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I found some issues with your design.
1) You did not drop voltage on SG3524 to 12V. 7812 voltage regulator can not work beyond 36V. A series transistor regulator ahead of 7812 will drop voltage to 24V for input of 7812 regulator . For example a setup like this will also provide supply for 12V relays (which are more common and cheap) and any other circuit such as status display. http://obrazki.elektroda.pl/838586830
If this is the FET then it is very good. A sigle pair can take 3KW of load.
Car head-light bulbs are between 60-100 watts(12V) . Use four of them in series so that they are not blown during a short circuit.
Similarly 50 watts 12V spot-light halogen lamps are common and available
I tried it with a 21watt and the output of the inverter is 127V AC. And I tried removing the 21watt and behold the inverter gave output of 227V AC. I tried connecting a I00watt bulb and the output was still 227V but the MOSFET started heating up because I didn't use a heatsink and also I tried it with a single MOSFET per bank. Is it normal for the MOSFET to heat up quickly.
At 100W load current is between 2A-3A. 3 x 66/1000 = 0.7Watts. The disissipation in FET is well beyond 1Watt. It would heat a little but take this load without a heatsink.
Firrst check current at no load. If there is a filter capacitor on 220V side, remove it. Now current should be minimal. The series bulb should not light or there will be minimal light. If this is the case connect meter with DC 10A range. The current should be less than 1A. Check this current then furher checking can be done.
The series bulb you made mention of, is it the motor bike bulb. If it is, when I used it the output voltage was 120V AC then I removed it and the output is now 220V AC. For now I am no longer using the series bulb. That time I used it, it was glowing, I don't know whether it is normal?
I tried connecting the 48V with 80N30W this morning and as soon as I connected the battery terminal, the bike lamp got burnt (even without powering the oscillator circuit).
Then I decided to still try the 24V with IRF3205 and I conducted the test. The MOSFET this time was as cold as ice cream for about ten minutes before I disconnected the power.
These are the multimeter readings:
1. When the bike lamp was in series with the positive of the battery and the multimeter in between them, the meter reading is 1.7A
2. When I removed the bike lamp the multimeter gave 4.8V
Is it normal? And how can there be flow of current even when the oscillator circuit is OFF (as I mentioned above).
There should be no current when power is not applied to driver board. It can only happen when +ve touches somewhere. thiscan also happen when ground connetion is loose and ground does not reach driver board. Run ground wire securly from common point of souces of Fets. Move up and down and tab the oscillator and driver board without applying power and with fuse or bulb protection of coarse. It should not happen again. For 48V, four bulbs are required, put them in series. Now they won't get fuse when connecte directly to 48V.
80N30 has 66m-ohm internal resistance while IRF3205 has 8 mili-ohm resistance in on condition ( Rdson). So there is 8 times less heat dissipiation.
21-Watt bulbs can pass only maximum 2A current. For these tests 10A is better. There are two fillaments in head-light bulbs. They can be paralelled to increase current.
Bulb glows when current is passed. Less current, less brightness. Most of current pass through FETs and transformer. When there is no drive, the FETs are off passing zero current. If at that time FETs pass current there are some unwanted pulses generated somewhere which can cause damage.
All these time I have been using the series bike lamp it has been fully glowing. But today I decided to remove the MOSFET circuit and test the othe circuit. I discovered that the series bike lamp didn't glow and the rest of the circuits are working perfectly.
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Do you still remember the transformer test I did above, I have a few quetion
When a 100watt bulb is connected in series to the input of a transformer and the output of the transformer is left open/unshorted,
1. If the bulb fully glows what does it mean?
2. If doesn't glow what does it mean?
3. If it glows small what does it mean?
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If the output is shorted,
4. If the bulb glows fully what does it mean?
5. If the bulb does not glow what does it mean?
6. If the bulb glows small what does it mean?
If you remove the link from IC output pins and input of gate drive or remove +ve supply of sg3524, there would be no current passing though FETs.
An ideal transformer wastes no enrgy. What power it takes is equel to what you draw from its output. Practical transmors are less (about 96%) efficient. Some power is waisted due to resistance in the coil and some in the megnetic core. Series bulb indicate the current with load (output is shorted, drawing maximum current) and without load. Without load the bulb is dim which indicate power loss is small.
Like I told you, when I tested the transformer without the output being shorted, the brightness of the series bulb was dim but when i shorted the output the brightness was full.
With this, I think my transformer is perfectly ok.
Please what next should I do?
As I early said.
Run ground wire securely from common point of sources of Fets. Move up and down and tab and shake the oscillator and driver board without applying power and (with fuse or bulb protection of course). There should never pass the current.
Remove the link from IC output pins and input of gate drive and connect +ve supply of sg3524, there would be no current passing through FETs.
I have conducted the test and in addition these are the things I did.
I removed the power going to the oscillator and I powered the inverter and the series bike lamp still glowed to its full brightness but when I added a 2.7k ohm resistor between the gate and the ground directly from the battery, the bike lamp didn't glow again.
I decided to abandon the main driver board and I connected a 39 ohm resistor between the output of the oscillator and the gate of the MOSFET with the 2.7k ohm connected between the gate and the ground.
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At first I didn't power the oscillator and the series bike lamp didn't glow but when I connected the oscillator, the series bike lamp glowed dim and there was 227V AC output and the transformer was humming.
When I removed the series bike lamp and replaced it with an AC fuse rated 6.3A 250V AC, at switch ON the fuse got blown, why?
Does it mean that if I connect without the series bike lamp the MOSFETs will be burnt?
All the steps I have discussed above were conducted with 48V DC supply and it is worth noting that the MOSFET didn't heat up.
This can happen if the frequrncy of the oscillator is lower than 50Hz. How do you check, if the frequency is right?
Another possibility is that the transformer is not for 48V but for less, for example 36V. This is easy be verified and you already be knowing. Did you check 48V on individual winding with multimeter while connecting 220V side with mains. The outer wires will show 96V AC.
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