I got an email from QS showing how he put the buck DC-DC downconverter right at the point where it plugs into the proto board. This is really the way to go to eliminate any IR drops from the resistance of the leads. There are two items which I am concerned about, one being the ripple current that may be coming out of the converter, and the other is if the proto board itself might have internal resistance that could cause IR drop. One has to remember that when we are working with 15 volts output and the ripple is a half volt, the ripple is only 1/30th or 3.3% of the output voltage. But when the output is 1.5V and the ripple is a half volt, the ripple is 1/3 or 33% of the output voltage – way too much!
I can address the first concern by using a linear regulator such as the LM317. Two issues here that have to be dealt with, one is that it cannot go below 1.25VDC. That’s okay for working with circuits designed to run on a single alkaline cell, but if the circuit is to work with a 1.2V rechargeable, then there has to be a way to go to a lower voltage. I’ve thought about this, and I figured that a 5 or more amp Schottky rectifier in series with the line would drop about 1/4 to a half volt, but it would not be well regulated. Another way would be to use a negative supply for the adjustment pot, but I have had major problems with this method.
The other issue with the LM317 is the limit on the current, generally 1 amp if it has a good heatsink, but less if the regulator gets too hot and shuts down. I can reduce the input voltage to 5 volts, and it might be able to handle the full 1 amp current or maybe more without too much heat.
My other solution to eliminate the ripple is to use an LC low pass filter on the output of the converter. This would be a choke of very heavy wire, and one or more low ESR capacitors. I can get these from the low voltage section of a power supply from a PC. I can also use some ferrite EMI suppressor sleeves over the wires – I have plenty of those. I’m not sure what the minimum output voltage is for the converter, but I believe is is a bit lower than the LM317, maybe 1 volt.
I downloaded the datasheet for the LM2596 switching converter. This document is on the TI website, but its author is National, which I find odd. The document tells all about how to design the circuit for this chip. I found the Application section and especially about the capacitors very informative. Also the inductor selection section. From what I read, the best way to reduce the ripple in the output is to put an inductor and another low ESR capacitor on the output, as I mentioned above.
I would like to have the power supply go down to zero volts, but I can’t do it simply with just a linear regulator or converter. I have some schematics for power supplies that go to 0 volts, but I think it’s cheaper and more effective to buy one already made. It cost me only $60 for a new Mastech 15 volt 3 amp power supply with a digital display. The disadvantage is that the Mastech weighs a dozen pounds and is too big to have close to the proto board.
The other thing QS showed was a digital voltmeter called a DPM of digital panel meter. The bright blue LED display really caught my eye, as did the two digits to the right of the decimal point. The Mastech has a cheap three digit red LED meter that can only give a single digit to the right of the decimal point, which makes it worthless for low voltages. That’s why just about everyone uses those cheap $4.00 Centech DMMs from Harbor Freight or wherever. I can read three digits to the right on the 2V range.
I have seen two types of DPMs, the kind that requires a separate isolated power supply or battery, and the kind that can use power from the same supply that it is measuring. I assume the one he has is the latter, it can run from the same supply it’s measuring. It has ‘only’ two digits to the right, not three like the Centech multimeters have. But then Centech meters are only accurate to about 1 percent, which means that the readings of the third digit to the right may not be accurate at all.
