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?

 

[buzzp]

The higher votlages at switching can be attributed to the voltage equation for inductance V=L di/dt. Since the current rate of change is at its maximum (limited by circuit parameters such as stray capacitance and inductance) when the switch is open, the voltage can be a lot higher than the applied voltage. However, it does not reach this value in an instant, it takes time due to parameters (circuit capacitance and inductance). Often the switching time of the diode is adequate to 'limit' (energy disipated in the diode and the coil before reaching maximum voltage of inductor) the voltage (before gets to peak value) before any damage is done. The standard diode series (1N400x) are generally more than adequate for obtaining the results you want. There is something being overlooked (other transients,other built-in flyback protection in the solenoid, etc) that has not been communicated in this forum. I would suggest some measurements with a scope or if possible, put the diode in the circuit without de-energizing the load to see if failure occurs. This test is highly unlikely unless the diodes are failing rather quickly.
Using strictly diodes in this configuration is very common for DC coils. You may want to visit some mfg sites to check through there app notes such as

http://www.onsemi.com,

or phillips, etc. The solenoid mfg is probably the first place to check to make sure you don't have something unique that requires special considerations. Good luck and please post your results.

 

[jbartos]

Suggestion: Visit

http://www.hvca.com/advprodsearch2.jsp

for 600V 25A diode and add a series resistor, or
find a diode manufactured for the higher voltage and current, e.g.

http://us.st.com/stonline/books/ascii/docs/3179.htm

 

[OperaHouse]

The problem is not with the diode or the solenoid circuit. The spike which causes failure is coming from somewhere else. A small resistor might delay the eventual failure of the diode junction from breakdown. Three diodes in series might also give you enough reverse breakdown to survive. A capacitor from the 125VDC to common would also help but the capacitor would also be destroyed unless you use a high quality snubber capacitor. These have specs that say something like 5A @ 20KHZ and are double wound (two capacitors in series) to avoid corona effects. Standard metalized film capacitors will suffer end weld failures and punctured dielectric. The circuit I described in the previous post with the resistor and capacitor will protect a standard cap from destruction and filter out the large spikes from the solenoid circuit. The relay will operate on a slightly lower voltage which is just fine. The capacitor will supply enough additional current at initiation for reliable operation. Most electro-mechanical devices have a pull in voltage to hold ratio of 5:1 or better. Definitely there will be no change in performance with a 10% drop. Of course, I assume this is not being rapidly cycled. Dropping the solenoid voltage after activation is a common practice in the gluing industry to prevent hot coils from changing the viscosity at the glue head.

 

[jbartos]

Suggestions to Warpspeed (Automotive) Jun 10, 2003 marked ///\\There seems to be a lot of confusion here folks !
///Please, be specific about a lot of confusion here.\\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.
///True, however, very elementary.\\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.
///Apparently, what is meant here is a 125V 290mA diode circuit, perhaps with a voltage drop resistor.\\The stored energy in the solenoid is 0.5LI^0.5
///The equation needs a clarification.\\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.
///True, especially, if there is a resistor in series with the diode.\\So the ratings of the specified diode should be far in excess of what is actually required.
///Please, clarify.\\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.
///Depending on the diode reverse voltage spike rating.\\ 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.
///Not quite external since the original posting is concerned with the diode sizing. The diode experienced two shortings according to the original posting.\\\

 

[Nandos]

Jcorb5:

The Joules available is = L * I^2 = 22 H * .28^2 = 1.728 joules per pulse.

Having 125 volts at 0.28 amps a 1N4004 will do the job and is just one amp diode, since it will take 30 amps for 8 milliseconds.

A diode 200 volts, 1 amp should be suficient

You may have another problem, since the diode will clamp the voltage to 125.7 volts.

Regards

Nando

 

[ScottyUK]

Just a thought here, from the back-to-basic principles of passive components.

The coil inductance will, on removal of the supply voltage, attempt to keep the current of 290mA flowing through the coil resistance, and will do so by collapsing the magnetic field of the relay coil and thus releasing the stored energy within it. the current circultaes throught he forward-biased diode. The diode should therefore see a decaying current which is initially 290mA and falls exponentially toward zero. The forward voltdrop is approx. 0.7V.

The diode in the 'relay on' state will be reverse biased by the supply voltage, and for the 1N400* series the reverse leakage will be in sub-uA level.

At no point will the diode see 22A. It might however see a high transient voltage if the diode is a 'slow' type designed for low frequency rectification. It might be worth considering a faster type such as one of the UF540* series, although I am doubtful that response time is the issue as the coil doesn't store enough energy to exceed the I^2T rating of a 22A rectifier and cause damage.

The R-C type snubber networks mentioned earlier are typical for AC switching to reduce arcing; diode clamps are standard for DC coil switching.

I must ask, why was such a large rating (22A!) chosen for a clamp diode? Is this the manufacturer's design?

The only other questions I would ask are:

Where is the diode physically in relation to the coil? Ideally it should be directly connected across the coil.

Are there any large loads on the DC bus which can dump energy into the system, such as regenerating motors, any heavy inductive switching, etc? I would consider getting a fast 'scope onto the supply rail and set the single-shot trigger level at, say, 200V and see what is happening elsewhere on the supply, as the problem sounds as though it is external to the coil / diode.

 

[jbartos]

Suggestion: When it comes to basics, the solenoid valve consists of inductance and resistance (and some stray capacitance); the diode equivalent circuit includes resistance and capacitance, visit

http://www.tpub.com/neets/book11/45n.htm

for a diode approximate equivalent circuit