DC Transformer?! Part 4 – Back to the present

Hello, folks,

it’s a beautiful sunny day here in Rome (caput mundi), Italy, as I get back to you after almost three months of  absence.

What happened in the meantime? Let me summarize it in a few concise statements:

1) I’ve been mostly busy doing something else. This is an unresolved issue of mine, I don’t spend enough time after these projects. I hope to change that for the future.

2) Remember the soldered breadboard? Well, I learned the hard way that simply using a digital multimeter to verify that there are no short circuits on the soldered tracks is not safe: despite having passed this test, our breadboard (shown below at an early stage of soldering) had some tracks that were not properly isolated from each other and didn’t work.

20150306_144557

3) Remember the two transformers (shown below)? Each of them had a 12 V primary winding and two secondary windings: the main 48 V secondary winding, and a demagnetizing 40 V secondary winding. The fact that the 40 V demagnetizing winding had fewer wire turns than the 48 V main secondary winding would have allowed each of the transformers to have a duty cycle greater than 0,5 , so that the ON states of the two transformers could have overlapped for a certain time interval. However, this required a complicated control circuit, and since we had to assemble the breadboard all over again, we chose to… GOTO point4

20150306_144630

4) We used Ockham’s razor (a fancy way to say that we simplified the system as much as possible): we ordered two slightly different transformers: each with a 12 v primary winding and two 48 V secondary windings: one to be used as main secondary winding and the other as demagnetizing secondary winding. This way, the control circuit to be assembled on the breadboard is greatly simplified: it uses just 2 Arduino digital pins (we chose pin 3 and pin 5), 2 transistors and 6 resistors. The updated DC transformer schematic is shown below, with the control and power circuits shown in different colors. Please note that both the control and the power circuit refer to a single transformer: since the system contains two transformers working in parallel, both the control circuit and the power circuit must be duplicated, and that’s why some circuit elements have two labels, one highlighted in orange for the first transformer assembly, and the second highlighted in yellow, referring to the second transformer assembly. Don’t worry, everything will be explained better as we build it…

DC transformer updated circuit diagram
DC transformer updated circuit diagram

5) Last but not least, our interest is shifting from this to the HomeMadeWatts project: we will try to finish the DC transformer project rapidly, so that we can work on HomeMadeWatts, in view of the Maker Faire Rome that will take place in October.

Have a nice day and see you soon!

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Trasformatore in corrente continua?! Parte 2

Buonasera a tutti, mi faccio sentire dopo tanto tempo ma oggi sarò telegrafico (perché me ne voglio andare a casa): ho finalmente completato la versione definitiva dello schema elettrico del trasformatore in corrente continua, che trovate di seguito. Tutti i componenti eccetto i condensatori sono già in mio possesso, ovviamente non ho ancora costruito l’aggeggio e quindi tanto meno l’ho provato, per cui potrebbe presentare ogni sorta di problemi (ed anzi esiste una regola generale nota come legge di Jenkinson che afferma semplicemente: “Non funzionerà”).

Buona serata a tutti ed a presto per la spiegazione…

DC-transformer-schema

DC Transformer?! Part 2

Hi everybody, this will be a really concise post: I’ve finally come up with the final electric layout of the DC transformer, which you find below. All of the components except the capacitors are already in my hands. I’m posting this without having built the circuit and tried it, so it might have all sorts of problems (actually there is a general and very simple rule known as Jenkinson’s Law which states: “It won’t work”).

Have a good day and stay tuned for the explanation…

DC-transformer-schema

Trasformatore in corrente continua?!

Flashback…

Anni ’90, ci troviamo all’interno di in un’aula della facoltà di ingegneria dell’università di Roma “La Sapienza”. Sto assistendo all’esame di elettrotecnica del mio amico e compagno di studi Paolo. Il professore gli ha appena rivolto una domanda sui trasformatori…

Paolo: “… Dunque, il trasformatore in corrente continua…”

Il professore (lo interrompe): “Prego?… Il trasformatore in corrente continua?!?”

