Tom's Life Bits

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===== DC-DC comments =====

1. Output V1 output must have a capacitor even if it's unused. V1 is used on the board as feedback for some regulation of output.

2. Outputs V2 and V3 can be made to have + outputs by reversing D3 and D4. Polarity of filter capacitor must be reversed also. If you only need one + output only that diode needs reversing. Diodes D3 and D4 are the two laying in parallel.

3. V1 cannot be converted to have a + output.

4. Adding up all of the loads from the spec sheet it appears that the unit provides about 360 mW. This means that any one load or combination should be limited to about 360 mW or at least until Q1 gets warm.
I've measured the DCR of the windings and it appears that the transformer can handle more current.

5. For more power output a transistor with better power capability may replace Q1. Q2 drives Q1 in a Darlington pair configuration. Just don't let the transformer get too warm.

6. It appears that the unit will handle input potential higher than +5 as specified. I don't know the characteristics of Q1 or Q2 so it's difficult to determine the maximum input. I wouldn't go above about 10 volts. Just keep the total load below 360 mW and Q1 cool.

6. I've drawn a diagram of the circuit and I'll send it to All Electronics. What they do with it is up to them.
Warren

A customer from STORRS CT USa
Excellent Circuit for Students to Build
I use this convertor as one of the first projects in a laboratory electronics class that I teach at UConn. I have noted a few things about the circuit that may be helpful:

1. In response to a question from a reviewer, the AC signal was measured to be a square wave with V= 5.98Vrms at about 120 kHz. The voltage is very stable but the frequency is not. The square wave is quite sharp with a transition time of about 100ns. If you try and measure the voltage using a meter that is not a true RMS meter, you will get an anamolous reading.

2. You can substitute higher values for the output capacitors (I recommend 20uF) to get less feedthrough of the 120kHz. The output voltages are identical (-10,-19,-26).

3. This thing gets really hot if you run it for a few minutes with any of the outputs loaded above the resistive loads that are part of the design. Also, leaving out resistors on any of the outputs generates even more heat. Be careful.

4. The most common student issue is that they forget that the outputs are negative and hook of the electrolytic capacitors backwards. Very exciting but other than overheating the circuit, none have been damaged.

5. The output circuits cannot handle any loads that are going to draw much more than a few tens of mA without droop. If you draw 26 mA from the 26VDC output it drops to 24VDC. But this is acceptable for many applications.

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