Speed Controller - error in Circuit

Schottky diode overkill surely? They have two benefits. One, the forward voltage is lower than standard junction diode, but only by quarter volt or so. But this will make very little difference in this circuit as Trevor's oscillograms show.

Second, when they go reverse biased they turn off very quickly compared to junction diodes as the stored free charge is small. This is very useful in a high frequency rectifier, where combined with the low forward drop it gives good efficiency. But in this circuit the diode is reverse biased most of the time and turns on only to clamp the inductive kick.

I reckon an ordinary 1n4148 would do the job perfectly fine, the main thing is to protect the MOSFET against avalanche.

Will a conventional silicon diode work ? Yes. But a schottky diode will do a better job for the reasons that John mentions.

I should however remind you the function of this diode is not just to clamp the inductive spike but also to redirect the energy into a productive use i.e. powering the motor. The schottky diode will recover more energy than the silicon diode.

For many of the uses of this type of controller the difference between the two types of diode is unimportant and either could be used. However, the schottky rectifier is better and these days is often cheaper the a conventional silicon rectifier of comparable rating.

For example for a motor running at 10A I would probably use a IN5822 schottky diode( cost 11p) Vf 0.5V @10a but you could use a silicon diode IN5401 (cost 14p) Vf 1.0V@10A.

Incidentally a !N4148 is a small signal diode rather than a rectifier and will not survive this usage (Vf=1.1V @0.2A).

Trevor

Just My few Amps worth.

Electronics is not my thing, Although I have built many projects over the years from published art.

Reading through this post has been enlightening it is clear there is a rich source of expertise within the membership.

How about debating and publishing a circuit right here that embodies the group consensus. It may take a few iterations before the dust settles but the result should be rather good, Professionally designed in fact.

I have enjoyed the debate so far.

Regards
John

Aw its gone awfully quiet, I was enjoying this !

general comments then some questions

General,

Yes do publish electronic from time to time, I find it interesting that this article has elicited so much debate whereas the 'wood butchery' does not.

If a comnon design could be a greed i'd like to see a group buy on PCB's

Questions

Firstly I can build circuits to a good standard but only have a basic knowlege of components & design - I can't be good at or have time for everything.

I have killed two 'bought a PWM speed controller on fleabay for £3.50, Its 555 driven FET with a schottky diod e and what look like a power diode for emf protection and but I'm reversing the polartity to the motor to allow reversing ( It goes through a zero position rotary switch) will that protection diode only work half the time i.e when the polarity is swtiched one way ?

If I buit the Inchanga design am I likley to blow that up too ?

My motor is 24v 1A brushed paravalux running @ 5000 is RPM but greared down and I'm running it at 24V as a powerfedd on my SX3 mill

Any help gratefully received

Tony

Tony,

Without knowing the failure mode of the two speedos you have killed its difficult to determine whether the covered design will fail the same way....having re read my posting and further looks at the schematic, Inchanga's design seems robust enough, but I'm no expert....

As for the Schottky diode, as long as it is is connected from the Drain to the supply rail and all you are doing is switching the motor round then the diode should not fail,IMHO.

You state the motor used is 24v dc rated at 1 amp, but is that it's unloaded current, full load running or stalled current....these make a difference....

Just looking at the text of the article, it would appear that the device chosen is quite conservatively rated, so you shouldn't have any issue

Good luck with the build.

Edited By John Rudd on 15/07/2015 14:12:40

Edited By John Rudd on 15/07/2015 14:14:47

Tony,

It would be interesting to see the circuit digram on the fleabay design.

All depends on how confident you feel on electronics. But if you feel able to trace out the circuit and put it online it would be interesting.

If it is the one I have just seen describesd as 6V/12V/24V 10A it should be possible to repair/uprate. What form did the failure take? Does the motor run only at full speed only or does it fail to run at all? I always doubt the specs on these Chinese controllers the are often hopelessly optimistic. As for tthe claim of over current protection this would need a microcontroller rather than a 555.

As John mentioned The 1A could mean idle current, full load current or stall current. For example I have a motor with 2A idle, 20A full load and 100A stall current. If you have the Parvalux model number it can be checked on the manufacturers website.

When you start a motor , until it starts to rotate it will pull the stall current, (if your power supply is up to it!) this will then drop off very rapidly as speed increases. If you then reverse the motor the instaneous current will be TWICE the stall current falling to the stall current as the motor stops before beginning to rotate in the reverse direction. This is a great way to fry your controller so it is a good idea to stop the motor before reversing it. Of course having a power supply too feeble to supply these huge currents can also offer some protection.

Trevor

Inchanga has sent me some calculations on how his design copes with the back emf which will be going into the next 'scribe a line'.

Neil

Any chance of seeing them before you commit to print again?

Murray

I to would like to see this. The issue has never been about back emf (just another name for the voltage generated by the motor due to its rotation) but rather about the inductive spike that occurs during switching. As both the back emf are and the inductive spike are well understood it is difficult to see what new insight there might be.

The inductive spike is well understood and intensively modelled by switch mode power supply designers. The buck regulator type of power supply, which is dependant on the capture of inductive spikes, is everywhere you look these days.

Trevor

I have to say I'm intrigued myself, as a power electronics engineer with 30 years experience!

