Check: Simple Circuits

[Devil_H@ck]

Ok, I'll try that. Why is it that it can't turn off? Is it because the "high" current is keeping the transistor open?

[amb]

Are you sure you're using a simple (rather than a latching-type relay)? Also, since the input voltage is high (9-12V) and the transistor will turn on at 0.6V or so, you might want to change the 100K ohm resistor to a much lower value (something like 1K ohm, so that it forms a voltage divider with the other 10K resistor to drop the input voltage.

-Ti

[Devil_H@ck]

Originally posted by amb
Are you sure you're using a simple (rather than a latching-type relay)? Also, since the input voltage is high (9-12V) and the transistor will turn on at 0.6V or so, you might want to change the 100K ohm resistor to a much lower value (something like 1K ohm, so that it forms a voltage divider with the other 10K resistor to drop the input voltage.

-Ti

It's a simple relay, I've used them at least 5 times before and they are just plain, normal relays. Nice ones too .

The input voltage is 12V. I'll try changing both resistors, but would there be any difference? Zero volt is zero volt, the problem isn't turning on, it's turning off. Even if the trigger voltage were 200 volts, once there isn't any signal at the trigger input, it's 0 volts, no matter what. Right?

I'll go find myself a darlington tomorrow & then try changing resistors if it still doesn't work .

[HDTVman]

Do you know that the base is at 0 volts? Have you measured it? The lower value resistor from base to grd will insure that the base gets pulled to grd even if there is a small amount of leakage in the transistor and remember, caps take time to discharge. Oh ya. (Have you tried the darlington yet? oops I didn't check the time stamp. sorry)

BZ

[Devil_H@ck]

I got 2 BC517s today, according to the storekeeper they are 1A darlingtons. I wanted BC317s, but he didn't have those and BC517 seems to resemble it close enough .

I'll try it tonight.

* Edit:

Ok, I looked the thing it up:
High current (max. 500 mA)
Low voltage (max. 30 V)
Very high DC current gain (min. 30000)

Should be fine.

[Devil_H@ck]

Ok, it works perfect! Thanks Elso!

Hm, well, almost. There's one weird malfunction kind of thing, but it isn't that important. When I switch the relay on & then off and then pull the power chord so that there isn't any power left, the relay closes for a very short time. Not that this only happens when it's in the off state & the power chord is removed from the power supply, when turn it on/off it works just fine. What could be the reason of this?

[amb]

Are you still using the 100K resistors or did you already change them to 1K?

-Ti

[Devil_H@ck]

Ah, yes, that's probably the problem. Let's see if I get it correctly:
- The relay turns on, the 47nF cap charges
- The relay turns off, the 47nF cap starts to discharge very slowly through the 100k resistor
- The power is removed, the 47nF cap tries to keep the voltage it was fed by at the base of the darlington, thus turning it on. In turn the relay turns on and drains all the power left in the supply's caps.

Hm, actually, rereading it, it seems wrong, since the cap would do the same when I just turn off the relay, right? So, please, tell me, why is it that it turns back on for a short time (untill the supply's caps are empty)?

[FredM]

Hi - Lets look at the theory - the transistor will turn off when the voltage on its base is less than Vbe (about 1.4V with a darlington) -

Provided the EN signals are all below (0.6V+Vbe), the time taken for the 100n b-gnd capacitor to discharge will be negligable - the voltage on this capacitor will not charge to more than Vbe anyway (it will be clamped) so it only needs to discharge a small amount for the transistor to turn off. with t=rc, 100n*100k=0.01 (10ms) but I would expect the time to be more like 500us given that rapid discharge occurs first, and that only a few mV drop in Vbe would be required.

I can see no mechanism for the transistor to be turned-on briefly as the supply drops - except if some spurious signals are coming from the EN inputs.

[Devil_H@ck]

Hm, ok then. It doesn't really matter, because it only happens when the power plug is pulled. The relay is used to conduct the AC to the torriods in an amp, so there won't be any more AC to conduct.

But thanks for the explanation, I always forget about the transistor voltage drops.