Speed Controller – error in Circuit

[MuzzerParticipant @muzzer]

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

Murray

[Tony RayParticipant @tonyray65007]

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

[Tony RayParticipant @tonyray65007]

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.

[John RuddParticipant @johnrudd16576]

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

[TrevorGParticipant @trevorg]

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

[MuzzerParticipant @muzzer]

• 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

[jason udallParticipant @jasonudall57142]

Like others.
Well done in that tracing.

What you have there is stage one..trace what seems to be going on.

Stage 2. Compare tracing against ” textbook” ( or other reference..)circuits and double check circuit board against tracing where difference occur.
In particular ..even if “wrong” record what the board is doing. It might have been “wrong” from day one.

This leads to stage 3..can it be fixed/improved.

Again well done for a beginner. .we have all been there.

Btw can you see the family resemblance to the speed control circuit in the mag?..
.

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

[TrevorGParticipant @trevorg]

Tony,

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

Trevor

[Tony RayParticipant @tonyray65007]

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.

[Tony RayParticipant @tonyray65007]

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.

[TrevorGParticipant @trevorg]

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

[MuzzerParticipant @muzzer]

The IRFZ44 is the bigger brother of the IRFZ24 with about three times the current rating but those are both ancient devices.

Nowadays you can get devices with Rds.on in the 1mR region. Looking through their selector, I see that Farnell stock the IRF2804 which is a 40V device in TO-220, 2.3mR, 280A (25C) for £2. That sounds like a reasonable candidate. You'd be doing well to blow that up, even with no flywheel diode fitted and no current limit….

Murray

[TrevorGParticipant @trevorg]

I doubt if this board would benefit from Rdson much below 6mR bearing in mind it is designed for 10A operation. Lower Rdson devices not only put the price up but also have much higher value of gate charge requiring more power from the drive circuits. More than the 555 could deliver without slowing the switching speed. Better to save your pennies and stick with a lower spec device.

[Tony RayParticipant @tonyray65007]

Thanks for the suggestions on devices. After losing the first board I fitted an IRF3205 Vdss 55V RDss(on) 8mR this did not fix it. As I understand it this device will generate more heat as the RDss is higher ? Anyway the output of the 7812 is 2.5 V and I can detect some heat coming off the board from somewhere with no motor connected. Does that mean the reg is dead or is something dragging the voltage down ? The PSU is sitting at its usual 22V or so volts. I checked the mosfet for a d-s short and it's ok. I didn't measure the 555 as the reg volts are out. I can put a scope on it if needed; I picked up an analogue 35meg dual trace Hitachi for next to nothing and can measure things even if I don't understand what the trace is telling me. I will try to find board 2 and measure that.

[TrevorGParticipant @trevorg]

I had a suspicion that it was not the MOSFET that was at fault.

We have two possibilities:

1) Regulator is dead

2) The load on the regulator is so high that the regulator is going into thermal shutdown. Which means that the 555 is taking to much power (possibly dead).

First check that the there is not a Gate – Source short on the MOSFET. Just possible this is the problem.

If thats OK then remove the 555 and check the o/p of the regulator is now correct. Only problem is that the 555 is in such an awkward position that the easiest way to remove it will be to cut the legs off. But this means replacing the 555 even if it is not faulty. An alternative might be to lift the output leg on the regulator but this may have similar problems to removal of the 555. You will have to decide!

Until you have 12v across a working 555 there won't be anything for a scope to see.

Best of luck.

Trevor

[MuzzerParticipant @muzzer]

Posted by TrevorG on 21/07/2015 23:21:33:

I doubt if this board would benefit from Rdson much below 6mR bearing in mind it is designed for 10A operation. Lower Rdson devices not only put the price up but also have much higher value of gate charge requiring more power from the drive circuits. More than the 555 could deliver without slowing the switching speed. Better to save your pennies and stick with a lower spec device.

The gate charge is about 160nC for the 2804 vs 20nC for the IRFZ24 ie a factor of 8. That's because modern devices have a much lower gate charge than those devices from The Ark.

The power required to drive the gate is 1/2 x Q x V x f (unless you know better?). With a 12V supply and switching at 1000Hz I reckon that's about 1mW – hardly enough to release the magic smoke. What speed do you think we should slow it down to so that the 555 is able to drive it?

If you are going to bother building some thing like this without any form of protection other than a fuse or a current limited PSU you might as well choose a robust FET.

Tony – the 3205 seems to be an 8mR device compared to the 70mR for the IRFZ24, so the losses should be correspondingly lower. The total gate charge is very similar to the 2804, so the 555 should be fine with it.

If you have got something dragging the output of the 7812 down to 2.5V, something must be getting warm (apart from the 7812!). Can you tell by using your finger? Or see where the current is flowing using a DVM to follow the track drops.

Try removing the 555 and see if the voltage recovers to 12V. If possible replace it in a socket. There's not a lot to go wrong really.

