Relays output AC - DC compatible ? - Page 1
Forum section: electronics / general

author: Michaelpier
author_rank: Contributor
timestamp: January 26, 2011, 11:49:13 pm
content: Hello,new as a member but not as a guest here.Iam new to electronics and trying to read as many as possible.I do have a question and i want to ask if is possible to use the output of these relays for dc loads.Does not mention anything for DC on the datasheet.It says for AC.Can you please tell me if this is possible or not and why?http://eu.mouser.com/search/ProductDetail.aspx?qs=g5ciJ0jwZaFPYkHVnREaNg==http://eu.mouser.com/ProductDetail/Continental-Industries/SVAA-6V50/?qs=sGAEpiMZZMtLEhJ5P/NsZ2UJUUj3dRbva/5UaztwcbU%3dThank you ,Michael

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 26, 2011, 11:53:56 pm
content: Triac output SSRs will latch on in a DC circuit. If that's not a problem, they can be used.

author: tecman
author_rank: Frequent Contributor
timestamp: January 26, 2011, 11:57:51 pm
content: Using a relay, or switch on DC has some limitations. If you have an inductive load, then there is usually a significant derating. Some devices will have DC ratings, but if not be very careful. Using a flyback diode will minimize most of the inductive nasties. The other factor is the DC voltage you are using. If the voltage is over 24 volts (as a value with no specific basis), the relay may have issues as the voltage goes up, and the device is not DC rated. As the voltage goes up, arcing may occur, and if the "contact open" spacing is not wide enough, it may continue to arc and ruin the contacts. With AC, the voltage zero crossings will extinguish the arc. DC rated relays generally have more daylight in the contacts when open.Not a short answer, but it mostly depends on the load and the voltage.paul

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 27, 2011, 01:08:08 am
content: I am sure somebody out there has a good reason but off the top of my head I don't know why anybody would want to switch a DC load with a solid state relay. It would more efficient and much cheaper to use a discreet transistor.

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2011, 05:33:33 pm
content: There are many reasons for switching DC with an SRR: motor control is an obvious application.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 27, 2011, 05:41:39 pm
content: Quote from: Hero999 on January 27, 2011, 05:33:33 pmOff the top my head, it seems like many motor control applications would require 4 SSRs (unless they had multiple poles but then you lose control over dead time). I would really have to be in a rush to choose that over transistors.

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2011, 05:49:53 pm
content: If the motor doesn't have to be reverse, only one switch is required. Transistors are fine but they don't give you any galvanic isolation.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 27, 2011, 06:02:59 pm
content: Quote from: Hero999 on January 27, 2011, 05:49:53 pmThat's only true for brushed DC (read: crap) but OK, fair enough.

author: tecman
author_rank: Frequent Contributor
timestamp: January 27, 2011, 06:25:17 pm
content: Just remember that there are tens or hundreds of millions of relays sold every year. They are not a glamorous as a MOSFET or IGBT, but they are usually remarkably cheap for their power handing capability, very simple to drive/interface, will take an incredible amount of abuse and have a high power density if you consider all of the thermals associated with solid state devices. And as mentioned, they have a high degree of galvanic isolation. They are still the device of choice in many applications.paul

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 27, 2011, 06:28:13 pm
content: Quote from: tecman on January 27, 2011, 06:25:17 pmI agree completely, we are discussing the merits of solid state relays.

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2011, 07:13:30 pm
content: Quote from: CafeLogic on January 27, 2011, 06:02:59 pmTo be picky there's no such thing as a brushless DC motor. What's marketed as a brushless DC motor is really an AC machine with an integrated inverter. Brushed DC motors are more rugged and abuse tolerant than sensitive semiconductors, same with mechanical vs solid state relays.

author: Unknown
author_rank: Unknown
timestamp: January 27, 2011, 07:45:10 pm
content: Quote from: Hero999 on January 27, 2011, 07:13:30 pmDepends on the type of abuse, IMO. Relays are probably more tolerant to voltage and current transients, but SSR's will probably survive more switching cycles, mechanical stress (eg. vibration) or external magnetic fields. Can't really say that one is more rugged than the other.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 27, 2011, 08:22:59 pm
content: Quote from: Hero999 on January 27, 2011, 07:13:30 pmWell, if you want to be really picky, there is no such thing as a DC motor. The point just made about mechanical wear-out holds especially true here, BLDC is more reliable in the long term.

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2011, 09:45:36 pm
content: You're both right of course there are two kinds of ruggedness: mechanical parts can take stress which electronic parts can't and vice versa.

author: Michaelpier
author_rank: Contributor
timestamp: January 27, 2011, 10:00:51 pm
content: Thanks everyone for the inputs.These relays i need them to create a switch that can isolate the circuits of my experiments from the pulse generator.I dont want the positive or the negative of the generator to be connected in any way with any circuit.I will be using this switch in many cases.@CafeLogicQuoteDo you know a circuit or have anything in mind so i can use transistors instead of ssr to achieve what iam looking for?Grazie

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 27, 2011, 10:12:02 pm
content: An SSR is nothing more than discreet components in a sealed package. My comment regarding DC was based on the typical ease of implementing those components yourself. Obviously, yes you can use a transistor based solution, but that sounds like a fairly decent reason not to want to deal with. Keep in my mind that SSRs have breakdown voltage limits just like transistors. If you wanted to go discreet, you could use an opto-isolator, which is what happens in the SSR. I don't know about your circuit, but you would probably need some kind of "snubber". You might be able to get away with just a diode to short negative pulses, or perhaps a diode plus an additional zener to short over-voltage.