Murray

Gents, thanks for the comments

Trevor,

I have made a start on tracing out the fleabay design, its a little tricky as I think its on a 3 layer PCB but the component count is quite low.

If anyone fancies blowing one up for themselves

**LINK**

The output device is not that common but I did find a datasheet for it eventually

I'll keep working on it and hopefully post in a couple of days.

I'll also dig out the Parvalux info for my drive motor

Tony

Fleabay PWM controller cicuit diagram and components

Took me a while hopefully its correct sorry if I have broken every diagram convention inthe book ! Component details are in the second image.

I think there are a couple of errors, this is meant to be constructive and not dismissive of your abilities!

Firstly, I think the 2 1000mfd caps should be in parallel across the supply.....

Secondly the Anode of D2 ought to be connected before the V reg.

Good effort.....trying to reverse engineer a pcb isn't an easy task

As John said a very good first effort. These things are very tricky especially if it was a 3 layer board. Things to check (for completeness I have included the points already raised by John):

1) The 1000uF caps are prebably in parallel as mentioned.

2) Anode D2 should be connected to the junction of the 7812 and the fuse.

3) Anode D1(I assume thats its designation seems to have been off the edge of the scanner!) should be connected to the drain of the MOSFET not the source.If the MOSFET is blown up the Drain and Source may well have shorted in which case you will need to do a visual check or else remove the MOSFET from te board.

4) I suspect that the 555 pins Thresh(6) and Trigger(2) are linked together. I can then see how the circuit works.

If you can check these points we can move on to working out what went wrong.

Two additional questions:

1) Was the fuse blown

2) Is MOSFET blown ie is there a short between Source and Drain.

Trevor

• You've drawn the lower electrolytic shorted out. I'd expect to see one of them on the input and one on the output of the 7812.
• The motor +ve is unlikely to be connected to the output of the regulator unless it's a very low power motor. More likely to be connected between the fuse and the 7812.
• The anode of the bottom half of D2 is likely to be connected to the drain of the FET / bottom of the motor.
• Haven't used a 555 for years but something is missing from the timing circuit, probably a resistor or a connection not shown.
• I notice there is no overcurrent protection beyond the fuse. 10A fuse isn't going to protect it a great deal but will prevent a fire.

Getting there!

Murray

Edited By jason udall on 20/07/2015 18:25:27

Tony,

I thought it might help to put the changes to your cct diagram into an updated diagram (always assuming I am correct !!):

Trevor

Trevor, et al

Herewith checked CCT -Thank you, The only change is the 1000uf to the output of the voltage reg. You were right pin 2 & 6 of IC1 are commoned - there must be a track under it that I can't see. I now don't think its a 3 layer board. Theres also a 1N4001 or similar which I think is meant to be for polarity protection but I can't find any track from the anode leading anywhere. I now have a better idea of how its meant to work too. I'll measure the dud boards as suggested and look up my motoers parameters and post them later.

Confession time,

No the 10A fuses have never blown

I checked the Schottky - OK

And there was no short on Q1 D to S

So I checked the output of the 7812 OK at 12.1 V and concluded that with all the other diodes ok unless the 555 was dead there couldn't be much wrong so I bypassed the C/O switch and ... It works The C/O switch is dead.

I definitely blew one up as I shorted the output, but perhaps the second one is ok and the switch fault was intermittent.

The motor is a Parvalux PG4568 rated at 1A continuous 14W, 21W 15 min , 17.5W 1hr. It barely gets warm when powerfeeding

In terms of the basic design I take the point about no over current but I'm using a laptop PSU which itself has in built protection.

Now some questions

If I wanted to fix the dead one what device would you recommend to replace q1 ?

If I did want to change direction whilst power-feeding is it vital to ramp down the PWM before switching through zero?

Is there anything else that one could add easily to further improve it ? To me it does not look that different to the Inchanga design apart from the bipolar drivers.

At this point I'd like to thank you all for getting me back to a working powerfeed.

Ok you now have one working, even if the problem is not on the board. Now for the second one. Firstly do the checks I suggested for the first board as there are various possiblities for the faulty component (and more than one could have blown). You also have the ability to check readings agianst those on the first board.

My general thoughts on the circuit design are that it is a considerable improvement on the circuit published in MEW. In particular the inclusion of the schottky diode D1 and the elimination of the bipolar drivers. Also the increase in the 555 supply voltage will reduce the RDSON of the MOSFET allowing it to run cooler.

The only serious flaw I can see is that if a heavy duty motor is started the supply voltage could drop very low. At this point the output voltage of the voltage regulator will be higher than the input causing current to flow the wrong way through the regulator. I managed to blow up a regulator this way many years ago. As a result I now put a diode between the positive supply and the regulator input which protects against this problem. It is possible that the 1N400x type diode was being used to provide this protection. However, I suggest powering up with no motor connected and checking the o/p of the voltage regulator is still about 12V. You should also check the voltage on pin3 of the 555 (connected to R2) it should vary as you turn the pot. Check the working board for comparison if you are in doubt.

The RU7088 appears to be a substantial device but if you need to replace it you may have difficulty finding a supplier. There are a lot of alternatives available. You need a N-Channel TO-220 MOSFET with a Vdss of >=40V and RDSON of 6mR or less. The current rating of such a device would almost certainly be adequate but check that it is a minimum of 30A. It will probably be much higher.

Trevor