"Voltrap" Transient Voltage Suppressor

[garyvin]

My company Currently has been using a device called a "Voltrap". From what I understand it is a bi-directional Selenium Diode. We use them in hazardous duty control cabinets to prevent a trasient voltage spike or flashover due to the inductive kick of a relay coil. These are getting very hard to find and are quite expensive. I have been hoping to find a cheaper alternative. The system voltage is usually 125 Vdc. Anyone have a better understanding of these and could point me in the right direction.

 

[Comcokid]

Use of Selenium transient protection devices dissapeared in the 60's and 70's. These consisted of a stack of selenium diode-type structure with a individual breakdown of about 25 volts.

Today, most transient protection is performed by MOV's (Metal Oxide Varistors) - a bi-directional type device, -OR- silicon transient protection devices (i.e. transzorbs)- available in unit and bidirectional configurations. Other available devices, but somewhat less common in use because they clamp the line once triggered consist of gas-tube and thysistor-tybe ("surgectors") devices.

I suggest you down-load some application notes on MOV type devices. MOVs are available in a wide range of voltages and power ratings.

 

[melone]

Can you put recirculation diodes on each of the inductive loads?

 

[itsmoked]

It would be good to ditch the Selenium diodes as they give off poisonous gas when they fail...

 

[sreid]

The 1.5KE series of TVS diodes would probably be an equivalent (or better) replacement.

http://www.vishay.com/diodes/protection-tvs-esd/pkg-1.5ke/

 

[felixc]

Do you still have the specifications of these Voltraps? It would be nice to have, to make a comparison of the response time and energy absorbing characteristics with other solutions.

 

[garyvin]

Thanks for the Information. I downloaded some spec sheets already and I see a couple that should work. I do not have any specs really on the Voltraps. Its one of those things where we have used them for years and years and noone knows anything about them. Its only now that someone brought it up since they are getting hard to find and more expensive because of it. Thanks again.

 

[garyvin]

Ok I got another question. I am looking over the spec sheets and I am having a hard time picking out what voltage to go with. The circuit will have 125 bolts DC. A couple of sources I found say to go 10% over rated. This would mean 137.5 volts DC. That seems a little close to me. Also I just found out the specs of a seal in realy pannel that almost does that same thing uses a MOV rated at 256 volts AC. That makes for aprox. 330 volts DC. I am unsure as to what to get. Is there a rule of thumb that would seen a little more practical here?

 

[Comcokid]

Your original selenium protection was not very quick acting or very accurate in clamp voltage. If it was sufficient for the design for such a long time, you probably have plenty of voltage margin.

MOV's subjected to frequent spikes tend to lower in voltage over time. thread248-62476 Your design should not be subjected to frequent spikes (I assume), but keeping the voltage higher than you think may be the way to go. If you had sensitive circuits, transzorbs or some multi-component approach would be the way to go so you could clamp accurately at a lower voltage.

The device using the 256 volt device may had done it so they could be use with in a wider range of applications, or to insure they could intermittently survive a high voltage situation such as an open neutral. Sometimes the approach of transient supression is to take the top off the transients and not to try to protect for the over-voltage situations. If the purpose is to protect from overvoltage, then high-power clamping with an inline fuse might be used. The designer of this 256 volt device may have not had that kind of protection in mind.

Without more information of what you are trying to protect, what the damage threshold of the devices on your 125 VDC power, and how likely a transient situation is in your application, it's a little hard to give specific suggestions. But, I would think a MOV with clamping in the range of 160 to 220 VDC might be what you would want.

 

[garyvin]

Unfortuntly I can't tell you too much about what we use it for. I can tell you the main purpose for it is to prevent a flashover of the relay contacts when they are tripped. This is used in a Hazardous environment so arcing is not only bad for the contacts but could be very dangerous. This is not something that will cycle many times. It will see maybe a few dozen trips throughout the life of the unit. Would this be bad for a MOV?

As for the recomendation I got for the 256V AC MOV I found out that is used in a device that can operate on many voltages up to 220V AC. That makes much more sense now. I do agree that something in range of about 180-200V DC makes good sense. I appreciate the help.

 

[sreid]

As this thread progresses and the questions go beyond simply replacing one "Zener" with another "Zener", I think you need to think about re-evaluating your needs. Arc supression and contact protection in HV DC circuits is more complicated than simple voltage clamping. The Book " Noise Reduction Techniques in Electronic Systems" by Henry Ott devotes a full chapter to this subject (Ch. 7). Simple, solid, practical advice fully backed by theory. Expensive but free by using an Interlibrary Book Loan and the "Where you Work Free Xerox Copying Perk."

http://www.amazon.com/exec/obidos/t...f=sr_1_1/104-2300972-6033505?v=glance&s=books

 

[itsmoked]

Yes and any kind of arc suppression add on in an incindiary environment is NOT GOING TO BE SAFE. All arc supression components are subject to explosive failure. You can not have arcing contacts that aren't blowing something up just because a zener or Voltrap gizmo is present!

Shud d d e r

 

[ScottyUK]

Sparksalot,

This equipment sounds like it should be in an ExD (flameproof) or ExP (pressurised) enclosure if it is spark-generating and within a hazardous area. As I understand the haz area regs in the UK - and probably the rest of the EU - prosecution under the ATEX legislation allows individuals to be prosecuted as well as companies and their directors. I'd be thinking long and hard before doing anything questionable in a haz area from a personal liability point of view, quite apart from the prospect of it going ka-boom one day.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[garyvin]

I understand all of your concerns and they have already been considered. The actualy device and relay are in an explosion proof enclosure. The problem lies in that the switch that controls the relay is not and cannot be. The main reason for this is to prevent the contacts of the switch from being welded together when it tries to open. The switch itself is sealed but there is always, as with anything however slim, the chance of dammage so a spark is not wanted. Again this is something that might trip only a handful of times in it life.