# Diodes in series to limit voltage.

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• Solar Expert Posts: 1,019 ✭✭✭✭
Re: Diodes in series to limit voltage.
NorthGuy wrote: »
Why did they use bridges instead of single diodes? I guess they wanted it to work in opposite direction too. So, it doesn't matter where you connect plus and where you connect minus. But they wouldn't go into the expense of doubling the number of diodes just to prevent a polarity connection error. Then why? The answer is simple - this clipper is designed for AC.

I think they used these bridge rectifiers because they are fairly cheap and available.

You get two diodes worth of voltage drop in each package and they can also be heat-sinked.

I bet that's it.

boB
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Diodes in series to limit voltage.
NorthGuy wrote: »
Why did they use bridges instead of single diodes? I guess they wanted it to work in opposite direction too. So, it doesn't matter where you connect plus and where you connect minus. But they wouldn't go into the expense of doubling the number of diodes just to prevent a polarity connection error. Then why? The answer is simple - this clipper is designed for AC.

A small point that some people may not understand is that to produce a bi-directional forward voltage drop clipper from a bridge rectifier pack, you need to use the two AC connections and also attach a jumper between the + and - connections.

If you are just trying to clip DC, you can just connect the + voltage to be clipped to the - lead of the bridge and the + lead of the bridge to the negative side of the voltage to be clipped. No jumper needed.
Either way you get only two diodes working in series at at time and therefore only two diode drops in voltage. But in the AC case, the heat is spread evenly over all four diodes over time while in the DC case it is running through two parallel sets of two diodes all the time.

Look carefully at Coot's diagram to understand the difference.
So a four diode bridge will give you the voltage drop of two diodes but the power handling capability of four diodes (provided you give them a good heat sink.)
SMA SB 3000, old BP panels.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Diodes in series to limit voltage.

maybe if they weren't diodes being depicted symbolically in coots diagram, but rather batteries or pvs in series/parallel then it may become clearer as to the arrangement within the bridge. once that concept is grasped then remember each is a diode running in that same 2x2 arrangement. when using the 2 in series you are also using the parallel 2 in series. i think of the < portion like a street sign showing direction.
• Solar Expert Posts: 1,913 ✭✭
Re: Diodes in series to limit voltage.
inetdog wrote: »
A small point that some people may not understand is that to produce a bi-directional forward voltage drop clipper from a bridge rectifier pack, you need to use the two AC connections and also attach a jumper between the + and - connections.

If you connect AC on the AC "side" of the bridge, you get it rectified at "+" and "-" terminal.

If you do as it is connected now, one pair of diodes will clip peaks from one half of the sinewave, and the other will clip peaks from the other half of the sinewave. You wil get clipped AC with funny waveform.
inetdog wrote: »
Either way you get only two diodes working in series at at time and therefore only two diode drops in voltage. But in the AC case, the heat is spread evenly over all four diodes over time while in the DC case it is running through two parallel sets of two diodes all the time.

Yes, but you get the same amount of heat anyway.
• Solar Expert Posts: 1,913 ✭✭
Re: Diodes in series to limit voltage.
boB wrote: »
I think they used these bridge rectifiers because they are fairly cheap and available.

I guess you're right. Sometimes what is the best thing commercially is contrintuitive. It might've been cheaper to use rectifiers rather than plain diodes even though in the rectifier two of the diodes are completely unused.
• Banned Posts: 17,615 ✭✭✭
Re: Diodes in series to limit voltage.

In at (-) out at (+) gives clipping by two diodes in series, plus a parallel set that will continue to function if one of the others gives up. Built-in automatic redundant operation.

Still messy to stack up a hundred rectifiers.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Diodes in series to limit voltage.

good thing it wasn't selenium rectifiers.
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Diodes in series to limit voltage.
NorthGuy wrote: »
... even though in the rectifier two of the diodes are completely unused.
They are not completely unused. See post #33.

The power handling will be limited both by the size and number of diodes in the package and the size of the heat sink.
One diode connected to one heat sink will not handle as much power as two diodes in parallel connected to that same single heat sink.
SMA SB 3000, old BP panels.
• Registered Users Posts: 1
Re: Diodes in series to limit voltage.

[USER="12530"]pechan[/USER], first of all, any circuit with a hundred bridges in series isn't something that is going to work for a long time. The circuit needs to be redesigned from scratch. Anyways, if it's working, the easiest way for you to get this circuit to drop 30V is to connect a zener diode or multiple zener diodes of specified rating. For example, connecting two zener diodes of 15V reverse biased potential drop will obviously drop 30V and that too without losing much power. Just make sure that the zeners you use can work on your required power ratings. Sometime, there are bad circuits and a circuit with a hundred bridge rectifiers in series is definitely one of them.

printed circuit boards
• Solar Expert Posts: 891 ✭✭✭✭
Re: Diodes in series to limit voltage.
serkan80 wrote: »
@pechan, first of all, any circuit with a hundred bridges in series isn't something that is going to work for a long time. The circuit needs to be redesigned from scratch. Anyways, if it's working, the easiest way for you to get this circuit to drop 30V is to connect a zener diode or multiple zener diodes of specified rating. For example, connecting two zener diodes of 15V reverse biased potential drop will obviously drop 30V and that too without losing much power.

Yikes! Series connection of zeners like that would drop a huge amount of power all the time i.e. it would drop power even when the wind is low. It would greatly reduce efficiency of the system, and thermal loads would be hard to manage. If the system were operating at 1kW, and dropping 30V (2x15V) out of 280V that's ~100 watts (50 watts per device) you have to dissipate. It is hard to get 50 watt zeners and hard to heat sink them well. They are also expensive (\$25 qty 1 from Digikey.)

Not a good idea IMO.
• Solar Expert Posts: 92 ✭✭
Re: Diodes in series to limit voltage.

Actually the limiter is working quite well, I took the advice in this thread and bypassed about 30-40 rectifiers and that has been working very well.
I think the design is quite good because it is simple, passive, and easy to modify.

Pechan