Murray

[Tony RayParticipant @tonyray65007]

Gents

I'll recheck for the source of heat and probably remove the 7812 as its a little easier to access apart from the retaining screw on the heatsink – I had to shorten a 25mm hex bit and use a right angle bit driver when I changed the mosfet.

No great loss if I need to destroy the 555, at 10 for £1.70 they really are cheap as chips (sorry).

Murray please can you explain how to do this ?

"Or see where the current is flowing using a DVM to follow the track drops."

As one measures current in series ( and can't do that unless one lifts a device leg) do you mean measure the voltage drop various points to referenced to ground ?

Good news on the remaining board – it is also OK again in must have been the C/O switch.

Thanks for the info re RDss.

Tony

[MuzzerParticipant @muzzer]

You need a sensitive voltmeter (200mV range?) and if you are lucky with the layout, you may be able to see the voltage gradually drop as you probe the track, following the fault current from the output of the regulator towards the return (0V ground) connection, with the other probe attached to ground. Most of that 2.5V drop will be across one device (most likely the FET, possibly the 555). If you see a track that has no measurable voltage drop across the length of it, the chances are that it isn't carrying much current.

It seems that the 7812 will fold back the short circuit current to about 1/4A in a dead short, so perhaps you are seeing only a bit more than that at 2.5V. That may not be enough current to allow you to follow track drops, depending on your meter. Given that there are so few compts in the circuit, it won't take long to remove them or cut / lift a few legs to locate the short.

Murray

[TrevorGParticipant @trevorg]

The 7812 can protect itself in one of two ways. Overcurrent fold back or thermal shutdown. From experience thermal shutdown is more likely but in any case the output voltage would drop significantly which would explain the 2.5V. Easiest way to prove this is to lift the the Output leg on the regulator. When powered up the output leg should have 12V on it. If not then the regulator is dead.

Then it is hunt the failed component. If there is a short on the Gate-Source junction of the MOSFET this could cause the 555 to lock up and overload the regulator. Otherwise you are down to a failed electroytic capacitor ( the one we called C3) or more likely a failed 555.

Trevor

[Tony RayParticipant @tonyray65007]

Gents,

I checked the MOSFET and it is OK so pulled the 7812 with the aid of the bit I described earlier

and soldered it to a bit of stripboard for testing. I put a led with resistor across it to pull a bit of current and the out put was around 2.9V so my conclusion is that the reg is fried. Before removal I tried looking for the source of heat, definitely not the 555 probaly the 7812 but the thing was only barely above ambient. I should have the new device in a couple of days & will put on in. I suppose I could power up the 555 with an external supply but its probably not worth the effort and I can change it if I need to. Anyway I'll report back whn I have some news

Murray,

Thanks for the explanation on the track drops I'll keep it in mind for the future.

Tony

[TrevorGParticipant @trevorg]

Good work. At least its a cheap and easily sourced component.

The question is what caused it to blow. They are pretty rugged devices. Unlikely to be simple overcurrent as they are well protected.

The only serious vulnerability is the situation where o/p voltage is higher than input. You mentioned shorting the motor terminals this could cause this situation to occur and there is sufficient energy in C3 to do the damage. The good news is that this will only have damaged the regulator. A repetition of the fault can be avoided by putting a diode in series with the input of the 7812 say a 1N4001 or similar.

Trevor

[Tony RayParticipant @tonyray65007]

Trevor,

New reg fitted and I'm a happy man – thank you and as you predicted the IRF3205 is fine. I will see how I can retro fit the protection diode.

I have learned a lot; not to jump straight to suspecting the output device and to check the simple things like voltages first.

I have had a quickl look at current limiting circuits but nothing stands out , any suggestions would be welcome.

Tony

[Michael GilliganParticipant @michaelgilligan61133]

It's stretching the thread topic somewhat, but I think this may be of interest:

Roger Amos [of Model Railway Electronics fame] has published his controller circuit here.

This is a clever combination of PWM and 'servo' Voltage control. … Although intended for model railway use; it may prove very useful for small tools.

MichaelG.

[Bodger BrianParticipant @bodgerbrian]

Sorry to stretch the thread even further but having seen Tony's (very good) attempt at reverse engineering of a PCB assembly & his comment about a 3 layer board, I thought I'd offer some advice to anyone attempting to do something similar.

I hesitate to use the word 'never' but even so, as someone who designs PCB for a living (and has done so on & off for over 35 years), I should point out that PCBs never have an odd number of copper layers – it's always even (it's all to do with the manufacturing process).

So if you look at the edge of a PCB and think you can count an odd number of inner layers, you've either counted too many or missed one.

Brian

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Bodger Brian on 11/08/2015 13:18:29:0I should point out that PCBs never have an odd number of copper layers – it's always even (it's all to do with the manufacturing process).

Single sided PCBs have an odd number of copper layers as far as I'm aware

Russell.

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