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2011, 10:26:44 pm
content: Here's one which uses MOSFETs. Note that the capacitors need to be Y1 rated for mains use and there needs to be 7mm of creepage on the PCB. You can of course replace the 74C14 with an oscillator made with other CMOS gates such e.g. CD4001, 4011 or 4069.http://www.discovercircuits.com/DJ-Circuits/fetssr.htmIf you need a lower coil voltage use logic level MOSFETs, a voltage doubler, HC ICs or all of the aforementioned, ask me and I'll show you how.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 27, 2011, 10:32:37 pm
content: That illustrates my point very nicely about SSRs and AC vs. DC. That looks like a giant pain in the Ass which would not be necessary for DC (although it would work). Also not isolated.

author: Michaelpier
author_rank: Contributor
timestamp: January 28, 2011, 12:16:59 am
content: Thank you all,now i have more options.Discreet is not a problem.Is for my own use so i dont mind.As i was trying to read about opto-isolators and photo mosfets as you suggested i found this ssr:http://opto22.com/site/pr_details.aspx?cid=3&item=DC200MPThat looks quite fast times for the load that can handle 1Amp and 200v or is not?.What do you think?I need to read more about opto-isolators and photo mosfets.Michael

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 28, 2011, 01:03:48 am
content: Photo-mosfet and opto-mosfet are marketing terms for SSRs in which the mosfets are driven by an opto-isolated interface. The turn-off time is so slow in proportion to the turn-on on that one you picked that it makes me think its just a P-mosfet with the gate passively pulled up by a resistor with the turn-on accomplished by an active pull-down with an n-mosfet. If that works for you then why not? However, if you can afford that 1ms switching time and you only need 1 amp, you might want to look into mechanical reed relays. They are cheaper, very simple, and are intrinsically isolated.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 28, 2011, 03:48:30 pm
content: Quote from: AcHmed99 on January 28, 2011, 11:59:44 amThe keyword is reed. 7000V, 2Ahttp://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=725-1032-ND

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2011, 07:03:45 pm
content: Quote from: CafeLogic on January 27, 2011, 10:32:37 pmNo, the circuit I posted is as good isolated for all intents and purposes.The two 100pF capacitors form a 200pF barrier between the DC control voltage and the mains, giving an impedance of >13M?@60Hz which will leak only 9nA@120V which meets all the regulations, is well below most commercial SMPS and in-line with old fashioned iron core transformers.The only mistake in the schematic is it only specifies 3kV capacitors which are not good enough to fulfil the requirements for double insulation; to do that they need to be class Y1 which can withstand 8kV or you could replace each capacitor with two 220pF class Y2 capacitors wired in series.It's a safe and cost-effective solution providing it's correctly designed and constructed. The 74C14 can be used to give up to three channels or its function could be integrated into a microcontroller, if logic-level MOSFETs or a voltage doubler are used.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 28, 2011, 07:43:49 pm
content: What I meant by not isolated is that it is not galvonically isolated.

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2011, 08:36:21 pm
content: What do you mean? Nothing is totally galvanically isolated, there's always some leakage.200pF of coupling between the contacts and coil is probably similar you what you'd expect from a large relay/small contactor. If you're not happy, with it, reduce the capacitor values and crank up the frequency: as shown the frequency is about 120kHz so reduce R2 to 10k and you'll have 1.2MHz so you could use 10pF capacitors which leak under 1nA, not much more than an SSR. There's no reason to bother though, 9nA is low enough for medical grade equipment.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 28, 2011, 09:03:21 pm
content: I guess you are right that qualifies as galvanic isolation, I just think of it differently in my head. I think of it as isolation via mutual inductance or optics because capacitors can fail short circuit but wikipedia says they have special rated capacitors for that. Learn something new every day.

author: Zero999
author_rank: Super Contributor
timestamp: January 29, 2011, 12:17:06 am
content: Quote from: CafeLogic on January 28, 2011, 09:03:21 pmThey are known as class Y capacitors which are used in circumstances where they're failure could cause a deadly shock. Class Y2 capacitors can be connected from live or neutral to earth and provide basic insulation. Class 2 capacitors can be connected from the mains to exposed metal surfaces and provide double or reinforced insulation. You can use two Y2 capacitors connected in series, rather than a single Y1 capacitor.Y1 capacitors are routinely used in SMPSes to divert the high frequency noise coupled to the DC side from the transformer back down the mains, where it's removed by class X capacitors, which can only be connected from live to neutral. This is why if you hold one side of the DC of an SMPS and touch a grounded metal object at the same time, you may feel a slight tingle but the current is too low to do any harm to you.Some SMPSes I've seen use 4.7nF Y1 capacitors which leak 340µA on 230V 50Hz. The circuit I posted will leak 14.5nA on 230V 50Hz which you won't even feel.Here's probably more than you want to know about suppression capacitors:http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Capacitors/FilmCapacitors/PDF/PDF__EMIGeneralStandard,property=Data__en.pdf;/PDF_EMIGeneralStandard.pdfhttp://www.okaya.com/images/noise/1100.pdf