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#16
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Re: victor 885 parts
Cirrus,
I believe there is only one voltage regulator, the other things you see are drive transistors for the FETs. You are correct on the the H bridge design, this is very common in motor controls. If you look closely you will also see the equalizing resistors in series with each FET. These help to equalize the current each shares with the other two in it's string. There are very low resistance so measuring them with a normal DVM won't tell you if they are shorted. The values of these resistors are about the same as the resistance of the leads on the DVM. Remember that depending on the intial failure of the FET, other devices connected to it may also have been damaged. For instance, a short from gate to drain may have attempted to draw full motor current through the drive transistor until it failed. Power supply traces on the circuit board may also have been damaged by the excessive currents. Use a bright light and a magnifying lens when checking for damage. Post pictures and questions if you have them. I love to see stuff that is blown up. |
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#17
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Re: victor 885 parts
Actually the one i was looking at last night was quite blown. Everything else on the board seemed fine except one resistor that i couldnt get a reading thru. but for the most part everything but the fets were fine i believe. I have about 4 blown fets on this one. i pulled one fet off the board already just for curiousity. out of the others that are blown i see one with just a small burn mark on the front casing but not actually blown apart. 3 that the casing split. and one more that i didnt count as burnt, altho it has to be because the gate and the drain have continuity.
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Thanks again |
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#18
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Re: victor 885 parts
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#19
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Re: victor 885 parts
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Your idea about added FETs is correct, theoretically adding as many FETs as are needed to make up the current. Some factors might get in the way of success such as turn on time and source to drain "ON" resistance but for the low switching frequency of the Victor the turn on time should not be a problem. Of course the circuit traces are sized for the expected current so you might have to redesign the way power and output leads are treated. All in all, they are pretty cool little devices for $129. |
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#20
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Re: victor 885 parts
Sorry guys that i havent got the pictures up yet.Ive been having major problems with my camera. But out of curiousity. I noticed researching h-bridges that they usually use both p and n channel mosfets. but the victors only use n channel.
Does anyone know the advantages/disadvantages of this? Also can anyone give me a link or explain how to build all n-channel mosfets into a circuit? Thanks Again Craig |
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#21
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Re: victor 885 parts
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To control an all N-channel bridge, the gates of the high-side devices must be driven with respect to the motor output terminals, because that's where their sources are connected. So the gate drive circuit must include a power supply that "floats" on the motor terminal. This is often implemented using a special purpose driver IC such as IR2104. |
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#22
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Re: victor 885 parts
So basically I need to look at whatever voltage is usually needed on the low side to turn on the mosfet. then take the supply voltage and increase it by this number to run the gate.
For example. If it takes 8v to turn on the mosfet on the low side. And my source voltage is 18v Then i need to run the upper gates with the sum of those voltages. Required gate voltage(top side) = supply voltage + Required gate voltage(bottom side). And that ic that was mention is im guessing used to help create this extra voltage? I think i got it but i may be totally wrong. Thanks Craig |
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#23
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Re: victor 885 parts
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A FET has three terminals called source, gate, and drain. To turn it on [i.e., to get it to conduct with a low resistance called R_DS(on)] you apply about 10V between the gate and source terminals. Please refer to the IRL3103 datasheet, conveniently hosted by FRC 358. So each of the gate drive circuits for high-side devices in an N-channel FET bridge must be referenced to the respective source terminal. This is accomplished by establishing charge on a capacitor by means of the special purpose IC mentioned in my earlier post. |
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#24
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Re: victor 885 parts
Many FET driver IC's need a resistor between the driver output and gate to dampen transients and ringing. This is especially important at higher switching frequencies. These maybe the small resistors you see on the board.
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#25
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Re: victor 885 parts
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#26
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Re: victor 885 parts
http://www.chiefdelphi.com/forums/at...tid=4638&stc=1
I know this isnt a picture of the blown up parts. this picture only shows where I removed one Mosfet. Im still having camera problems so this is a pic from a few weeks ago. Sorry for the delay. Im workin on more pics |
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#27
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Re: victor 885 parts
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#28
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Re: victor 885 parts
Very good photo. I have suspected this condition on many occasions; as Al says it is very likely the leading cause of Victor failure.
The moral of this story is: don't make chips and metal dust around the electrical system. You may think you've cleaned it up afterwards, but that's actually somewhere between extremely difficult and totally impossible. |
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