This is a DIY LED TV backlight tester. I found this on the internet. Could you please tell me the purpose of the two serially connected DIACs placed between the + and − terminals of the bridge rectifier?

 

Does it make a "bridge rectifier"?

 

Does it make a "bridge rectifier"?

The two serially connected DIACs placed between the + and − terminals of the bridge rectifier?

 

It is hard to tell what the internal connections might be, but it is possible that the internal connections might be different than the picture. If you search for the datasheet, it comes back as a bridge rectifier. I rest my case.

 

Perhaps to protect the display.

 

hi rana,
This is what LTSpice shows.
E

 

If one diac drops 14V (as per Eric's sim), then two in series will effectively create a 28V pulser in that post #1 circuit.

 

It is hard to tell what the internal connections might be, but it is possible that the internal connections might be different than the picture. If you search for the datasheet, it comes back as a bridge rectifier. I rest my case.

This is not something internal. these two have been connected externally

hi rana,
This is what LTSpice shows.
E
View attachment 367531
View attachment 367532

Thanks for the reply. but your diagram is wrong. pls see the pic.

 

If one diac drops 14V (as per Eric's sim), then two in series will effectively create a 28V pulser in that post #1 circuit.

Thanks ! this could be the answer. do two DIACs drop double ?

 

The DB3 DIAC has a breakdown rating of 28V to 36 V. The two diodes in series form a bidirectional 64 V -ish clamp across the rectified DC output.

The 120K resistors limit the shunt current.

file:///C:/Users/User/Downloads/db3.pdf

ak

 

The DB3 DIAC has a breakdown rating of 28V to 36 V. The two diodes in series form a bidirectional 64 V -ish clamp across the rectified DC output.

The 120K resistors limit the shunt current.

file:///C:/Users/User/Downloads/db3.pdf

ak

Thanks for the explanation.
With this setup, once the voltage reaches 64V, + and − will be shorted, and the DIACs may heat up, correct? and can I connect an electrolytic capacitor (10u, 400V) between + and - ?

 

hi ranatung,
Sorry about the previous circuit error, this is a corrected version.
R3 is for the simulator operation, also the simulator uses the Peak voltage of the 230Vrms
What are the details of the LED types being tested??

E

 

hi ranatung,
Sorry about the previous circuit error, this is a corrected version.
R3 is for the simulator operation, also the simulator uses the Peak voltage of the 230Vrms
What are the details of the LED types being tested??

E
View attachment 367548

Thanks for the information. any LED from 3v -36V can be checked

 

hi ranatung,
This shows the result for 3 Medium power LEDs in Series.
E

@ranatungawk

Note: I assume you are aware of the risks when using this test device, as there is no mains isolation. Using an isolation transformer is always the best option to ensure your safety.

 

hi ranatung,
This shows the result for 3 Medium power LEDs in Series.
E

@ranatungawk

Note: I assume you are aware of the risks when using this test device, as there is no mains isolation. Using an isolation transformer is always the best option to ensure your safety.

View attachment 367549

Thanks for advice and information

 

With this setup, once the voltage reaches 64V, + and − will be shorted, and the DIACs may heat up, correct? and can I connect an electrolytic capacitor (10u, 400V) between + and - ?

Not exactly shorted, but a low impedance with a minimum DC voltage across them. Because these are DIACs, there is no (or very little) current conducted through them until the voltage across them is greater than their trigger voltage.

DIAC power dissipation is limited by the two 120K resistors. Worst case (dead short) current is 960 uA (0.98 mA) RMS. If there is, say, 10 V across each DIAC when conducting, that is around 9 mW per DIAC.

Cap across the output - yes.

ak

 

This shows the result for 3 Medium power LEDs in Series.

Note that, as pointed out in #8, there are two DIACS in series.

ak

 

If you place a cap across the output be sure to choose one for the expected voltage with unloaded probes.

Personally, I wouldn't recommend it.

 

hi @ranatungawk
Added the 2 DIAC version circuit, with no LED load and with an LED load for comparison.

E

 

As expected, the LEDs clamp the output voltage below the DIAC trigger voltage, so the don't conduct anything beyond their leakage current.

ak