single-source emergency-lighting system-challenging problem

[abbeyromy]

Hello Seniors, really need your help here! I saw this circuit somewhere, played with resistor values and finally got something which looks like the circuit is working but then I am not able to do the math as to why and how. So need your expert guidance.
The circuit basically has a rechargeable battery, which once charge to full can deliver load current at times when main supply fails. Please see the circuit and attached pspice result.

Theory before proceeding with my questions for the attached circuit->

The circuit is a single-source emergency-lighting system that will maintain the charge on a 8-V battery to ensure its availability and also provide dc energy to a bulb if there is a power shortage. A full-wave-rectified signal will appear across the 8-V lamp due to diodes D2 and D1.

The capacitor C1 will charge to a voltage slightly less than a difference
between the peak value of the full-wave-rectified signal and the dc voltage across R2
established by the 8V battery. In any event, the cathode of SCR1 is higher than the
anode and the gate-to-cathode voltage is negative, ensuring that the SCR is nonconducting.
The battery is being charged through R1 and D1 at a rate determined by R1.Charging will only take place when the anode of D1 is more positive than its cathode.

If the power should fail, the capacitor C1 will discharge through D1,R1, and R3 until the cathode of SCR1 is less positive than the anode. At the same time,
the junction of R2 and R3 will become positive and establish sufficient gate-to-cathode voltage to trigger the SCR. Once fired, the 8-V battery would discharge through the SCR1 and energize the lamp and maintain its illumination. Once power is restored,the capacitor C1 will recharge and re-establish the nonconducting state of SCR1 as described above.

Test conditions:
At 120ms the Power supply fails, assume both capacitor and Battery are charged to maximum at this moment. Now that power supply has failed the capacitor will try to discharge and thus turning on the SCR at some time. SCR will deliver current to the Load of 100 ohms.

Questions

1. Can somenody explain the Voltage across capacitor and VOltage across anode-cathode of SCR? At 120ms capacitor is discharging and at 200ms it starts charging up. Maybe cuz SCR is ON and conducting but why is this hapenning?
2. PSPICE result shows that Anode current of magnitude around 80mA starts coming at 200ms. Why 200ms? How?
3. The supply failed at 120ms but the SCR circuitry only responds at 200ms? What is wrong.

Can somebody please simulate the results and tell what is exactly the math behind?
Also if you try chaing the lamp load to 1k circuit fails to work. Any idea why?

Please do help! Really appreciate your support!
-Hemanshu

 

[abbeyromy]

hey seniors.. please help

 

[keith1200rs]

Re: single-source emergency-lighting system-challenging prob

Sorry I haven't been back with a reply earlier. I bookmarked it on my phone so I could look later on my computer and only just noticed the bookmark.

My understanding is that when the power is off C1 discharges mostly through R2 and the load Rlamp. The charging of C1 is then through R3 but the potential divider with R2 means that it won't get charged to many volts: 1.04V maximum with an 8V battery. This gives enough voltage to turn on the SCR (the other end of C1 is grounded through Rlamp).

Once the SCR fires C1 gets charged again because Rload now has a positive voltage through the SCR and the other end is held low by R2.

The very slow turn on time of the SCR (200ms) is due to the CR discharge of C1 to a voltage very close to the SCR firing voltage so you are on the almost flat part of a CR charge/discharge. If you reduce R3 you can make it turn on a quicker.

I hope that helps, although I may just be telling you what you already know. I have put the circuit in my simulator so I can look at it though, if you have any more questions.

Keith.