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Woes with protection components
Woes with protection components
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Old 29th January 2009, 08:57 PM   #1
Atilla is offline Atilla  Norway
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Default Woes with protection components

So, I read a few things about mains filtering, inrush current protection, overvoltage surge protectors and the likes. So I went and prototyped myself a little something - a board that incorporates all three, which will be used in front of 2x120VA toroids to power a pretty standard LM3886 chipamp.

Well, I've had a lot of fun with this. Salvaged components were used, from power supplies big and small - I've a bajillion X rated caps. All varistors were new though.

Problem 1 - the NTC Thermistors for inrush current limiting. I've two of them, 5ohm, they look fairly big. And they're stone cold - they can't get warm with a soldering gun pressed against them, not to mention a few cycles of inrush current. So, they're just there, wasting power, I'd assume, not doing anything much. Can I even measure if there's any effect from them, without an osciloscope? What am I doing wrong with them? I do know I can live without them, because I've powered up the rig without them and it definitelly doesn't blow the fuses, despite the 240VA rating and the 40k uF capacitance. I know it's not too much to be concerned with, I just wanted to see how it works.

Problem 2 - surge voltage protection. I used an array of MOVs, rated for 230volts across live and neutral. The line is fused so on more than instanteneous overvoltage they'll short and vaporise the 2amp fuse. I'm fairly ok with that. The problem is that I decided to use an Epcos gas discharge tube to complement them. That one shorted immediatelly. So fast that the first time it literally evaporated the 2amp slo-blo fuses in the IEC socket, before I even realized what happened. I am obviously not using them right. They're paralel to the MOVs, located after them. I'm fairly clueless as to the functioning of those things however, if anyone has used them and knows what their deal is - any info is appreciated. By the way, the datasheet explicitly says they're meant to be used for mains applications, along with MOVs. But I definitelly didn't apply them right.
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Old 29th January 2009, 09:03 PM   #2
Atilla is offline Atilla  Norway
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By the way, last time i measured my circuits, before I added the filter, I had a 26v out of my 24v rated toroids and +-34.7 volts DC.

Today, with the filter, it was +-35.8 DC. On all four rails - 2 PSUs with 2 rectifier bridges.

I really havent changed anything else and I don't think my mains have been lower last time. Weird.
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Old 30th January 2009, 09:17 AM   #3
Bakmeel is offline Bakmeel  Netherlands
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1- NTC thermistors
Can be tricky components to use sometimes, but there are a few guidelines to use here and here and here. Most of these links are simply the application sheets from NTC manufacturers. To see the effect, try to see the difference with an analogue current meter (not a Digital Multimeter). If the inrush is significant enough, you should be able to see the spike with the needle flying to the corner A crude but effective means....

2- MOV
I have only seen gas discharge tubes in series with a resistor. I don't know the theory, but basically a gas discharge turns into a very low ohmic path once the ionisation of the gas is etablished. Think of gas discharge lamps... They always need a type of ballast to keep the discharge under control. Then, I have seen MOV's (or spark gaps as I call them) together with a series resistor, in parallel with the differential mode choke to eliminate current spikes.

Good luck and take care not to burn yourself
More Power Igor! More Power!
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Old 30th January 2009, 10:15 AM   #4
Atilla is offline Atilla  Norway
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Thanks a lot, looking at the extra datasheets you linked, it becomes apparent that I've misunderstood the way the NTC thermistor needs to be calculated. Mine are both of too low resistance and too high rated current to be of any effect. They'd be perfect in a massive SPS though That will be fixed.

The MOVs have different applications, but they do seem to work fine. The GDT is confusing, it is still supposed to trigger at higher voltage, however - you're correct. I've seen them in application in a surge protector and they did have series resistors connected. My datasheet doesn't say anything however, I'll need to consult a different one.
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Old 2nd February 2009, 03:42 AM   #5
TechGuy is offline TechGuy  United States
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One other protection device is the TVS diode also refered as Avalanche diodes that operate like Zener diodes but can handle far more current.