Fine del flashback.

Il mio amico Paolo quell’esame lo ha poi superato, ma certo ha rischiato grosso quando ha menzionato il trasformatore in corrente continua.

Questo perché, per farla semplice, il trasformatore è composto da un nucleo in ferro che crea un accoppiamento magnetico tra due avvolgimenti elettrici, come schematizzato sotto.

AC transformer

Quando una tensione alternata V1 (es. 230 Volt) è applicata all’avvolgimento primario, essa genera nell’avvolgimento primario una piccola corrente  I1m chiamata corrente di magnetizzazione, mentre una tensione alternata V2 si manifesta sull’avvolgimento secondario, con V2 = V1*N2/N1, dove N1 e N2 sono il numero di spire degli avvolgimenti primario e secondario. Un esempio dell’andamento nel tempo di V1, V2, e I1m è mostrato di seguito.

Transformer magnetizing current graph

Se invece si applica all’avvolgimento primario una tensione continua V1dc, una tensione continua V2dc (= V1dc*N2/N1) si manifesta sì sull’avvolgimento secondario, ma la corrente di magnetizzazione I1m inizia da zero e continua a crescere finché non produce un danno da sovracorrente, come mostrato nel grafico che segue.

Magnetizing current graph for transformer subject to DC voltageIn conclusione, il trasformatore non può essere utilizzato per trasformare tensioni continue, bensì solo per tensioni alternate. E questo spiega anche l’agitazione del professore quando Paolo ha menzionato il trasformatore in continua.

Detto ciò, la mia idea consiste nel prendere un normale trasformatore a frequenza di rete (50 Hz), e, aggiungendo un pizzico di interruttori a stato solido (es. MOSFET) e diodi, trasformarlo (scusate il gioco di parole) in un trasformatore in corrente continua, che il mio amico Paolo aveva previsto già tanti anni fa. Non è niente di rivoluzionario, adatterò semplicemente degli schemi di circuiti ben noti.

Che dire, buon anno e rimanete in ascolto!

DC Transformer?!

Flashback…

We’re back in the 90’s, on the grounds of the faculty of engineering of the university of Rome “La Sapienza”. I am watching as my friend Paolo is taking his electrical engineering exam. The professor has just asked him about the transformer.

Paolo: “… Well, the DC transformer…”

The professor (abruptly interrupting him): “Excuse me? The DC transformer?!?”

End of flashback.

Well, my friend did pass his exam after all, but certainly he was risking big when he mentioned the DC transformer.

Why? Because, to simplify it, a transformer is composed of an iron core which creates a magnetic coupling between two electric windings, as shown below.

AC transformer

When an alternating voltage V1 (for example 230 Volts) is applied to the primary winding, it generates a small alternating current I1m called the magnetizing current in the primary winding itself, and an alternating voltage V2 also shows up on the secondary winding, with V2 = V1*N2/N1, where N1 and N2 are the number of turns of the primary and secondary windings. An example of the time graph of V1, V2, and I1m is shown below.

Transformer magnetizing current graph

However, if a DC voltage V1dc is applied to the primary winding, although a DC voltage V2dc (= V1dc*N2/N1) does appear on the secondary winding, the magnetizing current I1m starts from zero and keeps getting bigger until overcurrent damage occurs. This is shown in the graph below.

Magnetizing current graph for transformer subject to DC voltageConclusion: the transformer cannot be used to transform DC voltages, but only to transform AC voltages. And that explains why the professor was a bit upset when my friend Paolo mentioned the DC transformer.

That said, my idea is to take a common 50/60 Hz AC line frequency transformer, add a few solid state switches (MOSFETs), diodes and so on, and turn it into a DC transformer (foreseen so many years ago by my friend Paolo). This is nothing revolutionary, I will simply adapt some well known circuit layouts.

Well, happy new year and stay tuned!