current transformer with SG3524

Discussion in 'The Projects Forum' started by onlyvinod56, Nov 17, 2010.

1. onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1
Hello.

I have designed an inverter circuit (500W) with SG3524. A shut down facility is given at the 10th pin of that IC. I want a circuit to monitor the load current and should send a pulse to the 10th pin when the load current violates the limit.

Now, i dont want to use a dedicated current transformer. See the schematic. I have a small 230/12V, 500mA transformer. I will use the 12v winding as primary (will be in series with the load) and the 230v section as the result of current sense. What will be the value of 'R' if the maximum load current is 2 Ampere.

and any precautions needed?
please dont tell me to use a ring type current transformer. Here i want to know whether a noraml transformer can be used like CT or not.

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2. debjit625 Well-Known Member

Apr 17, 2010
790
186
No,it cant take a load of 500W ,every wire (conductor) have an ampacity ratting which tells us how much current it could handle,and the 12V 500mA secondary's wire will not handle that much (500W with a load at 220V ).You dont need ring core CT what you need is Wound Primary CT ,they can measure currents from 1 amp to
100 amps.They are connected in the same way you were trying to do with the small step down transformer just they have the proper ratting.

Good Luck

3. SgtWookie Expert

Jul 17, 2007
22,183
1,728
I'm wondering how you think that you can put 38.334 Amperes of current through a transformer winding that is rated for 500mA?

230V/12V= 19.167 Amperes current required on the 12v side for 1 Ampere current on the 230V side.

You have no feedback from the secondary side to know when the voltage is proper.

You have no methodology to provide a sine wave reference to compare the secondary side to.

Your design simply will not work.

4. onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1
A general step down transformer rated 230/12v, 1Amp.
That 1Amp will be the LV side max current capacity. Am I correct? If so.
HV max current rating will be 1 x [12/230] = 0.052amp

So, if the load current of the inverter is 1ampere ( = current passing through the LV of the CT, as the LV is in series with the load), the secondary current of the CT (230 volts side) will be 0.052 amp. Then there is no question of high currents. I think its easy to handle the current.

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5. debe Well-Known Member

Sep 21, 2010
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Most inverters Ive built you dont monitor the HV side current, you measure the LV current draw across a resistor in the 12v feed to the output switching devices, this is set to shut it down when max is exceeded on the HV side.

6. onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1
Ok Debe...

see this

Is it possible to design with the common op-amps? or shall i go with the dedicated ICs?

7. debe Well-Known Member

Sep 21, 2010
955
185
Hi onlyvinod56, this is how the last inverter i built sensed overload conditions. Since im not a designer only a repairer or builder you may need more help here from more knowlegable people to work out posible values. DeBe.

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8. onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
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Hello,
see the attachment

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9. onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
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one more thing.....

how can i make the inverter to be an automatic?
i mean the inverter should back up immediately when the ac mains goes off.
Can a simple opamp and relay do this job?

10. debe Well-Known Member

Sep 21, 2010
955
185
R for current sensing in a 300W inverter is a piece of 1.6mm copper wire 60mm long in a coil. For 500w would need to be a bit thicker. If you want a 500W inverter to automaticly come on when the mains fails its cheeper to buy a ready made UPS. DeBe.

11. onlyvinod56 Thread Starter Active Member

Oct 14, 2008
362
1

I have seen some 0.1ohm, 50W resistors on net. It goes around 6\$.
and

I have designed my inverter. It is working good with a 100W lamp. I want to finish it with over load protection and and the automatic feature.

Can u help me here??

12. debe Well-Known Member

Sep 21, 2010
955
185
Cant help mutch more, about the limit of my axpertise on design. But not to sure about making the inverter in to an auto change over for mains power, this is a highly risky venture SAFETY wise. DeBe