# Explanation needed - 240v step down for 12v & 5v output

#### born2fly

Joined Dec 5, 2020
32
I have a control board for a hot tub that has a 240v input step down transformer that outputs 12v & 5v. Why would this be done when the standard voltage is 120? I'm not an engineer but am very curious why it couldn't have just been a 120v step down transformer. There are a few relays for the pump motor(high & low), heater & aux. There is also an LCD display that connects to the board.

thanks,
rich

#### AnalogKid

Joined Aug 1, 2013
10,125
Without seeing the label on the tub at the AC power input, and whatever lable or writing is on the transformer, it's hard to say, One possibility is that the secondary circuits are designed to regulate down to 12 V and 5 V when the input is 240 V, and the extra headroom is dissipated as heat. If you can handle the heat, this is one way of having a "universal input" power system without the complexities of a switcher.

Are the 12 V and 5 V regulators switching or linear?

Photos?

ak

#### ronsimpson

Joined Oct 7, 2019
2,399
My hot tub uses 240 which is a combination of two 120V circuits. Only small tubs can work at 120.

#### AnalogKid

Joined Aug 1, 2013
10,125
I wasn't going to drill down into this, but Ron brought it up.
I have a control board for a hot tub that has a 240v input step down transformer that outputs 12v & 5v. Why would this be done when the standard voltage is 120?
Is it?

Is the main AC input to your tub 120 or 240? They each take two wires plus a ground, so you can't tell just by looking. Neither option is a given.

ak

#### born2fly

Joined Dec 5, 2020
32
Without seeing the label on the tub at the AC power input, and whatever lable or writing is on the transformer, it's hard to say, One possibility is that the secondary circuits are designed to regulate down to 12 V and 5 V when the input is 240 V, and the extra headroom is dissipated as heat. If you can handle the heat, this is one way of having a "universal input" power system without the complexities of a switcher.

Are the 12 V and 5 V regulators switching or linear?

Photos?

ak
My hot tub uses 240 which is a combination of two 120V circuits. Only small tubs can work at 120.

There wasn't any info on the transformer. I was told by customer that the board runs on 240v. Seems the 12v regulator blew when they attempted to connect the pump when everything was 'hot'. The pump was bad, apparently Fuses didn't blow, the regulator took the hit. I was just curious why 240v. The pump runs on 120v. Possibly 240 for power management? Still have things to learn/ understand. Just had to ask ...why

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#### MrChips

Joined Oct 2, 2009
27,137
AC power equipment such as heaters and motors, household appliances, clothes washers and driers need 120VAC or 240VAC.
Control circuitry will activate mechanical or solid-state relays in order to energize equipment that require high power.

The control circuitry itself does not need high voltages. Low voltage circuits run on 3VDC or 5VDC.
It is common to step down 120VAC or 240VAC to 12VAC. From here the AC voltage is rectified and regulated to provide the DC voltages required by the control circuitry.

#### Jon Chandler

Joined Jun 12, 2008
390
An appliance like a hot tub or clothes dryer typically operates from 240 volts in the US. The heating elements will operate from 240 volts because of their higher power requirements. Motors will usually run on 120 volts, taken from one 120 volt leg to ground. The motors used are usually fractional horsepower, and readily available for 120 volt operation.

Control electronics, LED lighting and other low-power needs will usually be low voltage DC, provided by a 120VAC –> DC converter.

#### ronsimpson

Joined Oct 7, 2019
2,399
. I was told by customer
Did they move the tub to a new place? or has it run in this location for a while?

Why the questions. A pump and usually this transformer run from Line1 to Neutral at 120V. And perhaps a second motor runs from Line2 to Neutral at 120V. The heater normally runs from Line1 to Line2 at 240V. BUT There is often a jumper on the board to allow Line1 and Line2 to be connected together, and the pump will run from Line to Neutral at 120V. (The tub will heat slowly.) Sometimes the house does not have 240. If the tub saw set to 120/or 240 and you plugged it into 240/or 120 then bad things will happen. And sometimes we will never know what happened.

#### born2fly

Joined Dec 5, 2020
32
Thank you all for the replies. Maybe I need to clarify better. I am familiar that certain equipment can require higher voltage. What I don't quite understand is why a board that operates 12 volt relays needs to have a transformer that needs 240 volts. It just seems like overkill to me. The power to the actual equipment is connected from a different source and all this board does is act as a low voltage switch device.

#### MrChips

Joined Oct 2, 2009
27,137
Thank you all for the replies. Maybe I need to clarify better. I am familiar that certain equipment can require higher voltage. What I don't quite understand is why a board that operates 12 volt relays needs to have a transformer that needs 240 volts. It just seems like overkill to me. The power is connected from a different source and all this board does is act as a low voltage switch device.
Not quite sure I follow you.
Your house is supplied with 120VAC or 240VAC.
The usual way to bring this down to 12V is to use a step down transformer. Most houses with door bells and HVAC installations use a "bell transformer" to bring the voltage down to 24VAC or 12VAC.

#### born2fly

Joined Dec 5, 2020
32
Not quite sure I follow you.
Your house is supplied with 120VAC or 240VAC.
The usual way to bring this down to 12V is to use a step down transformer. Most houses with door bells and HVAC installations use a "bell transformer" to bring the voltage down to 24VAC or 12VAC.
Yes, I get that but why not not just have a transformer rated for 110v instead of 240v? That's a common residential voltage. I'm just trying to understand why it's wired that way.

