pulse transformer details for scr firing

Discussion in 'General Electronics Chat' started by danda suredra, Apr 25, 2011.

  1. danda suredra

    danda suredra Thread Starter New Member

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    i am using 4503 1:1:1 pulse transformer to generate the triggering pulses to trigger scr. scr is able to trigger when a resistive load is connected. but when i connect an highly inductive load it's not able to trigger the scr. scr used is tyn 410. and pulse transformer is driven by (sl-100) which is fed by 556 timer with 180us on time.
    can any one plese give the specifications about 4503 pulse transformer.
    thanks in advance.
  2. t_n_k

    t_n_k AAC Fanatic!

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    The likely reason that the SCR isn't turning on with the highly inductive load is that the gate pulse is not on for sufficiently long enough time for the SCR to reach latching current in the forward conduction condition. Increasing the firing pulse duration will probably solve the problem.
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  3. danda suredra

    danda suredra Thread Starter New Member

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    thank you. but how can i increase the firing pulse duration its all decided by the pulse transformer. and i am stuck with that. i dont know any thing about the pulse transformer it is mentioned as 4503 with 1:1:1.
  4. danda suredra

    danda suredra Thread Starter New Member

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    the input for the pulse transformer is around 185us pulse. and it has 24v bias supply for the input. and input resistance is 200ohms and gate resistance used is 220ohms. one pulse transformer is able to drive the scr.
    but other one of the same type is not able to drive it. but both are driving for resistive loads.
  5. t_n_k

    t_n_k AAC Fanatic!

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    There's most likely a solution for your problem, but without a schematic diagram no one can really comment or make specific suggestions. If your pulse is derived from a 556 timer then I would be very surprised if you can't stretch the pulse to something longer.

    The only limitation of the pulse transformer will be the Volt-μsec rating of the transformer. That should be known from the data sheet if you have access to the data sheet for that type no.

    If you can post a schematic of your existing circuit that would be very useful.
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  6. danda suredra

    danda suredra Thread Starter New Member

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    but i dont know the volt usec rating of the pulse transformer i even not able to find the data sheet of that one. its "pulsetek thyristor trigger RPTYPE 4503 pulse transformer isolation". i have attached the circuit. the ckt name file is fed from phase_shifter. where i wil get inverse cosine wave. and firing angle can be controlled by the -Vc.

    Attached Files:

  7. t_n_k

    t_n_k AAC Fanatic!

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    It looks like the SCR gating control is a pulse train derived from half of a 556 timer running in gated astable mode. The gating control comes from a 4538 monostable.

    There are timing capacitors which could be adjusted - C2,3,6,8.

    For instance in the top half of your schematic C2 & C3 are associated with either R4 (monostable gating circuit) or R5,6 & 7 (astable circuit).

    Do you know the values of C2, C3 & R4,5,6 & 7?

    Increasing C2 would increase the monostable gating duration and increasing C3 would increase the width of the gated pulses. If you know these values it would help if you posted them .....

    Unfortunately, not knowing the pulse transformer spec is going to make it hard to determine how much one might increase the capacitor values.

    Another option may be to introduce some capacitance across the gate-cathode terminals on the SCR's and perhaps also changing (increasing) the astable frequency at the 556 timer. In other words extending the pulse width at the gate terminals directly using an AC-DC converter action through the pulse transformer & secondary side diodes (D7A or D10A).

    Post the component values if you can and there may be some ideas forthcoming from other members as well.
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  8. danda suredra

    danda suredra Thread Starter New Member

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    c2 = c6 = 47nf ; R4 = R7 = 150k; for 4538b which is used as monostable multivibrator with 4.7ms pulse.
    c4 = c3 = c7 =c8 = 10nf; R7 = R20 = 27K; R6 = R19 = 1K; for 556 timer. which wil give 185us(approx.) on time pulses (12).

    can u please suggest any pulse transformer which i can know the rating in such away i can take care the req. gate current of the scr. i am using tyn410 scr. Igt = 15mA; Vgt = 1.5V(both are max.)tgt = 2us;
  9. danda suredra

    danda suredra Thread Starter New Member

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    if i keep a capacitor across the gate cathode terminals what does it wil do.is it increase the pulse duration across gate cathode terminals. rc time constant. please suggest a proper pulse transformer.
  10. t_n_k

    t_n_k AAC Fanatic!

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    Hello danda suredra

    I'll give some further thought to the values you have posted and a possible solution.

    One other thing that is worth knowing is the SCR load side power schematic.

    You mentioned you are attempting to turn on a highly inductive load. What exactly is the load (values) and high side power supply type & value.

    This is worth knowing because (you) the designer can then decide what the minimum gate pulse on time is required to ensure the load will be turned on. I notice from the data sheet that the TNY410 SCR has a latching current of 50mA
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  11. danda suredra

    danda suredra Thread Starter New Member

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    I am giving the rectifier output to the field terminal of the synchronous machine. synchronous machine is 230V,1KVA rating. Maximum field current is 1A. u may feel why i went for 10A rating scr. but even 4A rating scr has same gate current(Igt = 15mA).
    please Suggest me proper Pulse Transformer.


    Thanks for your valuable suggestions. waiting for pulse transformer details and further suggestions.
  12. t_n_k

    t_n_k AAC Fanatic!

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    The information you gave regarding the synchronous motor isn't really sufficient to determine the minimum pulse duration. I would need to know the field winding parameters - the inductance and resistance and AC supply voltage (is it 230V?).