MOVs and GDTs have longer response times and usually have very high clamping voltages. Gas Discharge tubes take microseconds, MOVs take tens of nanoseconds, and TVS diodes take less than a nanosecond.

MOVs also have a very limited number of cycles before they break. They are typically used because the are very simple and cheap. FWIW: I try to avoid using MOVs because of this issue.

Another surge suppression method is the crowbar circuit, that uses a semiconductor switch (such as a SCR), that is designed to clamp very high surge currents.

The problem is that I decided to use an Epcos gas discharge tube to complement them. That one shorted immediatelly.
Its possible that your input voltage is above or near the clamping voltage. Are adding the GDT after rectificationa and DC filter caps? If you are then then voltage will be much higher than the mains output it 230*1.414 = 325V.

GDT clamp when the input voltage causes breakdown of the gas, causing it to ionize. This rapidily causes the gas to heat up into plasma, which has a much much lower resistance.

keep in mind that if you have a large transformer and there is some load present or filter caps on the transformer output, its going to cause some inductance leakage spikes in the primary side. These spikes could also cause the GDT to trip. You could add a input inductor to reduce power-on current inrush to reduce spiking. Its also possible that the GDT isnt the cause. Perhaps your filter caps are creating a very large current inrush that is exceeding the fuses rating.

Today, with the filter, it was +-35.8 DC
Sounds like there is some added capacitance.
26V*1.414 = 36.76V
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Old 2nd February 2009, 05:01 AM   #6
Atilla is offline Atilla  Norway
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I'm adding the MOVs and the GDT first thing on the board, before anything else, so when they conduct there won't be anything in the way of the high current.

The system works perfectly fine with all the load connected, including the amp. It is definitelly not its fault for blowing the fuses. It also works with MOVs only, which are rated for the same voltage. I know the principles of their opperation are slightly different, as well as their time to react to a surge, so I assume it is possible that the GDT manages to activete at turn-on, while the MOVs do not. I found one GDT rated 240V, with explicit note about its use in 230v mains surge protection, so I'll give it a shot.

I'm not too concerned with this protection scheme, I was just trying out things, essentially. But I'd rather have a fuse blown on a power surge immediatelly, so I'd like to settle on some sort of protection that will do that.
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Old 4th February 2009, 09:52 AM   #7
wine&dine is offline wine&dine  Austria
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Originally posted by Bakmeel
I have only seen gas discharge tubes in series with a resistor. I don't know the theory ...[/B]
Oh do tell ... ;-)

You run GDTs off a resistor if you want them to emit light, as in power-on indicators. As protective devices, they are supposed to short out and carry kiloamps (no kidding) once they are triggered - and obviously they should not be triggered at nominal line voltage.

As the original poster found out, the NTCs in question were oversized. From his description, the GTD are undersized.

Take a look at http://docs-europe.electrocomponents...6b80027f47.pdf . While there are 230V models, that number refers to the DC trigger voltage. Remember 230 * sqrt(2) = 325 and choose the 350V model instead.
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Old 4th February 2009, 10:37 AM   #8
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
In the UK we have a nominal 240Vac supply.
The maximum supply voltage is 254Vac.
250Vac protections are not suitable for our supplies, we use 275Vac rated components, whether X or Y rated caps or MOVs or TVS.
Some are specified as DC rating, so take care to use mains peak voltage for these DC rated components.
240Vac +6% ~=359Vpk
230Vac +10% ~=358Vpk.
regards Andrew T.
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Old 4th February 2009, 02:54 PM   #9
Atilla is offline Atilla  Norway
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Absolutely correct - what I used was rated 230Vdc, which will trigger immediatelly and blow the fuses. Spectacular.

What does work is the 240Vac model, which has a DC breakdown rating of 425V. This is a Compgap GDT and it has a little more clear datasheet.

Thanks for all the useful input!
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