've looked at have a motor with an integrated speed controller, which uses an IC to sense the motor back-EMF and keep it constant.
 

Offline james_s

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #47 on: August 18, 2019, 06:09:05 am »
He is right. Delivering a constant current to a permanent magnet DC motor will not result in a constant speed, but a constant torque.

Voltage regulation is required to achieve a constant speed.

Even if you're wrong about the original application of the circuit, you're right that it is suitable for enabling a 12V relay to work with either 12V or 24V.

Would constant torque with a constant load result in constant speed?

Most or all turntables I have had worked with shaded pole motors which will slow down with increasing load but the variability of the load was so insignificant that it was considered constant. I think it would be the same with the capstan of a cassette player. The load would be pretty much constant .
Interesting point. I don't know whether the load would be constant or not. I would have thought that a turntable would have more friction when the stylus is on the outside and less when it's in the middle.


The load on a tape deck motor is not constant at all, it requires a fair bit of torque to drive, especially when you have things like fast-forward and rewind, some tapes have a lot more friction than others. The torque required to drive a turntable though is negligible, tracking force on the stylus is on the order of a few grams, if you can measure a difference in torque required depending on the position of the stylus then you must be hogging out a trench in the record.
 

Offline JDWTopic starter

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #48 on: August 18, 2019, 06:23:34 am »
Seems like I should have titled my opening post “... & tape decks”

 :palm:
 

Offline james_s

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Re: How to use the same RELAY on 12V & 24V vehicles
« Reply #49 on: August 18, 2019, 07:02:58 pm »
Your question was answered already wasn't it? It's not unusual at all for threads to wander into other topics once the question has been answered.
 
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