0
\$\begingroup\$

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 12V but can drop as low as 6V when fully dimmed. Can I use a 6V relay here to give 12V power to the accessories? According to this data sheet, it's max switching voltage is 16V so I think this should be safe.

https://www.mouser.com/datasheet/2/164/fbr51-52-472017.pdf

\$\endgroup\$
3
  • \$\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\$ – DKNguyen Feb 25 '19 at 16:11
  • \$\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\$ – Unimportant Feb 25 '19 at 16:11
  • \$\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 B Feb 26 '19 at 3:20
0
\$\begingroup\$

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.

\$\endgroup\$
4
  • \$\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\$ – WhatRoughBeast Feb 25 '19 at 18:32
  • \$\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\$ – C. Lange Feb 25 '19 at 19:07
  • \$\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\$ – Elliott B Feb 25 '19 at 23:23
  • \$\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\$ – C. Lange Feb 26 '19 at 13:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.