Diodes in the supply

A diode in the supply is the cheapest insurance in electronics and the most expensive to run. It survives a battery fitted backwards, and it charges you half a volt and a watt of heat on every day that nobody fits anything backwards.

The Diodes chapter covers what a diode is, what its 0.7 V costs and which end is which. This page is about the place a diode ends up on a board that runs from a battery — in series with the supply — and about how much that costs.

Which diode

Three families cover almost everything a project needs, and the difference that matters here is the forward drop.

Family	Drop	Reverse	Typical part
Silicon rectifier	0.7–1.1 V	slow, cheap, rugged	1N4001–1N4007
Schottky	0.3–0.5 V	fast, leakier, lower reverse rating	SS34, 1N5819
Zener	conducts backwards at a set voltage	that is the point	BZX55C series

A Schottky is the one to reach for in a supply. It drops less, it switches faster, and the two things it is worse at — reverse leakage and how much reverse voltage it survives — barely matter on a 5 V rail.

The diode tax, in volts and in watts

5.00 V in · 500 mA

Load current500 mA

Drop

700 mV

Load sees

4.30 V

Wasted as heat

350 mW

700 mV gone and 350 mW into the air. The drop hardly changes with current, so the heat rises in step with the load: double the current and you double the watts. At an amp a silicon diode is burning most of a watt in a package designed to dissipate about that much — which is why the next two pages exist.

Move the current slider. The drop does not move much, which is the whole problem: a diode charges the same toll at 20 mA as at 1 A, so the heat rises in a straight line while the voltage lost stays put.

The job it is there for

A supply fitted backwards is the ordinary failure. A JST plug that only fits one way until somebody makes it fit the other. A barrel jack whose centre is negative on this adapter and positive on the last one. A pair of screw terminals. A battery holder.

A supply fitted the wrong way round

correct

Load sees

4.60 V

Normal-day drop

400 mV

Reversed

blocked

Protected, for 400 mV. A series diode is the answer everybody reaches for first, and its bill arrives every single day the supply is the right way round: 400 mV the load never sees, and that voltage times the current as heat. On a 5 V rail you can afford it. On a cell that is already down to 3.4 V, you cannot.

With nothing in the way, current goes backwards through everything that offers it a path: through the regulator, through the ESD diodes inside every chip, through an electrolytic capacitor that is now reverse-biased and will eventually vent. Sometimes one part dies and saves the rest. Sometimes it is the whole board and it takes two seconds.

The bill

0.4 V out of 5 V is eight per cent of your supply, before anything has used any of it. Out of a lithium cell at 3.6 V it is eleven per cent, and it is eleven per cent that a 3.3 V regulator needed as headroom. This is why so many battery projects "work on USB and not on the battery".

Worse, the drop moves with temperature and with current, so the number is not even a stable one to design around. A silicon diode loses about 2 mV per degree of warming — helpful here, since it drops less when hot, and a nuisance everywhere else.

The part everyone actually uses

Switch the figure above to the P-channel MOSFET and look at the bottom line. Source to the supply, drain to the board, gate pulled to ground: with the supply the right way round the gate sits well below the source, the FET is fully on, and the drop is the current times a few tens of milliohms. Fit the supply backwards and the gate is no longer below the source, the FET stays off, and — the part that makes it work at all — its body diode points the wrong way to conduct either.

60 mΩ at 300 mA is 18 mV. Against 400. For about the same money.

What the P-FET still cannot do

It blocks a supply connected backwards. It does not block current coming back from the load, because once the FET is on it conducts equally well in both directions — it is a switch, not a valve.

That sounds academic until you put a battery and a USB port on the same rail, at which point the USB rail feeds straight into the cell through an uncontrolled path. Fixing that is what the next page is for.