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Transformers in series, is this possible?

Thevenin

Oct 27, 2009
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Hello, I have two transformers, onw was an old cellphone charger (5V, 1.2A) and the other was on old CD walkman transformer (4.5V, 500mA) I'm wondering if I can connect their outputs in series to have 9.5V.

Thanks.
 

ymiky

Oct 29, 2009
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Oct 29, 2009
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No,maybe 5V 1.2A is enough to get the 9.5V voltage
Note:5V 1.2A is just a rating,usually the output voltage is related to the load,without load,the transformer may output10V or more,it depends.
 

Thevenin

Oct 27, 2009
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I thought it was always going to give the same voltage, and what was going to vary was the current, always 5V and the current depending on the demand of the circuit and it depending on its impedance. Could you explain me that you said?
 

Tommya300

Oct 28, 2009
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You can connect them in series making sure that they both are an ac or a dc output device can not go anywhere with the AC riding on the potential of the DC the ac signature will be aprox. positive 5 volts higher or lower at plus or minus 10 volts depending on the dc polarity. and the opposite going wave will be aprox, zero. A good way of destroying a perfectly good device if connected it. Mahe a CD player into a small toaster just for a millisec or so.
The lower current rating will be your bottle neck of aprox 500 mA.
Really not the better of ideas Check it out on an O'scope.
 

Thevenin

Oct 27, 2009
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You can connect them in series making sure that they both are an ac or a dc output device can not go anywhere with the AC riding on the potential of the DC the ac signature will be aprox. positive 5 volts higher or lower at plus or minus 10 volts depending on the dc polarity. and the opposite going wave will be aprox, zero. A good way of destroying a perfectly good device if connected it. Mahe a CD player into a small toaster just for a millisec or so.
The lower current rating will be your bottle neck of aprox 500 mA.
Really not the better of ideas Check it out on an O'scope.

Hi Tommya300, I'm affraid I didn't understand what you said They're both DC output decives, one is 5V, 1.2A and the other is 4.5V, 500mA, but those other things... are you sure you axplained yourself well?
 

cj_elec_tech

Oct 7, 2009
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Hi Thevenin.
If both the units output DC, you can connect them together in series to increase the voltage (just make sure to get the polarity right, like you would with a pair of batteries). However, be aware that the units might not provide regulated DC outputs.
If this is the case, the voltage will change depending on the load you apply.
(Originally the phone drew varying current levels from the unit when it operated or charged the batteries and this will alter the output voltage of the power unit) - I think this is what Tommya300 is saying and ymiky too, and they're correct.
The output voltage change is due to the power supply's internal impedance (think resistance) and occurs primarily due to Copper losses, Iron losses and heating within the transformer.
But if one of the units is DC and the other AC - and you connect them in series - you'll end up with a sizable AC component 'riding' on top (added to) the DC voltage and that could be BAD for your circuit - Tommya300 says that too.

My advice is Test First!
Use a multimeter and a dummy load (resistors) to simulate your circuit load and verify the voltages you measure (I guess you don't really want your circuit to be killed, do you? ;) )
Let us know how you go.

CJ
 
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Thevenin

Oct 27, 2009
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Thank you very much CJ, now I can understand it, and yes they're both DC outputs, and yes also see that if they're one AC and other DC will be a problem, and as soon as I can I'll be testing it with the multimeter, but right now I don't have one.

So how possible is that the output voltages change? I thought that they were constant in that (as I said) and what varies depending on the load was the current, they can both vary?, if you can explain that to me or give me a link to somewhere I can get that explanation I would apreciate it very much. How can I know if they provide regultated DC outputs, testing right?

Thanks.
 

Resqueline

Jul 31, 2009
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So how possible is that the output voltages change? I thought that they were constant in that (as I said) and what varies depending on the load was the current, they can both vary?, if you can explain that to me or give me a link to somewhere I can get that explanation I would apreciate it very much. How can I know if they provide regultated DC outputs, testing right? Thanks.

It depends on wether they are stabilized/regulated or not.
The 5V one might be but the 4.5V one is probably not.
Unstabilized ones consists of just of a transformer, a rectifier, and a capacitor. Small ones might deliver 50% more voltage when unloaded than when fully loaded (which are the values indicated on the label). Consider this as a (+50%) perfect voltage source in series with a (internal) resistance - which will give you a voltage drop as you increase the current.
The regulated ones have active electronics and are usually light-weight. They deliver a (more or less) constant voltage.
Testing is the only way to tell unless you have the experience to "just know" it. There's rarely any giveaway on the label.
 

Thevenin

Oct 27, 2009
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Thanks for the reply Resqueline, now I can understand, so that's why the power supply of a PC has all that circuitry, right? it most be very stabilized to supply exactly 5V & 3.3V and the other ones that I don't know. And for a specific load.
 

Resqueline

Jul 31, 2009
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Yes, high performing digital circuitry needs well regulated supplies (+/-5%) to "stay on top of the situation".
PC's used to run on +/-5V & +/-12V. The negative voltages are not used in modern machines, instead they need 3.3V. The 12V is converted to 2.5V & 1.6V on the motherboards to run the RAM & the CPU respectively.

Simple radio/audio devices are usually very forgiving about their power supply, but sometimes they contain internal regulators for critical functions.
 

Thevenin

Oct 27, 2009
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Thanks, then I might have some only transformer, bridge, C filter & zener transformers right? but you said that those regulated ones are lighter, then a should ask, why? and say that one of my chargers is very light compared with the other one that is considerably heavier.
 

Resqueline

Jul 31, 2009
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Good old fashioned transformers that operate directly on 50 or 60Hz have to be made from lots of iron & copper.
Modern power supplies are made with switch-mode technology. They rectify the mains, chop up that dc and pass it through a small ferrite-cored transformer & then rectify it again. The high switching frequency (20-200kHz) enables the use of considerably smaller transformers with much less copper for the same power.

Only rarely does wall-warts use zeners for stabilization, and only low powered ones (less than 1W).
 

Thevenin

Oct 27, 2009
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Thanks for the reply Resqueline, I've seen those transformes (very small transformers, maybe those are wich you're talking about), and with a whole circuitry rather than only bridge, C filter and zener.

Thanks
 
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