How to use the same RELAY on 12V & 24V vehicles - Page 1
Forum section: electronics / projects

author: JDW
author_rank: Frequent Contributor
timestamp: August 08, 2019, 01:19:37 am
content: 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.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 08, 2019, 01:45:49 am
content: 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.

author: Dundarave
author_rank: Regular Contributor
timestamp: August 08, 2019, 01:47:20 am
content: 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.

author: JDW
author_rank: Frequent Contributor
timestamp: August 08, 2019, 02:07:40 am
content: Quote from: Richard Crowley on August 08, 2019, 01:45:49 amThank 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.Quote from: Dundarave on August 08, 2019, 01:47:20 amThank 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.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 08, 2019, 02:11:22 am
content: Use 12V relays with a 12V LDO.

author: floobydust
author_rank: Super Contributor
timestamp: August 08, 2019, 02:28:01 am
content: 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.

author: JDW
author_rank: Frequent Contributor
timestamp: August 08, 2019, 02:40:34 am
content: Quote from: floobydust on August 08, 2019, 02:28:01 amAbsolutely wonderful solution. That's the ticket. THANK YOU!

author: digsys
author_rank: Unknown
timestamp: August 08, 2019, 03:21:02 am
content: 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?

author: Psi
author_rank: Super Contributor
timestamp: August 08, 2019, 03:23:57 am
content: Another solution is a solid state relay with wide input voltage range.But the PWM solution will be cheaper

author: MosherIV
author_rank: Super Contributor
timestamp: August 08, 2019, 06:57:06 am
content: 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.

author: soldar
author_rank: Super Contributor
timestamp: August 08, 2019, 01:37:49 pm
content: Look at circuit #4 athttp://www.4qdtec.com/csm.html

author: Psi
author_rank: Super Contributor
timestamp: August 08, 2019, 01:47:10 pm
content: 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.

author: soldar
author_rank: Super Contributor
timestamp: August 08, 2019, 03:51:46 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: August 08, 2019, 10:02:48 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: August 08, 2019, 10:17:48 pm
content: 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?

author: Nusa
author_rank: Super Contributor
timestamp: August 08, 2019, 10:56:46 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 09, 2019, 12:26:35 am
content: 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 CB12V 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.

author: JDW
author_rank: Frequent Contributor
timestamp: August 09, 2019, 12:49:56 am
content: Quote from: soldar on August 08, 2019, 03:51:46 pmThat 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.

author: JDW
author_rank: Frequent Contributor
timestamp: August 09, 2019, 01:13:36 am
content: Quote from: floobydust on August 09, 2019, 12:26:35 amNot necessarily. Consider theLM9076 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 HongfaHFD27/003-H3V 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/RI = 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!

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2019, 03:42:34 am
content: 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.

author: BrianHG
author_rank: Super Contributor
timestamp: August 09, 2019, 04:27:53 am
content: 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)500mahttps://www.digikey.com/product-detail/en/texas-instruments/LMZM23600SILR/296-51378-1-ND/9745742Place your own inductor: (42v max)100mahttps://www.digikey.com/product-detail/en/texas-instruments/LM2840XMK-ADJL-NOPB/LM2840XMK-ADJL-NOPBCT-ND/3527199300mahttps://www.digikey.com/product-detail/en/texas-instruments/LM2841YMK-ADJL-NOPB/LM2841YMK-ADJL-NOPBCT-ND/2056741600mahttps://www.digikey.com/product-detail/en/texas-instruments/LM2842XMK-ADJL-NOPB/LM2842XMK-ADJL-NOPBCT-ND/2056742

author: JDW
author_rank: Frequent Contributor
timestamp: August 09, 2019, 04:55:41 am
content: Quote from: BrianHG on August 09, 2019, 04:27:53 amI've actually been pondering theLM5008Awhich has 6-95V Vin, 350mA output, and can produce a 3.3V output.Alternatively, there is theLM5018, 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.

author: BrianHG
author_rank: Super Contributor
timestamp: August 09, 2019, 05:12:28 am
content: Quote from: JDW on August 09, 2019, 04:55:41 amThe 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.

author: JDW
author_rank: Frequent Contributor
timestamp: August 09, 2019, 05:22:24 am
content: Quote from: BrianHG on August 09, 2019, 05:12:28 amYes, 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 theLM2841datasheet (Vin-min=4.5V), the typical Iq is 1.35mA with the device ON and no load. In contrast, theLM5008A(Vin-min=6V) has a typical Iq = 485mA.

