Maker Pro
Maker Pro

LED Circuit not behaving as it should

Danno

Aug 31, 2015
65
Joined
Aug 31, 2015
Messages
65
Hi,

Might seem a dumb question but here goes, I have an old micro controller and on the front are 3 status LED's. Normally when it is running the LED's are on steady, however I have sketched the circuit out and also run it on an online simulator but cannot get the LED's to light steady, only 2 of the 3 flash.

I'm I missing something obvious here? Thanks

LEDcct.GIF
 
Last edited by a moderator:

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,799
Joined
Nov 17, 2011
Messages
13,799
LED #3 cannot be on because the voltage at the output of the bridge rectifier is negative, therefore all current will flow through D3.
 

Danno

Aug 31, 2015
65
Joined
Aug 31, 2015
Messages
65
I see what you are saying but how I have sketched it is how it is wired up and all three LED's light up steady on the unit, but for the life of me I can't work out how.
 

Danno

Aug 31, 2015
65
Joined
Aug 31, 2015
Messages
65
Ah, silly me I have made a school boy error. The LED's are connected to the AC side. Amended schematic attached.

I guess then the LED's just flicker very quickly rather than stay on full?

Also what is the point of the zenner diode on the neg leg of the DC and what do the diodes accross the LED's do,

Thanks

LEDcct1.GIF
 
Last edited by a moderator:

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,799
Joined
Nov 17, 2011
Messages
13,799
I guess then the LED's just flicker very quickly rather than stay on full?
Yes, at mains frequency, 50 Hz in GB.

Also what is the point of the zenner diode on the neg leg of the DC
This zener will limit the output voltage of the negative DC leg. However, since the positive leg is uncontrolled, "DC out" will still not be stabilized. As shown the Zener diode and series resistor are quite unnecessary.

what do the diodes accross the LED's do,
They prevent the LEDs from high reverse voltage which would break down the LEDs.
 

Old Steve

Jul 23, 2015
734
Joined
Jul 23, 2015
Messages
734
Are you 100% sure that the cathode of the zener is connected to gnd, and not to the positive rail of the output voltage? That would make more sense.
 

Arouse1973

Adam
Dec 18, 2013
5,178
Joined
Dec 18, 2013
Messages
5,178
Are you 100% sure that the cathode of the zener is connected to gnd, and not to the positive rail of the output voltage? That would make more sense.

Hi Steve. Looks like the zener is for negative voltage regulation so that would be right as it is drawn wouldn't it?
Thanks
Adam
 

Danno

Aug 31, 2015
65
Joined
Aug 31, 2015
Messages
65
Yes, at mains frequency, 50 Hz in GB.


This zener will limit the output voltage of the negative DC leg. However, since the positive leg is uncontrolled, "DC out" will still not be stabilized. As shown the Zener diode and series resistor are quite unnecessary.


They prevent the LEDs from high reverse voltage which would break down the LEDs.


Thanks,
The DC does go off to a 1723 regulator so maybe the Zenner and resistor do serve a purpose?

Also the diodes across the LED's are not Zenners?
 
Last edited:

Old Steve

Jul 23, 2015
734
Joined
Jul 23, 2015
Messages
734
Hi Steve. Looks like the zener is for negative voltage regulation so that would be right as it is drawn wouldn't it?
Thanks
Adam

There are no 'negative' voltages in the circuit shown, and the ground connection is not used in the output. As shown, the output is effectively 0V and +V.
Edit: It must serve a purpose, but it's a very unusual circuit configuration, unless the ground connection also comes into play somewhere that is not shown.
 
Last edited:

Old Steve

Jul 23, 2015
734
Joined
Jul 23, 2015
Messages
734
Thanks,
The DC does go off to a 1723 regulator so maybe the Zenner and resistor do serve a purpose?

Also the diodes across the LED's are not Zenners?
No, they're just normal, everyday diodes, across the LEDs as mentioned to protect them from high reverse-voltage. (A LED can only handle a max of about 5V reverse-voltage.)
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,799
Joined
Nov 17, 2011
Messages
13,799
There are no 'negative' voltages in the circuit shown,
The 12V at the anode of the zener diode is in fact -12 V, have a look at the bridge rectifier diodes' orientation.

As shown, the output is effectively 0V and +V.
Nah, DC out is +14V (unstabilized) directly from the bridge rectifier vs. -12V, stabilized by the zener diode.

Edit: It must serve a purpose, but it's a very unusual circuit configuration, unless the ground connection also comes into play somewhere that is not shown.
I completely agree.
 

Old Steve

Jul 23, 2015
734
Joined
Jul 23, 2015
Messages
734
The 12V at the anode of the zener diode is in fact -12 V, have a look at the bridge rectifier diodes' orientation.
Sorry, I should have said "there are no negative voltages in the output".

Nah, DC out is +14V (unstabilized) directly from the bridge rectifier vs. -12V, stabilized by the zener diode.
With respect to ground, that's true, but not in the actual output.

I was really referring only to the DC 'output' in my comments - I should have made it clearer.

And isn't the DC output actually (26V x 1.414)-1.2V?
Edit: (I'm assuming that the 723 regulator that it feeds has a smoothing capacitor.) And I meant 'unloaded' DC output.

Edit (again): Actually, that's not quite right either, but you get my meaning. The bridge rectifier is fed by the full winding of the transformer secondary, which 'starts out' as 28VAC RMS.
Re-reading your replies, I guess that's what you were saying, in a different way. I'm very tired. I'll shut up now.
 
Last edited:

Danno

Aug 31, 2015
65
Joined
Aug 31, 2015
Messages
65
I'll add the 723 circuit to the schematic later, thanks for all replies it's appreciated
 

Old Steve

Jul 23, 2015
734
Joined
Jul 23, 2015
Messages
734
Sorry if I confused things. And I'll be interested to see where the ground connection and regulated 12V from the negative leg of the diode bridge comes into play.
 

Danno

Aug 31, 2015
65
Joined
Aug 31, 2015
Messages
65
Sorry if I confused things. And I'll be interested to see where the ground connection and regulated 12V from the negative leg of the diode bridge comes into play.


Well -12v goes to our old friend the 9410, that is the only place it goes.
 

Attachments

  • Snap 2015-09-02 at 19.34.55.png
    Snap 2015-09-02 at 19.34.55.png
    12.1 KB · Views: 93

Old Steve

Jul 23, 2015
734
Joined
Jul 23, 2015
Messages
734
Well -12v goes to our old friend the 9410, that is the only place it goes.
Ah, I was going to ask what the 9410's Vgg pin was connected to. A programming voltage perhaps, although a negative programming voltage seems odd. Maybe not though, I'm only really familiar with PICs in that regard. They use a higher positive voltage for programming.
 

Old Steve

Jul 23, 2015
734
Joined
Jul 23, 2015
Messages
734
Is this a type of crowbar circuit then?
No, a crowbar circuit typically shorts across the supply rails, blowing a fuse, to protect a circuit from over-voltage.

Crowbar.JPG

The diode across a LED just provides an alternate current path when the polarity reverses.
 

Danno

Aug 31, 2015
65
Joined
Aug 31, 2015
Messages
65
No, a crowbar circuit typically shorts across the supply rails, blowing a fuse, to protect a circuit from over-voltage.

View attachment 21719

The diode across a LED just provides an alternate current path when the polarity reverses.

Got it, it's a clamping diode. A diode in series with the LED would also do the same job I assume?

Thanks
 
Top