# What's wrong with circuit?

Discussion in 'General Electronics Discussion' started by Tha fios agaibh, Dec 29, 2016.

1. ### Tha fios agaibh

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Aug 11, 2014
I got this video link from bluejet for a remote control power circuit, but I wasn't able to get it working. It was originally posted under projects, but I'm reposting here because I could really use some circuit help here.

Here's a sketch of receiving circuit using values given in video.

What puzzles me is why I'm only getting 1.25v from the top of 1k resistor above 2n3906 to the bottom power rail.
Don't I need at least 1.5v to trigger the triac?

2. ### Colin Mitchell

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Aug 31, 2014
You are losing 1,000 volts across the first diode and dropping 0.7v across the second diode.

3. ### Tha fios agaibh

2,215
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Aug 11, 2014
Thank you Colin, but can you elaborate?

1,000v? I'm applying 120v and was expecting voltage in the single digits above transistor.
I should either be forward biasing the transistor and sending enough voltage to trigger the gate, or have a large Vce reading otherwise, right? Vce is only 0.5v and gate voltage is 0.03v at best.

What am I not seeing?

4. ### Colin Mitchell

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Aug 31, 2014
The N wire does not move at all but the L wire rises to 180v in the positive direction and then falls to minus 180v.
The circuit is constantly flipping over and back again at the rate of 60 times per second.
This what all the components are doing and that is the voltage they are seeing.
When the voltage rises, the second diode start to conduct when the voltage reaches 0.7v and the voltage across it does not increase any further.
At the same time, the first diode is actually a 1,000 v zener diode and no voltage will appear on the lower lead of this diode until the voltage reaches 1,000v. As you can see it only gets to 0.7v and if the second diode was not included, the voltage would only reach 180v. So, it never does its job.
Now, when the voltage goes in the negative direction, the whole circuit is flipped over and the lower rail now becomes positive and you can see the transistor conducts, the first diode conducts and something might happen.

Last edited: Dec 30, 2016
5. ### Colin Mitchell

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Aug 31, 2014
I cannot go any further because I cannot read any of the values of resistance.

6. ### Tha fios agaibh

2,215
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Aug 11, 2014

better?

Last edited by a moderator: Dec 31, 2016
7. ### Tha fios agaibh

2,215
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Aug 11, 2014
Thanks for explanation.

Oops, I found a solder point I missed between those two diodes.
Now, I have 17v at top of 1k and 5.3v at collector. That's more like it!

I'll fiddle with it tomorrow and see if it works.

Thanks so much for your help.

8. ### Colin Mitchell

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Aug 31, 2014
View attachment 31124

The 0.68u will pass about 24mA and charge the 220u. The transistor will be turned on and 24mA will flow through the 1k producing about 24v across it.
This is basically the DC conditions of the circuit and the timing (the point at which the transistor turns on) will depend on the effect of the timing components at the front of the circuit. The transistor has to turn on for a greater percentage of the time, otherwise the 220u will charge to 180v and blow up.

9. ### Tha fios agaibh

2,215
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Aug 11, 2014
May I ask how you got 24ma?
I was expecting an impedance of 4k across the 220R, 220K and paralleled .68 cap. giving me 45ma.

Oh, I was figuring the transistor was only switched on when high pass filter (caps on the left) allowed enough current to pass to the base and only then switching on triac.

If transistor has to be on a greater percentage of the time that means triac will too. If so, what keeps the triac from triggering when transmitter is not injecting its high frequency into the circuit?

10. ### Colin Mitchell

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Aug 31, 2014
I use 3.5mA per 100n for half-wave
The whole circuit works on 50Hz.
I don't see how the circuit starts and stops

11. ### Tha fios agaibh

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Aug 11, 2014
Thanks, I've never heard of that rule of thumb. Very cool.

Now I can't understand why triac is always on. Gate voltage is 0.81 to .83 depending on pot position. Turning on transmitter makes it jump slightly (0.02v) but something is wrong.
Checked transistor and diodes and all are working properly. Ideas?

12. ### Colin Mitchell

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Aug 31, 2014
"I use 3.5mA per 100n for half-wave" This applies for 240v AC. So the voltage will be about 12v to 18v.
You have drawn the circuit differently, NOW. The circuit above will BLOW UP.

2,215
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Aug 11, 2014
Better?

14. ### Tha fios agaibh

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Aug 11, 2014
I'm not sure what reference point I should be using here, but from bottom of 1k resistor below collector, I'm using as negative.

Voltage on base and emitter are the same and I'm not sure why.
Transistor should not be forward biased unless base is more negative than emitter, so why am I passing a current of 9ma to triac gate?

15. ### Tha fios agaibh

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Aug 11, 2014
My guess is the 220k biasing resistor needs to be lowered in order to fully saturate transistor that in turn would turn fully short gate voltage?

16. ### AnalogKid

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Jun 10, 2015
No, he isn't.

A 1N4007 diode is rated for 1000 V PRRV, peak repetitive reverse voltage. It is NOT NOT NOT a zener diode of any kind. That advice is so bizarre, it reduces the rest of your posts to somewhere between useless and dangerous. In a circuit like this that has no external earth ground reference, it is NOT the case that the N wire (Neutral) "does not move at all". With no external reference, it is equally true that the L wire does not move, and the N wire is powering the circuit ***from the circuit's point of view***. Notice that even though the transistor is a PNP, the circuit is drawn in a typical NPN style, with "gnd" across the bottom. You can analyze the circuit traditionally, as long as you keep in mind that the circuit's Vcc is a negative voltage.

ak

(*steve*) and davenn like this.
17. ### Alec_t

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Jul 7, 2015
Current through the 10uF cap?

18. ### Tha fios agaibh

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Aug 11, 2014
Yes, must be. (embarrassed I didn't notice the obvious)
I thought it was coming from collector side.

I'm trying to figure why triac is always on.

Do you agree that this is a biasing problem?
I believe the 220k resistor and pot should be low enough resistance to drive the base negative to fully turn on transistor which in turn pulls down gate voltage. Just strange giving values don't work on a proven circuit.

Btw, my original problem was a soldering mistake. Now corrected.

19. ### Tha fios agaibh

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Aug 11, 2014
On second thought, the 9ma must not be coming from the cap.
If we know there's 3v dropped across the 330R then we know the 9ma is flowing through it, and the gate current of the triac is probably that much. Right?

20. ### Alec_t

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Jul 7, 2015
Right. The voltage across the 220u cap (the 'supply' voltage for the transistor) is about 16V. The pull-down resistors on the transistor base keep it permanently switched on, drawing about 5mA collector current. Triac gate current (about 8mA according to LTspice) flows via the 390R resistor and the 1k collector resistor. About 14mA RMS and 2mA RMS flow through the 220u cap amd 10u cap respectively.
N.b. if you label your components it would make discussion of your circuit much easier.