Nuts and Volts - June 2010

QUESTIONS & ANSWERS

gate has 0.9 volts hysteresis, according to the datasheet so I computed the R-C values using the relation: dV/dt = I/C. In this case, the voltage on the capacitor will ramp up and down in 0.25 seconds ( 4 Hz period).

deflection plates of a 6AR8A.

— Craig Kendrick

Sellen

dV = 0.9V d T = 0.125 sec Set I = 0.1 mA = 3V/R3, therefore R3 = 30K (3V is 1/2 the supply voltage). C = Id T/dV = 0.1 mA*0.125S/0.9V = .0139 mF; 15 µF is a standard and close enough. I chose a tantalum cap for low leakage and temperature stability.

I chose the 6. 2 volt zener because it has a sharp knee and will hold a constant voltage over a wide current range. The five volt zener has high internal resistance and is not as stable. For improved temperature stability, you can put a silicon diode in series with the zener. The two temperature coefficients approximately cancel.

■ FIGURE 5

MAGNETIC QUESTIONS

Q1. In regards to N & V March ‘ 10, page 30: What size wire do I use to wind 82. 5 or 87.2 turns on the core of the RS 273-104? Can I round it off to the nearest turn?

2. Using the same RS 273-104 core, I need 2 IF transformers for 455 kHz. Can I do this on this core and how many turns will it need? IF transformers are getting hard to find and costly.

3. Again, using the same 273-104 core, I need a ratio detector or discriminator transformer. The driving tube will be 6BN11 or 6BW11. The secondary center tap in the middle and the two opposite ends go to the

A1. Wire size does not matter, as long as it fits in the opening. Keeping in mind that the inductance varies as the square of the number of turns, you may want to round up or down. Without measuring the inductance, it is only approximate anyway; #16 wire or smaller will fit.

2. I rather doubt that this core will be good at 455 kHz and there is no data on it. You will be better off to salvage a transformer from an AM radio.

3. I don’t know how the 6AR8A discriminator works — that is up to you — but the transformer is shown in Figure 7. R1 simulates the 6BN11 plate resistance, and R2 and R3 load the transformer at resonance to widen the bandwidth. I did not simulate the resistance of the coil; I don’t think it will be significant. There are three identical coils of 360 uH each. Each one has 45 turns of $20 wire or smaller (#16 wire would take up all the opening with no room for insulation). I assumed that the tuning cap would be 365 pF variable as found in most AM radios.

searched the past several years’ worth of issues but haven’t found it. Either I haven’t found the right issue or it was someplace else. Any ideas about the issue or how to build one?

I have an old “tube type” unit that doesn’t work — even after I replaced the 67 1/2 volt batteries with a bench supply. I’ve got a nice metal box, display meter, switches, etc., and a GM tube in a housing. I’d like to make something of this thing.

— Duane Haines

GEIGER COUNTER

AThe article that you remember was in March ‘05 (page 36) and May ‘05 (page 50). You can easily search back issues on the Nuts & Volts website ( www.nutsvolts.com) and click on the digital issue search ( www.nutsvolts.com/search-digital. php). If you want to repair the Geiger counter, first check the tubes. If you

QMany months ago, I saw an article in Nuts & Volts (I think) about building a Geiger counter. I’ve

FLASHING BRAKE LIGHT PARTS LIST PART DESCRIPTION Q1 PNP, 100V, 25A Q2 NPN DARLINGTON, 100V, 5A IC1 QUAD NAND SCHMITT D1 6.2V ZENER, 1W C1 15 µF, 20%, 10V R1 10 OHMS, 10W, 5% R2, R4 3.3K, 1/4W, 5% R3 22K, 1/4W, 5% HEATSINK 50 DEG C/30W (1.7 DEG C/W)

■ FIGURE 6

PKG MOUSER PART # TO-247 511-TIP36C TO-220 512-KSD560 DIP- 14 595-CD4093BE DO- 41 512-1N4735A RADIAL 74-199D16V15 AXIAL 71-RH10-10 AXIAL 291-3.3K-RC AXIAL 291-22K-RC