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Old 16th December 2012, 11:10 AM   #11
dmills is offline dmills  United Kingdom
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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.
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Old 16th December 2012, 11:14 AM   #12
freax is offline freax  Australia
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ah sweet thanks for that very useful information Dan, I'll be looking into both of them.
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Old 16th December 2012, 11:19 AM   #13
FoMoCo is offline FoMoCo  United States
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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.
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Old 16th December 2012, 11:26 AM   #14
freax is offline freax  Australia
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~~ 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?
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Last edited by freax; 16th December 2012 at 11:54 AM.
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Old 16th December 2012, 11:47 AM   #15
dmills is offline dmills  United Kingdom
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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).
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Old 16th December 2012, 11:58 AM   #16
dmills is offline dmills  United Kingdom
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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.
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Old 16th December 2012, 12:03 PM   #17
freax is offline freax  Australia
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Quote:
Originally Posted by dmills View Post
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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Last edited by freax; 16th December 2012 at 12:25 PM.
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Old 16th December 2012, 01:41 PM   #18
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MOVs are fairly fast but with limited overload cpacity. GDTs are slow but very robust so I like to use two stage protection with a GDT followed by a MOV, all fused
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Old 18th December 2012, 04:02 AM   #19
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Bigger diameter MOV will survive more abuse before it opens.

Gajanan Phadte
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Old 18th December 2012, 12:53 PM   #20
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Unfortunately bigger/higher rating leads to higher leakage and larger capacitance. These can be serious problems in protecting telephone lines and other impedance sensitive applications.
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