author: soldar
author_rank: Super Contributor
timestamp: August 09, 2019, 06:14:27 am
content: Quote from: JDW on August 09, 2019, 12:49:56 amThat 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.

author: JDW
author_rank: Frequent Contributor
timestamp: August 09, 2019, 06:18:15 am
content: Quote from: soldar on August 09, 2019, 06:14:27 amIndeed. Please see my last two posts!

author: soldar
author_rank: Super Contributor
timestamp: August 09, 2019, 06:32:35 am
content: 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.

author: Nusa
author_rank: Super Contributor
timestamp: August 09, 2019, 06:33:05 am
content: Quote from: soldar on August 09, 2019, 06:14:27 amOr 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.

author: JDW
author_rank: Frequent Contributor
timestamp: August 09, 2019, 06:37:51 am
content: Quote from: Nusa on August 09, 2019, 06:33:05 amBut 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.)

author: soldar
author_rank: Super Contributor
timestamp: August 09, 2019, 06:53:10 am
content: Dual voltage relays:https://www.newark.com/space-age-electronics/ssu-pam-2/relays-control-spdt-24v-7a-panel/dp/08N1908

author: JDW
author_rank: Frequent Contributor
timestamp: August 09, 2019, 07:03:03 am
content: Quote from: soldar on August 09, 2019, 06:53:10 amToo 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.

author: MosherIV
author_rank: Super Contributor
timestamp: August 09, 2019, 07:04:19 am
content: 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).My thoughts if it helps.

author: soldar
author_rank: Super Contributor
timestamp: August 09, 2019, 07:22:50 am
content: In case it is of use for others reading:QuoteProbably integrates the electronic current control.

author: soldar
author_rank: Super Contributor
timestamp: August 09, 2019, 03:27:21 pm
content: Quote from: soldar on August 08, 2019, 03:51:46 pmBy 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 09, 2019, 07:23:31 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 09, 2019, 07:42:58 pm
content: Quote from: JDW on August 09, 2019, 01:13:36 amHave 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.

author: soldar
author_rank: Super Contributor
timestamp: August 09, 2019, 10:16:20 pm
content: Quote from: floobydust on August 09, 2019, 07:23:31 pmI 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.Quote from: floobydust on August 09, 2019, 07:23:31 pmIt "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.

author: floobydust
author_rank: Super Contributor
timestamp: August 10, 2019, 12:47:12 am
content: 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:

author: JDW
author_rank: Frequent Contributor
timestamp: August 10, 2019, 02:56:58 am
content: Quote from: floobydust on August 09, 2019, 07:42:58 pmYes, 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.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 10, 2019, 05:00:51 am
content: Since it's a small signal, have you considered an opto MOS solid state relay?Quote from: floobydust on August 09, 2019, 07:42:58 pmIt's not that expensive to clamp those just like how kV level spikes are clamped on AC mains - use a MOV.

author: JDW
author_rank: Frequent Contributor
timestamp: August 10, 2019, 07:53:26 am
content: Quote from: NiHaoMike on August 10, 2019, 05:00:51 amIf 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)LTC3637and 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.

author: soldar
author_rank: Super Contributor
timestamp: August 10, 2019, 10:10:19 am
content: Quote from: floobydust on August 10, 2019, 12:47:12 amI 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.Quote from: floobydust on August 10, 2019, 12:47:12 amIt 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2019, 12:52:44 pm
content: Quote from: soldar on August 10, 2019, 10:10:19 amHe 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.

author: soldar
author_rank: Super Contributor
timestamp: August 10, 2019, 01:09:31 pm
content: Quote from: Zero999 on August 10, 2019, 12:52:44 pmWould 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 .

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2019, 06:11:38 pm
content: Quote from: soldar on August 10, 2019, 01:09:31 pmInteresting 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.

author: soldar
author_rank: Super Contributor
timestamp: August 10, 2019, 06:46:04 pm
content: Quote from: Zero999 on August 10, 2019, 06:11:38 pmYes, 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.)

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2019, 08:20:56 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: August 18, 2019, 06:09:05 am
content: Quote from: Zero999 on August 10, 2019, 06:11:38 pmThe 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.

author: JDW
author_rank: Frequent Contributor
timestamp: August 18, 2019, 06:23:34 am
content: Seems like I should have titled my opening post “... & tape decks”

author: james_s
author_rank: Super Contributor
timestamp: August 18, 2019, 07:02:58 pm
content: 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.

author: JDW
author_rank: Frequent Contributor
timestamp: August 18, 2019, 09:20:53 pm
content: Quote from: james_s on August 18, 2019, 07:02:58 pmMight as well talk religion, politics and space exploration too. Apparently, anything goes in this forum!

