Relay driver with holding current

[macboy]

Hi

I have used a coil with R=300Ohm (approximately) and inserted a series resistor of 560R, and tried a few caps. Here are pics with 470uF and 100uF - oscilloscope probe on the coil (working off battery so not earthed). Resistor divider now feeds 1.6mA to base.

Edit: on initial power up, the relay remains in the off position because there is no time to charge up the cap and create the spike that is needed. That is a problem that needs to be solved.

In the first post you said you want a holding voltage of about 5 to 8 V. Using a 560Ω in series with a 300Ω coil will give you closer to ~ 4 V across the coil. Try around 300Ω (270Ω, 330Ω). Also keep in mind that the voltage across the cap will be only the difference between the supply and holding voltage (roughly 6 V), so although it may "high" in capacitance, it can be a low voltage rating, so it can still be a physically small part. I have frequently used this R//C trick for relays without issue. It is simple and it works, you just need to tune the values to your particular relay.

 

[AndrewT]

add a short delay to the switching transistor.
A B to E cap does it. You can increase this cap to create longer delays.

 

[RJM1]

Just put the cap and the resistor on the other side of the relay so you won’t have to wait for the capacitor to charge.