    In any case you might try something before looking at changing the pulse transformer. It may be possible for the transformer to accept double the pulse width without saturation occurring.

    The pulse width will be determined by capacitors C3 & C8. If these are currently 10nF try increasing them to say 22nF - giving about 2 times the gate pulse width. If this doesn't work try 47nF.

    If this doesn't work then it may be necessary to find another pulse transformer &/or make some other changes.

    Do you have access to an oscilloscope to enable you observe waveforms at the various test points?

    NB: If you can find the field winding specification then would be of great use.
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  13. danda suredra

    danda suredra Thread Starter New Member

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    Attached files are the hardware setup. with R load of around 50ohms the output is shown.

    Attached Files:

  14. danda suredra

    danda suredra Thread Starter New Member

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    i dont know the exact specifications. but when i measured those terminals with multimeter i got around 87.5 ohms.(resistive mode). once
    the circuit is conducted. and i am able to flow 0.4A current through field
    with the same set up. but later one pulse transformer output dipped(prior it gave 5.5Vpeak) pulses but later it was 4.2V (peak) so its not triggering. but other 2 scrs are triggring. so it is acting like a half controlled bridge rectifier with the conduction till 150degrees.
  15. t_n_k

    t_n_k AAC Fanatic!

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    If you are working with mains voltages you need to be very careful.

    There is a policy on this site that advice is not given by members without them being confident of the electrical competency of a person to work with high voltages.

    To be honest with you your layout doesn't inspire me with confidence - if this equipment is running from mains voltages. Do you have formal training & qualification as an electrician or electrical engineer?

    Is this work you are doing for yourself or perhaps as part of a course of study. If the latter are you receiving appropriate supervision & direction by a person with the necessary technical knowledge & qualification?
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  16. danda suredra

    danda suredra Thread Starter New Member

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    i am attaching those test points as i observed previously with the oscilloscope. May be by seeing the schematic of the diagram u felt about me. I am doing this under the proper guidance of my sir with out his presence i am not simply connecting this set up to the machine fields. with out energizing the machine from supply we are observing whether it can able to trigger with the load. we are first checking whether it is giving continuous current or discontinuous current. Thanks for ur concern and advice and ur quick replays.

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  17. t_n_k

    t_n_k AAC Fanatic!

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    OK - I understand the situation.

    It was not quite clear to me as to what was your high voltage side setup.

    It seems you are developing a full-wave fully controlled bridge rectifier for excitation control of a synchronous machine.

    Depending upon the excitation winding inductance you are probably right on the limit for reliable operation.

    I've done some calculations / simulations. Using a DC winding resistance of 90Ω and inductance of 1.2 to 1.3H with a 240V AC supply and gate pulse width of 185usec the operation becomes unreliable. Doubling the gate pulse width allows operation with double the winding inductance [~2.6H]. This was based on a 50mA latching current for the SCR.

    Well I would try the changes to the capacitor values I suggested and see where that leads. Check that the pulses on the transformer secondary side are reasonably close in terms of pulse duration with respect to the primary side pulse width. If the secondary side pulses are not sustained much beyond the 185usec width then you will probably need a different pulse transformer with a higher volt-microsecond rating.
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  18. danda suredra

    danda suredra Thread Starter New Member

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    Thank you for ur suggestion. exactly i want to make a fully controlled bridge rectifier. If every thing works out we want to control the firing angle automatically by looking at the machine terminal voltage. today i will look at the pulse width. and when pulse height is around 5.5V it triggered with the same duration.one pulse transformer is working with that.so two devices are triggering. where as the same rating pulse transformer for other two scrs is giving 4.2V pulse height so its not able trigger the device.

    Can i give input for the pulse transformer with out any resistor.(In circuit i showed 100 ohm resistor where as i used a 200 ohm resistor(white bulged) in the firing circuit.
  19. t_n_k

    t_n_k AAC Fanatic!

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    It's not clear where you are measuring the 5.5V and 4.2V trigger pulse values - is that at the gate (G) to cathode (K) terminals of the SCR's? Or on the transformer secondaries?

    The SCR's should turn on with even lower Gate-to-Cathode voltages. The data sheet suggests 1.5V as typical for turn on.

    You need to post the schematic diagram of that part of the circuit which includes the signal path from the transformer primary side drive transistor to the SCR gate connection. Indicate all the values and, if possible, show the (loaded - gate connected) waveforms at the secondary side output and the gate.

    I'm also not sure if you are saying that the gate pulse magnitudes on one side of the pulse drive have mysteriously changed and there is now an apparent malfunction in the circuit - with only half the bridge working. If nothing else has changed - say a recent wiring error hasn't crept in - then one or both SCR's may be damaged or the gate pulse driver transistor may be damaged. Also, is this apparent malfunction occurring with the machine exciter winding connected as load or with a dummy load connected to the SCR bridge output?

    I understand it may be difficult to explain everything clearly in English but it is hard to offer advice when the facts of what is happening aren't 100% clear. The more complete the picture the better.
  20. t_n_k

    t_n_k AAC Fanatic!

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    For further clarification as well - If the large white power resistor is 200Ω and corresponds to the resistors denoted as R22 & R9 then you are losing quite a lot of the main driver 24V supply voltage.

    Are R10 and R23 in the gate circuit or are they being used as test loads for the gate pulse driver board? If they are in series with the gate drive what value are they?
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