|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
9th Oct 2018, 9:18 am | #1 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
|
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 |
9th Oct 2018, 10:34 am | #2 |
Octode
Join Date: Jun 2013
Location: Fleet, Hampshire, UK
Posts: 1,764
|
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 |
9th Oct 2018, 10:44 am | #3 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
|
Re: Miniature relay Suppressor
Yes, a cap or snubber would lengthen the life of the relay contact (and reduce RF interference).
|
9th Oct 2018, 2:14 pm | #4 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
|
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 |
9th Oct 2018, 3:25 pm | #5 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
|
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.
|
9th Oct 2018, 10:07 pm | #6 |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
|
Re: Miniature relay Suppressor
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.
|
10th Oct 2018, 10:13 am | #7 |
Octode
Join Date: Jun 2013
Location: Fleet, Hampshire, UK
Posts: 1,764
|
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? |
10th Oct 2018, 11:36 am | #8 |
Octode
Join Date: Jun 2013
Location: Fleet, Hampshire, UK
Posts: 1,764
|
Re: Miniature relay Suppressor
Saying that, current draw is going to be limited by the electric motor that's being driven...
|
10th Oct 2018, 1:24 pm | #9 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,549
|
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. |
10th Oct 2018, 2:12 pm | #10 | |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
|
Re: Miniature relay Suppressor
Quote:
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 |
|
10th Oct 2018, 8:41 pm | #11 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,172
|
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 |
10th Oct 2018, 9:08 pm | #12 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
|
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. |
10th Oct 2018, 10:37 pm | #13 |
Heptode
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 760
|
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
__________________
George |
11th Oct 2018, 7:24 pm | #14 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,172
|
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 |
11th Oct 2018, 8:03 pm | #15 |
Octode
Join Date: Jun 2013
Location: Huddersfield, West Yorkshire, UK.
Posts: 1,782
|
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.
__________________
Should get out more. Regards Wendy G8BZY Last edited by Wendymott; 11th Oct 2018 at 8:05 pm. Reason: Typo correction |
11th Oct 2018, 9:07 pm | #16 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,364
|
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 |
12th Oct 2018, 2:33 pm | #17 | |
Octode
Join Date: Jun 2013
Location: Fleet, Hampshire, UK
Posts: 1,764
|
Re: Miniature relay Suppressor
Quote:
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. |
|
13th Oct 2018, 12:48 am | #18 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,549
|
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 |
13th Oct 2018, 11:50 am | #19 |
Octode
Join Date: Jun 2013
Location: Huddersfield, West Yorkshire, UK.
Posts: 1,782
|
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.
__________________
Should get out more. Regards Wendy G8BZY |
14th Oct 2018, 2:58 am | #20 | |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
|
Re: Miniature relay Suppressor
Quote:
|
|