How to adjust for mains voltage 110V<->240V automatically? - Page 1
Forum section: electronics / projects

author: martin1454
author_rank: Regular Contributor
timestamp: September 07, 2020, 11:23:59 am
content: Hi there!A fellow friend have given me a challenge.Is it possible to drive a 2KW heating unit from either 110V/115VAC or 220/230VAC without setting anything on the equipment (like those small switches we often see at a power supply). His heating unit will consist of multiple small heating cartridges (somewhere between 50 and 100W)So far I have 3 possible solutions for him.Option 1:Use a few relays to switch between running 2 cartdriges parallel and seriel (eg, having 110V heating cartridges that will run parallel or set 2 of them in series if running 240V)Option 2:Using a PFC to change it all to 380V/400V DC and run it off that. (But I cant seem to find a reasonable price PFC for 2KW)Option 3:Some kind of magical transformer I dont know of yet that will transform 110V to 230V and 230V to 230V without adjustment (I dont belive there is?)Is there other / Better options you use for changing between the mains voltages? Since its a pure resistiv load, the inductance should not be an issue.And no, I can not just run the 240V through a diode to cut off half the voltage!

author: tunk
author_rank: Super Contributor
timestamp: September 07, 2020, 11:34:36 am
content: I don't really have a clue, but for the first option I wouldthink you need some logic/electronics to check voltageand then switch the relays.

author: MosherIV
author_rank: Super Contributor
timestamp: September 07, 2020, 11:36:52 am
content: Combination of 1 & 2.Use smps to convert to fixed dc voltage to run the heaters.Most smps can cope with either 110V or 240V.2KW smps is not going to be cheap though.

author: martin1454
author_rank: Regular Contributor
timestamp: September 07, 2020, 11:37:30 am
content: Quote from: tunk on September 07, 2020, 11:34:36 amNaturally - Either a AC->DC and scale down the voltage to something a uC can read, or a simple analog circuit that flips the relays.

author: martin1454
author_rank: Regular Contributor
timestamp: September 07, 2020, 11:42:36 am
content: I got close -TDK PF-1000A-360 looks good! I'll see if 1500W is enough, otherwise I have to go up in price.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 07, 2020, 11:43:14 am
content: Definitely #1 for such stupidly simple heater application.2kW of power conversion, either from SMPS or traditional transformer, is huge waste of space and money.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 07, 2020, 12:42:35 pm
content: Use 240V heaters that will provide enough heat if operating on 120V, then restrict the on time when running on 240V. For that, probably the easiest to separate them into 4 groups and switch each group on for one cycle out of 4 cycles, staggered so the instantaneous current doesn't get too high.Quote from: martin1454 on September 07, 2020, 11:23:59 amSeparate them into two groups with two diodes going opposite directions, then the DC offset will largely cancel out. You'll still get a factor of 2 difference in power but that should make things easier for the control algorithms.

author: tunk
author_rank: Super Contributor
timestamp: September 07, 2020, 01:54:44 pm
content: And maybe add thermal fuses as a safety precaution.

author: magic
author_rank: Super Contributor
timestamp: September 07, 2020, 02:38:30 pm
content: Definitely relays. Connect the heaters in series and add a DPDT break-before-make relay in the middle to optionally disconnect each heater from the link joining them and connect straight to a supply rail. Make sure that it starts in series mode and that mains isn't shorted out by the relay during transition

author: ejeffrey
author_rank: Super Contributor
timestamp: September 07, 2020, 03:37:06 pm
content: Quote from: martin1454 on September 07, 2020, 11:23:59 amDefinitely this except only use a single DPDT relay. That will ensure there is no configuration that shorts the supply rails. Power up in 240V/series mode then if the detected voltage is 120 flip the relay to put them in parallel. Think about what happens if your detection circuit malfunctions and flips to 120V mode at the wrong time. A fuse or thermal cutoff would be a good idea if they aren't already present.

author: vad
author_rank: Frequent Contributor
timestamp: September 07, 2020, 04:25:23 pm
content: Quote from: martin1454 on September 07, 2020, 11:23:59 amIs the product intended for airline crew of long haul flights, who would want to carry such a 2kW heater to different parts of the world couple times a week?If yes, I would go with Option 2. On such niche market your friend would have to put 5-digit price sticker on such a space heater to stay afloat anyway, so the cost of 2kW SMPS would be negligible.If not, then what is the business case here?

