Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Still, a resistor across the coil, as you mentioned, is not a bad place to dump the energy if one can tolerate some wasted power in the resistor. If the resistor has the same value as the resistance of the relay coil, the flyback voltage will never be greater than about twice the voltage applied across the relay coil when the transistor is turned on. The bonus with the resistor is the slow contact release time is largely reduced (compared to a diode) which can save the contacts from spot welding upon opening. And a resistor is less expensive than a diode or a capacitor.
This is a benefit of using a resistor.the slow contact release time is largely reduced (compared to a diode)
agreedTo clarify, ccurtis said the relay releases faster with a resistor compared to a diode.
This is a benefit of using a resistor.
You didn't model the diode + zener I mentioned they use in high end devices to achieve fastest release times?
With the zener as high as possible, say 36v zener when using most 45Vce small switching transistors.
hi,
This is with a 33VZ ans 1N4148 clamp....
Interesting ringing that happens after the voltage drops below Vz. Coil inductance vs Zener capacitance? Wont effect the relay, but might cause EMI.
hi,
I also noticed that when using R/C subbers, thought it may be an artefact from Spice...
Plus the supply voltage.A diode can also be placed in series with a resistor to avoid the wasted power in the resistor, yet still largely improve contact release time over a diode alone across the coil.
The peak flyback voltage will be the resistor value multiplied by the relay coil operating current.
Plus the supply voltage.
hi,
This is with a 33VZ ans 1N4148 clamp.
Its a fast release, used on 'old' paper tape punch pin solenoid drivers.
I think you are operating under a misconception.Probably too late to comment since last post on this thread was a month ago.... but...
I got into the habit of using a series resistance between the collector and the coil. I pick a small resistance such that the voltage across the coil is still above the min spec holding voltage, considering it forms a voltage divider twixt the coil resistance and the resistor. Consider also the voltage is max when the transistor switches on (since i=0 initially), and reduces as current increases -- just what we want. Now the flyback diode is sometimes inside a driver (like ULN2003 or 2803) where I may tie the cathode side to 12V, for instance, while the coil is a 5V coil. I like to make use of such built in clamp diodes and the series resistor and option to clamp at higher voltage is what make a simple diode clamp work better (vis a vis the slowing of contact opening due to back emf).
The advantages are that the coil is not overdriven (keeps down coil temperature and overall current draw) which may be important. Still the initial voltage across the coil is high and falls to only as much as necessary to hold the contacts together. Then when the transistor switches off, the flyback current is not short circuited through the clamp diode so that the contacts can come apart quicker (so make sure your diode anode is on the other side of this resistor) not to mention also clamping to a higher voltage, allowing that much more time for the spring to pull the contacts apart. By and large, the contacts are more damaged by arcing while pulling apart than anything else and to avoid welded contacts may well save other components in a system. Seems worth it to apply more thought than the usual habit of throwing 1N4148s across coils, which is what I used to mindlessly do until I finally started looking at it some more.
My $0.02.