diode for collapsing magnetic field

[Blank Stare]

I have a 12 volt circuit, switched by a relay. The (motor) load is 20 amps, when running, but I do not know what the start-up load is. I am going through relays faster than I can buy new ones. I have been told by some friends (who are NOT electronics professionals) that I need a diode to handle the spike in power, when the relay opens, and the magnetic field collapses.

Trouble is, I have no idea how to figure that out, as I am not an electronics savant. (I'm a painter, who likes to tinker...) Can anyone help me figure this out, so I can get the right diode?

I very much appreciate any constructive feedback.

~ Blank Stare

 

[wannabegeek]

Check out the link to the article below.

It's hard to guess how much back emf that relay will generate, but I suppose we can guestimate by knowing the inductance of the coil. Perhaps you can google the numbers on part and track that down. In fact, there's prolly a recommended diode from the manufacturer.

I'm thinking the back emf will could be on the order of 300 Volts....A diode has a max reverse voltage, that's how much of the back emf it can stand b/f breaking.
As the article suggests, you may want to use a germanium diode that has a smaller forward voltage drop, 0.2V instead of the standard 0.7V.

http://en.wikipedia.org/wiki/Flyback_diode

 

[(*steve*)]

Yes, you drew the diode in backwards.

The diode current will not exceed the motor current just prior to the relay opening.

However, the diode needs to go across the motor, not the switch.

 

[CDRIVE]

If relay contact destruction is the issue, then short of insuring that the relay is sufficiently sized, I would think a snubber would be more appropriate. Unless I'm mistaken and unlike a common inductor, I would expect the free wheeling EMF developed by a brush DC motor to be the same polarity as the source. Hence, not Back EMF.

Am I all wet?

Chris

Edit: I just looked at the schematic again and maybe I misinterpreted the function of the Diode. If the object of the Diode is to dump this EMF back into the source then yes a Diode does make sense. That is after it's flipped, as Steve said.

 

[Blank Stare]

OK, some additional details

I suspected I had drawn that in wrong, but my brain was about used up, when I saved it, and went to bed. I know how to wire it in, but I was unsure of the symbol orientation.

Yes, it is the relays that are getting trashed - the motor doesn't seem to be affected.

Sometimes they fail open, sometimes they fail closed.

OK, some details...

The relays I have been using are the sort you typically find in automobile fuse boxes.
It's a black cube with contact tabs on the bottom side.
Two of the tabs are for the remote switch. Of the remaining three, one is for power in, and two for power out. (One normally open, and one normally closed.)

I do not use the one that is normally closed, since I do not need to have two different amp settings in my circuit.

I have used 30 amp, and also 40 amp relays, and all of them have fried in a matter of a few uses, except the very first one, which lasted a few months. (I've inspected all of the wiring, and connections, all look to be fine.)

I even used a lawn tractor solenoid, which lasted ONE cycle.

The motor runs an air pump. If I bypass the relay, all systems are "go". That is say, that when the pressure drops to a preset psi, it turns itself on, and pumps air. If it is working correctly, it fills an air locker (tank), and then shuts down when it reaches pressure.

I guess I will look for information on the relays. I'm sure where to look, but I'm sure I'll find something, without too much searching.

Thanks again for your gentle guidance.

~ Blank Stare

 

[Blank Stare]

...I just looked at the schematic again and maybe I misinterpreted the function of the Diode. If the object of the Diode is to dump this EMF back into the source then yes a Diode does make sense. That is after it's flipped, as Steve said.

As I understand it, that is exactly the purpose of the diode.

I did a bit of google-ing around, and I have seen the EMF thing discussed, but I am not familiar enough with the subject to say with conviction that this is the fix. I have been assuming it is, because my friends (some electronics training, but not in the industry,) told me that it is the solution.

I am not closed minded about other possible solutions.

Is there any reason not to put diodes in both places? I'm not proud, I just want it to work again, and I am not afraid of spending a couple of bucks, and doing some soldering, and shrinking some tubing.

Thanks again,

~ Blank Stare

 

[Blank Stare]

here's a corrected schematic.

 

[CDRIVE]

Is there any reason not to put diodes in both places? Thanks again,

~ Blank Stare

I don't know what you mean. What other place, other than across the relay contacts?

Chris

 

[CDRIVE]

here's a corrected schematic.

Yes, like that.

Chris

 

[Blank Stare]

I don't know what you mean. What other place, other than across the relay contacts?Chris

Across the motor?...

However, the diode needs to go across the motor, not the switch.

 

[CocaCola]

here's a corrected schematic.

Look at the attached, as Steve suggested the location(s) of the diode in my picture will snub the motor (and relay), the relay one isn't necessary with a manual switch but it won't hurt... If the switch is 'electronic' like a FET or whatever it should be there to protect that electronic switching device...

Is there any reason not to put diodes in both places?

There is a reason, they are 'snubbing' their respective inductive loads, but it's not always necessary... And they should be installed parallel to that load...

Read the page wannabegeek linked or Google up flyback/snubber diodes and get an understanding of how and what they do...

 

[Blank Stare]

... If the switch is 'electronic' like a FET or whatever it should be there to protect that electronic switching device...

Read the page wannabegeek linked or Google up flyback/snubber diodes and get an understanding of how and what they do...

OK, I read the page several times.
Which, of course, means I am more confused than before

The linked page keeps referring to relays, but doesn't seem to be concerned with motors, or "loads", except for one small mention at the bottom of the page.