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2020, 04:37:03 pm
content: Quote from: martin1454 on September 07, 2020, 11:23:59 amWhy not? You can use a single DPDT relay, two large rectifier diodes and a simple circuit that pulls in the relay at 150VAC or so. Simply arrange the circuit so that half of the heating elements are fed thru each diode and they are opposite polarity. Actually, you can just use a DPST relay if you find one cheaper. Of course, this won't result in the same amount of heat in either case, so I don't know what your tolerances are there. Is it thermostatically controlled?

author: ejeffrey
author_rank: Super Contributor
timestamp: September 07, 2020, 05:10:37 pm
content: Quote from: vad on September 07, 2020, 04:25:23 pmThis is a bit of a silly argument. Even if any individual item will rarely if ever move between 120 and 240 V, having it automatically and safely select between them without requiring a switch that can be set incorrectly is a good idea. If nothing else, it avoids needing to have your supply chain make sure the switch is initially set correctly for the region of sale.Whether it is worth the small added cost depends on the product.QuoteThis is absurd. Why on earth would you use a 2 kW SMPS to replace a single relay. Such a product will not only be more expensive to manufacture, but more expensive to design, heavier, potentially less reliable, and probably require a fan.

author: bdunham7
author_rank: Super Contributor
timestamp: September 07, 2020, 05:28:48 pm
content: Quote from: ejeffrey on September 07, 2020, 05:10:37 pmSilly questions invite silly answers. Since the OP hasn't given a clue as to what the application actually is, we haven't the faintest idea what would be appropriate.

author: Pawelr98
author_rank: Regular Contributor
timestamp: September 07, 2020, 10:25:55 pm
content: Relay would be the easiest option.With analog circuitry it should be relatively simple.One DRC delay network for parallel operation with a single 431 for parallel activation.Another network with just diode + capacitor + resistive divider (no delay) for second 431.With 240V the second comparator locks out the first comparator and drives the relay for series operation.Triac phase control is another way to do it.Just fire it later when on 240V.Simple as that.Half-wave rectifier is not going to magically make 240Vac into 120Vrms.A half wave rectified 240Vac is roughly 170Vrms (1/sqrt(2)).The power does go down by 50% though since P=U^2/R.

author: TheMG
author_rank: Frequent Contributor
timestamp: September 07, 2020, 11:57:24 pm
content: Relays to select the appropriate combination of series/parallel heating elements would be, in my opinion, the most logical solution. Robust, reliable, and inexpensive to implement compared to the other solutions.All you need is a relatively simple voltage detector circuit, to switch the relays when the input voltage is below 130V. Note that I would design the circuit in a fail-safe manner, by making the normal state of the relays (de-energized) for 240V, and then energize the relays for 120V configuration. That would avoid smoking the heating elements should the voltage detector circuit fail when the unit is powered from 240V.I don't know what your intended application is, but do keep in mind that at 120V, 2kW is 16.7A. This, at least in most of North America, would require a 30A receptacle as that is above the maximum allowed for a continuous load even on a 20A circuit (80% of 20A is 16A).

author: magic
author_rank: Super Contributor
timestamp: September 08, 2020, 05:14:46 am
content: Quote from: ejeffrey on September 07, 2020, 05:10:37 pmQuote from: bdunham7 on September 07, 2020, 05:28:48 pmLOL, people. Save for some harsh environments where relays just can't operate (vibrations, whatever) I can't think of a single reason to bother with this APFC nonsense.Remember, power factor of a hotdog had been measured and is exactly one

author: ejeffrey
author_rank: Super Contributor
timestamp: September 08, 2020, 05:24:42 am
content: Quote from: magic on September 08, 2020, 05:14:46 amJust to be clear, I was saying that whether the relay was worth it vs, doing nothing depends on the application. Unless there is a very esoteric situation like you suggest that precludes a relay, using an APFC power supply for this is just pants on head silly.

