Question about diode across relay coil

[guitarmike2107]

While you’re looking at relays you may want to consider solid state relays.

No coils, no voltage flyback and no mechanical clicking.. just doing dpdt is a bit more tricky.

It was thanks to Steve that I found these:
Data Sheet - ASSR-401C and ASSR-402C Low C x R, Form A, Solid State Relay (Photo MOSFET) (400V/100

Here are some others that could work
http://uk.mouser.com/catalog/643/1850.pdf

I attach flying leads and then use them on turret builds, they are small enough to do so.

[J M Fahey]

Even if a mechanical switch is used , instead of a transistor, you need them.
The voltage generated at turn off may easily be 10x the coil rated one, or even much more , and produce arcing and pitting at the switch contacts, not forgetting pop.

[kg]

While you’re looking at relays you may want to consider solid state relays.

No coils, no voltage flyback and no mechanical clicking.. just doing dpdt is a bit more tricky.

It was thanks to Steve that I found these:
Data Sheet - ASSR-401C and ASSR-402C Low C x R, Form A, Solid State Relay (Photo MOSFET) (400V/100

Here are some others that could work
http://uk.mouser.com/catalog/643/1850.pdf

I attach flying leads and then use them on turret builds, they are small enough to do so.

no clicking???

how the hell am i supposed to troubleshoot?

(only half joking.)

[mooreamps]

Even if a mechanical switch is used , instead of a transistor, you need them.
The voltage generated at turn off may easily be 10x the coil rated one, or even much more , and produce arcing and pitting at the switch contacts, not forgetting pop.

and ah, bypassing the contacts would not abate some of the arcing ??

-g

[Jazz P Bass]

(according to National Instruments, the flyback diode should be no greater than 18 inches from the inductor).
Source: Wikipedia: flyback diode.
I do agree thogh that the diode right at the coil is a common layout.

[guitarmike2107]

ha ha, I was looking at a friends superdooper prosonical Fender recently, when you turned it on and off a number of relays were energized/released click click click click. Sounded weird actually

That was a good deal though as I got a 6 pack and box of chocolates for not fixing his non broken amp.. he did learn a good lesson always check your cables first!

[guitarmike2107]

If you stick with the mechanical relay, try and get one with the suppression built in.
If not, you may want to consider putting a resistor or zener diode in series with the flyback diode.
Depends on how robust you want to build the amp of course

[J M Fahey]

and ah, bypassing the contacts would not abate some of the arcing ??

-g

Yes, of course.
A capacitor across the switch contact will work; it's good practice though to add a small resistor in series with it (say, 100r) to dampen the resonant tank made by it and the unspecified coil inductance.
This turns the simple capacitor into a snubber , which is a fancy way to say that you add a lossy component (the resistor) to "eat" that energy pulse in a reasonable time.
This is mandatory on AC circuits (say a line switch feeding a power transformer) because diodes can't be used. You can also use Varistors or back to back Zeners to absorb voltage peaks.
As of the power of inductive circuits , whan I was building my "big" magnetizer my anxious partner wanted to try it, and hooked a Frankenstein movie type knife switch between the 6 diode, 380V, 3 phase rectifier and the monster magnetizer coil (think a fridge sized electromagnet), *without* the big diode I was buying in that very moment.
He was trying to interrupt *DC*; all 380V 60A of it feeding that coil.
He turned it "on" with no problem at all, but when he tried to turn it off, it would not.
An impressive arc jumped the air between the switch contacts, happily keeping it "on" ... and burning everything within 10 inches.
He had to *run* to the "street power cabinet" and pull the fuses there.
After that, we wired the flyback diode *and* switched the 380V AC side with a Siemens 100A contactor, which still works faithfully after 20/25 years.
We got tired of burning SCRs and the mechanical solution was best.
My "small" magnetizer (monophasic 220V) is also AC switched and the bridge rectifier acts as a "free" clamping diode.

[R.G.]

and ah, bypassing the contacts would not abate some of the arcing ??

I'm sure you think you understand what you think JM might have meant if he had said what you thought he might have said when he said it, but I'm not sure I think you mean what you might have meant when you actually said it when I thought you said it.

[GregS]

I'm using a 5V relay for switching in an amp I recently built with no diode across it. I have been using it for months without any issues or pops.

[mooreamps]

I'm sure you think you understand what you think JM might have meant

Ya, what ever......

[kg]

I'm using a 5V relay for switching in an amp I recently built with no diode across it. I have been using it for months without any issues or pops.

that may very well be true, but it doesn't mean there's no use in following best practices.

[Enzo]

Amen, getting away with something doesn;t mean it is right.

Gaz' original problem was not recognizing his circuit. The diode is NOT from power supply to ground, it is from power supply to the other end of the relay coil. It is only when the switch is closed that the anode of the diode connects to ground. But since the purpose of the diode is to shunt the coil's inductive spike when the current to it is interrupted, the diode must be across the coil not across the supply.

When someone says the diode must be wired across the coil, that means the two ends of the coil are connected to the two ends of the diode. it doesn't specify that the diode be right on the relay. In other words, "across the coil" specifies electrical position, not physical position. ANother refernce above states the diode should be within 18 inches, fair enough, but that is a separate thing from electrical wiring.

[R.G.]

Ya, what ever......

Wow! Clever reply! Oh, agony! I'm mortally wounded! Cut to the quick by the serpent's tongue!

I'm using a 5V relay for switching in an amp I recently built with no diode across it. I have been using it for months without any issues or pops.

Yes, what they said.

What that means is that something is absorbing the flyback energy. Sometimes a driver transistor can eat this kind of abuse, especially if it's at human speed, for lots of iterations without failing immediately. It may even last for a long (by human standards) time if it's not overheated by the avalanches. But it's not good for whatever drives the relay. Generally if what drives the relay is a transistor it lasts longer, as single transistors can take more abuse than the insides of an IC in many cases.

Using a diode means the diode is always shunting the flyback spike back into the power supply, and there are no long term worries (or sudden failures) with the driver. This is why it's just good practice to do this and not count on getting by with with it for an unknown period of time. Good design practice means never (OK, hardly ever ) having to say you're sorry after shipping the design.

[mooreamps]

Wow! Clever reply! Oh, agony! I'm mortally wounded! Cut to the quick by the serpent's tongue!

Don't worry.. He'll live... :|

-g