I have the following schematic:

PIC and pulse transformer schematic

RB3 is connected to PIC microcontroller digital output. I am wondering is it possible to know from the output voltage waveform is this pulse transformer in saturation.

enter image description here

2V/div - 500us/div

Blue signal is from the PIC and the purple one is output from one of the pulse transformer's secondary windings. On the first image, the PIC's signal is 50us wide. On the second image, PIC's signal is 500us wide and it can be seen that the pulse transformer stopped responding to the voltage on primary coil.

Are these images enough to say that this pulse transformer is in saturation? Unfortunately I don't have any datasheet for transformer, it's very old and manufactured by VAC.

  • 1
    \$\begingroup\$ I'd be interested to see the primary current, you need a changing current to generate a voltage, it's possible the transistor is saturating instead. If the primary current rises then plateaus then the transistor is probably the cause, if the primary current just keeps rising and rising but the output drops off, then the transformer's probably saturating. \$\endgroup\$ – Sam Jun 12 '16 at 0:14
  • \$\begingroup\$ @Tom Yes/No. Transistor saturation comment is correct. But if primary saturates then current rises to "what the market will bear". In an ideal case where the core saturates suddenyly then current jumps to I = V/R or infinity for no R. In real world inductors the current rises as a linearish ramp until saturation starts then curves upwards as core progressively saturates. This works quite well in practice as a means of visibly seeing a core go into saturation. \$\endgroup\$ – Russell McMahon Jun 12 '16 at 0:58
  • \$\begingroup\$ Is ningbo = Ningbo across the (wide) river mouth from Shanghai, or something else? [I have visited Ningbo 3 or 4 times)(And China 15 times (from NZ)]. \$\endgroup\$ – Russell McMahon Jun 12 '16 at 0:59
  • \$\begingroup\$ It would be good to see some dots on the pulse transformer windings to confirm phasing \$\endgroup\$ – Autistic Jun 12 '16 at 2:00
  • \$\begingroup\$ It could also be inadequate primary inductance, limiting low frequency performance. As Russell says, measure the primary current, that will give you useful additional information \$\endgroup\$ – Brian Drummond Jun 12 '16 at 10:57

Saturation looks likely BUT as Tom says - could be the transistor not having enough current sinking capability.

What happens if you change R3 to 1k?

How much inductance do you need for the transistor to have enough drive?:

At Vcc = 5V then I_R3 ~= (Vcc-Vbe)/R3 ~= (5-0.6)/10k ~~= 0.4 mA.
For V3 Beta = 100-200
I_V3 max = Beta x Ib = 40-80 mA.
Say 100 mA to be "kind".

I primary ~= Vp x t / Lp or Lp ~= Vp x t / Ip for core to just saturate in time t.
For 500 uS pulse, for core to just reach saturation
Lp = Vp x t / Ip = 5 x 500 uS / 0.1 A = 25 mH
So you need a 25 mH effective primary inductance to remain within the transistor's drive capability.

1N4007 is not very suitable in this application.
1N514x (x depends on voltage)or even 1N4148 would work better.


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