I want to latch a relay with a Teensy so just out of curiosity I latched both a 3 volt and 5 volt relay with 3.3 volts. I was surprised to find the 5 volt relay drew .04 amps while the 3 volt relay drew .12 amps. Why is that? (The 5v relay latched just fine with 3.3 volts.)
It's because the 5V coil has a higher resistance.
The 5V relay probably did not "latch just fine". First of all, it did not latch. Latching is only performed by special latching relays, it means that it will remain in one of two states even with no power applied. Second, just because a component operates doesn't mean everything is "fine". It only means you haven't exceeded the absolute maximum or minimum operating conditions. Those are not the same as the safe, normal operating conditions.
Also you asked a question about specific relays without telling us exactly which relays they are. The forum guidelines suggest that you do in cases like this.
If you look at the spec sheet for two relays from the same series... Identical except for different coil ratings, the power is usually the same, so the higher voltage version would require less current (Power = Voltage X Current).
Everything in electronics has a tolerance. A 5V relay isn't going to stop working at 4.99 or 4.9V. You'd have to check the specs but the relay is probably rated for +/- 20% or so... In that case the relay would work reliably down to 4V and below the minimum spec it might work, or not, or it might be unreliable. If the voltage is too high the coil will overheat and burn-out.
The 5V relay may be just fine in your project but if you were going "into production" it would be dumb to design a circuit where the relay was under-powered (or over-powered).
Thank you both for the replies.
Ummm, on my Vulcan 1500 12V motorcycle the ignition relay is 9V. They burn out a lot. But the reason they use 9V is so that the ignition will be connected when the bike is cranking in the cold. Sometimes there's a hidden design tradeoff. They figure selling relays is better then "won't start" trouble calls.
60% under voltage isn't horribly low for a magnet, as long as you get at least the threshold of magnetism it'll work - sometimes.
One fun thing that low voltage to a relay will do is make it vibration sensitive. Since the magnetic holding force is low, rapping or jarring it (like while riding) it can cause it to release. Fun to troubleshoot - especially on the side of the interstate ... at night ... in the rain.
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If you're using one of these, the pull-in voltage is 3.75V, so connecting it to 3.3V is out of spec and the relay may randomly fail to operate.
90mA isn't something an Arduino 20mA (or even 40mA) output can drive directly. Its 45% of the max 200mA for the board! So relay boards have an opto or transistor driver that you need if you roll your own.
...and you should have a flyback diode on the relay coil. Also normally provided on the relay boards.
Hi,
Measure the resistance of the 5V coil and the 3V3 coil and do some OHMs Law.
Tom... 
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