Component value change

[Gareth]

Hi

The motor decoder circuit calls for either UF4001 or UF4003 Diode. Unfortunately these are not readily available over the counter although RS sell in multiple of 50.

I can get the 1000v ver (UF4007). Will that be acceptable for the decoder?

Gareth

[domv]

you could use general purpose 1N4001/4002/4004/4007 available anywhere for the rectifier bridge

[Paul Harman]

You can not use a 1N4001 because it is not fast enough, it will get warm and put a high load on the track because both sides of the bridge will be conducting. You need a diode that is good for 10KHz - 1N4001 is OK for 50/60Hz industrial mains frequency.

Any fast 1A diode will do. UF4001 is ideal, but I have included UF4003 in the kits because they were available cheaply. I have not looked at the UF4007, but I suspect it will be OK. Sometimes high voltage diodes have a higher forward voltage so it might be worth checking that in the spec before you buy.

If you are really stuck you could parallel up a few 1N914 or 1N4148 - two in parallel should be good for a 600mA motor (three will give 900mA), but you will loose the overload protection built in to the L297 amplifier - the diodes will become fuses, and you will have to halve the rating on DC.

If you want to buy something over the counter, Maplin JA47B or AN07H should be suitable (JA46A is too low voltage), or any of N79CA, N80CA, N83CA. I have not tried any of them but the spec looks to be OK.

[Gareth]

Thanks

Living here in RSA we don't have direct access to all of the Maplins and Farnells that have in the UK. My local hobby shop only stocks a limited range of many items ie only the UF4007 but not the UF4001/4003. The local branch of RS do stock them but in quantities of 50 plus. We can get from Maplin etc but by post and our post is somewhat unreliable, things go missing.

I would rather try the UL4007 and not the 1N4148 as I want to try the decoder on some old X04 motors which I have measured at 750ma.

Gareth

[Jon Benn]

Am looking to optimise the circuit for my particular system and wondering what the optimum input resistor
values would be for track voltage of +/- 20V. Presumably best values vary for different track voltages. Anyone got a formula please?

[Paul Harman]

This is the formula for the minimum R1 and R2 value for accurate timing:-

R1(2) = R3(4) x (track voltage / 0.6)

Reducing the R1 value will improve decoding performance when there is poor track contact. It is all a bit of a tradeoff.

Using the formula for 20V gives 47K for R1 and R2 with 1K for R3 and R4.

The values used in the design will ensure that timing is most accurate when the track voltage is below 5V, but in practice the only important thing is that R1 = R2 very closely, and that R3 = R4 very closely hence the specification of 1% resistors.

[Jon Benn]

Excellent, thanks Paul....just waiting for my big bag of 1% resistors to arrive then can upgrade all the decoders I bodged together with components I had to hand. Great forum btw.