Hi,
In one of my projects I will add a DC-blocker according to this article from Rod Elliott:
https://sound-au.com/articles/xfmr-dc.htm
See Figure 8 - Recommended Design
I’m wondering if it’s possible to add LED’s to light up when DC is present (and blocked by the bridge).
Is this possible?
Regards, Gerrit
In one of my projects I will add a DC-blocker according to this article from Rod Elliott:
https://sound-au.com/articles/xfmr-dc.htm
See Figure 8 - Recommended Design
I’m wondering if it’s possible to add LED’s to light up when DC is present (and blocked by the bridge).
Is this possible?
Regards, Gerrit
Attachments
You would have to double up on the diodes so that the clamping is about 2.6V. Then a pair of red LEDs anti-parallel with about 1k in series should work.
Its unlikely to ever light up, significant dc on the mains is rare.
Excess mains is common, I measured 258V in my office last week - enough to make a 230V toroid buzz hard
Its unlikely to ever light up, significant dc on the mains is rare.
Excess mains is common, I measured 258V in my office last week - enough to make a 230V toroid buzz hard
Unfortunately "light up when DC is present" is imprecise.
How much DC must be present before you are willing to light up the LED? 1 millivolt? 10 millivolts? 100 millivolts?
I suspect what you really want is a full blown measurement system with a numeric LED or LCD display, showing the instantaneous value of DC-on-the-mains, and updating that display several times per second. You'll need an a AC-to-DC power supply, a mains DC sensing circuit, a low pass filter (perhaps in software), a microcontroller with A-to-D converter, and a (3.5 digit?) , (2.5 digit?) display.
How much DC must be present before you are willing to light up the LED? 1 millivolt? 10 millivolts? 100 millivolts?
I suspect what you really want is a full blown measurement system with a numeric LED or LCD display, showing the instantaneous value of DC-on-the-mains, and updating that display several times per second. You'll need an a AC-to-DC power supply, a mains DC sensing circuit, a low pass filter (perhaps in software), a microcontroller with A-to-D converter, and a (3.5 digit?) , (2.5 digit?) display.
I think so: this circuit (not tested) should do it:
It can probably be pruned/rationalized, by using a single transistor and a diode bridge for example, but it should already be operational as it is
You could also do this:
Now you have roughly 2-2.8 V available across the bridge. Plenty to turn on a 1.7 V base voltage red LED. Add 100 Ω in series with the LED so it doesn't blow. You'll need two LEDs in anti-parallel as the DC voltage could have either polarity.
This will work for high levels of DC voltage (a few volt). It won't work for mV level DC. But then neither with Elvee's circuit. His will be more sensitive though.
Tom
Now you have roughly 2-2.8 V available across the bridge. Plenty to turn on a 1.7 V base voltage red LED. Add 100 Ω in series with the LED so it doesn't blow. You'll need two LEDs in anti-parallel as the DC voltage could have either polarity.
This will work for high levels of DC voltage (a few volt). It won't work for mV level DC. But then neither with Elvee's circuit. His will be more sensitive though.
Tom
Here is the diode version:
Also untested.
By twisting one's noodle, a pure resistor-based version using no semiconductors should be possible, but it would be a proof that it can be done, just for the beauty of the feat
Also untested.
By twisting one's noodle, a pure resistor-based version using no semiconductors should be possible, but it would be a proof that it can be done, just for the beauty of the feat
Something worth noting: the blockers (and the proposed indicators) will only work at a low load current: the 50Hz impedance of 4700µ/2 is far from negligible, and the voltage drop may cause the partial disabling of the blocker, and an erroneous DC warning.
Using much larger caps, 22,000 or 47,000µ would be preferable, keeping in mind that 2.5 or 4V WV voltage is perfectly sufficient. 63V is a vast overkill
Using much larger caps, 22,000 or 47,000µ would be preferable, keeping in mind that 2.5 or 4V WV voltage is perfectly sufficient. 63V is a vast overkill
Rod Elliott says the same thing and proposes rule-of-thumb equations to help readers select capacitance value.
Hi Elvee,
I’m using 22000 uf capacitors, 25v Volts.
