back emf supression w/ diode across solenoid coil.
back emf supression w/ diode across solenoid coil.
(OP)
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?
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?





RE: back emf supression w/ diode across solenoid coil.
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.
RE: back emf supression w/ diode across solenoid coil.
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.
RE: back emf supression w/ diode across solenoid 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
RE: back emf supression w/ diode across solenoid coil.
RE: back emf supression w/ diode across solenoid coil.
RE: back emf supression w/ diode across solenoid coil.
RE: back emf supression w/ diode across solenoid coil.
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??
RE: back emf supression w/ diode across solenoid coil.
RE: back emf supression w/ diode across solenoid coil.
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)
RE: back emf supression w/ diode across solenoid coil.
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.
RE: back emf supression w/ diode across solenoid coil.
RE: back emf supression w/ diode across solenoid coil.
i.e. the parallel diode will have one forward VTG ( ~.7V)
accross and the coil's R will dissipate the power.
<nbucska@pcperipherals.com>
RE: back emf supression w/ diode across solenoid coil.
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
RE: back emf supression w/ diode across solenoid coil.
RE: back emf supression w/ diode across solenoid coil.
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.
RE: back emf supression w/ diode across solenoid coil.
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.
RE: back emf supression w/ diode across solenoid coil.
John Markus "Electronic Circuits Manual", 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.
RE: back emf supression w/ diode across solenoid coil.
RE: back emf supression w/ diode across solenoid coil.
RE: back emf supression w/ diode across solenoid coil.
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 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.
RE: back emf supression w/ diode across solenoid coil.
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
RE: back emf supression w/ diode across solenoid coil.
RE: back emf supression w/ diode across solenoid coil.
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.\\\
RE: back emf supression w/ diode across solenoid coil.
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
RE: back emf supression w/ diode across solenoid coil.
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.
RE: back emf supression w/ diode across solenoid coil.
http://www.tpub.com/neets/book11/45n.htm
for a diode approximate equivalent circuit