The need to drop some volts
I mentioned in a couple of previous posts that my giant toroid transformer that I want to use for my power supply was a little too beefy for my purposes. I mentioned in the most recent blog post that transformers are often rated at just below normal mains voltage to provide a “guaranteed minimum” and that once you combine this fact with your rectifier and filter, you actually end up with a higher voltage than you originally intended. Most of the time, this is great for ensuring you can overcome that pesky dropout voltage (around about 2.5V for most regulators to be on the safe side) but in some cases, it can get dangerously close to frying said regulators by exceeding the max input voltage (maximum voltage differential).
In my case, the chunky toroid, after rectification and filtering with no load gives me a rather beefy 42.6V when on paper it should have been 30V. Given that most common regulators have a maximum input voltage of 35-38V, it becomes obvious that I had to step it down a tad. There were a few options to do this:
- Unwind the secondary of the transformer a few winds
- Use series diodes to drop the voltage by 0.7V each
- Construct a pre-regulator to drop the voltage to safe levels
I have examined each of these in turn and came to to the following conclusions:
Thing of beauty, don’t want to wreck it
The toroid is a lovingly packaged beastie, professionally wound in crazy spirals, wrapped in plastic and nicely presented with leads. Since I barely know what I’m doing, it would be unwise in the extreme to mess with it. Why break something to make it work when other solutions can work just as well?
Isn’t that gross looking? Embarrassed to say I made that…
I did try the series diodes, and was able to get a voltage drop of 4.2V by bodge soldering 6 of them in series. As you can see from the photo, there are quite ugly and apart from appearances do not reassure me that they will hold together well and 4.2V is not nearly enough of a drop to be useful to me. Essentially, I made a mess of 6 diodes to only get down to 38.4V, barely outside the red danger zone. Also, the legs on the high-power diodes are quite chunky and would be a severe pain in the ass to get into a pre-drilled circuit board and I will not have them floating in the air inside the case in case heat/cracked joint/whatever causes them to fall apart starting a fire. Also, when I’m long dead and someone opens the case to see what I’ve built, do I really want them to see that bodgy mess? No way man.
The third option is one I considered before and after thrashing about with the other ideas, it has been revealed to be the best one. Difficulty is, with voltage regulators having the limitation mentioned above I couldn’t use them. As mentioned in a previous post, I am going to go with a simple, yet high powered idea of using a darlington transistor biased with a zener diode. I was able to acquire some 30V zeners (part: 1N4751A) so these two, plus a couple of biasing resistors is all I need.
Here, I ran up a simulation to illustrate:
Please ignore the 2N3904/3906 transistors, they are mere placeholders for the TIP142/147 darlingtons I intend to use since CircuitLab didn’t have either in their box of parts. Only me building it up would determine whether or not I have to adjust any of these values/parts or not. I like living dangerously.
I chose a 10Ω load resistor to simulate a current output of 3A at around 30V. The actual output voltage doesn’t need to be precise as it will be further regulated later, so long as it’s above 26.5V or so it will be fine.
The tricky bit is biasing the thing. I first assumed that I could use a 10kΩ resistor to power the zener and bias the transistor. I quickly figured out that this isn’t enough, especially when under load. The zener needs a minimum of 5mA to get that nice voltage drop going on and the transistor needs a proper current to regulate the voltage.
Through fiddling about, I settled on what I think is a good compromise. My zeners are 1W so there’s one limitation I have to consider. Though I am well within the max collector/base voltage limit of 100V, the max base current is 500mA, which will be hard to come close to, but worth noting.
Through messing about with the load, I noticed that it had a hard time regulating itself at higher currents and I kept having to drop the biasing resistance to keep it working. This presented a problem of higher currents both for the base and the zener. Though the base can handle it and then some, the zeners were fast approaching their power limit.
The compromise is the 470Ω biasing resistor. It seems, at that level, to keep it’s regulation up to 3A more or less which is what I chose as my maximum current output anyway. Any higher than that my current limiter will drop the voltage to keep it from melting itself.
This also keeps the zener/base current steady at 26.84mA. Though this seems like peanuts, given the voltage drop across the zener the power dissipation would be about 800mW or 80% of it’s maximum.
Though I realize some of that current is going into the base of the darlington transistor, I really don’t want to try lowering the resistance any further. Always better to have some headroom in case Murphy and his law show up. Ohm, and his law, also indicate that the resistor will dissipate a third of a Watt (0.338 to be exact) so 1W 470Ω resistors would be nice to have. As it turns out, I have one.
If I need to get higher power parts, this is also possible. A 30V Zener diode 5W (part: 1N5363B) exists and is cheap, and i’d nead a helluva resistor to go with it, probably a 10W 100Ω. I think I can avoid doing that though.
In a few minutes, I’ll head down to the lab and build it up.