12v Relay with selectable 12 or 24v DC supply - Page 1
Forum section: electronics / general

author: Ditch
author_rank: Contributor
timestamp: February 25, 2021, 04:45:30 pm
content: Hi There,I'm after a little guidance from you helpful people if you could.I'm going to make a controller with 3x 12v relays onboard. This will be used within the fire and security where security systems work on 12vdc and fire systems work on 24vdc.I don't want to be making 2 separate controllers for the 2 working voltages so my question is what is the approved way of driving the 12vdc relays from both voltages?I want to use regulators. I know I can drop from 24v to 12v and then drive the relays from the 12v supply but a) this will cook the 12v regulator and b) I'll get a voltage drop through the regulator when at 12v which will be on the borderline of the operating voltage of the relay.This controller will have a microchip (pic18f46k22 - I'll use another 5v regulator for this).I don't want to go down the buck converter route and through past experience, solid-state relays can cause problems with the monitoring of the fire alarm inputs so these cant be used.So, what is the easiest way to achieve this? Any pointers or guidance would be appreciated.Ian

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 25, 2021, 04:53:07 pm
content: If not buck converter then SEPIC?

author: madires
author_rank: Super Contributor
timestamp: February 25, 2021, 05:04:02 pm
content: In most cases 12V relays will also work fine at slightly lower voltages, e.g. 10V. Despite you don't like buck converters, they are the best choice to avoid hot linear regulators for a large input voltage range. If the circuit doesn't need much current an LDO could be an idea, i.e. it's a linear regulator with a low voltage drop and the power dissipation for a low current can be easily handled by a heatsink. For example, for 0.1A and 24V input it would be 12V * 0.1A = 1.2W.

author: Ditch
author_rank: Contributor
timestamp: February 25, 2021, 05:28:48 pm
content: Ok, thanks for your quick replies.By your answers, the buck convertor or SEPIC (had to google that!)method is the answer to this.I would like to possibly sell this commercially and would need these to pass the EU emissions which I assume would be difficult using a buck convertor for somebody like me!Thanks for your help though.Ian

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 26, 2021, 03:13:59 am
content: There are polarised type relays (they are usually low-current, like signal ones). They are more sensitive so they need less current to operate. So you may be fine with linear regulator (low-drop one). So you'll don't have a problem with emissions.

author: MarkF
author_rank: Super Contributor
timestamp: February 26, 2021, 03:41:03 am
content: Depending on the relay you may be able to drive the coil with either voltage.For instance, these 12V coil AXICOM relays have a coil voltage range of 9.6V to 28V.Otherwise, you could just power the relays through a 7812 regulator.So what if it drops regulation or voltage at 12V input.The relay coil isn't going to care as long as the voltage is above its pull-in voltage.Or, use 9V relay coils. The regulator would have enough headroom to drop both voltages.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 26, 2021, 04:40:51 am
content: Have the microcontroller check the input voltage and drive the relays with PWM for 24V.

author: ajb
author_rank: Super Contributor
timestamp: February 26, 2021, 04:57:27 am
content: You can drive lower voltage relays from higher voltage supplies via PWM. The relay coil is an inductor, so current does not change instantaneously, so you end up with a ~triangular current waveform with the ripple vintage set by the PWM frequency and inductance of the coil. Your MCU could measure the supply voltage and decide whether or not to PWM the coil (or at what duty cycle) accordingly. PWM can also be used to reduce power consumption via "peak and hold" drive, where a higher "peak" power is initially applied to actuate the mechanism before switching to a lower "hold" power to maintain its position, which could be useful to reduce the demands on your power supply, although I'm not sure it would help enough that I'd want to use a linear regulator for 24->12V in an application like this.Of course you will need to deal with the EMC implications of PWM, but that shouldn't be too difficult.

author: Psi
author_rank: Super Contributor
timestamp: February 26, 2021, 05:27:33 am
content: +1 for doing PWM when powering 12V relay on 24V system

author: Ditch
author_rank: Contributor
timestamp: March 17, 2021, 07:35:16 pm
content: Hi All,Sorry for raising an old post but I've only just checked this.I like the idea of using PWM. I'm already checking the input voltage so this will not be hard to do and seems like a good option.I've also just checked the datasheet for this Omron relay -https://docs.rs-online.com/8f03/0900766b8002b925.pdfThis says that I can drive a 12v at up to 130% which is a max of 27.6v. Is this an option or is it bad practice (and probably reduced life expectancy) if I was to do this?

author: ajb
author_rank: Super Contributor
timestamp: March 17, 2021, 08:37:45 pm
content: That would be 130% *of* the rated voltage, IE 12V*1.3 = 15.6V, not 130% *above* rated voltage. But even so that's the maximum rating, and in general you want to leave yourself as much margin as you reasonably can on the design to reduce typical stress on components and leave headroom for things like variations in supply voltage, plus margin for transients or other outside interference your device may experience.

author: Ditch
author_rank: Contributor
timestamp: March 17, 2021, 09:56:12 pm
content: Brilliant ajb, Thanks for your helpYou mention above EMC implications using PWM. I'd imagine switching the relay coil using PWM like you suggest could possibly be problematic for emissions testing?What would be the best approach for reducing this?

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2021, 10:17:26 pm
content: PWM can be implemented with discrete components too, which has the advantage of it varying with the voltage, without having to worry about the software.How about running the MCU of a buck regulator and using a 5V relay? That would also avoid the need for a separate regulator, or SEPIC.

author: SmallCog
author_rank: Frequent Contributor
timestamp: March 18, 2021, 02:27:24 am
content: I'd just go with Solid state relays rather than traditional onesI've got some some nice DIN rail ones here that trigger off 1.5-24VDC