ULN2003 and pulse transformer

[sandeepa]

Can I use ULN2003A to drive a pulse transformer ?
I have attached the circuit diagram.
The pulse transformer drives a 40A, 1200PIV SCR.The gate current is rated to approx. 150ma.

I have used this circuit in a number of devices.After running for a while, 2 of these devices stopped functioning.The ULN2003 had cracked in these 2 devices.

Is ULN2003 suitable to be used with a pulse transformer ?

Thanks.

 

[Roff]

What is the pulse width?
What is the frequency?

 

[sandeepa]

Pulse width is 55us.
5 such pulses are sent every 10ms.

 

[mneary]

What is the purpose of putting unlimited current into the pulse transformer? The only current limits in your circuit are the 0.35 ohm transformer and the ULN2003. A TRIAC gate doesn't need 70 amperes!! The ULN200x series are <500mA under ideal conditions.

A resistor of 470R in series with the pulse transformer would dramatically improve your reliability.

 

[Roff]

The low inductance of 680uH, combined with the 55uS pulse width, allows the collector current to ramp up to almost 2 Amps. As mneary said, you are exceeding the collector current spec. This is with the 100 ohm load shown in your schematic.

A 470R resistor in series with the primary will make your output pulse width short, with an exponential decay.

 

[mneary]

I should mention that the 470R should be 1/4 watt or larger, carbon composition or thick film. Beware: it will quickly burn up if your trigger software ever makes a sustained output instead of pulses. If your software could apply 24V steady to the resistor, use a 2 watt resistor.

A 1/4 watt resistor will be operating at a low duty cycle, above its ratings. Thick film and carbon comp can handle surges, but a thin film resistor won't last long in this circuit.

 

[Roff]

I should mention that the 470R should be 1/4 watt or larger, carbon composition or thick film. Beware: it will quickly burn up if your trigger software ever makes a sustained output instead of pulses. If your software could apply 24V steady to the resistor, use a 2 watt resistor.

A 1/4 watt resistor will be operating at a low duty cycle, above its ratings. Thick film and carbon comp can handle surges, but a thin film resistor won't last long in this circuit.

See my last post. The L/R time constant is too short.

 

[sandeepa]

I put a 180R is series with the primary.I measured the current in the primary.The multimeter shows only around 1mA.
This doesnt seem right, because the gate current needed is around 100mA.Pulse transformer winding is 1:1.

 

[mneary]

Did you see any smoke?

If you are using a multimeter then you have turned the driver to 100% duty cycle? Your resistor must be huge to withstand the power on a continuous basis. If you applied 24V continuous to 180R and it was rated at less than 2W then it burned up.

You say that normally the circuit runs at 3% duty cycle. A quality analog multimeter should read 300-350 µA. A cheap digital multimeter will display any numbers it feels like.

Your gate needs 100 mA? I thought the maximum was 150 mA and my suggestions gave you 20% of that to be safe. Oh, well, this means a lot more current in the driver. Then you must take into account the other sections of the ULN2003a. And you need to guarantee that the software won't leave the driver on for more than the duty cycle it's designed for. See Figures 14 and 15 in the data sheet. The ratings are less for the D package; so be sure to use the correct graph!!!

A 180R will receive 3.2W from 24V. It needs to be a 5W resistor unless you can absolutely guarantee pulse mode, then maybe it can be a 1W THICK FILM type.

 

[sandeepa]

I connected the multimeter in series with the 180R resistor, in the current range.That shouldnt burn up anything ?
The connection was
+24V-->ammeter(DMM in current mode)-->180R/1W-->pulse transformer primary-->ULN o/p
The resistor is 180R,1W.
The duty cycle is controlled by the PIC and doesnt beyond what i stated earlier.

I had connected a 100W lamp to the SCR o/p and it was glowing nicely, for a considerable time.Which means the gate is drawing whatever current it needs.Yet the multimeter showed only 1mA reading.

Surely I am missing something ?

Thanks.

 

[mneary]

Surely I am missing something ?

You are missing the fact that the current is only present 3% of the time.

What would you want the DMM to show? Peak? Average? RMS? Minimum? Expensive DMMs might say in the manual what they intend to display under these circumstances,. Cheaper DMMs (less than $200 US) show a number of its own choosing, likely none of the above.

An old moving-coil analog meter will display the average, If the current is 110 mA, the meter would show about (0.11 A * 0.03), or 3.3 mA. (Note that this is NOT the type of meter that you're using.)

n.b. In an earlier post I said 300-360µA. It should have been the numbers shown here.