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Dynamo output control

Hi all

Thought I would update on progress with the rheostat field control system. (see previous thread)

I managed to obtain a rheostat from Canada...
Including airmail it was cheaper than Maplins and RS, and is a new old stock military spec (5ohm, 150W), so Ebay is not so bad after all! It is a big old beast though, almost as big as the engine. I am going to fit it in place of the exisitng field resistance in series with the field coils (I dont have a standard light switch arrangement so wont need complicated switching).

Dynamo is converted back to 3 brush with new brushes and some repairs to insulation.

The Schottky diodes have been purchased from Maplin, rated at 30A they should be OK. As we speak the wife is in the Plymouth Maplins shop buying a list of parts! I have discovered that the diode is in fact two in a package with a common cathode. An important observation I have made is that the cathode is also electrically connected to the heatsink on the back of the diode package. I am therefore going to fit a thermal pad as used in computer CPU's to conduct the heat to the large aluminium heatsink I am using without making any electrical contact (I couldnt find a diode mounting kit anywere?).

If the parts are in stock at Maplins I may have something up and running this weekend?

So far it seems to be going together with a wonderfully Heath Robinson look which should get some conversation going if nothing else


Regards

Steve

Re: Dynamo output control

Hello Steve,

Glad you are making good progress.

I am not exactly certain what diode you finally purchased, but would have thought that it might be in the so-called TO220 package

If that is so Maplin Part No. RN91Y @ 0.1p and CHO3D @ 0.31p are the mounting grommet and insulator respectively for that size package. Together with some special heat transfer grease which I know Maplins have, these parts would be what you would need to safely mount a TO220 cased device to a heatsink.

I would be interested in knowing the phyical size of your diode pack if it is not a TO220,

Regards

Barry R.

Re: Re: Dynamo output control

Hi Barry

Diode case is a To247ad style?
The chap at Maplins is not over helpfull I'm afraid, he looked quite blank when I asked for the diodes. Another chap there swore blind that diodes are always long thin circular devices with two leads!! I had to explain the schottky diode package to him (after you had explained it to me with your helpfull diagram!).

RS components do seem to have more range BUT you seem to need an account to purchase?

With hindsight perhaps I could have tried harder to source the diodes you recomended which would have easier to obtain mounting kits? I might loook up the physical sizes of the T0220 package to see if it is similar to what I have, maybe the mounting kit would be usuable? I might buy one to see? I am still thinking (hoping?)that the thermal pad will do the job, although it has so called "phase change" characteristics when it gets to a certain temp so I presume it will still insulate electrically?

At least it keeps me off the streets!!!
(and the car off the road!)

regards

Steve

Re: Re: Re: Dynamo output control

Thanks for the part numbers Barry!!!

Just looked on the online Maplin catalogue and I'm sure that the mounting kit will do the trick!
I will pop in and buy on Mon

Shame the chap at Maplins wasnt more switched on as I could have bought at the same time!

Thanks

Steve

Re: Dynamo output control

Hi Steve,

I was under the impression, perhaps mistakenly, that anyone could buy ONLINE from RS, however if you wish to buy from one of their trade counters (unfortunately the nearest to you in the Plymouth area would be either Bristol or Southampton) you do need to have a business account otherwise they'll simply refuse to serve you. The advantage with RS in my experience is that their counter staff are very knowledgeable and extremely helpful. I have an account, so if you get stuck for anything let me know and I'll see what I can do - unfortunately my knowledge of electronics is particularly slim so I'd need input from yourself and Barry to identify the correct components .

Jeff

Re: Re: Dynamo output control

Thank you Jeff

I've just returned from a trip into Maplins (they open on sundays) I have now got the correct diode mounting kit, thanks to Barry's part numbers.

I have come to the conclusion that its easier to look at the catalogue in store yourself rather than rely on someone serving you , who cant realistically be expected to know every part they sell!

Just got to put it all together now!

Steve.

Re: Re: Dynamo output control

It's true that anyone can buy online from RS but you have minimun order levels and postage to contend with.
If you open an account those two barriers disappear. I opened an account last year without having to tell any lies about my status (I'm just a bloke who mucks about in my garage). I had no problems.

Charles