I want to power some accessories in my car using illuminated switches on the dashboard. These switches will be wired to the dash illumination circuit, which is normally 12 V but can drop as low as 6 V when fully dimmed. Can I use a 6 V relay here to give 12 V power to the accessories? According to this datasheet, its max. switching voltage is 16 V, so I think this should be safe.
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1\$\begingroup\$ What is 6V? Coil voltage has nothing to do with contact voltage. If you want to wire a 6V coil to 12V the just add a resistor to limit the current so nthe 12V can't push more than the rated coil current through the coil. \$\endgroup\$– DKNguyenFeb 25, 2019 at 16:11
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\$\begingroup\$ Are you sure it's a steady 6V when dimmed? There could be some switching action going on for dimming instead. A DMM will "average out" PWM for example. \$\endgroup\$– UnimportantFeb 25, 2019 at 16:11
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\$\begingroup\$ It just occurred to me I can switch ground instead of positive, then the relays will always get 12V and it will avoid this issue. \$\endgroup\$– Elliott BFeb 26, 2019 at 3:20
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\$\begingroup\$ Switching ground should work as long as the illumination lamps in the switches are not polarity sensitive. Bulb types should work, LED types may not. \$\endgroup\$– GodJihyoDec 27, 2021 at 1:19
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3 Answers
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The coil and contacts of a relay are electrically isolated from each other. Powering the coil with mechanically actuate the contacts. The voltages can differ between the two (e.g. 24VDC coil with 230VAC contacts).
The coil requires a specific voltage to turn the relay on and off. This voltage is selected on page 4 of the document you've provided. Looks like your options are 6, 9, 10, and 12 VDC. EDIT: It would be in the best interest that this coil voltage is not fluctuating, and instead, a constant voltage source. Powering a 6 VDC coil from a 12 VDC source will cause the coil to become quite hot.
The contacts are what will be providing power to your accessories. They have a max rating of 16VDC, 35A (page 3). So, 6 to 12 VDC on your contacts will not be a problem, so long as you have a constant voltage on your coil.
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1\$\begingroup\$ You have not dealt with the issue of coil power at higher coil voltages. Although, to be fair, the data sheet doesn't either. Using a 6-volt coil at 12 volts will dissipate 4 times the nominal power, and that is hardly ever a good idea. Using the "Coil Temperature Rise" graph, and figuring on about 2 degrees per 0.1 watt increase, a 6-volt relay driven by 12 volts will have a coil temperature of about 160 degrees C. Again, hardly ever a good idea. \$\endgroup\$ Feb 25, 2019 at 18:32
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\$\begingroup\$ That's a great point. What I was attempting to explain was that the contacts would be connected to this "dash illumination circuit" with the varying 6-12 VDC, and the coil should be connected to a consistent voltage. I'll edit your point in for clarity. \$\endgroup\$– user199402Feb 25, 2019 at 19:07
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\$\begingroup\$ Actually the coil will be connected to the dimmable 6-12VDC and the contacts will have constant 12V. Page 7 of the data sheet shows a coil voltage graph for the 9V model, but not for the 6V model. Looks like the 9V relay can tolerate 12.5V at 50ºC. \$\endgroup\$ Feb 25, 2019 at 23:23
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\$\begingroup\$ @ElliottB, that's a mistake on my part -- definitely misunderstood. Why can't the coil be connected to direct 12VDC? To me, you'd connect the dimmer voltage to the light on the switch, then 12VDC to the contact of the switch to the coil. Then another 12VDC to the contacts of the relay + accessories. \$\endgroup\$– user199402Feb 26, 2019 at 13:48
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According to the datasheet, there are two series of relays FBR51 and FBR52. Both are available in 6, 9, 10 and 12 volt versions.
For the FBR51 series, the coils are rated at 0.6 watts
6V^2/60Ω = 0.6W
12V^2/240Ω = 0.6W
For the FBR2 series it's 0.8 watts
6V^2/45Ω = 0.8W
12V^2/180Ω = 0.8W
See page 4 for the voltage and coil resistances.
What happens if you run the 6V version at 12V?
FBR51 series: 12V^2/60Ω = 2.4W
FBR52 series: 12V^2/45Ω = 3.2W
Since the wattage is related to the square of the voltage you see that doubling the voltage quadruples the wattage.
In layman's terms, they're probably gonna go up in smoke.
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The relay DC coil voltage rating on most relays is continious .Also the relay manufacturers have made allowances for practical lead Acid battery systems .For example if you put a relay with a coil nameplate rating of 12 VDC would be fine in a car that is driving along with a charging voltage of say 14 VDC .Normal relay coils have lots of turns of fine copper wire with significant thermal mass unlike certain fragile semiconductors .This means that they are reasonably idiot proof .If you overload a 6V relay with 12V it will take minutes to cause thermal problems . Long before the enamel insulation on the copper wire flakes out the plastic housing will distort which mucks up contact alignment causing fail .So in summary you can overvolt the coil for seconds without punishment. In other words a 24V relay is not going to explode if pulses of 48 V are applied .This means that you could use 6V relays in your 12V system with some RC scheme or some PWM scheme that allows tempory 12VDC but enforces steady state 6VDC.