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 19, 2019, 02:04:28 am
content: Quote from: JDW on August 18, 2019, 09:20:53 pmQuite to the contrary. The "topic-drift" is one of the better features of this forum. We read (and participated in) several possible solution to your problem, but also discussion of several related issues.

author: JDW
author_rank: Frequent Contributor
timestamp: August 19, 2019, 02:46:07 am
content: From "About Us": "The site is essentially a self moderated forum, and people lead by example."And the example appears to be, "ramble on, folks!"Who am I to stop to the party? Carry on!

author: james_s
author_rank: Super Contributor
timestamp: August 19, 2019, 04:35:13 am
content: Well you're welcome to duck out at any time should the topic of discussion no longer be relevant to you. Nobody is forcing you to stick around and listen to our chatter. You're as welcome as anyone to hang out here but you don't get to dictate what people talk about. Topic drift is par for the course, if you're bothered by that then this probably isn't the forum for you.

author: JDW
author_rank: Frequent Contributor
timestamp: August 19, 2019, 04:45:02 am
content: Sorry, but there's where I draw the line. "Nobody is forcing you to hear the chatter" is a ridiculous statement because I created this thread and therefore it's obvious I will follow it to see if someone ever posts a relevant-to-the-title reply that may interest me.Why not just start your own thread(s) and leave this one alone so that people who come to this threat with the understanding it will be on-topic can follow this thread more easily to its final conclusion?This is becoming an example of why most forums cannot moderate themselves because many people posting to the forums think they are always right going about it the way they feel is best. But I assure you, when there is conflict brewing, it's time to create your own thread. So please do that. Now.Thank you.

author: james_s
author_rank: Super Contributor
timestamp: August 19, 2019, 04:55:40 am
content: No, sorry, you don't get to make demands like that.One of the things you will learn real quick around here (and I'm quite sure Dave will back me up on this if he happens to see it) is that you own your posts and your posts only. You do not own the thread, you do not get to tell others what to discuss, you do not get to tell others not to post. If you don't like where a thread goes that's tough, you can politely request people stick to the topic at hand, or you can leave, or add certain members or certain threads to your ignore list. Start trying to throw your weight around and make demands and you'll not make many friends around here, that's not how it works.

author: JDW
author_rank: Frequent Contributor
timestamp: August 19, 2019, 05:08:41 am
content: Quote from: james_s on August 19, 2019, 04:55:40 amI respect your sentiments, but please consider reasonably who is throwing whose weight around. Do you consider the wording of your own previous post "friendly"?Logic itself dictates that rambling on and on within someone else's post is showing oneself rather unfriendly. The evidence that is true exists in pretty much every other forum out there -- hijacking threads is almost always prohibited to prevent conflicts like this. And while I am aware no such prohibition exists here, and while I for the most part don't mind that free-for-all way of thinking, it is greatly appreciated when others would be friendly enough to so kindly consider the heartfelt wishes of the individual who started the thread. It's merely a common courtesy. That is what I have been pleading with you gentlemen about for the last several posts. But until now, my thoughts have been trampled under foot and I am considered to be someone who can no longer make friends here. That's quite a punch below the belt.Friends, if I am totally in the wrong in thinking the way I do about this particular thread, I humbly ask for everyone who has posted in this thread thus far to share your opinion telling me so. I am happy to abide by the group feeling on this matter. For now, I still feel the current dialog is cluttering up the thread and makes it more difficult for other people in the future to find useful information in this thread. As such, it would be greatly appreciated if those who wish to continue the current off-topic discuss could please open a new thread. And I promise I will not post in your thread to mucky it up. This is how friends behave toward one another. I hope I can obtain your kind understanding about this.Thank you for your kind cooperation and understanding.

