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Sanity check - FET H-Bridge circuit
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danners430:

--- Quote from: exe on September 12, 2018, 02:28:54 pm ---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).

--- End quote ---

I think you may be wrong (but it could also be me!) - from my perspective, an H-Bridge would always need 2 P-Channel and 2 N-Channel devices - the P-channels to source the current, and the N-channels to sink the current... but at the same time, I was under the impression that all enhancement mode FETs were normally-off, with V+ "switching" the device "on"...
Alex Nikitin:
If you have a somewhat higher voltage available, here is a very simple bridge circuit, safe to operate. The choice of MOSFETs / diodes is up to you, the IRL530N and MBR1100 are used only as an example, many other devices would be suitable.

Cheers

Alex
Zero999:
Depletion mode MOSFETs would be no good anyway. The P-channel devices, on the top would need a higher voltage, than the motor supply to turn off and the N-channel devices, would need a negative voltage to turn off.

I doubt 25mA would be enough to power a motor. If that's all you need, you're better off just using the IO pin of the MCU directly or a logic buffer, such as the 74AC240.
http://www.mouser.com/ds/2/149/74ACT240-888464.pdf
danners430:

--- Quote from: Hero999 on September 12, 2018, 03:09:12 pm ---I doubt 25mA would be enough to power a motor. If that's all you need, you're better off just using the IO pin of the MCU directly or a logic buffer, such as the 74AC240.

--- End quote ---

Believe it or not, 25mA was the figure I was quoted by DCC Concepts when asking them... however, they also say that current increases as voltage increases... hang on!?

At the same time, however, I can't afford to drive anything directly from the MCU pins, as the MCU I'm using can only supply a total of 20mA per pin, and 70mA total for the whole device. I still need to be able to drive a whole host of other devices, so I'd risk going over the power limit.

However, some of the products from DCC Concepts use a higher current, sometimes as high as 75mA and above, and sometimes it's necessary to connect multiple devices together, hence my preference for FET drivers.


--- Quote from: Alex Nikitin on September 12, 2018, 03:06:34 pm ---If you have a somewhat higher voltage available, here is a very simple bridge circuit, safe to operate. The choice of MOSFETs / diodes is up to you, the IRL530N and MBR1100 are used only as an example, many other devices would be suitable.

--- End quote ---

The motor would be driven off a rectified 16V supply, but the LEDs will be driven by the 5V supply, as I really want to minimize the "spread" of the higher voltage over my PCB... but it could work.

Also, I'm afraid you'd have to explain that circuit... I can't seem to fathom how it works!
Zero999:
The problem with a series resistor is the voltage drop. You have a 250R, so the maximum current your circuit can supply into a short circuit at 5V is 20mA. When the voltage across the load is 2.5V it can deliver just 10mA.
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