Power Amplify Circuit

[ScuzZ]

I thought I might be able to simulate this circuit to teach myself how to amplify voltage, but I can't seem to get it to work correctly.

RL1 is a 12v Relay for those attempting to assist me.

I found that even with the 'darlington pair' the voltage is still not enough to activate the relay.

Why is it the more transistors that I place in the array the less the voltage?

How could I get this circuit to work?

 

[eng1]

The relay is in the wrong place. It must be connected between 12 V and the collector of the transistor (you need just one). The cathode of the battery and the emitter must be tied to ground (insert a GND symbol). The resistance depends on the collector current, you probably need some kohms.

 

[ScuzZ]

I understand what you're trying to do, but even when I do those things I still can't get the relay to switch.

I can however get it to switch using 3 transistors, like an extended darlington pair.

 

[Nigel Goodwin]

That's because 10M is too high a value, adding a further transistor works because you've then got so much more gain.

 

[ScuzZ]

The 10M resistor is to simulate water.

 

[Nigel Goodwin]

The 10M resistor is to simulate water.

So why did you choose 10M?, water is likely to be far lower than that.

 

[ScuzZ]

That's what I measured the resistance at.

 

[Nigel Goodwin]

And I bet you used a digital meter to do so!.

 

[ScuzZ]

Well I done a fair bit of googling first, but couldn't find anything.

So yeah, poured a glass of water and put the probes of my digital multimeter in.

What's wrong with that?

 

[evandude]

why don't you try fixing your circuit arrangement (moving the relay), changing the 10M resistor to a much lower value, and then using that to get an actual idea of how the circuit is supposed to work, and then put something together and just trying it out?

Your measurement of 10M ohms seems high, unless you are using distilled/deionized water, and/or a terrible electrode setup, in which case your simple transistor setup is probably not going to cut it anyway... A pair of electrodes very close together (or better, overlapped finger pairs on a PCB) would probably be much better.

 

[Nigel Goodwin]

Well I done a fair bit of googling first, but couldn't find anything.

So yeah, poured a glass of water and put the probes of my digital multimeter in.

What's wrong with that?

Digital meters are pretty useless at measuring resistance, 10M is far too high, unless it's distilled water (and probably too high then as well).

Get the two probes from your darlington, and place them in the water, ensure the relay is in the collector of the transistors, and not the emitter - see if it works then.

 

[Hero999]

Digital meters are pretty useless at measuring resistance

Perhaps you should throw out your meter. I've never had a prolem with measuring resistance on a digital meter, all the resistors I've measred read true to their value and I've even measured 10M reesistors without any problem whatsoever.

 

[audioguru]

My Fluke digital meter measures resistance perfectly. Its max is 40M and on its lowest range it can automatically subtract the 0.4 ohms of its leads.

My tap water measures 225 ohms. Acid rain might measure higher. Distilled water is nearly an insulator.

 

[Nigel Goodwin]

Perhaps you should throw out your meter. I've never had a prolem with measuring resistance on a digital meter, all the resistors I've measred read true to their value and I've even measured 10M reesistors without any problem whatsoever.

Measuring a resistor out of circuit is fine, but measuring water (or resistors in circuit) is a completely different thing!.

My tap water measures 225 ohms.

And you don't think that seems rather low?.

 

[ericgibbs]

hi,

Using a DVM, measured tap water as 13K to 15K for 1cm probed separation, insertion depth about 1cm.

Measured filtered water [ Brita ] about 1K lower ???

EricG

 

[Hero999]

I would also expect acid rain to be a lower resistance rather than higher because it's more ionic and I would expect filted water to be higher because the ionic compounds should have been removed.

I don't see your point Nigel, why should an analogue meter should be any better at measuring the resistance of water than a digital meter, it makes absolutely no difference what meter you use providing it's callibrated.

 

[Nigel Goodwin]

I would also expect acid rain to be a lower resistance rather than higher because it's more ionic and I would expect filted water to be higher because the ionic compounds should have been removed.

I would agree!.

I don't see your point Nigel, why should an analogue meter should be any better at measuring the resistance of water than a digital meter, it makes absolutely no difference what meter you use providing it's callibrated.

You're obviously too young to have come from the pre-digital meter days

Their method of measuring resistance is completely different, and they behave in completely different ways. Measuring a simple resistor out of circuit works fine on both, but with anything else you have to consider that the reading may bear no resemblance to the actual value. In 'general' I would suggest an old analogue meter is 'better' for measuring ohms, because it pushes a higher current through the circuit.

The 'mistake' in this thread was measuring the water with a digital meter, rather than simply sticking two probes from the darlington into the water!.

 

[audioguru]

I thought my tap water's resistance of 225 ohms was low when I made my Plants Watering Watcher-2 projects. Then I added salt to the water and it measured about 20 ohms.

The soil in my house plants measures 47 ohms when damp and 50k ohms when fairly dry. The probes are a little thicker than 1mm, 0.7" apart and 5cm is in the soil or water.

 

[Hero999]

I do remember analogue metera and appreciate the difference between them and digital meters. I understand that is a DC voltage is bobbing up and down an analogue meter is often best at rejecting the AC ripple.

I still don't understand why analogue meters should be any better at measuring the resistance of water, resistance is resistance as far as I'm concerned and it doesn't matter what meter to use to measure it.

 

[Nigel Goodwin]

I still don't understand why analogue meters should be any better at measuring the resistance of water, resistance is resistance as far as I'm concerned and it doesn't matter what meter to use to measure it.

But water ISN'T a resistor, and it doesn't behave as such - like measuring skin resistance, try it with an old analogue meter and a digital one, the readings will be completely different.