choosing a varistor for 120/240vac - Page 1
Forum section: electronics / beginners

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 04, 2017, 08:20:08 pm
content: Hi All,I am trying to add a varistor to my circuit and not sure how/where to start with. my circuit is designed to work on both 120 and 240VAC. any help would be appreciated. With the knowledge i've gained i am guessing this (https://www.digikey.com/product-detail/en/stackpole-electronics-inc/RV471K07T/RV471K07TCT-ND/7312420)would be a good choice.

author: SeanB
author_rank: Super Contributor
timestamp: November 04, 2017, 09:25:59 pm
content: D2 is definitely not a component you need on the AC side, it will blow fuses at a minimum on the first power on, if it dies not explode as well. The VDr you are looking at will work, and clip transients well enough, though putting a 170v version across each 120VAC winding would work even better, providing protection in both input voltage ranges as opposed to the single version providing poor protection at 120V selection.

author: Zero999
author_rank: Super Contributor
timestamp: November 04, 2017, 09:33:49 pm
content: I agree with Sean: put a transient suppressor across both 120V windings and it should be fine.Another option is to add a transient suppressor diode to the DC side of the supply. A 20V to 30V unit will do.You've also drawn the AC:DC part of the schematic, the wrong way round, which makes it more difficult to understand. Start with the input voltage on the left, and output voltage on the right. Even if it makes the schematic larger, it will still be easier to read.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 05, 2017, 06:30:41 am
content: Thank you seanB but D2 is a TVS diode though the symbol is not correct. I shall add more mov to the 120VAC as well.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: SeanB
author_rank: Super Contributor
timestamp: November 05, 2017, 06:55:51 am
content: Do not use a TVS on the mains input, it is not designed to handle the energy it will get, and will likely fail very quickly. Just use one on the secondary, but the MOV on the primary, and a class X 100n to 330n 275VAC capacitor in place of the TVS will work better.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 05, 2017, 06:58:16 am
content: Thanks during I learned that the diode would suppress minor transients and mov would take care of the larger voltage.How does a cap work here ? Any link to the product ?Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 05, 2017, 07:21:04 am
content: QuoteWould this be necessary ? And would this be in series to gnd ?Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2017, 10:51:41 am
content: Quote from: anishkgt on November 05, 2017, 07:21:04 amIt would be more effective than a MOV on the mains side. The transformer will be able to handle large voltage spikes, without any damage. The LM7805 will be easily damaged by high voltage transients. Putting the transient suppressor on the secondary side of the transformer is more effective, than having it on the primary, because the transformer's internal impedance will limit the current and DC transient suppressor diodes are faster than MOVs.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 05, 2017, 11:21:07 am
content: Quoteinteresting! i guess learning from forums and taking advise from one person is not the best way to learn something new. I was told it would better to have it at the primary as the MOV can only protect against high votlage only and having a TVS diode and an MOV is a good thing.So a Bi-directional TVS DIODE of 20-30V before the bridge, correct ? isn't that a bit high for 9VAC transformer ? and 100n or 300n how do i know the which to choose from digikey ?

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2017, 09:21:18 am
content: Quote from: anishkgt on November 05, 2017, 11:21:07 amIt doesn't matter whether you use a bidirectional diode before the bridge, or a unidirectional diode after the bridge.The diode needs to breakdown (start conducting) at a higher voltage than the nominal peak voltage, plus a fair margin and keep the voltage below a level which would damage the LM7805 whilst the diode is conducting.I'd go for the P6K24A. It's guaranteed to not conduct, until the voltage reaches 20.5V (higher than (9V+50%)*1.414) and the clamping voltage is 33.2V, which is under the maximum voltage rating of the LM7805.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 06, 2017, 09:40:41 am
content: Thank you. And I guess the cap would be 0.1uf 275VAC. I’v seen those rating in other appliances.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2017, 10:03:02 am
content: Quote from: anishkgt on November 06, 2017, 09:40:41 amWhich capacitor? There isn't one on the mains side of your circuit.If you're talking about the capacitor on the DC side, then such a high voltage rating is not necessary, although no harm will come from using a capacitor with that rating.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 06, 2017, 11:45:54 am
content: I was referring to the class X cap suggested in post#5.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 06, 2017, 05:05:34 pm
content: Wouldn't it be ideal to have the TVS after the fuse and before the bridge ? so that during an over-voltage/current the fuse would blow. On the other hand if the TVS was placed after the bridge then the bridge would have to deal with the over-voltage/current before the fuse blows.