Switching Power Supply on North American 240v AC (split phase)

[trevers]

I have been doing a fair bit of research on this question, and I cannot get a clear answer. I have searched this forum and have completed extensive Google searches.

Preface -
I am building a Wi-Fi connected programmable thermostat for an electric forced hot air heater that I recently installed in my home. Why am I building it when there are so many commercial thermostats available? (Nest, Ecobee, Honeywell, etc.) It seems that there is a distinct lack of programmable (let alone remote controlled) products that will control US mains AC powered heating. To make it worse this heater is rated at 4000 watts which in the US requires a 240v 20A circuit to power it. In my search I found exactly one unit that would control a device like this.

Problem -
For my project I need an AC/DC switching power supply that will accept 240v AC in and output 12v DC. I will be using the 12v to control a DPST relay to switch the AC, and to power a buck converter outputting 3.3 volts to power the uC (in this case an Onion Omega2). I know that you are thinking that almost every switching power supply available today will do this, and I agree. Here is my conundrum.
Unlike most of the reset of the world, in the USA 240v AC is delivered over two "hot" 120v wires and a single ground, meaning there is no neutral wire.

https://en.wikipedia.org/wiki/Split-phase_electric_power#North_America

When I read datasheets on available power supplies they indicate that they require the AC input to be in the form of L - N - GND where it is assumed that there is potential between the L and N terminals, and also a ground reference.
For example, here is one possible supply that I have found for my use - https://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=249143994&uq=636519489417303528
You can see the terminals on the datasheet - http://www.us.tdk-lambda.com/ftp/Specs/ls.pdf

Can I safely use a supply like this with both the L and N being at 120v with only the ground as a neutral? Will it even work? I would just buy one and try but messing with mains voltage and amperage is not something that I do without extreme caution.

Thoughts?

[nick1911]

I know this isn't directly addressing your question, but...

Assuming you're using a commercially produced furnace, why not tie into the existing 24vac control system?  The control transformer is always hot on these units.

Also, why are you not using the built in sequencer, relays, etc?  There's also thermal fuses that should be considered.

[IanB]

I am building a Wi-Fi connected programmable thermostat for an electric forced hot air heater that I recently installed in my home. Why am I building it when there are so many commercial thermostats available? (Nest, Ecobee, Honeywell, etc.) It seems that there is a distinct lack of programmable (let alone remote controlled) products that will control US mains AC powered heating.

This is weird. Why would a home heating solution not be compatible with industry standards for thermostatic control? Is it not designed for home installation perhaps?

Can I safely use a supply like this with both the L and N being at 120v with only the ground as a neutral? Will it even work? I would just buy one and try but messing with mains voltage and amperage is not something that I do without extreme caution.

Yes, it should work. 240 V is 240 V, and if the module is installed according to code and relevant electrical standards (especially paying attention to the ground connection), then my unqualified opinion is that it should be OK.

[Richard Crowley]

When I read datasheets on available power supplies they indicate that they require the AC input to be in the form of L - N - GND where it is assumed that there is potential between the L and N terminals, and also a ground reference.

Are you assuming that the products are designed for low/zero voltage between "Neutral" and "Ground"?  Else, what is the issue with both L and N being 120V above Gnd?

Do you really have a furnace with NO green-wire safety ground?  Perhaps that is an EXCELLENT excuse to add one.
I agree that it would be dangerous to use any SMPS on North American "split-phase" 220VAC without a proper ground.
My HVAC system operates on 220VAC, but it has a green-wire safety ground.  I've never seen one without it.

[trevers]

This is an in wall unit. There is no control circuit. The only control is a basic mechanical thermostat that is wired directly to the mains and the coil.

https://www.marleymep.com/products/fahrenheat/fzl-fan-forced-wall-heaters

Yes it is perfectly legal. (In the US)

[IanB]

Also, it is quite common for the 240 V to high power devices like ovens, heaters or dryers to be delivered over a four conductor cable with L1/L2/N/G conductors. This enables control electronics to get 120 V from one of the hot conductors and the neutral. If the circuit powering this heater is a fresh install it could be set up that way, or the circuit could be upgraded to that configuration?

