Originally Posted by audioguru

The old IRF510 mosfet is made by International Rectifier, not Maplin. So guess where to go to get the best datasheet??

It is spec'd with a 10V gate voltage, not anything less.

Its threshold voltage is the gate voltage where a few of them barely turn on (0.25mA).

if the motor current is lower than 1A, 10 V of gate drive is wasted, because the FET channel has become well 'ohmic' with gate voltage @ 5V



According to the datasheet, 5 V of gate drive is just good for 1 A motor drive current.

You look at "typical' devices on the graphs so I guess you test each of your Mosfets to make sure they do not have minimum specs that are printed in text on every datasheet.
Or maybe you know somebody at the factory who supplies you with "typical" Mosfets.

The max on-resistance of an IRF510 Mosfet is 0.54 ohms when it is cool and has a 10V gate voltage. Then when it is warm and with 1A of current it has a voltage loss of almost 1V.
Your "typical" mosfet will have a little higher voltage loss when its gate is 5V.
Is a loss of 1V and more acceptable?

Okay lots of replies so sorry if I don't respond to everything. I'll just try to summarise some of the main information I've discovered so far:

Motor voltage: 3V (the 6V battery pack is actually divided such that two of the D-Cells power 2 motors and the other two power the remaining three).

Motor current: Measured at 200mA when a single motor is being driven (so can drop to 66-100mA without experiencing many noticable issues).

Regarding bipolar transistors: I originally planned to use these, but marcbarker pointed out some shortfalls. And with the output of the uC being 20mA max, that may be an issue.


I was thinking about the option of putting transistors between the uC and the MOS-FETs, but as already pointed out this would further increase the cost and I'm not exactly experienced yet either; ending up with 40 of various kinds of transistors, as well as a load of resistors, possibly diodes to isolate the ground circuits, etc., sounds extremely complicated... and expensive.

Bipolar transistors I can get for about 12p each from maplin if I order in the quantities which would be required, but I don't know of many other electronics suppliers, and maplins don't do any logic-level MOS-FETs. I'll take a look at the site suggested by someone in one of the posts on here

spec'd at 10V

In the OP Application, the needed current as understood is much less than 1 A, so it'll be a waste of time driving the gate with "10V" if 6V would still do virtually the same. "Vgs=10V" is preferred of course it is, but 10V isn't available in this application, luckily, with some careful analysis with a low motor current, 6V would work, only just.

IRF510s are old school nowdays early generation devices. There are many lower Rds(on) and higher gm newer generation fets available that's better suited, that a 510 wouldn't be picked, so it's not really worth wasting effort going that deep into the analysis. But I will...

You can see from the IRF510's typical curve I posted, the 'typical' Rds(on) it has (@ Vgs=6V) = 0.6 ohm. 6V is the battery voltage. The effect of parametric spread on the shape of the 'typical curve' will take some imagination, based on worst case datasheet figures, but in practice in the real world it's actually much less when real samples are measured.

At the drain current of less than 1A, the fet is so well enhanced (Vgs=6V) into the 'ohmic conduction' part of the curve, that a fair bit of variation of actual Vgs(th) from 'typical' could be tolerated before the fet starts to drop out of ohmic conduction mode. In practice, Vgs(th) doesn't vary that much from typical, if it does more than 1V that's very bad luck, or the fet is an original 1980's one. Actual perfomance can be better than typical as well as worse than typical, and modern fets often out-perform the (old) datasheet.

What often happens with fets is that people don't understand the curves, temperature dependence and production spread variance properly. They'll get 'bitten' by a project that went wrong, then after that they mis-trust the datasheet blaming 'typical' data as 'mis-representative;, then as this advances, they cover themselves each time by interpreting all of the 'worst-case' data they can find and applying it all together at the same time (which is totally invalid of course, but it does minimise the risk of them ever making a mistake in their design).

Anyway, it's all academic really, because even though an IRF510 would work with 6V of gate drive, it would be better to use instead later generation automotive 1.5V Vgs(th) fets that have milliohm Rds(on), which are the same price.

Originally Posted by giftiger_wunsch

Regarding bipolar transistors: I originally planned to use these, but marcbarker pointed out some shortfalls. And with the output of the uC being 20mA max, that may be an issue.

Bipolar transistors seem better than MOSFETS now, in the light of new information: the motor current being so low. Since bipolar drive current= motor current / hFE, the max output current of the uC isn't a concern any more.

A h-bridge made out of npn & pnp transistors seems to me a good option now.

Or maybe utilise a tri-state buffer?

Okay, given the current information, I've drawn up another schematic which is hopefully closer to what it should look like now. I'm not sure if I have connected the ground (green) correctly; I tried to connect it such that when PA1 is high and PA2 is low, PA2 acts as ground for PA1, and vice verca. That prevents a short-circuit if PA1 and PA2 are ever both driven high.

Any comments are welcome.


The transistor which seems most appropriate is the BC337-16 model transistor. Maplin doesn't provide a full data sheet, but the Ic is 500mA; I'm not entirely sure how to interpret the rest of the information.

The upper transistors in your H-bridge are emitter-followers with a 0.7V or more voltage loss.
Then the 3V motor gets only 2.1V minus the voltage loss across the 12 ohms resistor.
The diodes do nothing.

Sorry if it's already been mentioned but you need to use an IRL540 because it's cheap , is logic level and has more than the required current drive.

Originally Posted by audioguru

The diodes do nothing.

But without them there would be a short circuit between the PA1 and PA2 pins, and virtually no current would flow through the BJTs since that's the more resistive path.

Originally Posted by audioguru

The upper transistors in your H-bridge are emitter-followers with a 0.7V or more voltage loss.
Then the 3V motor gets only 2.1V minus the voltage loss across the 12 ohms resistor.

Is there a way I could avoid this? Or will I simply need to increase the voltage supply?


I had a feeling it was too good to be true that I may have finally found a solution...

What about this, people?

The 12 ohms resistor robs the 200mA motor of most of its voltage. 2.4V is gone and is making heat. The motor will not start running. What is the function of the resistor?

The PNP upper transistors are fine when the motor's power supply voltage is the same as the supply for the micro-controller.
But the 330 ohm base resistors provide a base current of only 6mA. 20mA is needed.

Would a SN754110 or TC4427 work? They make great half bridge drivers for small motors and are not expensive.

Okay I don't understand most of this now and I don't see any obvious solution so I think I'll give up. It was worth a try. Thanks for all the help.

Originally Posted by giftiger_wunsch

Okay I don't understand most of this now and I don't see any obvious solution so I think I'll give up. It was worth a try. Thanks for all the help.

Huh? What was wrong with using the SN754110? One IC can drive two small motors.

I don't know what they are, I probably wouldn't understand the datasheet, and don't know enough about electronics to be able to build my circuits. I started this project expecting that the wiring would be prety simple, and I've found I barely understand the principles behind it. Maybe I'm better off sticking with software development.

Thanks anyway.