| Electronics > Beginners |
| Sanity check - FET H-Bridge circuit |
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| danners430:
Hey guys, I've done a bit of research, and a normal relay only draws 2 LEDs worth of power - I can survive that :-) I'll use that for the motor, but would still like to use an H-Bridge for the LEDs - if nothing else to gain experience using them. To go back to my original question, would that circuit work in limiting shoot-through currents, while controlling a bi-colour LED (in place of the motor) with only a single I/O pin (if I replaced two FETs with depletion mode devices)? |
| Kjelt:
A normal relay has an activating current (usually the nominal Voltage / coil resistance) and a holding current ( a lot less than the activating current). Now the trick is to experimentally find out the holding current and add a safe margin on top and keep it at that value after activation. In the 90s Elektor had a clever schematic for it in one of their summer issues , using just two transistors but I can not find/remember it, if anyone has it please post it. |
| exe:
--- Quote from: danners430 on September 12, 2018, 01:14:05 pm ---with only a single I/O pin (if I replaced two FETs with depletion mode devices)? --- End quote --- Uhm, it may work. But what depletion fets do you keep in mind? Are you sure they are easy to buy? The world is ruled by "normal" mosfets, exotic devices are not widely on sale... I'd say a common approach is to use only jelly bean parts unless absolutely needed. But for fun and experiments I think any parts is fine. |
| danners430:
--- Quote from: exe on September 12, 2018, 01:24:30 pm --- --- Quote from: danners430 on September 12, 2018, 01:14:05 pm ---with only a single I/O pin (if I replaced two FETs with depletion mode devices)? --- End quote --- Uhm, it may work. But what depletion fets do you keep in mind? Are you sure they are easy to buy? The world is ruled by "normal" mosfets, exotic devices are not widely on sale... I'd say a common approach is to use only jelly bean parts unless absolutely needed. But for fun and experiments I think any parts is fine. --- End quote --- OK, you got me... just checked after seeing your post, and I can't see any P-channel FETs out there :-( I suppose I could always use a PNP transistor to "invert" the gate voltage... or, of course, a traditional NOT gate... |
| exe:
--- Quote from: danners430 on September 12, 2018, 01:34:20 pm ---I can't see any P-channel FETs out there :-( I suppose I could always use a PNP transistor to "invert" the gate voltage... or, of course, a traditional NOT gate... --- End quote --- Yep, this whitepaper says "P-channel depletion mode devices are not generally available": https://www.aldinc.com/pdf/IntroDepletionModeMOSFET.pdf . Concerning inverting, etc, I think it's a viable solution. All four mosfets would be identical which is nice. Or, may be, you can just use a bridge with mixed P-channel and N-channel mosfets? So, they will open at oposite input levels (but please check, I can be wrong). |
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