back emf supression w/ diode across solenoid coil. 5

[jcorb5]

I am looking into the failure (shorting across) of an MR760 diode that was installed in parallel with a solenoid valve to suppress back emf. The solenoid coil is 35.1 watt, 465 ohms, 35.1 watt and 22 Henries. What I am trying to do is find out how to size a diode for this application. The MR760 diode has shorted across in 2 different instances when being used in this application. What critical parameters should be used to select the right diode. Also should I be using something other than a diode to suppress the back EMF? Maybe a transorb?

The contact opening time of the Struthers Dunn relay that controls the solenoid is on the order of 1-2 milliseconds. The calculated EMF for the solenoid is –6206 VDC for 1 ms contact open time, -3102 VDC for 2 ms time contact open time, -2069 VDC for 3 ms contact open time, and 1551 VDC for 4 ms contact open time.

The MR760 diode is rated 1000 volts peak repetitive reverse voltage, working peak reverse voltage, DC blocking voltage. It is rated 1200 volts non-repetitive peak reverse voltage (half wave, single phase, 60 HZ peak, and 700 volts RMS reverse voltage.
It is rated 22Amps for average rectified forward current, 400Amps non repetitive peak surge current for 1 cycle,

Any thoughts on properly sizing a diode for this application?

 

[37pw56gy]

A resistor having a value between 1-5x the coil resistance might be a good alternative to the diode snub.

Advantages: Quicker, more positive release of the solenoid; simple resistor less susceptible to sudden failure caused by surges; no tendancy to rectify stray AC energy.

Disadvantages: Resistor size, power dissipation much greater than a diode.

MOV's are also a good, low-cost alternative. Do not consider use of a capacitor snub in this type of application.

 

[moturcu]

37pw56gy:
Depending on the size of the coil resistance, inductive effect of the coil resistance will create another di/dt effect. Some applications may require another clamping diode accross resistance.
So bottom line, well sized diode would be better solution.

 

[electricpete]

I don't think I have ever seen a diode by itself... always a diode in series with a resistor connected accross a dc coil.

Without that resistor you at least one problem - full current flowing through the diode for long period of time until it dissipates in the coil resistance. Also the resulting slow dropout time may not be good for some applicaitons

 

[electricpete]

Wow, I didn't see that is a hefty diode. 22A continuous rating? It must be a big one.

 

[electricpete]

Your normal current is on the order of SQRT(35w/465ohms) ~0.3A? Are you sure there is a 22A diode in there?

 

[cbarn24050]

Hi, the diode only needs to be rated at the supply voltage and coil current.

 

[jcorb5]

Electripete,
thanks for the reply.
you are correct, the coil typically draws .28 amps (calculated). the diode is connected in parallel to the coil
The diode normally is reversed biased when the coil is energized (125 VDC) and then the maximum reverse current is between 25 microamps (junction @25 degrees C) and 1 milliamp (jxn at 100 degrees C).
The maximum forward current is 22 amps.
When the contact opens and the coil collapses the voltage is reversed and the diode is forward biased, at this time it can handle 22 amps to "bleed off" the back EMF.
To repeat myself, the only time the diode is forward biased at 22 amps is when the upstream contact is opening and the coil field is collapsing and bleeding through the diode.
does that make more sense? do you have any advice on sizing the diode and resistor for this particular coil??

 

[electricpete]

You have exceeded my knowledge. I have always seen the resistor in series but since you have huge margin in your diode continuous current rating it doesn't seem necessary.

 

[jbartos]

Suggestion: Visit

http://www.leachintl2.com/english/english2/vol6/properties/how7.htm

http://www.surgex.com/importance.html

http://www.brunel.ac.uk/depts/des/bureau/publish/danpic/danpic.htm

(for a signle diode 1N4001 suppressing back emf)

 

[moturcu]

jcorb5

First of all, you should determine what kills the diode. is it over current or over voltage. From the number you have given, both are possible. Over current and related over temperature kills diode quitely but over voltage kills it with an explosion. This might be the clue.

If over voltage kills, based on your back emf calculation you have to go with a higher voltage rated diode. If you you do not like it you should look for other snubber solutions adapted from power electronics suchs RC accross the switch etc.

IF the over current is killer, after switch opens current changes as
I=Io(1-exp-(R/L)t).

