guys, if i need to run a 12v. acc. what is recommended? john
What is the power source? Is it a bttery or the Magneto?
Edward R. Levy
That 12 Volt accessory will not need AC, like the Magneto provides. It will require DC, like a battery provides.
Hence the title of the post "rectifier circuit"
In that case, a small rechargable 12 volt battery could be used with a separate circuit for a single accessory and recharged with a rectifier circuit and diode pack from the Magneto, like Seth H. was offering last week. That charging current would only flow at highway speeds for most Mags. John's message is kind of cryptic. He don't say if his T is already wired for and using 12 volts.
It really depends on what John needs 12 volt power for and also whether or not he intends to run Ford coils off the magneto at the same time.
The practical limit is about three amps with half-wave rectification, but there will be less than 12 volts below about 1,000 rpm. If 12 volts is required at low engine speeds, Jim is correct in that a storage battery will be necessary.
yes, i didn't add much info, was looking for a circuit to build that would run an oooga horn. thought about a bridge rectifier and a voltage controler. solved the problem i was facing today, went to the spring swap meet car show and found a 6 v. oooga horn. so i bought it. now to make a mount and install it. a couple of pics from the show. john
Bridge rectifiers won't work on a stock T magneto!
Sure they will Noel, you'll still have just half-wave rectification with two diode drops and two of the diodes will be wasted. For catching the positive waves, you'd connect the mag to the (-) terminal, the AC terminals would be left unconnected, and the output would be the (+) terminal.
But, the same thing could be achieved with one diode with only one diode drop.
Thanks for the pictures John. I really like that model A five-window coupe!
Or you could connect the AC terminals together and hook them to the magneto. The plus terminal would be the DC. Ignore the minus terminal.
Yes you could, and you could have heated seats with the minus terminal.
seth, didn't take many of any one car. john
You and I have personally discussed this subject at length.
To use a bridge in this application is purely bad design practice.
I am surprised you feel it was necessary to "correct" me!
would one of you explain why a full wave rectifier doesn't work on a t magneto. thanks, john
The magneto output is ground referenced AC meaning one end of the thing is already connected to ground internally. Thus its AC output is a positive pulse above ground and a negative pulse below ground. Put another way the output if bridged rectified with AC connections to MAG and Ground would result in a DC voltage on the positive terminal that is positive and a DC voltage on the negative terminal that is negative. You cannot use the minus voltage for much of anything since the battery has the negative terminal grounded already so you can use the positive output and ground but the negative output cannot be connected also at that time to the battery negative are you are simply shorting out one leg of the bridge. I you unground one end of the magneto internally and bring that end out of the motor then you can use the magneto to bridge rectify for full output but then the coils won't run because the coils need one end of the mag ring to be grounded so that the mag output is plus and minus. Not sure if you followed all that but if not then buy a bridge and hook it up and learn that way. You will quickly see it won't work without modifiying the mag or ungrounding one end of the battery.
We are all well aware of the “John Regan” and “Seth Harbuck” Solutions to getting some DC from the AC Magneto. Both are “half wave” solutions. There is a full wave solution that does not require modifications to the magneto and does not interfere with the normal operation of the coils on AC but provides a Full Wave rectified DC output.
The problem as stated above is that the magneto is grounded. To overcome this, the negative side of the bridge rectifier must be physically isolated but electrically connected to ground. This can be accomplished with a high value capacitor.
I used two electrolytic “beer can” capacitors. Electrolytic capacitors are polarity conscience – that is they have a plus and minus terminal and the do tolerate AC or being connected backwards (they usually explode). However, there is an old trick that eliminates these limitations. That is to connect two of them in series – back to back. I Connected the bridge minus DC terminal to first capacitor minus, first capacitor plus to second capacitor plus, second capacitor minus to ground. The order is - + + - across the capacitors, + - - + will work just as well.
I did some experimenting with a HCCT. The output is a full waverectified DC. When I get some time to get back to this experiment I will see how much power I can get from this configuration.
Now we can add the “Be_Zero_Be” solution the list
OOPS - Here is what I ment to describe on how I hooked up the capacitors. That is what happens when we trust our memories.
There's nothing "wrong" with using a bridge other than being wasteful. You said they won't work and left it at that - that's all I was correcting. Bridges are in a rather large package so cooling would be enhanced so IMO, that's not purely bad design practice.