[Richard Crowley]

This is an in wall unit. There is no control circuit. The only control is a basic mechanical thermostat that is wired directly to the mains and the coil.

https://www.marleymep.com/products/fahrenheat/fzl-fan-forced-wall-heaters

Yes it is perfectly legal. (In the US)

Step 4 of the installation instructions clearly show connecting the ground wire from the distribution panel to the green-head screw on the metal enclosure of the appliance.  https://www.marleymep.com/system/files/node/file/field-file//fzl_wire.pdf  Page 7.

Are you saying that your unit is illegally installed without a safety-ground wire?

If it does not have the specified ground wire, then adding your power supply, etc. will make it no more dangerous than it already is.  (which is to say that I wouldn't touch any metal part of it)

[trevers]

No I am not. It is installed exactly like the instructions.

In fact I marked the white line with black masking tape as instructed to signify that this is a hot wire and not a neutral.

See attached screencap from the instruction manual.

[jmelson]

Practically any wide-range input power supply will run FINE on US (balanced) 240 V power.  I've done this on occasion with no problem.

Jon

[Zero999]

Yes, a switched mode power supply, rated to 240V, will be fine run from 120V-0-120V split phase.

How much power do you need? Small switched mode power supplies, which are class 2 (double insulated) are available if you need something which doesn't require an earth connection. The mains connections are also non-polarised.
https://www.digikey.com/product-detail/en/triad-magnetics/JSU120-1500/237-2268-ND/7389458

[Richard Crowley]

Can I safely use a supply like this with both the L and N being at 120v with only the ground as a neutral?

No, NOT "with only the ground as a neutral".  With the ground as Ground!

[Gregg]

Please use a genuine contactor to switch the 4000w heater element(s) and not a small cube relay.   Look for a definite purpose contactor rated for more than the actual current; a 30 amp two pole would be sufficient.  A 20 amp contactor would do, but a 30 amp will last twice as long in real world use.  They are available in various coil voltages and often you can find a genuine name brand new old stock one on fleaBay rather than gamble on a cheap Chinese one.  Also, 240 to 120 volt control transformers are not expensive and may solve your grounded neutral dilemma. 

[nick1911]

Please use a genuine contactor to switch the 4000w heater element(s) and not a small cube relay.   Look for a definite purpose contactor rated for more than the actual current; a 30 amp two pole would be sufficient.  A 20 amp contactor would do, but a 30 amp will last twice as long in real world use.  They are available in various coil voltages and often you can find a genuine name brand new old stock one on fleaBay rather than gamble on a cheap Chinese one.  Also, 240 to 120 volt control transformers are not expensive and may solve your grounded neutral dilemma.

That's really good advice.

Contactors do tend to be loud though, an SSR might be a better option, although at a higher cost.

[Richard Crowley]

While I completely agree with the advice for current over-rating, note that the load is purely resistive, so using a "contactor" is not really necessary here.  A proper contactor is designed to break an inductive load (i.e. a motor) so it is designed differently from an ordinary relay.  But there will be little or no inductive component of the load here and an ordinary relay (mechanical or solid-state) should be adequate.

20A SSRs which are rated up to 480V are available for less than $10.  AND they feature built-in zero-crossing switching control.  I even saw a 3-phase, 30A SSR for $32  https://www.ebay.com/itm/191211092216

If the heater doesn't have an independent over-temp cutoff switch, I would certainly take the opportunity of adding one.  It will provide an extra level of protection against failure on several levels.

[flynwill]

Another option you might consider is to refit the heater with and off-the-shelf 240VAC - 24VAC transformer (stock item at any HVAC supply house), and contactor / SSR to switch the unit.   Now you can connect any standard thermostat with whatever features you like.  This is a very common modification to small wall heaters like yours.

[Zero999]

Please use a genuine contactor to switch the 4000w heater element(s) and not a small cube relay.   Look for a definite purpose contactor rated for more than the actual current; a 30 amp two pole would be sufficient.  A 20 amp contactor would do, but a 30 amp will last twice as long in real world use.  They are available in various coil voltages and often you can find a genuine name brand new old stock one on fleaBay rather than gamble on a cheap Chinese one.  Also, 240 to 120 volt control transformers are not expensive and may solve your grounded neutral dilemma.

That's really good advice.

Contactors do tend to be loud though, an SSR might be a better option, although at a higher cost.

Another disadvantage of an SSR, compared to a mechanical relay, is a higher voltage drop and greater power loss on the circuit being switched.

[IanMacdonald]

The main difference is that a center-tap supply needs a fuse in both feeds. A PSU designed for a L/N supply will only be fused in the live. This it would have no overcurrent protection if a short develops on the other live. The other issue is one of labeling - a terminal labeled 'N' would be assumed to be safe by any subsequent guy working on it. I guess you could correct that.

Since the PSU should be isolating, it makes no difference to the output.

Although, why not just use a 120v PSU between one live and neutral?

[Richard Crowley]

Although, why not just use a 120v PSU between one live and neutral?

In North America it is uncommon to find a neutral wire in a 220V branch circuit.  At least in existing cases. Perhaps modern wiring has added Neutral.  Of course, you could always CHEAT and use the green-wire safety ground (BrEnglish: "protective earth") but that is risky and illegal.

[Zero999]

Although, why not just use a 120v PSU between one live and neutral?

In North America it is uncommon to find a neutral wire in a 220V branch circuit.  At least in existing cases. Perhaps modern wiring has added Neutral.  Of course, you could always CHEAT and use the green-wire safety ground (BrEnglish: "protective earth") but that is risky and illegal.

That also might trip an RCD (GFCI in US).

[thobie]

Another disadvantage of an SSR, compared to a mechanical relay, is a higher voltage drop and greater power loss on the circuit being switched.

Shame to lower the total efficiency of the electrical heater. Might even get cold at winter.

[Richard Crowley]

Another disadvantage of an SSR, compared to a mechanical relay, is a higher voltage drop and greater power loss on the circuit being switched.

Shame to lower the total efficiency of the electrical heater. Might even get cold at winter.

    

[Gregg]

Another disadvantage of an SSR, compared to a mechanical relay, is a higher voltage drop and greater power loss on the circuit being switched.

Shame to lower the total efficiency of the electrical heater. Might even get cold at winter.

Hmmm!    I wonder where the inefficiency of a SSR might be manifested. 
Could it possibly become heat??  And what is the device producing, maybe heat.   
Entropy may not actually be a waste in this scenario. 
I know I’m being a little sarcastic, but a true engineer has to look at the whole system as well as the components.
I personally would use a quality contactor in a proper electrical enclosure to keep from modifying the heater in any way that could possibly come back to haunt me.  The proper place for an SSR would be on a good heatsink in the incoming air flow with probably a fan fail defeat.

[IanB]

Shame to lower the total efficiency of the electrical heater. Might even get cold at winter.

Hmmm!    I wonder where the inefficiency of a SSR might be manifested. 
Could it possibly become heat??  And what is the device producing, maybe heat.   

Dare I say, "Whoosh!" as a response to this post?

Seems like the joke went right over your head... 

[T3sl4co1l]

Although, why not just use a 120v PSU between one live and neutral?

In North America it is uncommon to find a neutral wire in a 220V branch circuit.  At least in existing cases. Perhaps modern wiring has added Neutral.  Of course, you could always CHEAT and use the green-wire safety ground (BrEnglish: "protective earth") but that is risky and illegal.

240V circuits without neutral are deprecated and not recommended for new installation (may even be illegal, I don't know).

In any case, easy to go look it up in the code to be sure. No need to spread potential misinformation on a public forum.

Shame to lower the total efficiency of the electrical heater. Might even get cold at winter.

Hmmm!    I wonder where the inefficiency of a SSR might be manifested. 
Could it possibly become heat??  And what is the device producing, maybe heat.   

Dare I say, "Whoosh!" as a response to this post?

Seems like the joke went right over your head... 

I mean it's not like it's a heat pump, where the efficiency is over 100%.

Tim

[cowasaki]

I have home automation throughout my new home.  I'm in the UK so use 240v for everything.  The ones I'm using are Energenie products https://mihome4u.co.uk/devices.  They do a 240v relay for about £30 but you would need the controller.  They are controllable via RF433 modules as well as across the internet, mobile, remote controls, google assistant etc