DC transformers

[Special K]

I took apart a small digital alarm clock and noticed a device on the PCB that looked like a tiny transformer. This clock is powered by an AA battery only (no option for AC). I did a search on google for DC transformers and found a few pages but nothing that gave a simple explanation about how these devices work. All I could find was a description of a circuit that accepted an input voltage and switched a transistor on and off at a high frequency (kHz range) to give the effect of a different DC voltage across the load (there were other components in the circuit, but that was the basic idea). However it seems to me that these devices would be ICs and look nothing like an AC transformer. The device I saw looked just like an AC transformer. Does anyone know what this device might be (or could tell me more about DC transformers)?

 

[sgtroyer]

You're right, a DC-DC converter shouldn't look like a transformer. However, one of the components of a DC-DC converter is an inductor, which I suppose could look a lot like a transformer. I'm not sure why an alarm clock would need a DC-DC converter, though, so I wouldn't assume that's what it is. Do you have a close-up picture?

As for how a DC-DC converter works, here's a rough sketch. A transistor is connected in series with an inductor. When the transistor is turned on, current flows. When it's turned off, the inductor tries to force the current to keep on flowing. It can't go through the transistor, so it spills onto a capacitor (in parallel with the transistor). A diode ensures that current can charge up the cap, but not discharge it. Each switching cycle increases the voltage on the cap.

 

[Special K]

I'll see if I can get a picture of it today or tomorrow. The circuit you describe is exactly what I saw in the description I mentioned above.

 

[Mark R]

DC-DC convertor circuits come in two sorts.

Isolated (e.g. in your PC PSU) where the output is electrically seperate from the input. This is achieved by using a transformer. (This is used for safety reasons - A typical PC PSU operates on 340V DC derived directly from the mains)

Non-isolated (e.g. in mobile phones or laptops) where the output is connected to the input. Here the conversion is achieved with an inductor.

The construction of an inductor is essentially identical to that of a transformer - they will look virtually identical. The difference is that the core material used in a transformer is designed not to store energy, whereas the core in an inductor is designed to store it. There are other minor differences - a transformer must have at least 2 seperate windings. An inductor only needs 1 winding but may have more.

 

[FrankSchwab]

The reason for the DC-DC converter in the clock is simple -
Most (all?) of the electronics are incapable of running from the voltage of a single battery. Even with two AA's in there, you're only getting 3.0V, and that only when the batteries are brand new. The DC-DC converter is there to generate the higher, regulated voltages needed by the IC, the LCD drivers, and the backlight.

/frank

 

[Special K]

OK, I've put some pics up at the neffing website:

PIC 1

PIC 2

PIC 3

Can anyone tell me exactly what that thing is?

EDIT: apparently I'm linking to other pictures now...

 

[Krakerjak]

It is just a coil used to step the voltage up to a required level for the components from the 1.5 or 3 volts the batteries would supply.

 

[Special K]

Originally posted by: Krakerjak
It is just a coil used to step the voltage up to a required level for the components from the 1.5 or 3 volts the batteries would supply.

Yeah but you can't use a standard transformer with the DC voltage that the battery supplies. That's why I was asking about the components of a DC transformer.

 

[dkozloski]

The reason you can't find information on a DC transformer is that there is no such thing. In fact the subject was a joke in my basic electronics school 40 years ago like a left handed monkey wrench or green lamp oil. In order for a transformer to function it requires that a magnet field expands and contracts thus the lines of force can cut wire "turns"and induce current flow in the secondary. This is by definition AC current. DC sources such as batteries can power switching circuits that produce a varying DC current that feeds the primary side of a transformer.

 

[syberscott]

Originally posted by: SpecialK

Originally posted by: Krakerjak
It is just a coil used to step the voltage up to a required level for the components from the 1.5 or 3 volts the batteries would supply.

Yeah but you can't use a standard transformer with the DC voltage that the battery supplies. That's why I was asking about the components of a DC transformer.

There is really no such thing as a DC transformer. The DC battery voltage is switched at a high frequency, then fed through the transformer and then rectified back to DC power. By switching the DC power from the battery very fast it makes the transformer "think" it's getting AC power.

 

[Mark R]

The device shown in the pictures, could be either a transformer or an inductor. It is impossible to tell from the photographs, although given the application it is very likely that it is an inductor.

A photo showing the connections to the device would be very helpful - if there are only 2 connections, then it is without doubt an inductor.

Transformers can be used in DC-DC converters, but it is essential that the input voltage be switched. The switching action, acts similarly to converting the DC into AC. If the switch fails (stuck on) then the transformer action will fail resulting in rapid destruction of the switching circuit.