Two 110Volts transformer on 220 Volts

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lilimike

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I have this design for North America:


Note: my drawing is not perfect, P1 is the power source from the house 110V.

The purpose of this circuit is to plug a motor or other devices in P2 or P3 and get the Amps reading while monitoring the supplied voltage.

Now I need to reproduce this for example: Dubai on 220 Volts.
Questions:
1 - Can I connect two 110V transformer's primary in serial and still get the 12 Volts secondary from each?
2 - Do I need to double the shunt resistor to get accurate Amp readings?
3 - Does the 50hz affect anything in the circuit?
 
Anybody have a way to buy UK wall plug/switch here in Canada?
I believe I need type G, searched Digikey for type G, wall plug, UK plug but nothing is coming up.
 

1- Only if they are two identical transformers and the secondaries feed identical loads. You might get away with it, but 220VAC transformers are readily available. In your case I would get a single 220Volt transformer with two isolated secondaries. Search for brand name Signal Transformer, DPC or DST series and choose one with the appropriate power your displays need. Wire the two primary windings in parallel for 110VAC, or in series for 220VAC. Use the two secondaries individually for your two displays.

2- Only if you change the scaling on the Amp display too. The voltage across the shunt for a given current won't change just because the system voltage doubles.

3- In most cases, no.
 
I have found a voltmeter that doesn't need to be powered from a low voltage so only the Ammeter will be powered. One of the transformer will be there just to allow for 220V.
Replacing the transfos by one that accept 220v is not an option due to cost issues.

I guess I can test here by using the 2 110V phases.
 
Don't do it.

As I said before, you can only series two identical primaries if the outputs of both are loaded equally. If one output is loaded and the other isn't, then they will have different primary currents. This mismatch will show up as an imbalance when you try to series connect them, resulting in different voltages across the two primaries.
 
Both my transformers are identical, If I connect the primaries in series and drive the ammeter with just one transformer, when you say it will create an imbalance are we talking about a few volts or enough to blow the transformers?
The ammeter requires 8 to 12 volts AC so if the voltage drops by as much as 4 volts I am still ok.
If not, can I calculate the current used by the ammeter and connect a resistance as a dummy load on the other transformer?
 
You can't to it. It will have an adverse effect, even if there's only a slight imbalance. The only way to use two identical transformers' primaries in series is to also have the secondaries in series (or parallel) so they can share the load.
 
What panel meters are you using? I know you mentioned that you can't use an alternative transformer due to cost concerns, but it is the only sensible way forward, plus you would be replacing 2 transformers with a single one so should work out a fair bit cheaper in the long run
 
crutschow said:
You can't to it. It will have an adverse effect, even if there's only a slight imbalance. The only way to use two identical transformers' primaries in series is to also have the secondaries in series (or parallel) so they can share the load.
Click to expand...
I only need one source of power so if I connect the secondary in parallel am I good to go?

I am using this ammeter:
**broken link removed**
This link is for a DC ammeter, mine is the same but configured for AC measurements.

I bought hundreds of small transformers at $1.50 each, using two is costing me $3.00 I could not find a 220V primary, 8 to 12V secondary and buying from brand name I found nothing under $15 I have to make between 50 to 100 units so it adds up...
 
Last edited:
I have a few similar LCD type panel meters that have a similar power requirement. For one of my projects a few years ago I just powered them with a simple capacitive coupler type transformerless power circuits. If I remember correctly even with the back lighting on mine didn't need much over 50 ma at best.
 
Without trying too hard, I came across quite a few 3W transformers with with a 9-0-9 or 12-0-12 out and 220 primaries for less than $2 from the same place, i.e the far east on aliexpress. I suspect that in the quantities you would be looking for there's a negotiation to be done on price. You only need 15mA, so you could go even smaller on the transformers and if you could find a source of manufacture, you also have the opportunity of a dual output winding saving you the cost of one of the transformers. These were small split bobbin types, perfectly safe in your application. I appreciate it's a pita having to take one for the team so to speak, having already purchased the transformers that you have, but if your project takes off in a big way, it makes much more sense to be better prepared to make some money from it and give yourself less headaches. Some potential cost savings might be, for example, only a single transformer, only one to mount in an enclosure, wiring, construction time etc. You also have an opportunity to use the transformers that you have in some other project, so it's not like you will never see a return from them
 
I bought hundreds of small transformers at $1.50 each
Click to expand...
If that is the case, instead of putting the two transformers in series with the secondaries providing separate supplies, think about this:

Where you have to power the meter from 240v, use two transformers in each position. ie a total of four transformers for each unit.
Connect the primaries in series to give the 240v operation, and connect the secondaries in parallel to give the 12v for the meter.
While this looks to be an extravagant waste of transformers, if you already have them at low cost, and there is sufficient space in to box to accommodate them, why not use them?

JimB
 
lilimike said:
I have found a voltmeter that doesn't need to be powered from a low voltage so only the Ammeter will be powered.
Click to expand...
The voltmeter I have is powered directly from the live where it gets its sampling from and supports up to 500Volts. Therefore I would keep my original 2 transformers, connect the primary in series and secondary in parallel.
I would use up the stock I have and then move onto something like this:
https://canada.newark.com/myrra/44159/transformer-2-3va-230v-1-x-12v/dp/46P4912
and add a small PCB or find equivalent panel mount.

Does this makes sens or am I still playing with fire?
 
If the voltmeter works directly from the supply, then using two transformers for the ammeter makes sense to me.
I don't think that you will have problems with smoke and flames!

JimB

PS, I like your cat.
 
I am looking at a next revision for my project using this transformer: https://www.farnell.com/datasheets/92146.pdf
I will make a small PCB with two headers and heat shrink tube.

The specs call for 230V primary with no indication of max. If fed with 250V I am safe with an extra volt on the secondary but can the primary take the extra 20 Volts?
In my original design I have a fuse in line with the transformer as I was using surplus stock, for this new transformer based on the indication "INHERENTLY SHORT CIRCUIT PROOF TRANSFORMER" can I safely remove the fuse?
 
There is a transformer design called "Energy limiting" and a short on it's output causes the transformer to heat up a bit, but no damage occurs. You sometimes find these on Furnaces.
 
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