Nuts and Volts - September 2008

and switching the other off. IC4 is a FET driver; it can supply a lot of urrent for a short time, which is what is needed to charge the input capacitance of the FET. R3 and R4 limit the peak current that IC4 has to supply. The FET is rated 100 volts and 22 amps so it is well within its rating, even though the input current will exceed 20 amps under full load, because each one is on 50% of the time.

For the transformer, I used a 12 pin bobbin that was on hand. I tried a smaller one but could not get enough turns on it. I tried to wind #16 wire but that was impossible so I used four #27 wires in parallel. Using parallel wires is an advantage because the current runs on the surface and multiple wires provide greater surface area. The transformer layout is Figure 4. By starting on pin 1 and finishing on pin 11, and starting on 2 and finishing on 12, I can run power straight across from 2 to 11

for the center-tap, leaving 1 and 12 for the transistor drain connections. On the secondary, winding from pin 9 to 4 then 6 to 7 allows me to jumper 4 and 6 for the center-tap, which simplifies the layout. The bobbin is Magnetics #PC-B3515-L1 and the core is Magnetics #OF43515EC.

A big problem when operating in an automotive environment is protecting the circuit from transients which can reach 60 volts when starting, or when the battery is disconnected while running. I have not tried the transzorb approach before but if it works as advertised, the peak voltage should be limited to 15 volts. The output has multiple filter caps to handle the ripple current and to reduce the ripple voltage. Figure 5 is the parts list.

can increase C1 for longer time. You can substitute another MOSFET but it must have a logic level rating. I did not put a diode across the relay because it turns off slowly and the 100 volt rating of the MOSFET is unlikely to be exceeded. The Mouser part number for the MTP10N10EL is 863-MTP10N10ELG and the 0.33 μF cap is 581-BQ024D0334K ( www.mouser.com).

POWER TRANSFORMERS

RELAY DRIVER

QWhat I lack now is information on the physics of constructing my own power transformers. Do you know of an outlet to purchase components to make a power transformer, i.e. the core, bobbin, and magnet wire? Also, what is the deal on toroidal transformers? Any information on constructing those? Notes on what gauges of wire produce what voltages would be helpful also.

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— Tyre Daniely

10T

21T

QI need a circuit to key a five volt SPDT relay using the external speaker output from a GMRS radio. Once keyed, it needs to remain keyed as long as a signal is being received. Use transistor or op-amp circuits if possible.

— Rileys Electronics

FF

S

F

6 to 12 volt xfmr, bottom view
12 June 2008
Each winding consists of 4 #27
wires in parallel.

■ FIGURE 4

AThis circuit (Figure 6) will work as long as the input signal is one volt peak or more, but don’t exceed five volts without a reverse diode to ground at the NPN base. The turn-off time is about 1/2 second, but you

■ FIGURE 5

SIX TO 12 VOLT CONVERTER PARTS LIST

PART DESCRIPTION MOUSER P/N ■ IC1 555 Timer, CMOS 511-TS555IN ■ IC2 4013 Dual D Flip/Flop 595-CD4013BME4 ■ IC3 4081 Quad 4 Input AND 595-CD4081BE ■ IC4 MC34152 FET Driver (SMD) 863-MC34152PG ■ Q1, Q2 N-FET, 60V, 22A, TO-220 512-FQPF30N06L ■ D1 Zener, 5V, 1/4W, 1N751 512-1N751ATR ■ D2 Schottkey, Dual, 100V, 12A (SMD) 844-12CWQ10FNPBF ■ Z1 Tranzorb, 10V, 1.5 k W 625-1.5KE10CA-E3/54 ■ C1 100 pF, 50V, 5%, NPO 140-50NS-101J-RC ■ C2 .1 μF, 50V, X7R 140-CC502B104K-RC ■ C3-C8 270 μF, 16V, Electrolytic 75-94SP277X0016FBP ■ C9 10 μF, 16V, Tantalum (SMD) 80-T494B106K016 ■ R1 39.2K, 1/4W, 1% 271-39.2K-RC ■ R2 14.7K, 1/4W, 1% 271-14.7K-RC ■ R3, R4 5 ohms, 1/4W, 5% 291-5.1-RC ■ R5 47 ohms, 1/4W, 5% 291-47-RC

ADesigning a transformer is a very complex undertaking. I don’t know how to do it even though I have built some transformers that worked.

I will try to provide some foundation and you can study catalogs from Magnetics, Inc., and Micrometals, Inc. ( www.mag-inc.com and www.micro metals.com). That being said, it is more cost-effective to buy a power transformer than to build one; mainly because there is no source for small quantities of the core or bobbin. Most manufacturers will give you a sample if you want to build one anyway. I have taken a laminated core transformer apart to rewind it, but that is a pain that I prefer not to repeat. Enameled magnet wire is available from any electronics supplier (Mouser and Allied, for example).

Whenever a current flows, a magnetic field is produced around the conductor. The magnetic field increases as the current increases and, as the magnetic field increases, it generates a voltage in the conductor that opposes the force voltage. This conforms to Newton’s third law: For