Speed Controller - error in Circuit

Unfortunately the circuit drawing on page 32 of MEW 229 shows TR2 drawn as an NPN transistor.

A BC327 is in fact a PNP device and should be shown with the emitter and collector the other way round.

Apologies to Inchanga - it was drawn correctly on his original, and I didn't spot the error when checking as the transistor is the right way round for an NPN.

We'll put a corrected drawing in the next issue.

Neil

Yep, spotted that one, thought it might have been a 'deliberate' mistake to sort the men from the boys. Unfortunately there is a misunderstanding in the text too. The RFI potential of a waveform is related to the edge speed, not the fundamental frequency. The frequency simply determines the spacing of the harmonics.

Andrew

Glad to see you spotted the most obvious problem with the circuit.

There are other, perhaps more arguable, problems with the circuit:

1) The case for TR1 and TR2 doesn't hold for for a MOSFET with such a large ON resistance and consequently low value of Miller capacitance (65pF).

2) Similarily D3 is unlikely to be neccessary but in any case needs to be a Schottky diode such as a BAT48 rather than a IN4148/1N914 which are slower and have a higher Vf.

3) Personally I would leave out R4/TR1/TR2[/D3] and increase R5 to 200ohms connecting it directly to pin3 of the 555.

4) There should be a high current schottky rectifier between the positive supply (cathode) and the drain of TR3. this will act as an efficiency diode and avoid overheating the MOSFET. It also renders the snubber redundant.

5) Any brushed pm motor should have interference supression capacitors fitted directly across the brushes.

Trevor

I think arguable is the case - there area zillion ways to skin this particular cat. My own approach is to use a logic-level FET fed direct by a 20mA push-pull capable output.

I sought a second opinion and put several points to the author, who responded with both their extended professional experience in motor control design and electronics and the relevant datasheets to back up certain points, including the case against a diode as in your point 4...

Neil

So is this open season on designers of projects to pull their designs to pieces and critique in some way on a public forum?

Not in the least, as Neil has pointed out the MOSFET driver error was one of translation in the production of the magazine. I see no point in commenting on the design as I am perfectly capable of designing my own circuits. However, the point in the article about RFI and frequency is incorrect.

To pick up on a couple of points made by Trevor; personally I think that total gate charge is a more useful parameter when eatimating the drive current needed for a given switching time than Cgd. The MOSFET in question is avalanche rated so, depending upon the motor being driven, the Schottky diode may well be redundant. It will come down to component count, ie, cost, versus efficiency.

Andrew

Neil,

When I said arguable I was not referring to the diode in my point 4 or indeed the capacitors in point 5. I would be interested to know why anybody would leave them out.

I would like to know what these points backed up by the datasheets are. As I have suggested I find some of the design decisions difficult to understand.

Trevor

Andrew,

I was using Cgd because I was refering to Miller effect. For design purposes I to would look at Qg (total gate charge).

For a small enough motor with a low drive current the Schottky diode would be optional. However, this is meant to be a general purpose design. My problem is not that the MOSFET body diode will not conduct when faced with a high current pulse but that it will. When this occurs regularily the MOSFET can get very hot. Much better to push the energy back into the motor with a Schottky diode.

In fact if the reversing switch is not required I would put the Schottky diode acros the terminals of the motor.

Trevor

Hi

I bought a speed controller from the net for less than it would cost to build one, £3-50p delivered. While one would expect it to be built down to price it does have a dedicated voltage regulator, LM317T, mounted on heat sink for the 555 chip and circuit rather than a simple zener diode arrangement. The MOSFET is directly driven from the 555, there is no snubber network but there is a 10A Schottky diode on the output, again this is mounted on its own heat sink.

I'm not saying that it is is better or worse than the design presented in MEW but it is in line with the suggestions made by TrevorG .

Cheers

Martin

I have to agree that the lack of a flywheel diode across the motor is a major issue. My fag packet calculations suggest that the repetitive avalanche rating (4.5mJ when warm) will be exceeded once the load current reaches about 1.2A, even at this low frequency - in a circuit that would otherwise be good for a load current of something like 5-7Arms. Beyond that, it's in very dodgy territory.

Omitting any form of soft start or current limit is one thing but leaving out the flywheel diode is a no-no. You really need to get work like this reviewed before you publish it.

Murray

I am in no position to criticise the author or the editor/proofreader or whoever, but I do think the definition of PID being described as Proportional Integral Drive is incorrect. In stands for Proportional-Integral-Derivative and I am sure that was what the author was referring to.

Ian P

Ok,

I did get it reviewed.

I did get the author's feedback on a point by point basis.

Two main points:

1 The author has many years experience working on DC motor drives for a defence contractor.

2 His case is that motor drive is very different from driving a relay, because the motor keeps rotating and generates back EMF in a very different way to switching a coil on and off. The maximum back EMF up to a 30V supply will be within the FET's safe ratings, so the diode is not necessary, and he states that if included it will cause an element of regenerative braking whenever the FET is turned off causing greater drive to be required.

I'm not going to further defend this article on a point by point basis, but if others can do better they are welcome to put their heads above the parapet and submit their own designs.

However, given the disproportionate criticism that any article involving electronics receives, I may just stick to mechanical articles in future...

Neil

.

That seems a great pity; given the number of basic electronics queries posted on this forum.

MichaelG.

Please don't give up on the electronics, I really enjoy the crossover between mechanical and electronic projects. I feel that some people set a very high standard for what are hobby projects. We do not need the cutting edge of electronics in any design. If it works , that suits me fine and I don't care if the purists nit-pick over some obscure theory.