author: martin1454
author_rank: Regular Contributor
timestamp: September 08, 2020, 06:33:31 am
content: Quote from: vad on September 07, 2020, 04:25:23 pmNo no no..... It is a product that should be sold in limited quantity, but to all over the world, and is specified not to have that little 110V-240V switch you see on a lot of products.

author: martin1454
author_rank: Regular Contributor
timestamp: September 08, 2020, 07:25:57 am
content: Quote from: magic on September 08, 2020, 05:14:46 amLol.....The application is to heat up a large chunk of metal for drying some kind of liquid.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 08, 2020, 08:13:52 am
content: Option 4: a phase control dimmer.You're probably going to need some regulation anyway, you can incorporate this in your design with a dimmer.eg: at 220V you run 0-50%, and with 110V you run 0-100%.

author: vad
author_rank: Frequent Contributor
timestamp: September 08, 2020, 11:10:05 am
content: Quote from: martin1454 on September 08, 2020, 06:33:31 amThe most economical and the most reliable way of meeting such requirements is to have multiple models of the product, and resolve the “little switch” problem at the point of logistics. For example, have 120V / 1,440W model for North American distributors and 240V / 2,000W for the rest of the world.PS. As have been mentioned earlier by TheMG, 2kW appliance requires 30A receptacle to meet U.S. National Electric Code. If the space heater is intended for residential use, you would want to sell the product with 15A plug that is rated for 12A current draw / 1,440W maximum power.

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2020, 01:43:36 pm
content: Quote from: ejeffrey on September 08, 2020, 05:24:42 amTrue, but this whole thread seems to be that way. No question is too simple for me--I'm not all that advanced myself--until that question comes from someone actually designing a commercial product, not a beginner tinkering in their basement.

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2020, 02:14:05 pm
content: Quote from: martin1454 on September 08, 2020, 06:33:31 amOK, here's the issue. You haven't answered my question about thermostatic control. If (and only if) your heating elements are plain constant-resistance (like nichrome) that operate at 120V and they can be arranged into two banks of exactly equal power and there are no per-element thermostatic controls, you can solve your problem easily with one DPDT relay with a 240VAC coil. Just make sure its pull-in voltage is above 150VAC.If that isn't how your system works--if the heating elements have their own individual controls or vary in resistance, then this won't work. You could use the same strategy with an appropriate transformer, but it would be a huge and expensive transformer.

author: Zero999
author_rank: Super Contributor
timestamp: September 08, 2020, 02:30:46 pm
content: Quote from: ejeffrey on September 07, 2020, 05:10:37 pmThere are some advantages of using an SMPS, over a simple relay. An SMPS will regulate the power, which will be the same, irrespective of the supply voltage. The relay option will give two voltage operating modes 110VAC to something like 125VAC and 220VAC to 240VAC. A switched mode power supply will give a range 100VAC to 250VAC and anything in between. There's always the risk with a relay, the voltage could be 170VAC, which would give double the power dissipation, if it's set to 120V mode, or half the power dissipation, if it's in 240V mode.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 08, 2020, 03:04:02 pm
content: Definitely relays, but NOT a single DPDT one - you need two seperate relays - one to turn on for 110v, the other to turn on for 230V. Plus a short delay between opening one and closing the other.The reason is that on a changeover relay contact, as the contact opens, an arc can form between the contacts, and may still be burning by the time the common contact hits the other side, so you now have an arc connected right across the mains input.Break-before-make may well not prevent this at the sort of heater currents you're talking about.below is a circuit I used to do exactly this job many years ago, using two 115v heaters.RL1 closes for 230v, or RL2 for 115VIt also includes the voltage detection circuit - the output of optoisolator U2a produces pulses at either 50hz or 100hz depending on the supply voltage.U3 is for heater-open detection.

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2020, 03:42:05 pm
content: Quote from: mikeselectricstuff on September 08, 2020, 03:04:02 pmThat's a nice circuit, the OP should employ you as a consultant. However, the problems you cited can be solved by simply choosing the correct relay. Non-inductive AC loads are by far the easiest and least arc prone to switch. Obviously a single relay would be a crude solution, resulting in a significant inrush of 4X at the higher voltage. It would still work and I've seen worse in the real world. Crudeness has its virtues.