It appears that the "accessory 12v" intends to be grounded (connected to the car chassis), however a full wave bridge, if left isolated from ground is more efficient in extracting magneto power. There is no need to connect the negative rectified power to ground....assuming the eventual using device is not grounded.
bob, are the diodes correct in your diagram? also would an isolation, 1 to 1 transformer achieve the desired results. got me going to the old memorys place now. cathodes and anodes and all. john
seth, i think i see what you are saying, your explaination and bob's schmatic are helping. thanks, john
correct that to John R. for the explanation, john
The diodes are correct on Bob's diagram. However, when the magneto is putting out a negative pulse or wave, the upper left diode will conduct and short out the magneto. I don't think there will be any negative pulses for the coils. John's idea of an isolation transformer should work. Finding one to operate at the magneto's voltage range may be a push.
You are correct. It was late last night when I posted. Here is the corrected diagram. It is as I described in my text above.
bob, it appears to me that both sides of the bridge end up at the caps to ground with no positive output at +DC. the cathode has to be negative, the anode, positive. john
ok, i see it, but i still think the diodes need to be changed. on one wave the mag is shorted and should burn out the diodes, soon. john
It's been over a year since I did this experiment. I will repeat the experiment soon and post the results with output waveforms.
Since John is running a 6V motor driven horn he needs a battery. The magneto won't have enough power to run the horn's motor and the engine coils.
A magneto 6V charger from http://www.funprojects.com would do the job of keeping a 6V motorcycle battery charged so the horn could be used.
By all means post voltage AND current waveforms for your circuit. Should be interesting.
John is correct a transformer with properly chosen impedance and winding ratio would allow a full wave recovery of magneto power. I say "properly chosen" since if not of the right impedance and careful attention to transformer losses, you might end up with about the same amount of output power with a full wave transformer inserted as you would get from a simple diode half wave circuit.
There is yet one more way to do this but it is a bit hairy. What you do is put the bridge to the magneto and a modest filter capacitor from bridge + terminal to bridge - terminal in the "typical" full wave bridge with filter arrangement. NOW the only problem is that the negative side of the filter capacitor cannot be grounded so you have this "floating" DC full wave rectified source. You then put a switching circuit in there that also "floats" to switch the DC source of the capacitor at a high rater of speed at say 40,000 Hz. Now you pass this switching voltage AC waveform through a VERY SMALL transformer with a full wave rectified output but since this SMALL transformer has the isolation you want - the output of this secondary rectified output and filter can be grounded and like magic you have your physically small but pretty powerful FULL WAVE power source with NO mods to the magneto. This is based on sound engineering practice but is NOT easy to design since the voltage output of the first bridge stage is a widely varying DC voltage and if you draw too much power from this source - you cannot run coils at the same time. Thus the secondary switching circuit must in fact be a switching REGULATOR to give you stable output at 6V, 12V, or switcheable easily between them. It is all doable and I have designed up a circuit that should work but never had the time to build it and perfect it. It is difficult but not impossible to do it this way. Once debugged it shouldn't be a hard thing to replicate. I simply don't have the time to pursue it at this juncture. If someone has an understanding of what I am talking about then they would be able to follow this lead and produce the thing. I will NOT provide any schematic nor act as consultant for this project since it would be faster for me to just do it myself and I don't have that time. I will say again - it is doable this way but not easy with some exotic control required to limit the power the circuit takes so that the coils will still run while at the same time then taking all available EXTRA power away to charge the onboard battery which would of course be the ideal ground referenced source your seek.
The basic problem is that one end of the Mag is at ground. To effectively use the bridge rectifier for full wave action the grounded end of the Mag coil would have to go directly to the bottom of the bridge circuit, as drawn. Then the capacitors could move to the other side and filter the rectifier output.
That would work but the coils would no longer work. The best solution was stated earlier above, that is an isolation transformer. It would have to have the correct impedance, frequency response and bandwidth to handle the variable frequency of the magneto. Not impossable by any means.
The other problem is that the Mag output does not closely resemble a pure sine wave and would not go through a transformer as well as could be expected or desired.
Could you pick up a rectifier up for an old motorcycle and use it?
james, you don't have to even have dc to go thru a transformer. the rule says, change of currant and change or time. as far as what comes out of the mag, i will leave that to those that have observed the wave form on an oscilliscope. john
I have seen the waveform photos, but no actualy live waveforms. They all looked rough to me. Change is definitely very important to a transformer. DC just won't go through. Smoother is better, but not an absolute requirement.
Then too, neither is 12 volts. A good coil will produce 1.3 amps of good sparks with 2.5 volts of AC from that same Mag.
james, correct dc to ac in my above post please, and yes dc will go thru a transformer if there is a change of currant. once again the rules for inductance is change of currant, change of time and ac does not enter into the equation, as with a vibrator for radios. those units pulsed the currant and fed it to a transformer and everything worked just fine. john
Those vibrators did not work without a buffer capacitor to smooth that square wave into a pseudo-sinewave. I know about those, I repaired them for quite a few years, always replacing the buffer capacitor with the vibrator and for a good reason.