But it does seem to talk directly about the destruction of the relay, due to arcs "across the...contacts".

If the current can only flow through the air, the voltage is therefore so high that the air conducts. That is why in mechanically-switched circuits, the near-instantaneous dissipation which occurs without a flyback diode is often observed as an arc across the opening mechanical contacts. Energy is dissipated in this arc primarily as intense heat which causes undesirable premature erosion of the contacts.

That sounds like what I am experiencing.
If I was having "fly back" at the motor, wouldn't I be experiencing motor failure, rather than relay failure?

Sorry if I sound ignorant ( I am, you see), and please don't think that I am being arrogant (I'm absolutely not). I just like to understand what makes things tick, and I always want to know what's in the "black box".

Thanks again,

~ Blank Stare

 

[Blank Stare]

Look at the attached, as Steve suggested the location(s) of the diode in my picture will snub the motor (and relay), the relay one isn't necessary with a manual switch but it won't hurt... If the switch is 'electronic' like a FET or whatever it should be there to protect that electronic switching device...

There is a reason, they are 'snubbing' their respective inductive loads, but it's not always necessary... And they should be installed parallel to that load...

Read the page wannabegeek linked or Google up flyback/snubber diodes and get an understanding of how and what they do...

OK, I just took another look at your modified version of my schematic. I misunderstood, and now realize that you HAVE suggested two diodes, rather than just the one. I'm not sure why one isn't needed across the high amp side of the relay, but I am willing to admit I am beyond my depth here.

Therefore, I accept your premise, and subsequent logic and placement of the diodes.

So that potentially solves the problem, bringing me back to my original question: "which diodes should I use?"

Wiki suggests Shotsky, either 1N4004, 1N4005 or 1N4007 power diodes as flyback diodes. How do I determine the best one for this application? I have no clue how to do the math, so I'm looking for a "gimmee" here...

Thanks,

~ Blank Stare

Oh, and what the heck is a "FET"?...

 

[(*steve*)]

The diode across the motor should be rated for something in excess of 12V and 20A. A 50V diode should be more than sufficient. Doesn't need to be anything really special. It will almost certainly be a type of diode that is intended to be mounted on a heatsink, however that will not be required in this instance. Here is one selected almost at random (I just searched for through-hole diodes 20A or more and 50A or more and this one came up first).

Beware of diodes with metal tabs or cases as they are most often connected to one end of the diode or the other and you don't want to short things out.

The diode across the relay can be something a lot simpler. 50V 1A or greater. A 1N4001 would be fine.

The diode is not needed across the relay because it is the motor that is generating the voltage spike and the diode essentially shorts it out.

No need to know what a FET is (it's a Field Effect Transistor, sometimes used to switch the relay or even the motor. It is much more easily damaged by spikes than a switch)

 

[CDRIVE]

If relay contact destruction is the issue, then short of insuring that the relay is sufficiently sized, I would think a snubber would be more appropriate. Unless I'm mistaken and unlike a common inductor, I would expect the free wheeling EMF developed by a brush DC motor to be the same polarity as the source. Hence, not Back EMF.
Am I all wet?

Chris

Edit: I just looked at the schematic again and maybe I misinterpreted the function of the Diode. If the object of the Diode is to dump this EMF back into the source then yes a Diode does make sense. That is after it's flipped, as Steve said.

I don't want to confuse the OP any more than he already is but after reading the replies since I posted this quote I can't help but reiterate it.

Chris

 

[BobK]

V = L dI/dt

The voltage across the inductor is related to the rate of change of the current. So it is opposite of the direction of the voltage that drives the motor, because when you cut off the supply to the motor, dI/dt is negative, i.e. the current is decreasing. That is why the diode has to be backwards in order to conduct.

Bob

 

[CDRIVE]

V = L dI/dt

The voltage across the inductor is related to the rate of change of the current. So it is opposite of the direction of the voltage that drives the motor, because when you cut off the supply to the motor, dI/dt is negative, i.e. the current is decreasing. That is why the diode has to be backwards in order to conduct.

Bob

Bob, that's what I'm questioning. A DC motor isn't a simple inductor. If the source is removed while the armature is rotating I would expect it to operate as a generator with the polarity identical to the original source. One of the most basic DC motor experiments involved driving the motor from another mechanical source and using it as generator to light a lamp.

Chris

 

[BobK]

Presumably there are both effects going on, but it is the collapsing field that kills electronics, it can create an extremely high voltage, enough to arc across the relay contacts in this case.

Bob

 

[CDRIVE]

While I won't discount that BEMF is causing the failure I'm skeptical because a starter motor can pull > 300A and I know of no diode used with them to protect the solenoid contacts.

What does the control circuit look like? I'm talking about the relay control voltage. Is it a simple air pressure (mechanical) switch that supplies the relay coil? Have you measured the coil voltage? Is this pump an after-market device? Is this an air horn, air brakes, ..other? If it's an after-market device, can you provide a link to the product description?

Chris

 

[CocaCola]

While I won't discount that BEMF is causing the failure I'm skeptical because a starter motor can pull > 300A and I know of no diode used with them to protect the solenoid contacts.

Well to be fair a starter solenoid is not a run of the mill relay, kind of an Apple to Oranges comparison... The contacts and lugs on a starter solenoid are huge and made to take the abuse, not so with most conventional relays...