Nuts and Volts - November 2010

software. I use Raptor at college ( ITT-Technical Institute) and it is available at http://raptor.martincarlisle.com. Best of all, it’s free and really small.

— Joseph Fulton Dear Russell: Re: Soldering Iron Timer, March ‘ 10, Page 28.

— Anonymous Dear Russell: Re: Mystery Component, July ‘ 10, Page 25.

Response: Thanks for the feedback, Warren.

In your column, a person asked about a mystery component in a microwave oven. The component inside the transformer is a thermal fuse. It blows under two conditions: over current and over temperature. If this fuse is blown, the transformer is considered bad. I used to repair microwave ovens and the only way to get this transformer is to buy a new timer unit. Putting any part in there will compromise the UL safety.

Response: Thanks for the feedback, Randy. I would forward your comments to Jon Carter if I could find the original email!

Dear Russell: Re: Power Supply, July ‘ 10, Page 24.

According to the Hammond transformer website — www.hammondmfg.com /pdf/5c007.pdf — IDC output = . 62 x IAC for a FWB with cap input power supply. That means that their 3. 8 amp transformer will only produce an output of 2. 3 amps DC when used in your circuit.

— Mike Eck

Response: Thanks for the feedback, Mike. You are right, that transformer will overheat if loaded to three amps. Your comment brought to my mind the question: How much inductance — in series with the rectifiers — will produce a continuous current such that full VA rating applies? Trial and error simulation indicates that 5 mH will do it but I can’t find an inductor rated 5 mH at three amps. One could connect seven 680 uH inductors in

■ FIGURE A

QUESTIONS & ANSWERS

series (Mouser PN 542-2322-V-RC) if you want to do it.

Dear Russell: Timer, July ‘ 10, Page 25.

Thank you for your suggestion for building a two-hour timer. I am enclosing some pictures which I took of the finished units (Figures A and B); two were made. Your circuit was used almost exactly as you presented it except that I omitted the trimpot. I used a larger series resistor (560K) paralleled with one determined using a decade box.

The units as originally built controlled the AC line using a 4015L2 thyristor/MOC3020 solid-state relay combination. However, I found that the power was not being turned off with the very small load of a small “wall-wart.” The voltage dropped to only 65 volts! Only then did I think to check the specs of the 4015L2; the minimum current is listed at 0.8A! One of those obscure facts you learn with experience. As you can see in the pictures, a relay was substituted.

Since only two were required, the circuit was assembled on a Vero board rather than an etched one. The timing capacitor was metallized polyester rather than an electrolytic for long-term reliability. I thought that you might like to see how one of your “babies” turned out. Once again, thanks.