author: floobydust
author_rank: Super Contributor
timestamp: August 19, 2019, 06:15:26 am
content: I go off-topic if I know (with facts) a technical example or statement is misleading, such as linear constant-current control not contributing to global warming, or is used for motor speed control... Usually that doesn't derail a thread, but it does seem to take the piss on a decent discussion if people react and get cranky and defensive, unleash a wall of text or start up the flame thrower. I see it happen frequently- not just here, but on many Internet forums. Mixing noobs, professionals, enthusiasts, old farts, and jerks along with different language and culture - there will be clashes and it's bound to be noisy.In social media, using up/down votes was supposed to improve the signal/noise ratio in threads, but bots and brigading etc. corrupted that idea. It's possible to manipulate votes.Nothing better has been found. It all seems to rely on the manners of the community, or the hostile members get banned by the mods.I think best is to ignore the few posts that wandered off-topic and adding new (relevant) posts to a thread seem to keep it going.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: August 19, 2019, 10:37:49 am
content: The lack of sympathy and trolling from some members, plus 50% of topics going off... topic. That indeed characterizes these forums, but it is certainly not a feature.The eevblog forums are on the extreme of the bell curve of forums, as atypical they are (perhaps head-to-head with reddit). Now talking about bells, I have an urge of hearing Anita Ward.Kind regards, Samuel Lourenço

author: ogden
author_rank: Super Contributor
timestamp: August 19, 2019, 11:09:49 am
content: Changing subject of thread to meaningless whine is much worse "off-topic" than posting irrelevant to original question, post. Completely agree that OP does not "own" the thread. As soon as thread is going and people invested their time answering question or just sharing their opinions, it becomes **common** knowledge of community of this forum. Show respect to those who did stick to original subject and be so kind - change subject back to original.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2019, 11:25:41 am
content: All threads drift off topic, but as long as it's vaguely technical, I don't see the problem. In this case someone posted a circuit originally designed for something else, but might suit the original poster's application. Inevitably the topic wandered off to the subject of that circuit. If you feel as though your question has not been answered, simply provide more information, rather than complaining about it.Anyway, back on topic. The circuit Soldar posted can be modified into a crude switched mode power supply by adding a couple of RC circuits. It will self-oscillate at around 1kHz, when the current exceeds a certain threshold, determined by R1, which will give about 60mA, in this case. The inductance of the relay will smooth the current so it doesn't chatter. At low supply voltages, Q1 will simply be solidly on, with no oscillation. The efficiency won't be great, but it's better than a linear current limiter.The circuit can be driven from a 5V microcontroller output if R2 is reduced to 2k2 and its positive node is connected to the output, rather than 24V.

author: ogden
author_rank: Super Contributor
timestamp: August 19, 2019, 11:29:59 am
content: TLDR; - Did anybody suggest to use switching supply to create 5V rail for 5V relay and LDO for 3.3V rail?Now I technically posted off-topic because thread is about "Re: A thread that has gone off-topic". Whatta...

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2019, 11:33:14 am
content: Quote from: ogden on August 19, 2019, 11:29:59 amI'm not sure if those exact solutions have been proposed, but I think a switched mode power supply has been mentioned before.EDIT:Quote from: ogden on August 19, 2019, 11:09:49 amI did and you changed it back. This reply was solely to change the topic back to the original again. Please don't change it back again.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: August 19, 2019, 11:40:49 am
content: Quote from: ogden on August 19, 2019, 11:09:49 amNeither you apparently. Do you want to know who owns the thread? Whoever owns the forum (Dave). Anyway, drifting off-topic is disrespecting the OP.Kind regards, Samuel Lourenço

author: ogden
author_rank: Super Contributor
timestamp: August 19, 2019, 11:43:52 am
content: Quote from: Zero999 on August 19, 2019, 11:33:14 amLOL. You did not change subject of the *thread*, just your post. Also - I did not answer to your post.

author: ogden
author_rank: Super Contributor
timestamp: August 19, 2019, 11:46:45 am
content: Quote from: bloguetronica on August 19, 2019, 11:40:49 amDave does not own thread same way as Google/Alphabet does not own e-mails sent by me with gmail.QuoteYour current post is off-topic according your logic as well, yet you openly chose to disrespect OP. How's that?

author: bloguetronica
author_rank: Frequent Contributor
timestamp: August 19, 2019, 11:55:32 am
content: Anyway, I don't want to feed disruptive behavior, and I'm purposefully ignoring some previous posts. I think the OP should change the title back.Kind regards, Samuel Lourenço

author: EEVblog
author_rank: Unknown
timestamp: August 19, 2019, 11:55:44 am
content: Changing the thread topic to something completely different is silly and doesn't help anyone, please don't do that. I changed it back.I have not followed this thread, but it's quite common for threads to drift off-topic a bit or into other areas of a related problem when the question has been answered.If anyone wants to bring a thread back "on topic" then you do that by posting on-topic stuff again, then people will respond to you, etc.