Doug

Ignore the know- it- alls, keep the electronics coming. Many of us are clued up on machining but are interested in learning new areas.

I agree with Doug and Duncan, keep the electrical/elctronics content coming.

I think that some have missed the point with this design in it being aimed at readers who know little or nothing about electronics but would maybe like to try and build a simple cheap motor controller.

I have seen quite a few circuits designed by a local electronics mag that needed reviewing after publishing because of mis- prints or sometimes component specs between manufacturers differ a little causing problems. usually they just publish the modifications in the notes and errata section and life goes on.

There is also a section for the readers to submit improvements to the circuits published by the magazine - after being reviewed by the technical staff and there is some scope for this project to be modified , improved or added to by the readers and forum members.

Neil took the correct path in notifying of the error but i didn't see anything in his OP asking for a technical review of the project.

If you can improve on the design then do so and submit it to Neil for publishing and i have yet to see a decent design for 180v- 230v permanent magnet DC motors as it seems to me that no one wants to take a bite at that cherry for some reason , there are plenty of controller circuits/ projects for brush type AC motors .

My local Model Engineering mag has a section - sparks 'n' arcs that covers all sorts of electrical things and seems to be very popular especially amongst the Electric Loco builders .

I felt the article was easy to understand , got to the point without wandering off on some mathematical tangent that some electronics magazines tend to do a lot and didn't try to bamboozle the reader with technical guff that they really didn't need to know anyhow - i thank the Author for this and i thank Neil for whacking it In the Magazine- MORE OF PLEASE !!!!

Rant over!

Off to the workshop now to fit my new DRO kit and Tacho to my X2 mill - replacing the cheapie magnetic type DRO with glass scales + display as the cheapie set drove me mad with its constant failure in the accuracy department and fitting a new Tacho to replace the unit i built ten years ago because it will give a neater fit out - it is small enough to fit into the control box on the head above the emergency switch , the old one was in a jiffy box mounted to the column, worked ok but just ugly!

Ian.

I think the responses here have made sure most are unlikely to submit their own design. Surely the real test should be "does it work reliably"

Edited By Clive India on 22/05/2015 10:32:26

Please keep the electronics coming.

Intended purely as constructive criticism, the case for a flyback, efficiency, catch (call it what you will) diode explained.

Motors are not the same as relays. As a simplified model, sufficient for this argument, the brushed dc motor may be modeled as a voltage source in series with an inductance - the voltage source represents the back emf and will be non-zero whenever the shaft is turning. The inductance is the inductance of the motor windings, denote this 'L'. Yes there is also resistance, inter-winding capacitance and other factors, but we only need to consider the voltage and inductance here.

This is what happens. Assume the motor is powered from a battery of 'Vbat' volts, and is rotating such that it is producing some back emf 'G' (for generated) volts. The switch TR3 closes (ignore on-resistance) and so applies a voltage of (Vbat - G) volts across the inductance L. Call this voltage 'E' such that E = Vbat - G

Rearranging the textbook inductance formula E = -L di/dt and ignoring the '-', the current in the inductance L will change at the rate (di/dt) Amps per second, which equals E/L.

The current flowing through the inductance increases linearly (ignoring effect of resistance) while switch TR3 is closed.

The flyback diode provides a path for this current when switch TR3 opens. The current flow in an inductance can not change instantaneously, and the voltage across the inductance will tend to oppose any change in current. This means that, when switch TR3 opens, the polarity of the voltage across the inductance reverses, and without some constraint, will soar off towards infinity. The only constraint in the circuit as drawn is the avalanche breakdown voltage of the TR3 device, as alluded to by 'Muzzer' above, and as he implies, this is safe enough for small loads. However, including a flyback diode, the current flow in the inductor is returned to the supply. This has the great benefit of reducing the average current drawn from the battery as energy from the inductor is returned to the supply rather than being wasted heating up TR3. Also, the current flowing in the inductor is the current producing torque. Returning this current via a flyback diode, the voltage across the inductor with TR3 switched off is far less meaning the rate of current fall is less, which is why this diode is sometimes called an 'efficiency' diode.

Jon Freeman

Neil, Please don't be put off. I was pleased to see an electronics project directly applicable to model engineering and don't see why the occasional article should not be included. There is much interest in spark erosion, DRO displays, tachometers etc. As a further example, I would be interested in a DC power supply project that I could use to power the auxillary milling spindle I am (slowly) building.

Andy

I am sorry Neil feels beleagured by the comments on this and other electronics articles. I would encourage him to see it as expression of great interest in this type of article. My posts, and many of the others, have not stopped at pointing out the problems with the article but have pointed out the solutions. This is something I would always try to do.

Obviously there have been problems with the current article. It would seem the the choice of reviewer has let us down. So this needs to be sorted out before another article is published.

So lets have more electronics articles. They are of great interest to MEW readers.

Back to the subject of the Schottky Diode across the motor. Jon's description is very good the only clarification I would add is that the current goes back into the motor rather than the supply.

I have produced some jpeg's which show the effect of the diode unfortunately I can't see how to upload them to this forum if anyone knows how this can be done please let me know.

They show the effect of a diode when a 545 motor, taking ~1A, is being controlled from a 7V supply without the diode there is a 35V spike limited by the MOSFET avalanche breakdown. With the diode the spike is limited to little more than 7V.

Trevor