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Old 9th Oct 2018, 9:18 am   #1
cathoderay57
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Default Miniature relay Suppressor

Hello,

I have built a timer circuit that switches a miniature 12v relay (RS Components 793-3777). The relay contacts are turning a 230v AC motor on and off. The relay contacts are rated at 10A whereas the motor is drawing a steady state load of less than 0.5A as measured on the AC range of a multimeter. Question is, bearing in mind this is an inductive load, is it worthwhile putting a suppressor capacitor with series resistor across the relay contacts? With a purely resistive load I wouldn't bother but with an inductive load I thought it prudent to seek advice. The RS datasheet recommends use of a diode suppressor for DC13 loads but is silent on other loads. I do not know precisely what type of motor so I can only deduce it is in the AC-7A or AC-7B load category. Cheers, Jerry
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Old 9th Oct 2018, 10:34 am   #2
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Default Re: Miniature relay Suppressor

I would personally add some snubbing, if only just a capacitor across the contacts. The transients can be very high on switch off for inductive loads, and as I understand it as the contacts disengage the transient high voltage will be enough for a spark to jump the gap, and over time carbonise (if that's a word!) the contacts. You can see the effect displayed quite nicely on even a piddly little record deck motor (On the Garrard decks which have switch contacts under the platter with transparent covers..)

As always It will be interesting to see what learned forum folk can offer on this topic
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Old 9th Oct 2018, 10:44 am   #3
G8HQP Dave
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Default Re: Miniature relay Suppressor

Yes, a cap or snubber would lengthen the life of the relay contact (and reduce RF interference).
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Old 9th Oct 2018, 2:14 pm   #4
cathoderay57
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Default Re: Miniature relay Suppressor

Thanks for the replies. I have some spare Class X2 0.1uF 275v AC caps. Is one of those across the contacts likely to do the job?

Cheers, Jerry
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Old 9th Oct 2018, 3:25 pm   #5
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Default Re: Miniature relay Suppressor

I'm sure it will help. Just be aware that it will pass some current when the relay is off, so beware if fiddling around the motor connections.
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Old 9th Oct 2018, 10:07 pm   #6
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Default Re: Miniature relay Suppressor

Quote:
Originally Posted by cathoderay57 View Post
Thanks for the replies. I have some spare Class X2 0.1uF 275v AC caps. Is one of those across the contacts likely to do the job?

Cheers, Jerry
One experiment you could do, after you have used it for a year, depending on how often the motor goes on & off, is to check the uF value of the X2 capacitor. In this application (if it is a typical metalized paper type) you will probably find the uF capacity has dropped down, because with each snubbing event the film is often slowly destroyed. I have given up on these now and I would simply use a 0.1uF 1000V or 1500V DC rated poly or MKP cap as the snubber, or a 1200V 0.1uF capacitor rated for use in a colour TV's line output stage as a tuning capacitor , is a very good choice as they have a very low ESR. These caps will all be bigger and bulkier than the X2 cap but they won't degrade with time. A 1000V rated oil filled capacitor is also a good choice.
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Old 10th Oct 2018, 10:13 am   #7
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Default Re: Miniature relay Suppressor

Id have thought the requirement for Class X is not as clear here, as they're primarily designed to be across the line, where a dead short can be incredibly dangerous and a fire risk.

I suppose a non class X could develop shorts over time and generate significant heat to be a mild fire risk or melt said capacitor, but if in a metal or other non flammable enclosure and the application fused as conservatively as possible this all mitigates that risk. What do you think Argus? Perhaps such a short would burn itself out?
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Old 10th Oct 2018, 11:36 am   #8
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Default Re: Miniature relay Suppressor

Saying that, current draw is going to be limited by the electric motor that's being driven...
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Old 10th Oct 2018, 1:24 pm   #9
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Default Re: Miniature relay Suppressor

I would be more tempted to put a voltage surge suppressor in parallel with the motor coil.
That would be out of circuit when the relay contact is open so that the capacitor life problem is eliminated along with the spark erosion problem.
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Old 10th Oct 2018, 2:12 pm   #10
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Default Re: Miniature relay Suppressor

Quote:
Originally Posted by PsychMan View Post

I suppose a non class X could develop shorts over time and generate significant heat to be a mild fire risk or melt said capacitor, but if in a metal or other non flammable enclosure and the application fused as conservatively as possible this all mitigates that risk. What do you think Argus? Perhaps such a short would burn itself out?
Whether caps are X2 rated or not and supposedly have fire "risk mitigating properties" as far as I see it, its a bit of a have.

All plastic capacitors, X2 rated or otherwise can burn up, if the current is not limited the energy is there. I have seen X2 caps do it.

So I simply go for very high voltage rated caps >1000V for any mains filtering and have fusing and don't get any trouble.

But a cynical view is this:

The world wanted cheaper and smaller capacitors, so they went to plastics and films. This was a downhill step for everything other than size & economy. The reality is, you cannot beat the metal enclosed oil capacitors from yesteryear for what amounts to zero fire risk and near total reliability.

So if you selected a metal cased cap like this, of a high voltage rating, you have both a much lower fire risk than any modern X2 capacitor and total reliability in a 230V mains application:

https://www.ebay.com/itm/Capacitor-O...t2gP:rk:2:pf:0
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Old 10th Oct 2018, 8:41 pm   #11
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Default Re: Miniature relay Suppressor

Hi Gents, a capacitor directly across the relay contacts is not a good idea on DC circuits and is problematical on AC. A resistor is series with the cap is a better idea.

If at the instant of contact closure the AC is at its peak value and the cap is fully charged, there is a great danger of the contacts welding due to contact bounce as they close.