author: EEVblog
author_rank: Unknown
timestamp: August 19, 2019, 12:02:04 pm
content: Quote from: bloguetronica on August 19, 2019, 10:37:49 amYou are not helping the situation with your posts.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: August 19, 2019, 12:08:25 pm
content: Now, my answer. The quick and simple solution, provided your relay requires a very low current ( below 20mA) is to just use a 12.6V Zener diode in series. Above that current, you could use an LDO. There are also 24V to 12V DC-DC modules that are transparent enough to allow 12V operation without any drop.Anyway, whatever solution you use, your relay will require a freewhelling diode so it doesn't damage whatever is controlling it.Kind regards, Samuel Lourenço

author: EEVblog
author_rank: Unknown
timestamp: August 19, 2019, 12:10:24 pm
content: Quote from: JDW on August 19, 2019, 04:45:02 amAnd you are also doing that now by insisting that threads stay completely on-topic with zero side-banter and technical tangents.This forum has grown into being one of the biggest and best engineering forums on the internet partly because itdoesn'tinsist on strict moderation like that.I've seen technical forums die because everyone tries to play forum-cop and moderate the forum into oblivion, I won't let that happen here.People can "moderate" threads by politely asking if people could keep it on topic, or better yet, simply post on-topic stuff themselves and lead by example.

author: EEVblog
author_rank: Unknown
timestamp: August 19, 2019, 12:13:28 pm
content: Quote from: JDW on August 18, 2019, 09:20:53 pmFYI, religion and politics are essentially banned here. Start talking that and threads get locked. Do it consistently and you'll get banned.Space exploration talk is technically interesting and is basically an engineering related problem, so it's good.

author: mariush
author_rank: Super Contributor
timestamp: August 19, 2019, 12:17:27 pm
content: A 12v battery could be below 12v but 12v relays would work still.did anyone mention bi-stable relays?You could then use a ldo for the 2-3s pulse needed to switch the relay...but then you'd have to hold some charge so when power is off you can reset the relay to other position, if that's important.

author: ogden
author_rank: Super Contributor
timestamp: August 19, 2019, 12:26:36 pm
content: Quote from: bloguetronica on August 19, 2019, 12:08:25 pmAre you sure such solution will work not only on 24V supply but on 12V as well? How? Please explain.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2019, 12:48:06 pm
content: Quote from: ogden on August 19, 2019, 12:26:36 pmIt won't work. All a zener will do is reduce the supply voltage by 12.6V.Quote from: ogden on August 19, 2019, 11:43:52 amThat's strange. When I changed the title of my post, the name of the thread also changed, but then it reverted back, when you posted. I apologise for accusing you of doing it intentionally.Anyway, here's another idea, based along the same lines. The switching is harder and faster so it should be lower loss and it has better current regulation, as the sense resistor is directly in series with the relay, but requires more parts.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 19, 2019, 01:10:09 pm
content: Remove C1 and replace it with a series RC combination from the base of Q3 to the collector of Q1, and also add some resistance in series with the base of Q2. Now you have a very simple switching converter.Or it might be possible to just adapt this simple converter circuit:http://romanblack.com/smps/a03.htmNow I wonder if a LED driver chip might be the best solution. The buck kind generally is already designed to have the load connected on the high side and it would have an enable pin to allow the microcontroller to turn it on and off.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2019, 01:23:19 pm
content: Quote from: NiHaoMike on August 19, 2019, 01:10:09 pmWell it is a very simple switching regulator. Yes, there are other ways of getting it to oscillate. I agree about adding a base resistor to Q2. Omiting the base resistor was an error.QuoteYes that's a good enough idea, but it requires an additional inductor.QuoteI agree. You can get some cheap buck LED driver ICs very cheaply.

author: floobydust
author_rank: Super Contributor
timestamp: August 19, 2019, 09:14:47 pm
content: Quote from: mariush on August 19, 2019, 12:17:27 pmI find latching relays not so good in something portable or a car, anywhere they get bumped- they can unlatch. Then firmware wrongly assumes the relay is latched when it isn't, which causes more problems.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 20, 2019, 02:06:53 am
content: Quote from: floobydust on August 19, 2019, 09:14:47 pmUnless you have a spare set of contacts that can be monitored by the microcontroller.