Hi, I've just bought 10x of Vishay MOV's PN: RMV10-230K and now I'm stuck in a bind on how to use them.
Here is the datasheet: http://www.datasheetcatalog.org/datasheet/vishay/rmv10.pdf
I've read that wiring them in series is fine if you want to double or triple the clamping voltage, is this true?
And that wiring them in parallel requires matched sets.
Now what I want to know is, what clamping voltage should these 230K models be set to if I am going to be using them on the quite variable 210v-258v AC mains that I have here?
I've used the calculations on this site: How to Size a MOV for Surge Protection | eHow.com
And come up with a clamping voltage of 1414.427 v
Here is my working:
250v RMS / 0.707 = 353.6067892503536
353.606 x 4 = 1414.427
Now does that mean that I will need to use 3x wired in series of the 230K model for a clamping voltage of 1785v which I think is too high, or should I consider buying other types/models? Or maybe even just using 2x in series for a clamping voltage of 1190v? Or will that be too low?
Thank you on your advice.
Here is the datasheet: http://www.datasheetcatalog.org/datasheet/vishay/rmv10.pdf
I've read that wiring them in series is fine if you want to double or triple the clamping voltage, is this true?
And that wiring them in parallel requires matched sets.
Now what I want to know is, what clamping voltage should these 230K models be set to if I am going to be using them on the quite variable 210v-258v AC mains that I have here?
I've used the calculations on this site: How to Size a MOV for Surge Protection | eHow.com
And come up with a clamping voltage of 1414.427 v
Here is my working:
250v RMS / 0.707 = 353.6067892503536
353.606 x 4 = 1414.427
Now does that mean that I will need to use 3x wired in series of the 230K model for a clamping voltage of 1785v which I think is too high, or should I consider buying other types/models? Or maybe even just using 2x in series for a clamping voltage of 1190v? Or will that be too low?
Thank you on your advice.
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A MOV does not operate like a Zener.
It does not "clamp" at a voltage.
The MOV presents a very high resistance at normal or rated voltage.
When the voltage exceeds the rated voltage by the specified amount, the resistance plummets to a very low value that allows the kA snubbing current to pass.
Will this type of resistance behaviour actually work when series connecting two or more devices?
230k is for supply voltages that never exceed 230Vac. That implies usage on supplies that are less than a nominal 216V .
275k would be used on 240Vac. These nominal 240Vac supplies can rise to 254Vac. The MOV must not operate when the supply reaches 254Vac. 250k is not suitable for 240Vac supplies.
It does not "clamp" at a voltage.
The MOV presents a very high resistance at normal or rated voltage.
When the voltage exceeds the rated voltage by the specified amount, the resistance plummets to a very low value that allows the kA snubbing current to pass.
Will this type of resistance behaviour actually work when series connecting two or more devices?
230k is for supply voltages that never exceed 230Vac. That implies usage on supplies that are less than a nominal 216V .
275k would be used on 240Vac. These nominal 240Vac supplies can rise to 254Vac. The MOV must not operate when the supply reaches 254Vac. 250k is not suitable for 240Vac supplies.
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The eHow page suggests 4x peak. The manufacturers datasheet works on the basis of 2x peak. On that basis even using two in series you are still clamping at a much higher peak voltage than the manufacturer suggests. I guess it depends on what you are protecting; how sensitive is it to overvoltage?
The usual problem is things like rectifier diodes, which are usually recommended to be chosen to cope with a 2x normal peak on the mains. This fits with the MOV datasheet.
I think the correct MOV is the 250k. Given that you have already bought the 230k, you could use two of them provided that the equipment you are protecting can cope with 3x or 4x overvoltage. Or use one 230k (protected by a fuse), and expect to have to replace it from time to time.
The usual problem is things like rectifier diodes, which are usually recommended to be chosen to cope with a 2x normal peak on the mains. This fits with the MOV datasheet.
I think the correct MOV is the 250k. Given that you have already bought the 230k, you could use two of them provided that the equipment you are protecting can cope with 3x or 4x overvoltage. Or use one 230k (protected by a fuse), and expect to have to replace it from time to time.
no matter, I'm sure I can reuse the 230K's for radio antenna protection and just buy some 250K's
The load to be protected is a 20 amp 13.8v linear power supply with a 600va transformer and soft start circuit based upon project 77: http://sound.westhost.com/project77.htm
The load to be protected is a 20 amp 13.8v linear power supply with a 600va transformer and soft start circuit based upon project 77: http://sound.westhost.com/project77.htm
I should imagine that a fuse placed before the MOV would blow in the event of a MOV conducting if its flow current is sufficiently high for a long enough period?
What would be designed for a sustained overvoltage? A Gas Discharge Tube?
Surge protector - Wikipedia, the free encyclopedia
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For radio antenna protection I usually favour a GDT with a static bleed resistor or (if not massively broadband) an inductor, MOVs can be rather capacitive at HF.
You can usefully place a high voltage GDT at the aerial socket with a much lower voltage one on the rx path after the T/R relay (if a trancever).
A good place for further protection is after the rx BPF bank and just before the first mixer as the BPF will limit the rise time of the energy at this point.
