capacitor in *series* with AC relay coil, is that common? - Page 1
Forum section: electronics / projects

author: JeanF
author_rank: Regular Contributor
timestamp: March 16, 2021, 06:08:57 pm
content: Hello,the other day, while disassembling an old Gardena THS 400 hedge trimmer that had its motor rather badly burned, I found this module in it along the universal motor:with only one small difference, on mine the two jumpers are actually inductors.TheDPDT relayis used to brake the motor when power is removed, as nicely shown in thispatentWhile the relay has a 220/240 V coil, mains is not directly connected to it, there is in fact a 0.1 µF class X2 cap in series with the coil.There's also a 0.047 µF one directly across mains input. As it turns out, the inductance of the coil (from datasheet) is about 100 H, which makes an impedance (at 50 Hz)of about 31.4 kOhm, while the impedance of the series cap is about 31.8 kOhm. The LC circuit is near resonance and that actually boosts the current through the coil! (I've measured around 50% increase).It’s the first time I see a cap in series with a relay coil like that. While trying to find references of this “trick” online, I found that my Google skills, albeit generally OK, are not good enough this time, I could only find (millions of) tutorials to use a cap to make a time delay with a DC coil relay.I can only guess it may be done like that to make sure the relay will stay closed even if the user has dozens of meters of extension cords, causing more than 20% of voltage drop (as the must-operate voltage of the relay is 80% of rated voltage) under load. But of course I can be wrong...Have you seen a cap used like that before? I’m curious to know whether that’s “genius” from the designer, or an age-old thing, or just odd.

author: penfold
author_rank: Frequent Contributor
timestamp: March 16, 2021, 06:28:11 pm
content: Its relatively normal to have suppression caps across coils to limit the flyback voltage in AC circuits... how significant is the DC resistance of the coil? It sounds a bit odd to be that close to resonance, I wonder if it also filters out some mains transients and prevents them triggering the brake circuit during operation

author: TheMG
author_rank: Frequent Contributor
timestamp: March 16, 2021, 06:35:44 pm
content: The capacitor across the AC line is for EMI suppression (the arcing occurring on the commutator of universal motors generates a fair bit of EMI if unfiltered).As for the capacitors in series with the relay... educated guess says they are simple DC-blocking capacitors. As the motor coasts to a stop, it acts like a DC generator, the capacitors prevent the DC generated by the motor from keeping the relay open, so the braking circuit can engage immediately as AC power is removed.

author: JeanF
author_rank: Regular Contributor
timestamp: March 16, 2021, 06:57:55 pm
content: Thanks. I've attached a small snippet of diagram to make it clearer. Indeed C1 must be there for filtering EMI from the motor, I was referring to C2Interesting point about the motor generating DC when braking. I can't test that with the original motor (broken beyond repair) and I don't want to tear apart a poor innocent drill just for that... Besides, I read somewhere that not all universal motors can be braked like that; the construction has to be such that enough residual magnetism remains (I don't know the proper terms, sorry) when power is removed.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 16, 2021, 07:12:01 pm
content: That's a cute trick to get higher current through the coil, but seems likely it wouldn't be worth the added cost vs. a relay with a slightly more sensitive coil.It could somehow be a DC blocking cap, but I'm not sure where the DC would come from. My other thought (not well formed) is that it could be a variant of a snubber cap to get the contacts to open quickly while limiting the voltage kickback to the power plug which could potentially be exposed to the user?

author: ajb
author_rank: Super Contributor
timestamp: March 16, 2021, 07:37:38 pm
content: Electric hedge trimmers are quite likely to be used with very long, low quality consumer grade extension cords--possibly even several connected in series--so it's possible that the increased current provided by the series cap is the whole point, to ensure more reliable operation with a possibly-significant voltage drop. Looking at the website for a typical big box store looks like 3-4.5A@120V is a typical rating for plug-in trimmers in the US, but I assume it's a pretty uneven profile given that the motor load will vary substantially and relatively rapidly as the cutting edges engage with the shrubbery. Based on that I wouldn't expect 240V trimmers to have much of a problem with voltage drop with a reasonable length of even the cheapest extension cords, but when it comes to consumer devices planning for only 'reasonable' usage is generally not a recipe for success.