# 5v vs 12v dc pcb mounted relay selection

I'm newbie and just got stuck at design decision whether to go for 5 volt dc relay or 12 volt dc relay in power circuit.Both relays equally mentions power dissipation as 170mW according to their datasheets.

Originally I selected the 5v one since mcu and other wireless controller are 3v3 ones.But then one of my friend suggested that current consumption in 5 volt would be higher.

Since circuit will be running 24X7,I intend to keep heat dissipation as low as possible.I have also attached possible design combo.Should I go with 1 or stick with 2.

• Could you link in the datasheet of both relays? – KarlKarlsom Jun 25 '16 at 13:01
• What about 12V SMPS and 3.3V LDO? With LDO you get noise free voltage, while the buck may give you troubles in terms of noise for MCU. The MCU usually needs low power, so the dissipated power on LDO is low. The best would be to look the schematics of available MCU boards, how the 3.3V is obtained. – Marko Buršič Jun 25 '16 at 13:19
• You have not mentioned how you intend to drive the relay coils from the MCUs. This is perhaps the most critical part of driving the relays, since your MCUs cannot safely directly drive either a 5V or 12V relay, and your driving circuit will need to deal with voltage spikes from the relay coils when they are switched on/off. – AndyW Jun 25 '16 at 17:15
• Yes,you are totally right on noise thing of buck converters. – Maverick Jun 25 '16 at 17:16
• @AndyW I appreciate your concern about driver interface.They'll be driven by opto coupler/darlington IC.Also there are various components in final circuit such as transient and noise filters with capacitor networks to handle peak powers.But I have omitted them in order to focus on relays and smps design. – Maverick Jun 25 '16 at 18:20

... that current dissipation in 5 volt would be higher.

Current isn't "dissipated" - it flows through the relay coils. The power is dissipated in the coils according to $P = VI$. Since the power required to pull in the relay is 140 mW (a constant, whatever coil voltage is selected) then if the voltage is reduced the current must go up.

If 12 V is available I would tend to power the relays from that just to keep the switching noise away from the 5 V or 3.3 V logic and to reduce the load, size and heat on the 5 V regulator.

Since circuit will be running 24/7,I intend to keep heat dissipation as low as possible.

The power dissipated in the relay coils will be 140 mW either way. The power dissipated in the switching transistor, however, will be proportional to current so the 12 V relay will be lower.

Your decision will probably be based on available supply voltages.

As each relay will dissipate 140mW, the power dissipation will be the same for either part.

You mention in your second paragraph 'current dissipation'. That is a nonsense, you perhaps mean current consumption?

Your two suggested power supply configurations are both reasonable, but are not directly comparable, as one uses an LDO to get to 3.3v, the other a buck SMPS. Any advantage one has over the other in power dissipation will be marginal, and down to how well the 12v->3.3v buck is implemented. It might do better than a 5v->3.3v LDO if well designed, it may not. Either way, this is a small difference in power, not a 2:1 ratio in overall dissipation.

You may have other reasons to choose a 5v or 12v supply, perhaps future expandability to other components?

• Sorry I do meant consumption.Just edited my question. – Maverick Jun 25 '16 at 17:28