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Is there a voltage drop across a relay?
As the output switching device of a solid state relay is a semiconductor device (Transistor for DC switching applications, or a Triac/Thyristor combination for AC switching), the voltage drop across the output terminals of an SSR when “ON” is much higher than that of the electromechanical relay, typically 1.5 – 2.0 …
How does a 12V relay work?
So, here’s how you wire a relay:
- Connect the wires to your relay’s coil.
- Connect the relay’s common terminal to your battery supply, which could be the same supply providing power to the coil.
- Finally, connect the equipment’s power lead to the normally open terminal of your relay.
Can a resistor drive a relay from 12V?
Yes, a resistor would work to drive this relay from 12 V. The resistor needs to drop 7 V with 12.5 mA going thru it. From Ohms law it needs to be (7 V)/ (12.5 mA) = 560 Ω. Your calculation is therefore correct. You should also consider the power dissipation of the resistor.
How much voltage do you need for a 5V relay?
The resistor needs to drop 7 V with 12.5 mA going thru it. From Ohms law it needs to be (7 V)/ (12.5 mA) = 560 Ω. Your calculation is therefore correct. You should also consider the power dissipation of the resistor. It will be (7 V) (12.5 mA) = 85 mW.
How does the voltage drop in a MOSFET?
If you substitute a logic-level p-channel MOSFET for Q2, you could make the drop even lower under favorable conditions. For example, a 5m Ω Rds (on) power MOSFET would have a drop of only 50 μ V at 10mA. The emitter voltage is limited by the base voltage since ~Vbe must be greater than ~0.6V to turn on the transistor.
What should the voltage drop be on a Q2 transistor?
Q2 should saturate when on and give you tens to hundreds of mV drop from the 4.7V rail depending on the transistor and the load current. If you substitute a logic-level p-channel MOSFET for Q2, you could make the drop even lower under favorable conditions.