ECN Forum Forums General Topics of the Trade General Discussion Area Elementary Q... DC Xformers?

Just proof that there's no dumb questions...

Especially compared to some of the ones I come up with!

My GPS operates on 3 VDC. My cell phone charges at 6 VDC. My truck obviously supplies 12 VDC to the "cigarette lighter".

Without some complicated inverter/transformer/rectifier setup, how does one reduce the DC voltage? Resistors?



I should mention that this is a curiousity Q, both my GPS and cell phone work just fine plugged in as is!

I just don't know how it works!

[This message has been edited by sparky66wv (edited 09-24-2002).]

well i know DC can be 'inverted', but i've never seen it 'transformed'...


[i]then again i live a sheltered life in the puckerbrush......

... A TRANSFORMER!

The simplest way to reduce dc voltage is a transformer. Radioshack has a few wrapped up in black plastic for you here . Trandsformers work the same for AC and DC. You could use a series resistor but you would have to match it to your load.

Well, I'm confused then...

What's all this about no induction in DC and Tranfomers working by induction?

Do they work magnetically? Like an electromagnet?

Was Edison simply duped by Tesla?



Hey! I suggested that it was a dumb Q!



Strangedog,
Linkie no workie...

[This message has been edited by sparky66wv (edited 09-24-2002).]

Yep, the simplest way to reduce the voltage is to insert a series resistor; just calculate the required value based on the current using basic Ohm's Law.

There are a couple of problems with this simple approach. First, if you are dropping a large proportion of the supply voltage you waste a lot of power, e.g. 3W 3V load running off of 12V results in 9W wasted as heat in the series dropper. And you've got to dissipate that excess heat, of course.

The other problem is that the series resistor isn't very good at maintaining a stable voltage with varying loads. Ohm's Law obviously dictates that as you increase the load the voltage across said load will drop.

Another possibility is to use a potential divider of two resistors. This can work with reasonable voltage regulation in very low-power applications, e.g. where the load resistance can be much higher than that of the lower resistor in the divider so that changes in load will have negligible effect. But for any sort of sizeable power the current flowing through the potential divider ends up being much higher that that through the load, so it's just not practical.

Going beyond simple resistors, a fairly standard electronic voltage regulator circuit works in the same way as a simple potential divider but with a zener diode in place of the lower resistor. The zener diode effectively alters its internal resistance to maintain the voltage across it at a near-constant value. It has the advantage that it will regulate both for changing load and for changing supply voltage. (The pre-semiconductor equivalent was the cold-cathode voltage stabilizer tube which worked in the same way, from an external point of view.)

The basic zener diode can only operate within a certain current range, however, so the most usual arrangement in regulated DC supplies is for the zener to act as a reference voltage which feeds into a transistor wired as an emitter-follower. The transistor provides no voltage gain in this configuration (hence the output voltage follows that of the zener) but it does provide the necessary current amplification so that a small zener current can regulate a much greater output current to the load.

Many regulated DC supplies incorporate op-amps and other much more sophisticated circuitry, but the basic principle remains the same. The series-pass arrangement still results in the dissipation of some heat, but it provides good voltage regulation.

As for changing DC levels with a normal transformer, it won't work. The DC step-up converter modules commonly sold these days use the supply to drive a high-frequency oscillator, the latter is then rectified back to DC (i.e. basically the same arrangement as an inverter-rectifier or UPS).


[This message has been edited by pauluk (edited 09-24-2002).]

The simplest way is a voltage regulator circuit. The extra voltage is disipated as heat. If you have a device that draws a high current you need a heat sink. You can use zener diodes aas well for low current divices.

Thanks again, guys...

I expected a simpler answer, now I don't feel so stupid!

I wondered how those "DC cigarette lighter adapters" worked...

No desire to make my own, though!

Cheers Paul!
to the rest: http://www.twysted-pair.com/

A transformer with DC well thats a new one.The only way I have ever heard of a transformer working with DC is if the DC is switched, with a transistor or some other means.
The old ignition systems of cars were DC and a set of points (contacts)the points broke the circuit everytime the high spot in the distributor came around.
For a regulated output they used to put a zener in the base-emitter circuit of a power transistor and take the output from the emittor side of the transitor but again these were filtered power supplies, but the output was always about .7 volts higher on the output because of the forward biased emmitter-base junction.
But as far as using pure DC with a transformer can not be done.

Mark

Technically there is a fly in the ointment. You can transform DC, but not change the voltage - You transform it into heat