I’ll take a closer look at all diagrams.
Regards, Gerrit
I’m using 22000 uf capacitors, 25v Volts.
I’ll take a closer look at all diagrams.
Regards, Gerrit
Perhaps two anti-parallel polarized capacitor can be use to save some cost if it can withstand reverse voltage not more than 1.5V?
... assuming they split the charge evenly, which they won't due to parts tolerances. 6.3 V types are not expensive, so why not just use those?
Tom
Tom
If you go through that route, you do not even need two capacitors: a single one having twice the value would do the job.
At room temperature, a polarized capacitor will withstand 1.5V, but just, and at elevated temperatures, it will fail
Congrats, but as I said it is not tested. It should be easy to test beforehand, with a point to point setup.
It is always preferable to make tests before committing to a hard design.
Good luck anyway, it should work, and I can try to make a sim, to determine the parameters of operation.
The basic scheme should be OK, but in order to work, it might need unrealistic components values.
If you can post exactly what you ordered, I can make the sim more accurate
It is always preferable to make tests before committing to a hard design.
Good luck anyway, it should work, and I can try to make a sim, to determine the parameters of operation.
The basic scheme should be OK, but in order to work, it might need unrealistic components values.
If you can post exactly what you ordered, I can make the sim more accurate
Last edited:
Hi Elvee,
I’m well aware of that, but I needed come components anyway. I will start with the diagram posted above #7.
I will start testing with a variac.
Regards, Gerrit
I’m well aware of that, but I needed come components anyway. I will start with the diagram posted above #7.
I will start testing with a variac.
Regards, Gerrit
Hi Elvee,
My initial built is ready and unfortunately not working as expected (or hoped).
I built according to your diagram with a single transistor and 4 schottky diodes. The schottky diodes are type SBX30403G since I needed TTH parts.
My observations:
1. The blue led starts burning faintly with only 10 Volts AC (using a variac). With a higher voltage it reaches it's normal brightness.
2. Whether the "output" is loaded or not makes no difference at all. The brightness of the LED doesn't change.
Capacitors are 22000 uF, zener is 1N5355B, transistor is BC547B. Base resistor is 2 x 10K instead of 22K, but that will not make a big difference.
Any suggestions?
Regards, Gerrit
My initial built is ready and unfortunately not working as expected (or hoped).
I built according to your diagram with a single transistor and 4 schottky diodes. The schottky diodes are type SBX30403G since I needed TTH parts.
My observations:
1. The blue led starts burning faintly with only 10 Volts AC (using a variac). With a higher voltage it reaches it's normal brightness.
2. Whether the "output" is loaded or not makes no difference at all. The brightness of the LED doesn't change.
Capacitors are 22000 uF, zener is 1N5355B, transistor is BC547B. Base resistor is 2 x 10K instead of 22K, but that will not make a big difference.
Any suggestions?
Regards, Gerrit
I do not see the smaller diodes on your prototype (BAT54) .
Anyway, I am going to simulate the circuit, to see how it behaves
Anyway, I am going to simulate the circuit, to see how it behaves
Hi Elvee,
First of all thanks for your generous help.
I used SBX30403G instead of BAT54, because I needed Through The Hole diodes.
The SBX30403G are supposed to have a very low voltage drop too.
Regards, Gerrit
First of all thanks for your generous help.
I used SBX30403G instead of BAT54, because I needed Through The Hole diodes.
The SBX30403G are supposed to have a very low voltage drop too.
Regards, Gerrit
OK, it seems to work when properly implemented.
This is the situation when no DC is present:
There is no current through the LED.
Now, with 1V DC present:
You can see spikes of 300µA, which is sufficient to light a blue LED.
Note that if you use schottky diodes for D1 to D4, the voltage will become too low to turn on the transistor, it will not be able to overcome the Vbe + the Vf of D5 to D8
This is the situation when no DC is present:
There is no current through the LED.
Now, with 1V DC present:
You can see spikes of 300µA, which is sufficient to light a blue LED.
Note that if you use schottky diodes for D1 to D4, the voltage will become too low to turn on the transistor, it will not be able to overcome the Vbe + the Vf of D5 to D8