Setting things up so that the recever is disconnected from the aerial socket when the rig is powered off is a good move (The BPF switching relays will help here), as is setting up the T/R relay such that when it is powered off the aerial is connected to the PA, this usually being more robust then the first mixer.
On MOVs, dont use ones rated too close to the nominal mains supply, and remember that in a conventional linear power supply, transformer leakage inductance and possibly saturation will tend to limit the level of transients making it through the iron anyway.
Regards, Dan.
You can usefully place a high voltage GDT at the aerial socket with a much lower voltage one on the rx path after the T/R relay (if a trancever).
A good place for further protection is after the rx BPF bank and just before the first mixer as the BPF will limit the rise time of the energy at this point.
Setting things up so that the recever is disconnected from the aerial socket when the rig is powered off is a good move (The BPF switching relays will help here), as is setting up the T/R relay such that when it is powered off the aerial is connected to the PA, this usually being more robust then the first mixer.
On MOVs, dont use ones rated too close to the nominal mains supply, and remember that in a conventional linear power supply, transformer leakage inductance and possibly saturation will tend to limit the level of transients making it through the iron anyway.
Regards, Dan.
Long term overvoltage is what core saturation and a fuse is for.....
Actually you can get overvoltage relays which will drop out on excessive line voltage, they are usful in things like power distribution for temprary events where accidents involving the loss of neutral on a three phase supply are not exactly unheard of.
Regards, Dan.
Actually you can get overvoltage relays which will drop out on excessive line voltage, they are usful in things like power distribution for temprary events where accidents involving the loss of neutral on a three phase supply are not exactly unheard of.
Regards, Dan.
One thing to keep in mind: An MOV does not always offer good protection against overvoltage.
For one they usually fail open and you don't know it until the piece of equipment fails from lack of protection. How many people check their MOVs?
A senior engineer that I worked with always stated that the only purpose for an MOV was so that we could tell that the customer overvoltaged the equipment. If the MOV was blown to pieces we'd know that it was overvoltaged. I'd tend to agree with his observations.
I'm not trying to convince you to not use an MOV, but keep in mind that they have limitations.
For one they usually fail open and you don't know it until the piece of equipment fails from lack of protection. How many people check their MOVs?
A senior engineer that I worked with always stated that the only purpose for an MOV was so that we could tell that the customer overvoltaged the equipment. If the MOV was blown to pieces we'd know that it was overvoltaged. I'd tend to agree with his observations.
I'm not trying to convince you to not use an MOV, but keep in mind that they have limitations.
~~ Level 1:
Its better than nothing, if the power supply is operating during a thunderstorm and a lightning strike hits the power line (rare but could happen) then I don't want the main transformer to be scorched.
That is basic level 1 protection, anything else (as described below) is a bonus.
~~ Level 2:
The power supply doesn't need to be operational during one of these failure modes, it just needs to be reversible and resettable to its previous state, even if it involves replacing MOV's.
I don't know what exactly the power source will be, it could be anything from a 2-stroke generator to a 4-stroke generator to a high-end 50kW generator to a hydroelectric power station.
And it could vary by a great degree from 100v during a sustained brownout to anywhere up to including a sustained 300vAC.
I want to provide this project the best possible protection within a restricted budget against anything that those supplies can throw at it, because whats on the other side of this power supply is very sensitive 13.8v DC equipment.
And the cheapest method to do this seems to be to construct a circuit which will catch the fast high voltage transients (MOV's + GDT's) and shut off the supply to the transformer during a sustained undervolt or overvolt situation by sensing what the voltage will be on the mains and taking appropriate action/throwing relays and waiting for a while.
~~ :Level 3:
Electrostatic protection of the final transistors.
As is with any lightning strikes no doubt some of that high voltage is going to make it through to the chassis of the power supply and meet up with the very sensitive TO-3 packaged 2N3771 transistors mounted onto a heatsink.
I was thinking that inorder to provide a high level of electrostatic protection that I should be able to float the entire power supply's regulation transistors and voltage regulator from the chassis with the aid of insulating the legs of the transistors from the heatsinks and providing mats to go underneath the PCB and having a significant distance to-from each board's spacer (on perfboard) through the use of plexiglass and nylon spacers.
Or maybe even building the etnire project ontop of plexiglass and using point to point wiring for the control circuitry....
And these transistors could be damaged by a buildup of static charge on the chassis, especially if the earth leg is all of a sudden floating (taken out by a lightning strike), or even better yet, comes up the earth lead and back into the power supply...
This is very likely if for example in the situation where a generator is used and the soil is inadequate to provide a level of protection needed, a thunderstorm rolls in and builds up a large static charge on all of the equipment, now how to protect those transistors against that? which one of their legs might be earthed through the DC side connected equipment.
Putting MOV's on the output would only protect during the event that the chassis is earthed, however floating the transistors and all associated circuitry would give the static electricity time to discharge slowly and not destructivley (and taking the transistors out in the process).
I'm hoping to also combat this by earthing the chassis seperatley, however the transsitors might not make it anyway, so I was hoping to provide a level of insulation between the chassis and the transistors.
What do you guys think?
