Re: Bridge rectifier

Thu Nov 08, 2012 11:15 pm

Bus Driver, I have a slew of the HF multimeters around here. I have found as they age, the 9 volt inside deteriortes slightly, the meter failing to produce the same reading twice in a row although a crisp LCD display is still maintained. Probably not the highest quality storage cell to begin with.

With so many of them you lose track of the newest one with the freshest battery. I started numbering them with Sharpies, I should just pitch them if they weren't so handy to have in every nook and cranny.

Re: Bridge rectifier

Sat Nov 10, 2012 4:44 pm

While I was focused here on battery chargers, one here with broken ammeter caught my attention. Really old, Allison brand, a minimal device in every way. 6/12 volt, 6/2 amps. Belonged to my Father. But it always has worked, even with the smashed ammeter. Found 10 amp ammeter that could be purchased from China, shipped to my mailbox, for the total of $4.22. That will not buy the gas for a round trip to the nearest Advance or O'Reilly's. And it was close to the desired size.
So the charger was modified to mount the ammeter. The original ammeter had no polarity markings. So the Positive battery clamp line was installed to the Positive on the new ammeter as the first hookup. To limit the amperage if the hookup was backward, a 33 Ohm resistor (used for alternator conversions for the exciter circuit) was used between the battery clamps and the charger was set on 6 volts. That would be about 0.18 amperes, enough to just make the needle wiggle. That worked perfectly-- the first guess was correct.
The original ammeter was truly the most minimal ( excuse my bad grammar) I have ever seen. The new one is high quality in contrast.
A new charger would be $40.00 and up and no doubt would be from China. And the time I spent on it would have been unpaid anyway.

Re: Bridge rectifier

Tue Nov 13, 2012 12:29 pm

Landreo wrote: It takes 0.6 volts to activate a diode,


This may seem like a piece of mindless trivia but is the key to your finding with the meter. The meter is trying to read a resistor that follows ohm's law. Diodes and all semiconductors are non-ohmic conductors, they do not follow ohm's law.

In the wrong direction the diode should not conduct and your meter will read infinity or a few hundred megs. In the correct direction the meter has to activate the diode and it may not have enough voltage or current or both to do that, the diode unactivated will be a few hungred megs or infinity. If the diode is activated then it will have a voltage drop of 0.6 volts no matter the voltage or current. Not completely true but close to completely true. The meter sees that 0.6 voltage drop and assumes it is due to a ohm's law following resistance and gives a resistance of around 600 ohms. The real resistance is likely less than one ohm. 0.1 volts into a diode gives zero out, 0.8 volts into a diode gives 0.2 out, 10 volts into a diode gives 9.4 volts out, 100 volts in gives 99.4 out. you always have that same voltage drop regardless of the voltage or current once over the 0.6 volt threshold. The meter does not know that and assumes the voltage drop is due to a resistor but it is not.

One meter may not have enough to activate the diodes and you will get infinity both ways, another meter may have enough to activate the diode and will read infinity in one direction and some other amount, commonly around 500-600 ohms, in the other. Your diodes seem to be OK. Some meters, my Harbor Freight cheapo, have a diode range which actually will read the forward voltage drop and will show something like .599, which is the actual voltage drop across the diode.

If you plotted the volts on the x axis and current on the y axis you would get a 45 degree line for a normal resistor, more voltage equals more current. For a diode, nothing would happen on the current axis until 0.6 volts then the line would jump almost straight up. At that point the resistance is near zero.

You can go to the company's website and download something called spice data which should have the actual resistance of the diode, the diode equation for that particular diode. In the old days I had to actually make a circuit, now I just put the spice data into a program, draw the circuit, and graph the results. Saves alot of smoke pouring out of the wires.


Bottom line, your bridge measurements seem normal. The 1000 volts may be the PIV rather than the working voltage

Re: Bridge rectifier

Tue Nov 13, 2012 12:56 pm

I sure am glad you folks know what you are talking about. I read the whole post, then had to go get a fresh cup of hot coffee :big smirk: :big smirk: :big smirk: :big smirk: :big smirk: :big smirk: :big smirk: :sick: