I am connecting five 12V relay coils in parallel to switch five different loads. Is it necessary to have fly-back diode for each relay coil or just one 1N400x diode in parallel is sufficient? Each coil is 300 ohms.
-bhavj
snipped-for-privacy@gmail.com schrieb:
Hello,
the current through one coil is 40 mA, the five coils take 200 mA. A
1N400x is good for 1 A. If these five relays are not far away from each other, only diode will do in my opinion. The current flowing through the diode should not be much larger than the current flowing through all coils together.Bye
Make sure you use the low-voltage one, 1N4001. Higher-voltage diodes of that series have long turn-on delays, so they aren't much good for suppressing inductive kicks.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
The relays stacked together and trace length is approx 70 mm from 1st relay coil to the 5th relay coil. So, do you think it would be a better option to place the diode across the coil of the middle (3rd) relay?
Thanks. Would it be better to use 1A schottky than normal diodes like 1N400x?
The 1N4001 should be fine, but do use at least a 60V transistor to drive the relays.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
BCX56 is the transistor. Relay coils are connected between emitter and ground. 12V is applied at the collector.
Oh, that's easy then. When the emitter goes low, current will continue to flow without interruption until it decays. No spikes to worry about at all, no diode needed, as long as you don't open-circuit the base.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
One more thing: Do watch out for oscillation. Putting 100 ohms or so in series with the base will usually cure it. (A transistor with its collector and base at the same voltage is still in normal bias, and can still oscillate.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
It doesn't matter where the flyback diode is located.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
I always use a transistor or MOSFET with internal diode. No need for an external diode.
-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
may be of interest
The OP should post his schematic. I've seen interesting things, like putting TTL levels into a base and expecting 12 volt swing at the emitter.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
I was sort of assuming that it was CD4000 CMOS, but you're right, you never can tell.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Here is the schematic I quickly drafted. All the 5 coils and fly back diode are in parallel and will be connected to 12V_RLY_COIL.
The idea is for the diode to protect the switching device (xsistor, FET, whatever) from the switching spike. The diode is usually placed near the coil connections so that the current produced by the spike does not couple to nearby circuits, as it would if the diode were placed on or in the switching device. If the switching device and the relays are all located very close together, it probably doesn't matter. However, if the relays are scattered all over the PCB, it might be better to suppress the spikes at the source, near the coils.
In my checkered past, I helped design an HF automatic antenna tuner. There were about 20 relays that would cycle a collection of inductors and capacitors through a binary search pattern until a suitably low VSWR match was found. Tuning speed was important, so some of the relays sometimes sounded like buzzers. The now ancient microprocessor used (Intel 8748) was rather sensitive to noise. However, it wasn't RF that was causing temporarily insanity in the uP. It was switching transients traceable to back EMF from the relay coils. Nobody had mentioned to the PCB layout person that the flyback diodes needed to be close to the relay coils. The next PCB revision moved them much closer, which produced far less noise. The rest of the noise was caused by trying to open or close all 20 relays at once (ground bounce), which required adding some filter caps and some slight delays in the firmware. I don't think this will be a problem with only 5 relays in parallel unless there's sensitive circuitry nearby.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Sorry, you have just been demoted to SCI Electronics Basic! Harry
I was thinking the same thing with a NPN and the collector tied to the
12v rail, you'd need to take the Base above the 12v rail to turn it on.Cheers
Is that supposed to be funny?
Harry is right.. there are multiple issues.
1) Can we assume you're trying to drive the relays with a 3.3V CMOS output, and 3.3V in should turn the relays "on"?1a) If so, is the common for the 3.3V supply same as the ground shown?
2) Any particular reason you want a high-side driver (so one side of the relays is grounded)?3) Is this an automotive 12V system with the associated transient issues?
4) What's with the 100uF cap across the relays, and the 0.1uF cap, for that matter?--sp
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.