(Circuit for meter details at bottom)
This was written by Brian Clarke VK2GCE and appears (to me anyway) to be the clearest description of how to do it with minimal kit. My notes in bold after each section.
Testing audio transformers
© Brian A Clarke VK2GCE
1 Test equipment required
Multimeter – analog or digital
Audio signal generator – fixed frequency or variable (I have a cheap Maplin kit that does sine, saw, ramp and square waves from about 10Hz to about 25KHz, I added a little speaker so I can monitor the tone and a little frequency counter as well but they are not nesessary). If don't have one then look for a circuit for an AF generator, it really doesn't have to be complex and could be build ugly style.
Variable resistors, 1off 25k to 50k and 1off 10 Ohm to 500 Ohm (Carbon, I found wirewound will add errors to measurements)
Connecting leads
Audio voltmeter- preferably calibrated in dBm, sensitive down to –60dBm (I use an analogue meter, dad gave me a while back no markings or who made it. Audio philes may hate me saying this but I just want rough tests and not HiFi testing so this is also true, audio is AC, as long as your meter doesn't have any unusual restrictions about frequency response (most DVM don't work well at AF), you should be good to go. Set up a 1khz tone (just a bit higher than a normal 700-800Hz cw side tione) and plug it into the AC inputs of the voltmeter and see what you get, if you get almost no response you can't use it, that simple). You could attempt to build an Audio voltmeter too, at bottom of post I include one possible circuit, or use an oscilloscope like this one http://m1kta-qrp.blogspot.com/2009/12/oscilloscope-kit.html but I was trying to limit the kit list!
2 Identify windings
For an output transformer, the high resistance winding, single ended or center-tapped is the primary
For a microphone, line or an inter-stage transformer, the higher resistance winding is usually the secondary
For centre-tapped windings, the two halves may not be exactly the same resistance, especially on E+I cored transformers
There may also be a screen winding, which may or may not be connected to the frame.
3 Measure dc resistance
4 Measure impedance
4.1 Output transformers
Connect the 100 Ohm variable resistor across the low resistance winding; initially, leave one wire open;
Connect the 25k variable resistor in series with the high resistance winding and the output of the signal generator; set the resistance to 0 initially;
Set the signal generator to 1kHz and near maximum output, which may be say, 20V rms
Use the audio Voltmeter to measure the input and output Voltages; write these down
Calculate the turns ratio = VIN/VOUT
Connect the 100 Ohm resistor in circuit
Reduce this resistor till the output Voltage falls 3dB (i.e. halves!)
Increase the 25k resistor till the output Voltage falls another 3dB (i.e. halves!)
Without disturbing the settings of the variable resistors, disconnect one lead from each
Use the multimeter to measure the settings of these two resistors – these are the impedances of the primary and secondary – you may need to replace the 100 Ohm
variable resistor with a lower value to get an accurate measure (I replaced with a sliding pot and fixed value resistors getting a rough value)
Compare the turns ratio with the impedance ratio – do they agree? Allow for the dc resistance of the windings
4.2 Microphone, line and inter-stage transformers
Follow the procedure for output transformers, except:
Connect the 25k Ohm variable resistor across the high resistance winding; initially, leave one wire open;
Connect the 100 Ohm variable resistor in series with the low resistance winding and the output of the signal generator; set the resistance to 0 initially;
Set the signal generator to 1kHz and low output, which may be say, 20mV rms (I used a Maplin AF sig gen)
Warning: do not exceed 20mV rms for microphone transformers
As above
As above
-
Connect the 25k Ohm resistor in circuit – it may have to be a higher resistance;
As above
Increase the 100 Ohm resistor till the output Voltage falls another 3dB (i.e Halves)
As above
As above
As above
5 Measure frequency response
Note: this section can only be done with a variable frequency signal generator.
Reconnect the variable resistors without changing their settings
Connect the audio Voltmeter across the secondary (I used the meter I mentioned, some DVM will not work!)
-
Reduce the frequency till the output falls another 3db compared with steps 8 above; this is the lower 3dB point (My audio sig gen is brilliant for this you just press a button and the signal drops and you watch the meter)
Increase the frequency till the output falls another 3db compared with steps 8 above; this is the upper 3dB point (My audio sig gen is brilliant for this you just press a button and the signal rises and you watch the meter)
-
Write this information down – perhaps attach it to the transformer. (Wrote on mine with CD pen)
6 Measure power response (I never did this bit)
If you want to measure power response of output and inter-stag
e transformers, you will need a very good quality power amplifier, high power non-inductive load resistors and a CRO.
In place of step 1in section 4.1 above, use a variable high power load resistor to achieve the value found in step 10
In place of step 2 in section 4.1 above, use a high power variable resistor to achieve the value found in step 10
Connect the audio Voltmeter across the primary;
Set the input power to about 1W
Connect the CRO across the secondary. Set the signal generator to 1kHz
Increase the power input until the image on the CRT just starts to flatten – this is probably 3dB over the rated power of the transformer
Reduce the power input
Change the input frequency to the lower 3dB point identified in step 3 of section 5 above
-
repeat step 6 at this lower frequency – the low frequency 3dB point may be at a much higher frequency than the lower 3dB point at low power [section 5] – it depends on how powerful the accountant was with respect to the design engineer in choosing how much core of what quality to use.
Parts:
R1_____909K 1/2Watt 1% Metal Oxide Resistor
R2______90K9 1/2Watt 1% Metal Oxide Resistor
R3_______9K09 1/2Watt 1% Metal Oxide Resistor
R4_______1K01 1/2Watt 1% Metal Oxide Resistor
R5_____100K 1/4W Resistor
R6_______2M2 1/4W Resistor
R7______82K 1/4W Resistor
R8______12K 1/4W Resistor
R9_______1K2 1/4W Resistor
R10______3K3 1/4W Resistor
R11____200R 1/2W Trimmer Cermet
C1_____330nF 63V Polyester Capacitor
C2,C3__100΅F 25V Electrolytic Capacitor
C4_____220΅F 25V Electrolytic Capacitor
C5______33pF 63V Polystyrene Capacitor
C6_______2΅2 63V Electrolytic Capacitor
D1-D4___1N4148 75V 150mA Diodes
IC1_____CA3140 Op-amp
IC2_____CA3130 Op-amp
SW1_____2 poles 5 ways rotary switch
SW2_____SPDT switch
J1______RCA audio input socket
J2,J3___4mm. output sockets
B1______9V PP3 Battery
Clip for PP3 Battery
Notes:
Connect J2 and J3 to an DVM-meter set to maximum amp range
Switching SW2 the four input ranges can be multiplied by 5
Total fsd ranges are: 10mV, 50mV, 100mV, 500mV, 1V, 5V, 10V, 50V
Set R11 to read 1V in the 1V range, with a sinewave input of 1V @ 1KHz
Compare the reading with that of another known precision Millivoltmeter or with an oscilloscope
The oscilloscope reading must be a sinewave of 2.828V peak to peak amplitude
Frequency response is flat in the 20Hz-20KHz range
If you have difficulties in finding resistor values for R1, R2, R3 & R4, you can use the following trick:
R1 = 10M + 1M in parallel
R2 = 1M + 100K in parallel
R3 = 100K + 10K in parallel
R4 = 1K2 + 6K8 in parallel
All resistors 1% tolerance
Another option is here http://sound.westhost.com/project16.htm
This is all for AF.
No comments:
Post a Comment