Noobie question about using a diode to protect power to a Looper pedal

[Nigel Goodwin]

Nigel, here is a diagram of the Lace Looper from Mammoth Electronics, available in 3, 4, or 5 loops and lots of powdercoat options.

On that diagram, you insert the diode at the bottom left hand corner, in the red lead labelled 'wire to main 9V+', with the stripe on the diode pointing towards the right.

Assuming that the DC input socket isn't a PCB version?, you simply unsolder the red wire from the socket, solder the striped end of the diode to the wire, and the other end of the diode to the socket. If it's a PCB socket, you simply cut the track near the socket, and solder the diode across the cut.

 

[Nigel Goodwin]

At the risk of being burned I'll explain how its done commercially with 0 volt drop (and this has already been mentioned).

Put a diode across the supply terminals of the looper in reverse, so the silver strip goes to the +, and the other side to -, then you put a fuse, or fusible resistor inline with the supply (before the diode).
Then if you power the device in reverse the diode will turn on and blow the fuse.
Obviously a fuse or some means of disconnecting the supply is important here.

Again, this ISN'T the way it's done, it would be an absolute disaster for this project (and most others). There you are on stage, nice big gig, hundreds of people in the audience screaming away - oops, connected battery wrong way round - no pedals any more.

It's done with a series diode, because it's completely safe whichever way you connect the power, and has no bad effects regardless.

The parallel diode and fuse is commonplace (or at least was, no idea if they still do it?) for car radios etc. - where you don't really want the extra loss of a series diode, and it's a 'one off' connection, not a connection every gig.

For effects pedals the 0.7V drop is insignificant, and is the perfect solution.

 

[GromTag]

Even less forward drop with a Schottky, forward voltage drop can be less with higher rating of current pass in most applications, tho some Schottky types can be rather slow to block, U seconds to milliseconds depending of device.

 

[crutschow]

Schottky types can be rather slow to block, U seconds to milliseconds depending of device.

seconds to milliseconds??
Schottky diodes are the fastest diodes available.
Where did you get that misinformation?

 

[GromTag]

Hmm, miss information, ok on that one for me not stating under devices max rating with reverse in the mA range for some.

Maximum instantaneous reverse current at rated DC blocking voltage typical of higher temperature, example 100 °C.
Lesser IR at lower temps

The range of milliamps at range of 300Us per IR for the ones I tend to use.

Data, SB330-E3/54 Vishay Semiconductor Diode.

Some circuits may not prefer the momentary reverse before blocking IF the voltage is high with the temperature in the higher range, and when I say some circuits, practically non existent. But not impossible. I have no examples.

Sorry.

 

[dr pepper]

Ouch..., burned.

I helped a mate who started a business for a while fixing car radios, just about all had a diode in reverse across the supply in to protect from reverse polarity, they either went short or blew a trace when some boy racer wired them up wrong.

A long time ago I had a cry baby waa waa for repair I think that had a diode in reverse too, and no fuse.

 

[crutschow]

Hmm, miss information, ok on that one for me not stating under devices max rating with reverse in the mA range for some.

Maximum instantaneous reverse current at rated DC blocking voltage typical of higher temperature, example 100 °C.
Lesser IR at lower temps

The range of milliamps at range of 300Us per IR for the ones I tend to use.

Still have no idea what you are talking about.

 

[Nigel Goodwin]

I helped a mate who started a business for a while fixing car radios, just about all had a diode in reverse across the supply in to protect from reverse polarity, they either went short or blew a trace when some boy racer wired them up wrong.

Yep, common in car radios - particularly bearing in mind that historically cars could be either +ve or -ve chassis. Then there's also the habit of car manufacturers to use non-standard colours, with the +ve radio feed quite often been a black wire