Club 80-90 Forums

The way I see it the -ve side of the bridge has no path to earth under any conditions. Remember it's an SCR not a TRIAC and it's in "the wrong way round".

Aside, this is going to get confusing because some folks are discussing it on the other thread, despite my suggesting not to, as it would get confusing. I will go over there and suggest the conversation should be here.

Noted Ghost.
I did wonder if a load made a difference so put a cigarette lighter on. It still only received 9V.
Sorry, I should have mentioned that before.
Also, I have checked to see if the physical orientation of the components and yes, the diagram does show them in the correct orientation.

A picture paints a thousand words! I'm trying to up load a photo using my phone but it's to big. I don't know how to resize it!

Copied from the other thread:-

dave friday wrote:But,but,but! at switch on the zenner will have a negative voltage* supplied through the resistor.
*With respect to battery negative.

Thinking......

Yes, but surely a zenner works the other way round, it's a "reverse breakdown" device, not forward biased. In this circuit it would just conduct any voltage to the earth, just as a normal diode would.

at switch on the zenner is the right way round..as soon as the scr is switched on by the 5.1 volt ,the scr stays on until the circuit is switched off.
That's my reading of it.
ps,as has been said it would be good to have the Zig infront of us,then we could disconnect the scr and see if we get 5.1 volts on the zenner!

I often used to have to think hard when dealing with the negative voltage side of balanced power supply circuits. I suspect, although this is not really a balanced supply source, there may be, as Dave suggests, elements of negative voltages going on here I hate trying to work out electronic flow charts backwards

dave friday wrote:at switch on the zenner is the right way round..as soon as the scr is switched on by the 5.1 volt ,the scr stays on until the circuit is switched off. That's my reading of it.

I think you've got it Dave

As I said above, my slight issue with negative voltage circuits was blocking my chain of thought

Though am I right in thinking when you say

as has been said it would be good to have the Zig infront of us,then we could disconnect the scr and see if we get 5.1 volts on the zenner!

It would be -5.1 (minus) Volts reference to earth?

When I first came across negative grid bias on a push-pull valve amplifier,i had to go and sit in a darkened room for a long while!
Yes 5.1 negative with respect to earth [with the scr disconnected]

I've just been out to the garage. I put the cigarette lighter on and measured some readings. I measured from the rectifier output to the chassis. I got 9.51dcv AND 240acv!
I could have sworn that I'd checked for ac power! The comment about the battery acting as the capacitor may make sense now.

I fear I may have miss led you all.
If I'm right, the RMS (dc equivalent) of 20.1acv (x 0.707) is 14.21.
The two voltages would be enough to charge a battery?
I'm sorry that this revelation has come so late on.

It seems "we" are getting there

Re the 240 Volts AC on the output of the bridge though

Thinking.....

Bubble Meister wrote: If I'm right, the RMS (dc equivalent) of 20.1acv (x 0.707) is 14.21.
The two voltages would be enough to charge a battery?

Yes

Bubble Meister wrote:I'm sorry that this revelation has come so late on.

No worries, this is fun

I still don't understand what that circuits function is though What would be wrong with simply connecting the -ve side of the bridge directly to earth? (as in most unregulated power supplies)

Yep. Having ac after rectification goes against my current understanding. I only did electrics as part of my mechanical studies.

Bubble Meister wrote:Yep. Having ac after rectification goes against my current understanding. I only did electrics as part of my mechanical studies.

I wonder if it's just a tiny bit of leakage through the transformer and your meter (digital I presume) is accurate enough to register it?

I think care is needed here.

Thinking......

Measuring across the dc terminals of the rectifier with the cigarette lighter on I have 22.7acv!

Ah, no, it's not necessarily AC. Without a "proper" capacitor, the 50Hz pulesed DC will cause your meter to read when set to AC due to the way the meter's circuits work (rectification and smoothing) so I don't think you need to even consider that.

Still thinking (and worrying slightly) about the 240 V AC measured on the output of the bridge though.