Its better than nothing, if the power supply is operating during a thunderstorm and a lightning strike hits the power line (rare but could happen) then I don't want the main transformer to be scorched.
That is basic level 1 protection, anything else (as described below) is a bonus.
~~ Level 2:
The power supply doesn't need to be operational during one of these failure modes, it just needs to be reversible and resettable to its previous state, even if it involves replacing MOV's.
I don't know what exactly the power source will be, it could be anything from a 2-stroke generator to a 4-stroke generator to a high-end 50kW generator to a hydroelectric power station.
And it could vary by a great degree from 100v during a sustained brownout to anywhere up to including a sustained 300vAC.
I want to provide this project the best possible protection within a restricted budget against anything that those supplies can throw at it, because whats on the other side of this power supply is very sensitive 13.8v DC equipment.
And the cheapest method to do this seems to be to construct a circuit which will catch the fast high voltage transients (MOV's + GDT's) and shut off the supply to the transformer during a sustained undervolt or overvolt situation by sensing what the voltage will be on the mains and taking appropriate action/throwing relays and waiting for a while.
~~ :Level 3:
Electrostatic protection of the final transistors.
As is with any lightning strikes no doubt some of that high voltage is going to make it through to the chassis of the power supply and meet up with the very sensitive TO-3 packaged 2N3771 transistors mounted onto a heatsink.
I was thinking that inorder to provide a high level of electrostatic protection that I should be able to float the entire power supply's regulation transistors and voltage regulator from the chassis with the aid of insulating the legs of the transistors from the heatsinks and providing mats to go underneath the PCB and having a significant distance to-from each board's spacer (on perfboard) through the use of plexiglass and nylon spacers.
Or maybe even building the etnire project ontop of plexiglass and using point to point wiring for the control circuitry....
And these transistors could be damaged by a buildup of static charge on the chassis, especially if the earth leg is all of a sudden floating (taken out by a lightning strike), or even better yet, comes up the earth lead and back into the power supply...
This is very likely if for example in the situation where a generator is used and the soil is inadequate to provide a level of protection needed, a thunderstorm rolls in and builds up a large static charge on all of the equipment, now how to protect those transistors against that? which one of their legs might be earthed through the DC side connected equipment.
Putting MOV's on the output would only protect during the event that the chassis is earthed, however floating the transistors and all associated circuitry would give the static electricity time to discharge slowly and not destructivley (and taking the transistors out in the process).
I'm hoping to also combat this by earthing the chassis seperatley, however the transsitors might not make it anyway, so I was hoping to provide a level of insulation between the chassis and the transistors.
What do you guys think?
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One other thing worth having if running off smallish rotating machines is an under frequency trip on the distribution gear.
This protects against an underspeed condition caused for exampe by a partially blocked fuel filter causing the transformer core to saturate.
One source for such things: BENDER - Products - Protective Relays - Voltage and Frequency Relays
Note that these are not about transient protection but about sustained error conditions in the distribution network and are probably best built into the distribution gear rather then the final load. I have mine in a box fitted with a shunt trip breaker that gets fitted between the generator and my downstream distribution gear.
For catastrophic PSU blowups a crowbar on the output is probably a good plan after the output fuse, that way the most common destructive blowup in a 13.8V supply (Shorted pass transistor) stands some chance of being caught.
Regards, Dan (Who uses 50V for his radios, much more reasonable drain impedance that way).
This protects against an underspeed condition caused for exampe by a partially blocked fuel filter causing the transformer core to saturate.
One source for such things: BENDER - Products - Protective Relays - Voltage and Frequency Relays
Note that these are not about transient protection but about sustained error conditions in the distribution network and are probably best built into the distribution gear rather then the final load. I have mine in a box fitted with a shunt trip breaker that gets fitted between the generator and my downstream distribution gear.
For catastrophic PSU blowups a crowbar on the output is probably a good plan after the output fuse, that way the most common destructive blowup in a 13.8V supply (Shorted pass transistor) stands some chance of being caught.
Regards, Dan (Who uses 50V for his radios, much more reasonable drain impedance that way).
See W8JIs site for much discussion of grounding for lightning protection...
I think you are going the wrong way, the best apprroach for transient and surge protection is almost always to firmly bond everything together so that current can flow without inducing large transient voltages.
All cable screens should be bonded to mains earth (and each other) at the entry point, where there there should also be spark gaps installed to limit transient voltages, and this plate should then be used as the earth connection point for the protected gear.
IME any attempt to get clever with floating various bits of the circuit just causes pain, better to firmly bolt everything together.
Regards, Dan.
I think you are going the wrong way, the best apprroach for transient and surge protection is almost always to firmly bond everything together so that current can flow without inducing large transient voltages.
All cable screens should be bonded to mains earth (and each other) at the entry point, where there there should also be spark gaps installed to limit transient voltages, and this plate should then be used as the earth connection point for the protected gear.
IME any attempt to get clever with floating various bits of the circuit just causes pain, better to firmly bolt everything together.
Regards, Dan.
Understood, thank you.
I've checked out his site and have dedicated the rest of tonight to reading it.
http://www.w8ji.com/station_ground.htm
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