I have looked at your picture of the solenoids, Christian. Is it possible to undo the insulation round the soldered joints? If so, I suggest you make little terminal posts of 1mm thick copper wire and drill 1mm holes in the wooden rail, and push the terminal posts in. The wires and one side of the diode can be soldered to the terminal, The other (marked) end of the diode can be soldered to the positive copper rail. I shall be giving a "talk" about MIDI, springs and about these little solenoids at our meeting at Rollcutter Rendezvous. You might decide that little springs could be more effective than weights for fast repeating notes. I will demonstrate that they are quite easy to make. Best wishes. Bob

Hello everybody! Thank you, Bob for advices. After Rollcutter meeting I'm going to undo all the organ, just keeping a few good things. I think we can learn with mistakes, don't we? I found the responsible of the 2 crashes (yes there were 2): just a screw. Take a look at the blog http://orgautomatix.blogspot.com/ and see pictures, you'll understand immediately! I believe the card is now definitively out, but I'll bring it to give to J Wale, if possible. next week. See you later Chris

Pallet magnets and diodes
As I now build Johns boards for him I can tell you that there is actually inbuilt protection against back emf from solenoids, and the early boards shouldn't really need diodes either. I have early boards (no diodes on magnets) and the only time they are affected is if I am sharing AC power with someone and then an idiot goes and unplugs their device from the mains supply whilst the boards are on in the organ (understandable) My PCB's are connected to a switchable fusebox so it's easy to reset them. As for fitting diodes on across the magnet, you need to remember that the positive wire is the 'common' for JW's boards and that you indeed connect this to the positive end of the diode (the ringed end). The negative lead "floats" until a signal is received and then the board pulls it to ground completing the connection for the solenoid to activate. I would recommend that solenoids/pallet magnets (call them what you will) be in the upright position in the windchest. The pressure heps them to keep shut not gravity. When the magnet is powered, the electrical current pulls it down, when released, the air pushes it shut. Shouldn't need to rely on gravity really. This is the standard way that they are used by many builders. Hope this helps...

The ringed end of the diode should go to the positive supply - but the ringed end of a diode is the "Cathode" or "negative end" of a diode not the positive end. The anti-emf diode should always be connected "back-to-front" with the polarity of the supply going to it. If you connected the positive end of the diode to the positive supply, you will just have created a short across the solenoid - probably with disastrous results !! Of course we are talking about connecting the diode in parallel with the solenoid here and not in series with it - in which case connecting the positive end of the diode to the positive supply would be OK but wouldn't really achieve the desired result. Stephen.

??
All the diodes I have ever used are positive to the ringed end. For example if you put a HT rectifier in a power supply the other way around, the smoother capactitor would explode. As stated in the previous post, with JW's boards these are not necessary anyway, and, should a diode go short (and they do) the ULN chip for that output will fry. Richard

Hello! I just received my PCB card, kindly repaired by J Wale, and I think to -*test*-('") it when I have finished my new valve chest. You'll see it soon on my next blog. I understand that it is not necessary to put diodes, but must I put fuse on each solenoïd, or somewhere else? I bought 40 diodes IN 4007, what can I do with? Or perhaps somebody is interested in? :-) Many thanks Chris

Solenoid fuses
Hi You do not need to put fuses on the solenoids (or diodes) as the John Wale boards have protection on-board against back emf from the coils. What I would say is that you must ensure that there are absolutely NO shorted wires anywhere as the ULN chips will fry and stop it working. Even just a "hair" of wire is enough, so do make sure your wiring is good both ends. For your positive wire I would suggest using quite a thick gauge as this has to carry the current for all the solenoids. The negative doesn't have to be as thick, but I would suggest that both positive & negative are insulated just "in case". If one of the ULN chips (each outside edge of the board) does blow, don't be tempted to just put another in, trace that particular output and check for a short. If you don't it will only blow again. Hope this helps you and good luck in your project. Richard

Chris, just ignore the previous posts and install the diodes on each solenoid. Reverse spikes radiate RF signals and can (and do) corrupt the MIDI datastream. The RF radiation takes place down the wires to the board, which act as arials. The freewheel diodes built into the ULN2803 drivers are there to snub the remainder of the reverse voltage. There is no substitute for clamping the solenoids with diodes directly at the source of the back EMF. Yes, there will always be people who claim they don't use clamping diodes and get away with it. Strange then that all automotive devices use them for modern managed engines. They are a commonsense and prudent way to design circuits. Bob Essex

Ignore what?
I hope as an organ builder Bob, you do not think that you are the only one who has any knowledge on this subject .. I manufacture all of John's boards and I have been using them without any diodes for three years without a hitch. Wires do not act as RF aerials and RF is very high frequency and this is not the case with back emf from a solenoid coil. I think it is quite rude to ride roughshod over other people's posts and trash everything they say, and tell others to ignore it just because you have a point of view. There are ways to put your point across without offending others, and I thought that was what this forum is about, not weilding one's position in the organ building world. Back emf is literally a back push of electrons back up the wire and not "radiating" as you suggest, and as for cars well, organs do not have spark plugs and enough volts to make a spark jump across a gap. Those boards adequately protect, and I would suggest that if there is a problem, it is not down to diodes or radaited interference, more the build quality and the wiring layout and position.

I'm really sorry. I did'nt want to begin such a debate! Chris

Don't worry about it
Hiya Pls don't feel you have to apologise. You are most welcome to mail me direct if you wish. I will try to give you honest help wherever I can and I wish you well in your project. It is always a great feeling when the organ fires up. That's what it's all about don't you think. Kind regards Richard

Hi there All I have to say that I have built quite a few MIDI systems, and have done quite a bit of -*test*-('")ing on Johns MIDI boards one way or another. I have tried the same boards both with back blocking diodes and without. I have to say that I didn't have a problem either way. I have run magnets for many hours without the diodes, and I did not have any burned chips, or errors. The problem I did find was interference from external sources. Try going to a rally and having a thumping great motorbike with a magneto running close by. Alternatively try setting up next to the Puff, Bang and Fart brigade - sorry, that should read stationary engines. That can throw the MIDI system completely. The only thing I suggest is screening the magnet wires if they are of any length. I use a multicore screened cable to the magnets if they are to be long. For best results the boards should be in a screened box. The power supply should be situated close to the boards. This should cut down on interference. You should be using an earth rod anyway with your generator, and that will help the screening of the MIDI data wire. I also found that a stabilised power supply helps cut down on interference. The use of extra diodes on magnets if you are using long unscreened wire does work slightly better for external interference. If you have a small organ with a MIDI system running on battery power, then you can have great problems with interference. Use an earth rod and try putting a capacitor on the suppy terminals of every board. Cheers Peter Griffiths

I stand by everything I have said in this thread. Having spent a lifetime in industrial electronics and knowing about RF, and having -*test*-('")ed myself these inductive circuits with an oscilloscope it is clear that RF is generated. This is due to the inductance and internal capacitance of the windings forming a tuned circuit. RF is radiated from the wires to the board. If you don't believe me, then please discuss this with John Wale, who is also familiar with this effect. He and I have discussed this at length many times. This interference can, and does, cause MIDI problems, particularly with the earlier MTP boards. Clamping the coils of inductors with reverse-biassed diodes is standard industry EMC practice. The internal diodes in the driver chips were never designed to fully clamp reverse spikes from inductors, just to reduce, or snub their effect on the transistor switch. The spike travelling down the wires to the board induces stray currents in the other ajacent wires, which also need to be clamped. You are right to assume that I am an amateur organ builder, but my electronics credentials are quite another matter. Bob Essex