Electronics question

Colbaker

Colbaker

Colbaker

Associate
Joined
8 Dec 2004
Posts
1,970
Location
Paignton, Devon
Hi all,

I have at max 22AC output from a small generator and want to convert it to DC, I am pretty sure i need some Diode's to do this (ampage is not that high although i am unsure of how to test the AC amps on my multimeter), What sort of Diode should i use? and how can i configure them?

Thanks
 
As above you will need a rectifier to convert the AC to DC.

In order to read the current you will need to either take a voltage reading across a known load (I = V/R) or place the meter in series with the load.
 
2 Diodes for half wave rectification, 4 for full wave. You have two options, you can buy some diodes, or you can buy them already in a package like a KBL005.

The diodes you want are silicon rectifier diodes, 1n4001 (upto 1amp) or 1n5400 (upto 3amps) are good options.


Be careful when measuring current, do not, DO NOT connect it across the circuit like you are measuring voltage!! Break the circuit and measure in series, your multimeter will go pop if you do it wrong.


Edit; Don't forget your smoothing capacitors after the rectifier, otherwise you have a bumpy DC load, you want the caps to make it nice and even.
 
Last edited:
neodude said:
What you need is called a Wheatstone Bridge
Click to expand...
No it isn't,
this is the second time I've heard this?

Shadez said:
In order to read the current you will need to either take a voltage reading across a known load
Click to expand...
Slightly misleading,
the known resistor (or shunt) needs to be in series with the load, giving you a measurable 'voltage drop', from which you can calculate the load current.
Measuring the voltage across any load would just give you exactly the same voltage the generator supplied (cable resistance aside).

To get an idea of the maximum load an AC generator can supply then you need a variable load. Assuming that whatever is powering the generator has a fixed torque, then the generators output frequency will start to reduce at its maximum loading (ie when you decrease the load impedance).

De-rate that by 20% to get some kind of safe maximum operating power.


The smoothing capacitor needs to be rated according to the generators output, so need to know what the OP is doing with this


Need more info to suggest anything else, OP is a bit vague :)
 
Last edited:
Assuming this is a single phase output then a bridge rectifier and smoothing capacitor will convert to DC.

Be careful of capacitor polarity and be aware that once rectified your output voltage is equal to the peak AC value, so you may convert from RMS AC to peak by multiplying by the square root of 2. In this case 22V * 1.41 = 31.1V; absolutely do NOT use a 25V rated capacitor for this reason, even 35V is too marginal, go for 50-63V. There are losses of approximately 1.2-1.4V through the bridge rectifier too but the exact output voltage will be load dependent anyhow.

To determine capacitor value we need an idea of output power and/or expected load. If a particularly smooth/low-ripple supply is needed an CRC or CLC arrangement can be helpful.
 
Thanks for all the replys lads, very helpfull.

a little more info, I am basically teaching myself electronics by creating small projects, this project involves using a pc fan re-wired to generate a voltage when the fan is spun, now I measure the AC which tops out at 22 volts on full push, obviously this current makes the attached LEDs flash so I wanted to convert to it to DC first, then as you have said I should add a small capacitor, eventually I may try making a voltage step up circuit.

The fan is rated at 0.18amps, is this the likely output of it being used as a generator? I don't quiete get the suggestions on measuring the amp output in AC. Can someone "lamen" it up for me please :).
 
Current is measured with the meter in series with whatever your load is. So if you have a length of wire powering your circuit between the power source and the load you have to break it and put the two probes to the two new ends while putting the meter in the correct mode and holes.

The meter actually measures a voltage drop across a known resistor and that's why you often have to switch holes.
 
Thanks for the help, its up and running now, I qm thinking of putting a couple of AA rechargeable batterys on it which I am hoping will trickle charge from the turbine.

can I just wire these batterys up to the DC output? I assume I would need to some of protected circuit?
 
You must log in or register to reply here.
Back
Top Bottom