I've just finished a rectifier assembly to plug my growler into for magnet charging. Simple case of putting a bridge rectifier inside an IP56 box along with a heatsink and a momentary switch. Seems to be a good way of sucking evey iron thing up off the bench so far;)
Duty cycle on the growler isn't high owing to the heat build up in the windings, but all my screwdrivers are now well magnetised!
Does anyone remember what sort of test weight I should be aiming to pick up with a freshly charged Ford magnet?
Congratulations - It looks like you created a very nice implementation on my Growler magnet charger idea. It looks very nice. You should be able to pick up at least 3 pounds after you charge the magnet. I use a cast iron piston and wrist pin with some extra weight added to make a 3 pound test weight
Anthony: when I charged my magnets off the flywheel,I can pick up about 6KILOGRAM,
is that about 15 POUNDS???
When the engine is running I have 30 - 35 volt AC
Greetings from Holland
1 KG = 2.20462 pounds,
We use 2.2 for figuring as it's easy and it's very close.
Easy way to convert from KGs to pounds is to double the KGs and then add 10%.
Example: 6 KG X 2 = 12, plus 10% (1.2) = 13.2 pounds.
What is the source for the bridge rectifier? Part number?
What is the source for the bridge, part number and wiring diagrame?
Here is a link to a source in the US.
Select one with a voltage rating of at least 400V for 120 volt AC input or 800V for 220 volt AC input. The amperage rating should be at least 15A.
When I get home from work tonight, I will post a wiring diagram if someone doesn't beat me to it.
Can you guys also include a diagram on how to make a magnet charger?
Radio Shack also carries the bridge rectifier. It's labeled for AC input and DC output. Just be really careful, using very short bursts of "on" time as Anthony suggests. AC impedance and DC resistance are two very different things (John Regan, jump in here...) You can easily overheat the coils enough to melt the insulation in just a few seconds. I'd hate to see someone ruin a growler by taking a sip of coffee while the switch was on.
The bridge recitfier that looks like the ones pictured above from Radio Shack is only rated at 50 volts. It is NOT suitable for this application.
Here is a sketch of the circuit. A Bruce says, be carefull to not leave this on more then a second or two. You will be applying about 170 Volts DC to the growler.
Sketch.pdf (44.2 k)
Sounds like you guys are gonna have some fun but be careful. Diodes rated nominally at 15 amps are NOT rated at 15 amps without a heatsink. Usually the understood heatsink is an infinite heat sink which essentially says that the diode (or bridge diodes) can handle the 15 amps so long as the package does not heat up in the process. Since you are putting DC on a growler winding that was designed to have AC on it - you need to measure the DC resistance of the total winding and then use Ohm's law to determine the amount of current you have. There is also a SURGE rating on diodes that tells how much current you can put through it "one time" as you power up something. With a magnetic device as your load the surge rating is likely NOT going to be exceeded but being an inductive load - WHATCH OUT for inductive "spikes" of voltage when you swithc this thing ON and OFF. These can kill your diode bridge. Get as high a voltage bridge as you can get. It only takes a mere fraction of a second to charge the magnets but if you use s pushbutton or switch to make/break the connection from the bridge to the growler winding - lookout for "spikes" and possibly big ones. If you find your diode bridge has shorted - that is what probably killed it. When diode voltage is exceeded - it will fail SHORTED. Since the duty cycle is very low as someone already stated, you can get by with less of a heatsink but you really should slap that bridge to SOMETHING metal that is massive as you can get so that there is no heat build up on the bridge. Finally - put a fuse in the circuit to blow if the bridge rectifier shorts out. Fuse the whole thing at about 25-50% more than the current you get when you power the thing up and all is normal. Don't HAY WIRE something together that can give you a shock since most people that are "electrocuted" are in fact usually killed by shock that causes their heart to start beating screwy. Electric shock is dangerous.
I dont think radio shack carrys a bridge heavy enough for that application at all.
I would check a large battery charger if I had to scrounge 1.
To help that bridge live a longer life,I would at least vent that plastic box,perhaps install a old computer fan.The best thing would be to attach it to a large metal heatsink,like aluminum or copper or something to disappate heat away from it.Even standing out in still air,it will get to hot quick.
That degree of cooling would be a little over-kill. Remember, the duty cycle (the amount of time it is on verses the amount of time it is off) is very small. A second or two of ON time and a minute or so of OFF time. Mounting the bridge to a metal plate that is several inches square is adequate. Take the time to build a neat and tidy unit with solid tight connections like Anthony did. John has some very good comments about safety that should bee paid attention to.
Yeah, I stand corrected. I checked and Radio Shack now has either 400V-4A or 50V-25A bridges, neither of which is adequate for this. Also, John's quite right about the inductive kick-back this circuit can produce. Without a snubber, it can be many times the input voltage when the coil goes open. I used to run into this even with little 6VDC gearmotors.
Your words are wise indeed. I hadn't considered fusing the DC side of he circuit but that makes quite some sense.
Just as an aside, would half wave rectification be any better or worse for this job do you think?
I have used a piece of aluminium angle for a heatsink and a spring loaded momentary toggle for operating this thing... with a red missile switch cover of course!
The I don't have the bridge handy for the part number but what I asked for was something capable of 800PIV, and the best available would carry 35A. Handy too that with such a rating it came with lucar spade terminals;)
Bruce I understand the potential of an inductive spike but what sort of "snubber" would you use? A high value resistor... doide... capacitor??
A diode works well as a snubber. In this application I would use a 1N4007 (1 Amp, 1000 PIV) connected across the DC side of the bridge. Radio Shack has a 1N4005 (1 Amp, 600V, Catalog # 276-1104) that will work OK. Connect the banded end to the positive DC leg of the bridge.
To answer several questions with one thread - here goes:
Half wave or full wave probably doesn't make a lot of difference since the duration of the application of the DC power needed to magnetize the magnet is a very short time indeed. I think I calculated it once as being like about .004 seconds or some very small number like that but that was using magneto ring in the car. A half wave at 60 Hz is .008 seconds so a single half wave will actually do the job unless the growler winding has a lot of inductance in it and the peak current takes longer to get there than the inductance will allow to build up in that short a time. There is NO benefit from heating up the winding with excess current nor to heat up the bridge rectifier (known among wise guy engineers as a full wave bridge rectum finder). After you get it all built - try disconnecting one leg of the bridge (2 diodes) and see if it makes any difference in the strength of the magnet. If it does not then leave it disconnected since that will reduce the DC current by half while not affecting the peak current and it is the peak current that will establish the peak magnetic field.
I totally agree that a diode across the magnetic device is the best way to reduce the transient to near nothing. Interestingly since the "spike" is applied FORWARD across this suppressor diode, the breakdown voltage of the diode need only be at least twice the APPLIED DC voltage. I would probably opt for a heavier and hopefully FAST diode like one of the FR series FR=Fast Rectifier and they make them in 3 amp too which is probably what I would look for. After zapping a few magnets - shut off ALL POWER and feel the parts and see if anything is getting hot. If yes then think about using a heavier part there and also reducing the frequency of application. MORE IS NOT BETTER once the magnet is charged.
As an aside - Have you ever put a diode backwards across the points on your BATTERY HORN? Watch how the sparking almost disappears when you do that. Saves the points but does reduce the horn volume a wee bit but not much. It is the same idea as here except in the case of the horn motor winding the "spike" causes arcing at the points of the horn.