Dim bulb tester, build one in five minute.

[Pano]

What is the reason(s) of using a Variac and DBT for?

The Dim Bulb Tester isn't really meant to reform capacitors, but to limit current inrush and damage if there is a problem inside the device that you are working on. If you put a 60 watt bulb in series, then no matter what is wrong inside the old device, it will never go beyond 60 watts. You could use a small fuse to protect the device, but the current limit of the bulb is more convenient and does allow some good visual feedback. The bulb also allows you to sit all day at a limited current, whereas a fuse won't. It will either work or pop.

Electrolytic caps can be reformed, but when 40-60 year old caps are bad, reforming is not what they need. Replacing is what they need. Running an old circuit with limited voltage and current will probably keep old caps from bursting - as they can do when hit with full voltage after sitting dormant for decades. So you might get the old device up and running for testing, but old bad caps are just old and bad and should be replaced.

Bottom line. The DBT is a safety measure in case there are faults in the device. It will not fix anything, it just protects against damage.

 

[chrisng]

The Dim Bulb Tester isn't really meant to reform capacitors, but to limit current inrush and damage if there is a problem inside the device that you are working on...

Thanks Pano for your reply and explanation, much appreciated!
And this is what I've learned from Antique Radio Forums:

If an electrolytic capacitor is placed in long term storage, there is no current or electrolysis to replenish and maintain the dielectric layer, so it deteriorates. As a result, if full working voltage is abruptly applied, the leakage current will be very high. This can have at least two consequences. One, it may raise the internal temperature of the capacitor sufficiently to cause it to vent, ending its useful life. The other is, if the current is high enough, the electrolysis may occur so rapidly that hydrogen bubbles form in the capacitor. Under the extreme electric fields that exist at the dielectric layer, scintillation (molecular sized sparking) can occur and ignite the hydrogen. This makes a popping sound that may be heard. Usually when the hydrogen bubbles explode, the plates are instantly punctured and short out, and that's the end of the capacitor.

So from this we can see that the keys to successful reforming are first, there has to be enough electrolyte still left in the capacitor to carry it out, and second, the current has to be limited so overheating or scintillation do not occur. Limiting the current is easy if a variable voltage power supply and a meter are used; for most axial and radial lead capacitors, reforming current should not exceed 2 mA. A series current limiting resistor must always be used to protect the meter and the power supply in case the cap shorts out. For larger can electrolytics, it is possible to reform at 5 mA if desired.

To determine how much electrolyte is still in a cap, allow it to reform until the current falls to a minimum at half of the rated working voltage, then turn the voltage up to full WV quickly. If the cap has an adequate supply of electrolyte, the current will increase when this is done but it will then rapidly settle back to a low value. If a cap is low on electrolyte, the current will go up and stay up for a long period of time. A low electrolyte cap may still work a while longer but it is on its last legs and replacement will soon be needed.

That's the reason I use two steps DBT and a "kill a watt" meter to monitoring the V,I and P.
I could be wrong, but I think this is a safe method to test a old radio were in storage for twenty some years.

 

[manniraj]

Using a Yellow Jacket 2177N 4 outlet power block to make a DBT:
1. Unscrew four screws on both sides of the yellow jacket to remove the metal bottom cover.
2. Unscrew six screws on the inner box (4 on top, 2 on bottom) to expose the wiring.
3. Cut the two wires (see 3rd picture) and connect them together with a wire nut.
4. Assemble everything back together.
5. Plug in the bulbs, Done!
View attachment 1059990View attachment 1059627View attachment 1059628
View attachment 1059636View attachment 1059844

Thanks for this @chrisng simple way to build one in the US. I see that its easy to get a 40w or 60w incandescent bulbs in home depot, so if using 2 of the bulbs in each of the socket makes it in parallel as I need a 100w to limit the current? I think the 3 pin sockets are in parallel with each other.

 

[chrisng]

so if using 2 of the bulbs in each of the socket makes it in parallel as I need a 100w to limit the current?

Yes, use any combinations to match the load.

I think the 3 pin sockets are in parallel with each other.

Yes, please see attached drawing

 

[TonyTecson]

dim bulb testers are a life saver to your expensive power transistors...

 

[Brentdavis99]

Does it matter which outlet you plug one bulb into and where you plug in the load in this set up?

 

[chrisng]

Since the bulb and load are in series, I believe either way will work. I like to connect the hot wire to the bulb and then connect the load, so if the bulb burns out or is removed, the load is disconnected from the hot wire.

 

[Brentdavis99]

Thank you! I’m a newbie and going to try out your design. Much appreciated.