Sunday 10 March 2024

Why don't you use a bridge rectifier?

This question has come up again, so I thought I would finish off a blog post I started quite a while ago.

The humble bridge rectifier.

Four diodes arranged to take an AC signal and convert it to DC.

A simpler circuit with only a single diode gives half wave rectification. The negative half of the waveform is cut off. It does give DC, but only half the power.

The bridge rectifier solves this problem by reversing the polarity of the neutron flow negative half wave and flipping it to be positive, giving a DC waveform with two positive peaks for every AC cycle.

That is not the application here.

The power input to the Minstrel and Mini PET kits use a 2.1mm DC jack. This then goes to a jumper block allowing the user to select centre negative or centre positive supplies.

In theory, the jumper blocks could be replaced by a bridge rectifier. No matter what the input polarity, the bridge rectifier would flip it to be positive.

Sounds great, but why then do I not do it?

"Why don't you just fit a bridge rectifier?"

Well, I have put together an example of why this is a bad idea.

Let's start off with the simple solution, you have a 9V DC power supply, and a Minstrel 3.

Power comes in, goes through the jumpers and to the circuit.

With the jumpers set for centre negative (the preferable option), that looks like this.

The alternate configuration is with the jumpers vertically, for a center positive supply.

Several people have suggested that fitting a bridge rectifier in place of the jumper block would be a better solution.

Two of the diodes in the bridge will be always conducting, and two will always be reverse biased and not conduction. But it would allow DC of either polarity to be used. (or in theory AC if a much larger reservoir capacitor was fitted)

A bridge rectifier is traditionally drawn as four diodes in a diamond shape as above, but I have rearranged that into four parallel diodes, the circuit is just the same.

If the centre negative 9V DC supply is attached, two of the diodes conduct and power flows into the circuit. No current flows in the other two diodes, so they can be ignored. Redrawing the circuit shows the two active diodes.

Again for simplicity, I am showing the voltage drop as 1V, in practice it depends on the diodes and the current flow. I normally use 1N4001 diodes. The voltage drop across those varies from 0.6V off load, up to 1.1V at 1A.

From a 9V input, the circuit gets around 7V. This is enough for the 5V regulator, but only gives a 2V dropout voltage. On the positive side, some of the heat is dissipated in the diodes, reducing the heat generated by the regulator.

On the Mini PET 40/80 and Minstrel 4D, I got rid of the jumpers and just added a protection diode in case someone plugged in the wrong polarity of supply. That adds a single diode voltage drop, around 1V.

That all seem fine, so why don't I just use a full bridge rectifier?

Well, the problem comes when you have two or more things connected to the same supply. You get the same issues with two mains referenced (or actively earthed) power supplies. For simplicity, I will consider the more obvious problem of a Y cable being used to power both the Minstrel 3 and a monitor.

That should be fine, they both get 9V DC in and do what they want with it. (OK, so I have drawn centre negative on all the diagrams, but the monitor is centre positive, so I swapped the jumpers on the Minstrel 3 to centre positive to make it work, but I am not going to redo all the drawings)

There is no problem with the polarity jumpers, but if there was a bridge rectifier, there is an issue.

I have added the video jacks to the drawing. In both cases connected to the internal ground rail.

If the Minstrel 3 has a bridge rectifier, it's ground rail is actually at 1V relative to the power input.

If the monitor is wired directly (as most things are), it's ground rail is at 0V.

Not a problem if they are running in isolation, but if you then connect a video cable between them, you tie those two ground rails together you effectively short out the diode drop.

"That's fine", you say, "all the more voltage for the circuit".

Well, no, because the video cable is providing the short circuit path.

The Minstrel has two paths to back to the power supply ground to chose from. One is via the diode, but there is a 1V drop that way. The other is through the video cable to the ground rail of the monitor and then on to the power supply. OK, there is a bit of resistance in the video cable, but not as much as 1V, so it goes that way.

Here I am just showing the ground rail, you can see the two options.

That diode is doing nothing, so what you actually have is this.

You didn't like my drawings of ammeters? Or you don't trust my workings?

OK, I have tried to model that, this is the Minstrel only powered via a bridge rectifier, with a small resistance added for it's power leads.

Now when there are two paths, the current is split. In most cases, most of the current with flow through the power cable from the Minstrel. In the worse case with a bad power cable (say 0.1Ω) and a really good video cable and power lead on the monitor is (say a total of 0.2Ω), you may still get some of the current flowing down the video lead (the amount depends on the relative resistances of the video cable shield and the power cable negative wire)

However, the big problem is when a bridge rectifier is used and this adds a diode drop in the negative side via the normal power cable. Then suddenly the video cable is a more attractive path for the ground return, and it all flows down the video cable and not the power cable. 0mA down the power cable, and 82mA through the video lead.

So, in conclusion, in this situation, if you have a bridge rectifier, then there is a significant current flow through the video cable to the monitor, and back to the power supply via it's power cable. This is not a desirable situation, and should be avoided.

"Yes, there are two paths you can go by,

but in the long run,

there's still time to change the road you're on"

This is the same issue you sometimes get when you plug in cables to live AV equipment and you get a tingle as you touch the plug and socket at the same time, just before connecting them. Sometimes you may see a tiny spark. If you put a meter there, you will often find half mains voltage between the two devices. This is why sometimes when you have lots of AV equipment connected between devices you can unexpectedly fry video leads or power supplies etc.


Remember the Mini PET 40/80 input that has a single diode and capacitor at the input?

Would that work with an AC input? In theory, the diode will block the negative half of the AC waveform and charge up the capacitor to give a DC rail. The capacitor is only 100uF, so that may not be enough.

I got a 9V AC supply and tried it out.

The yellow line shows the input, currently around 14-15V, so that looks promising. The green line shows the 5V rail, currently inactive because the soft power switch is off. Both are relative to the bottom line on the scope.

When powered on, you can see the 5V rail comes up and looks good for a bit, but eventually the voltage across the capacitor drops too low, and the 5V starts to drop out. The decrease continues for a few more cycles, until it drops too low to hold the soft power on, and switches off. It then goes back to looking OK.

I have separated the waveforms, so you can see that the yellow one does indeed take bites out of the green one.

So, as expected, 100uF is too low. Let's try 470uF.

It starts OK. And looks like it would be a good option, as there is now less overhead, less excess voltage to convert to heat.

However, add a heavy load, such as rewinding a tape that is already at the start, and it can't cope.

I increased this to 1000uF and tried again. That reduced the ripple quite a lot.

That does drop a little, but there is now a lot of headroom over the 5V rail, and even the 6V rail with the heavy loads.

If anything it is a little too high, might be a little too high, the 9V DC supply normally only has 4 V dropout to convert to heat, now closer to 8V.

If you can find a suitable 1000uF 25V, or larger capacitor that will fit under the perspex (max 13mm diameter), you can run the Mini PET 40/80 on AC.

That does indeed work. A bridge rectifier would halve the ripple, so would help, but then what if someone plugged in a DC supply.........


The full range of Minstrel and Mini PET kits (none of which have bridge rectifiers) are available form my SellMyRetro store.

All the links can be found here:


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