Ed
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Old 10th Oct 2018, 9:08 pm   #12
cathoderay57
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Default Re: Miniature relay Suppressor

Hi Ed, If I'm using a 0.1uF capacitor what value resistor do you suggest? Perhaps I would be better putting the filter across the motor (mains live to neutral) rather than the switch contacts. Do you reckon that would work?
Cheers, Jerry

Last edited by cathoderay57; 10th Oct 2018 at 9:15 pm.
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Old 10th Oct 2018, 10:37 pm   #13
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Default Re: Miniature relay Suppressor

If space is a problem and you don't mind the extra expense than look at snubbers on Farnell website e.g.
https://uk.farnell.com/roxburgh/re12...-20/dp/2336109
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Old 11th Oct 2018, 7:24 pm   #14
Ed_Dinning
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Default Re: Miniature relay Suppressor

Hi Jerry, a rough and ready guide would be to give the C-R time constant a value of about 1/10th of a mains 1/2 cycle, say 1mS, so with 0.1uF we get 100R. Use a carbon comp resistor, or you can use a WW as the inductance will not matter much at 50Hz.

Ed
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Old 11th Oct 2018, 8:03 pm   #15
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Default Re: Miniature relay Suppressor

In the 80's our company was requested to look at the problem of Beer Cooler compressor motors killing the relay switch module. The Peak voltage was horrendous as the contacts opened, killing the relay drive transistor, thus keeping the relay on, thus the Ice bank becoming solid ice.
We had little room to fit an R/C snubber circuit, thus we bought many 1000 VDR's (voltage dependant resistors)... I cannot remember the value at which they conducted, but Farnells were well happy. We drilled two holes one either side of the relay/ motor contacts.
As I say we retro fitted many 1000, NO returns... Eventually IMI paxman who made the coolers, had them fitted at source.
The hardest part of the whole operation was removing the pcb that was mounted in a tight fitting plastic case, a lot of sealant was used in some versions, and none in others..
I seem to remember the VDR's were rated at 300/400V.... but as I say.. its a long time ago.
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Last edited by Wendymott; 11th Oct 2018 at 8:05 pm. Reason: Typo correction
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Old 11th Oct 2018, 9:07 pm   #16
cathoderay57
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Default Re: Miniature relay Suppressor

Hello,
Thanks for all the responses - useful tips sand guidance. I'm going to try George's suggestion of the Farnell snubber, which incorporates a resistor the same value as Ed calculated. I'll connect across the load and see how it goes.
Cheers, Jerry
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Old 12th Oct 2018, 2:33 pm   #17
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Default Re: Miniature relay Suppressor

Quote:
Originally Posted by Wendymott View Post
In the 80's our company was requested to look at the problem of Beer Cooler compressor motors killing the relay switch module. The Peak voltage was horrendous as the contacts opened, killing the relay drive transistor, thus keeping the relay on, thus the Ice bank becoming solid ice.
We had little room to fit an R/C snubber circuit, thus we bought many 1000 VDR's (voltage dependant resistors)... I cannot remember the value at which they conducted, but Farnells were well happy. We drilled two holes one either side of the relay/ motor contacts.
As I say we retro fitted many 1000, NO returns... Eventually IMI paxman who made the coolers, had them fitted at source.
The hardest part of the whole operation was removing the pcb that was mounted in a tight fitting plastic case, a lot of sealant was used in some versions, and none in others..
I seem to remember the VDR's were rated at 300/400V.... but as I say.. its a long time ago.
So they were installed across the relay contacts. I assume these are metal oxide varistors (MOVs)? I wonder what kind of service life such a component would have as they apparently break down gradually with use. Your comment suggests they last very well indeed with repeated cycling of the compressors.

I used one in a similar application (but across the load) for a fridge compressor with a solid state relay. I was concerned about longevity though so removed it. Touch wood, the solid state relay has held up just fine. Though it has an integral snubber and is rated for industrial use (50 amps), its also zero crossing. Been running for months in this configuration and no problems yet

Last edited by PsychMan; 12th Oct 2018 at 2:59 pm.
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Old 13th Oct 2018, 12:48 am   #18
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Default Re: Miniature relay Suppressor

MOVs are cheap and I have never heard of one failing in normal use.
I have seen a 110V one that had blown up when connected to 240V.
https://uk.farnell.com/vishay/vdrs07...1187048?st=VDR
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Old 13th Oct 2018, 11:50 am   #19
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Default Re: Miniature relay Suppressor

Hyas PsychMan and Refugee.... Yes MOV's.... / VDR's The back EMF when the contacts opened, due to the compressor motor and maybe the mains at full peak... was a heck of a flash across the contacts.. Cured in an instant.
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Old 14th Oct 2018, 2:58 am   #20
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Default Re: Miniature relay Suppressor

Quote:
Originally Posted by PsychMan View Post
Id have thought the requirement for Class X is not as clear here, as they're primarily designed to be across the line, where a dead short can be incredibly dangerous and a fire risk.

I suppose a non class X could develop shorts over time and generate significant heat to be a mild fire risk or melt said capacitor, but if in a metal or other non flammable enclosure and the application fused as conservatively as possible this all mitigates that risk. What do you think Argus? Perhaps such a short would burn itself out?
A class X may develop shorts as well, most important requirement is that it shouldn't go up in flames too enthousiastically and of course that it should be rated for mains voltage. Most series are not rated for anything besides across the mains applications by their manufacturers (except of course RIFA MP, which are equally suited to produce smoke in every possible application). In any case, you should use an impulse rated polypropylene capacitor in applications like this, 1000VDC working voltage should be plenty. X2 ones (be it paper or polypropylene) will "self heal" which is in this case equivalent to self destruct.
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