I am trying to rectify a signal that has amplitude 240, and a frequency of 55Khz. I am not sure which rectifier to choose though. I have an NTE581 rectifier right now which is a general purpose fast recovery, but I don't want to blow it up if it wont work. The specs on the back of the package say Prv = 400Vmax, If(av) = 8Amax, Ifsm = 150A, Vf = 1.2@3A, and trr = 150ns. If possible to explain what the parameters mean, and what the parameters should be if this rectifier will not work. Thank you!!

 

What kind of signal is it?
Will you make a half or full wave rectifier?

 

It is a sine wave. I am going to make a full wave rectifier but the part I listed above is for half wave. Just wanted to try it out to see if it would give me half wave rectification or not. I looked at the online help from here about the rectifier terminology, but without looking at at figure with a signal and the parameters labled it is hard for me to visualize.

 

You cant use this diode because its Peak Reverse voltage is only 400V. If you use it in a half wave rectifier the reverse voltage the diode will see it will be the peak voltage across the load plus the peak voltage of the sine wave. This is 240+240=480Vpeak (in the worst case), so you have to use a diode with a reverse voltage greater than 500V.

 

Thanks mik3, so if I were to use four of these to create a bridge rectifier then they would work ok?? Also will the trr = 150ns be good enough to use with 55Khz?

 

Yes if you create a bridge rectifier it will be ok. The reverse recovery time is pretty fast for the 55 KHZ frequency.

 

If I was to use that had a larger peak reverse voltage then I could still use that in a bridge rectifier configuration???

 

Yes you can.

 

Well I tried using a NTE580 which is a fast recovery, 600V, 200ns rectifier. When I apply the signal, for a split second it shows the wave half rectified, but then it goes back to a sine wave that is smaller in scale than the input. Not sure what the deal is with this. Just using this rectifier in series with a resistor and taking the measurement across the resistor with the scope. Do fast recovery rectifiers have a large resistance in the forward direction? When I measure with a DVM I get about 4K ohms in the forward direction, and a few M ohms in the reverse direction. I thought that the resistance in the forward direction was really small.

 

Without a capacitor at the output of the half wave rectifier you can use the 400V one. Try to take a measurement with the 400V, maybe the 600V is not good.

 

Tried using the other rectifier and am seeing the same problem. Also the resistance I measure with the other rectifier is about 3.8K Ohm in the forward direction, and .5G ohm in the reverse direction. Is the way I am hooking up the signal wires correctly?? This is what I have....

wire 1: ________ 240Vpeak at 55 Khz
wire 2: ________~10Vpeak at 55 Khz
wire 3: ________240Vpeak at 55Khz
wire 4:_________~10Vpeak at 55 Khz

I am using wire 1 to the anode an wire 2 to the cathode.

 

Well I tried using a NTE580 which is a fast recovery, 600V, 200ns rectifier. When I apply the signal, for a split second it shows the wave half rectified, but then it goes back to a sine wave that is smaller in scale than the input. Not sure what the deal is with this.

Just using this rectifier in series with a resistor and taking the measurement across the resistor with the scope.

How many Ohms is your resistor?

Do fast recovery rectifiers have a large resistance in the forward direction? When I measure with a DVM I get about 4K ohms in the forward direction, and a few M ohms in the reverse direction. I thought that the resistance in the forward direction was really small.

Semiconductors have conductance instead of resistance (MOSFETs are an exception to this). Your DVM when set to measure resistance is actually sending a small current between the probes, and measuring the voltage between the probes. R = E / I.

Silicon diodes will have a voltage drop across them when a current passes through them. A 1N4007 1kv 1A diode might have a 0.7v drop at 10mA, 0.8V drop at 100mA, and 1v drop at 1A. It's a non-linear response.

 

Tried using the other rectifier and am seeing the same problem. Also the resistance I measure with the other rectifier is about 3.8K Ohm in the forward direction, and .5G ohm in the reverse direction. Is the way I am hooking up the signal wires correctly?? This is what I have....

wire 1: ________ 240Vpeak at 55 Khz
wire 2: ________~10Vpeak at 55 Khz
wire 3: ________240Vpeak at 55Khz
wire 4:_________~10Vpeak at 55 Khz

I am using wire 1 to the anode an wire 2 to the cathode.

What are you using for a reference? Ground?

What are these signals supposed to be driving? CCFL's?

 

The signal is going to drive a relay coil, but I need to make the signal dc because the frequency is too high. I am using a breadboard to put the components on, so my reference ground is the ground on the breadboard.

 

Can you post a schematic please. Where do you generate this 55 KHz signal from?

 

There is a big piece of equipment that is automated to take a bunch of tests, and coming from the machine are wires that control a microscope light. In the past we were using a fluorescent light, that would turn on/off during the automated testing procedure at times dependent on the software (software is done by company who makes this machine, and I can't change it). Now we are using an LED light. I want the new LED light to be automated the same way the fluorescent light was. So the only way is to use the signal wires that were used to operate the fluorescent light before to control a relay switch. When I spliced the wires (that once energized a fluorescent light, and are coming from the back of the machine) and tested them with a scope I get the results as explained before. So the 55Khz signal, and amplitude is generated by this particular machine (which is made in Japan). I will post a schematic later tonite. Is it possible to have a rectifier that has to fast a recovery time?? It is weird how the output signal is rectified for a second then it goes back to the original waveform. Also the output of the rectified wave is scaled way down. Not sure why either. Hope this clarifies more, and I will post a schematic shortly. Thanks

 

Sounds like it might be a driver for a cold cathode flourescent tube. High voltage, very low current.

You might need to wind a toroidal transformer to step down the voltage and step up the current.

What's the phase relationship between the two wires that are carrying 240v?

 

The phase seems the same when I measure the signals. Here is a schematic
http://img149.imageshack.us/img149/3802/schematicmg1.jpg Resistor is 1K and the diode is 400V, with 150ns recovery time. I try putting different combinations of the wires to the inputs of the circuit but am having no luck.

 

OK, what is the value of the resistor? If you don't know that, what are the color bands on it?

 

I have tried all sorts of different resistor values. 1K, 10K, 2.7K, ect. What value should I be using?