#### MrChips

Joined Oct 2, 2009
27,137
Yes, I get that but why not not just have a transformer rated for 110v instead of 240v? That's a common residential voltage. I'm just trying to understand why it's wired that way.

A device rating and its requirements are two different things. For example, a switch can be rated for 240VAC but can still be used to switch 120VAC.

A transformer is different. A 240VAC to 12VAC is 20:1 step down. You cannot alter this. It can be used to step down 120VAC to 6VAC.

You cannot do the opposite with a 120V transformer.
A 120VAC transformer that steps down to 6VAC cannot be used to step down 240VAC to 12VAC if this exceeds the rating of the transformer.

If your house is supplied with 120VAC and the appliance runs on 120VAC then you are good to go if the wiring and breakers can handle the load current.

If your house is supplied with 120VAC and the appliance needs 240VAC then you are stuck. You would need to hire a qualified electrician to resolve this problem.

Finally, it is common practice for manufacturers to design their appliances to run on either 120VAC or 240VAC. This simplifies manufacturing and stock inventory. For high power appliances this requires changing some connections at the input transformer. It is critical that this is set properly before power is applied.

#### born2fly

Joined Dec 5, 2020
32

A device rating and its requirements are two different things. For example, a switch can be rated for 240VAC but can still be used to switch 120VAC.

A transformer is different. A 240VAC to 12VAC is 20:1 step down. You cannot alter this. It can be used to step down 120VAC to 6VAC.

You cannot do the opposite with a 120V transformer.
A 120VAC transformer that steps down to 6VAC cannot be used to step down 240VAC to 12VAC if this exceeds the rating of the transformer.

If your house is supplied with 120VAC and the appliance runs on 120VAC then you are good to go if the wiring and breakers can handle the load current.

If your house is supplied with 120VAC and the appliance needs 240VAC then you are stuck. You would need to hire a qualified electrician to resolve this problem.
Yes you are correct however what I am unclear on is why a low voltage control board needs 240v to step down to 12v. It could be just as easily accomplished using a transformer that only requires 110v. The equipment gets its power in a different way, not from the controller board. All the board does is close the relay when a call goes to it to turn on the device, it does not supply the actual power for the device.

#### MrChips

Joined Oct 2, 2009
27,137
I added this at the end of my post.

Finally, it is common practice for manufacturers to design their appliances to run on either 120VAC or 240VAC. This simplifies manufacturing and stock inventory. For high power appliances this requires changing some connections at the input transformer. It is critical that this is set properly before power is applied.

#### ronsimpson

Joined Oct 7, 2019
2,399
Yes you are correct however what I am unclear on is why a low voltage control board needs 240v to step down to 12v. It could be just as easily accomplished using a transformer that only requires 110v.
I think we are in agreement. I am certain the transformer steps 120 down to 12 & 5. I know you were told "240" but the transformer only sees 120.
Same for a dryer where the heater is 240 but the light bulbs are all 120 and the control board gets its power from a 120 to 12V transformer.

#### AnalogKid

Joined Aug 1, 2013
10,125
There wasn't any info on the transformer.
Then how do you know it "runs" on 240 V?

I was told by customer that the board runs on 240v.
Not to pick on your customer or anything, but how confident are you in their information?

ak

#### BobTPH

Joined Jun 5, 2013
5,773
I think your misconception is that it “needs” 240 to step down to 12. It certainly could have used 120 when both are available, as in the US. In much of the rest of the world, only 240 is available. So if you make a board needs to work both places, you would use a 240 transformer.

#### Lo_volt

Joined Apr 3, 2014
275
Per AnalogKid's latest reply, it might be worthwhile to confirm that it is indeed necessary for the control board to be supplied with 240 volts. If I were designing a product, I would certainly allow a universal supply (120 to 240 volt input range) to be considered as a possibility.

#### born2fly

Joined Dec 5, 2020
32
Per AnalogKid's latest reply, it might be worthwhile to confirm that it is indeed necessary for the control board to be supplied with 240 volts. If I were designing a product, I would certainly allow a universal supply (120 to 240 volt input range) to be considered as a possibility.
Yes, for universal use, I can accept that. That would definitely make sense. I also get that if the device needs 220v it would be a part of the 'system'. But then again, you could split the feed to get 110v, if it was needed for 110v transformer. <grn>

#### MarcosKDC

Joined Sep 10, 2021
67

A device rating and its requirements are two different things. For example, a switch can be rated for 240VAC but can still be used to switch 120VAC.

A transformer is different. A 240VAC to 12VAC is 20:1 step down. You cannot alter this. It can be used to step down 120VAC to 6VAC.

You cannot do the opposite with a 120V transformer.
A 120VAC transformer that steps down to 6VAC cannot be used to step down 240VAC to 12VAC if this exceeds the rating of the transformer.

If your house is supplied with 120VAC and the appliance runs on 120VAC then you are good to go if the wiring and breakers can handle the load current.

If your house is supplied with 120VAC and the appliance needs 240VAC then you are stuck. You would need to hire a qualified electrician to resolve this problem.

Finally, it is common practice for manufacturers to design their appliances to run on either 120VAC or 240VAC. This simplifies manufacturing and stock inventory. For high power appliances this requires changing some connections at the input transformer. It is critical that this is set properly before power is applied.
How do you get the other 120VAC phase? I must have read something but I don't remember now.
In my country we only work with 230V for monophasic systems and we can have 400V when we're supplied with three phases, but usually only industrial appliances do that.