By integrating this equation over a 5L/R distance, you can get average current by which your diodes needs to be sized.

I guess surge current spec is irrelevant for your case because inductor current will always change slowly and it is only 0,28A rated.

 

[cbarn24050]

hi jcorb5, why do you think 22 amps flows through the diode, and where do you think it comes from?

 

[nbucska]

When the switch opens the current tries to remain unchanged
i.e. the parallel diode will have one forward VTG ( ~.7V)
accross and the coil's R will dissipate the power.

<nbucska@pcperipherals.com>

 

[jcorb5]

Cbarn24050.
Thanks for the reply. My posting above was poorly written.
the 22 amps is the manufacturers rated value for forward current. (so are the other values) I believe when the coil field of a few thousands volts is collapsing that the 22 amps rating may be exceeded. When the coil field collapses the back emf, which is opposite polarity of applied voltage, forward biases the diode causing alot of current to flow.
The below hyper link which was provided by BrianR contains manufacturer technical data sheets for the failed diode.

http://www.glencoe.com/ps/ee/bsee/electricity/6e/student/data_sheet_pdfs/MR750.pdf

you may need to copy the address into your address box

http://www.glencoe.com/ps/ee/bsee/electricity/6e/student/data_sheet_pdfs/MR750.pdf

 

[cbarn24050]

hi jcorb5, so now youve switched from an imaginary 22amps to an imaginary few thousand volts! Where do you think that comes from? This is all very basic stuff, allmost any diode will work in this application.

 

[RickOzone]

It seems to me that the diode is certainly capable of passing the surge current since the solenoid current is only 0.28 Amps. Therefore it seems more likely the problem is an over voltage condition. Perhaps the recovery time of the diode is an issue. Maybe a faster diode would work better. Also, the contact opening may be chattering causing multiple transients.

A high voltage zener in parallel might help such as the 1N5388 (200 V). An RC snubber across the switch contact would probably help too.

 

[Warpspeed]

There seems to be a lot of confusion here folks !

What happens is that with the solenoid energized there is 125 volts applied, and about 290mA flowing through the solenoid. The diode sees 125v across it in the reverse direction.

Now when power is removed, the 290mA flows through the diode, which has about an 0.6v drop. So in an ideal world you would only need a 125volt 290mA diode to do the job.

The stored energy in the solenoid is 0.5LI^0.5
0.5 x 22 Henries x 290mA squared or 0.925 Joules.

So 0.925 watt seconds is dissipated each time the solenoid releases into the dc resistance of the solenoid. This heating energy does not go into the diode.

So the ratings of the specified diode should be far in excess of what is actually required.

BUT there is one other factor not included in the above.

It is assumed that the 125 volt supply is only 125 volts. What if there are very narrow high voltage spikes in excess of 1Kv on the 125 volt rail ?

When the solenoid is energized these spikes could easily puncture the diode. Spikes like this can easily be generated by series inductance in the power supply, and the fast disconnection of a heavy load elsewhere in the system.

I might suggest you either fit a transorb instead of the diode, or a capacitor right across the 125v supply, or fix the overvoltage transients at the source. There is nothing wrong with the solenoid, or the diode. The problem is external.

 

[jbartos]

Suggestion: Reference:
John Markus &quot;Electronic Circuits Manual&quot;, McGraw-Hill, 1971 includes several circuits where a diode is applied to suppress the voltage spikes of a switched inductance and to dissipate the inductive load stored inductive energy.

 

[cbarn24050]

Hi jbartos, I think you have misread the reference you cited. Warpspeed's explaination is 100% correct.

 

[OperaHouse]

This is an example of taking a bad idea and beating it to death. If this diode is failing it is because of something else in the circuit besides this solenoid coil. I've found that these higher voltage DC control systems are usually found in old crane systems and the power is extreamly noisy. Most likely cause is a voltage spike that breaks down the diode when the solenoid is engaged. I would suggest that you place a 47 ohm resistor in series with the power to this solenoind with a .47 uf cap at the solenoid end to the common. This will basically filter the DC supply to the coil removing any spikes. A 1A 1000V diode is more than sufficient in this application. The company I worked for sold tens of thousands of these potted in epoxy. Adding a resistor in series with the diode is optional and is only used to increase speed. A MOV might work but it might have to be sized excessivly large to take the spikes from whatever else is on the line producing the noise.