variable resistor question

Hello, let's say a project is asking for a 470k resistor, but I don't have that specific one. Can I instead use a 1million ohm variable resistor then use my multimeter to dial it to 470k ohms?

Yes, you can use a potentiometer adjusted to the mid-point. (Pots have 3 terminals, and you'll only be using the wiper and one end. And make sure it's linear pot, not an audio pot, which it probably is because audio pots are usually not 1M.)

But... There's a very-good chance that a 1M resistor will work. If you show us a schematic we might be able to tell.

Or use two 1Megohm resistors in parallel to make that value (500k is close enough).
Leo..

Time to buy an assortment of resistors :wink:

Hello, this is the schematic for a PUT 2n6027 transistor. I currently have bought an assortment of resistors in the past but still couldn’t get exact, but I got close enough to the values. I am specifically using a 2n6027 110 transistor, which I got 10 of them from eBay for a couple of bucks. I have the circuit hooked up the same as the schematic and I have tried different LEDs, capacitors, and different PUT 2n6027 110s and still cannot get the circuit to work. Now when I looked online I saw in 2 different forums were people specifically had problems with the 110s, but the said when they used a 2n6027 b09 that it worked perfectly.Do you guys have any experience with the 110 or have any suggestions. Thank you very much.

0_20171218_234007[1].jpg

To fix my error. I meant the schematic contains a PUT.

A PUT is very old technology.

If you need an oscillator, look at an astable CMOS 555 timer cct. or set up an Arduino to output the needed square wave.

.

tjones9163:
Can I instead use a 1million ohm variable resistor then use my multimeter to dial it to 470k ohms?

The answer is...... yes. That will work. Although.... somebody indicated a pair of 1Mega Ohm resistors in parallel should work too. Just depends on the application.

Never heard of PUT, it looks like a reverse of SCR?

wilykat:
Never heard of PUT, it looks like a reverse of SCR?

It is close to an SCR, it is a programmable unijunction transistor PUT

The ratio of the 15K and 27 K determine the zeta point, that is the percentage of the supply voltage where the “diode bit” of the symbol will breakdown and conduct exactly like an SCR. Depending on the LED’s forward voltage you might want to change the value of this zeta point.

can I use regular voltage divider equation to determine that breakover voltage?

tjones9163:
can I use regular voltage divider equation to determine that breakover voltage?

Yes.

tjones9163:
Hello, let's say a project is asking for a 470k resistor, but I don't have that specific one. Can I instead use a 1million ohm variable resistor then use my multimeter to dial it to 470k ohms?

Varying the value of the 470k Ohm resistor will change the rate at which the LED blinks in that circuit. Using a potentiometer there is probably more educational than using the fixed resistor. The caveat is that allowing the "470k resistor" to go to zero could potentially damage the transistor or LED, so it would be wise to have a limiting resistor (in the range 1k to 20k) in series with the potentiometer as a substitute for the "470k resistor".

The 470k resistor and 2.2µF capacitor set a time constant - if you replace the 470k with 1M, you
can keep the time constant about the same by halving the capacitor value to 1µF. Time constant
is the product of R and C.