Bridge Rectifier

[Wizard564]

Hello,

I am a novice with electronic circuits, but attempting to expand my knowledge. I am trying to convert 14VAC to 12VDC with about a 15A load on a riding lawn mower for LED lights. I installed a 200V 60A Bridge Rectifer (NTE53016) and my output voltage is only 4VDC at full rpm on the stator. I also installed a 4700uf capacitor across DC + and -. Am I correct in assuming my bridge rectifer is too large for my project? Would a GBU1510 be a better choice to achieve my 12VDC output? Kind Regards!

 

[tronitech]

The bridge rectifier is way over rated but that doesn't matter. It will take a maximum of 200V at 60A. If you full bridge rectify 14VAC you should get approximately 20VDC. Something wrong somewhere?

 

[gophert]

Hello,

I am a novice with electronic circuits, but attempting to expand my knowledge. I am trying to convert 14VAC to 12VDC with about a 15A load on a riding lawn mower for LED lights. I installed a 200V 60A Bridge Rectifer (NTE53016) and my output voltage is only 4VDC at full rpm on the stator. I also installed a 4700uf capacitor across DC + and -. Am I correct in assuming my bridge rectifer is too large for my project? Would a GBU1510 be a better choice to achieve my 12VDC output? Kind Regards!

Make sure the bridge is connected to ~ symbol and output is + and -

otherwise, something is wrong with your mower, your bridge or other.

 

[Wizard564]

Thanks so much for your input!! I have the 14VAC rectifier terminals wired to frame and the other to the AC output of the stator. DC output of the rectifier - to frame and + is reading 4VDC. 4700uf Capacitor between + and - DC output. Can’t envision anything missing in this circuit.....

 

[tronitech]

Can you provide a simple drawing of your wiring?

 

[Wizard564]

Sure! I think I had a bad ground connection. I’m getting 12.8VDC across the DC output term. Of the rectifier now. I am still getting flickering of my LED lights even with the 4700uf cap. Wired as seen in the diagram. Wonder if I need a bigger cap for this circuit??

 

[gophert]

Sure! I think I had a bad ground connection. I’m getting 12.8VDC across the DC output term. Of the rectifier now. I am still getting flickering of my LED lights even with the 4700uf cap. Wired as seen in the diagram. Wonder if I need a bigger cap for this circuit??

your schematic does not match your text description above. Above you say that one lead from the AC source is connected to the chassis AND the negative output is connected to chassis (ground).

 

[tronitech]

You've posted a standard full wave rectifier circuit that doesn't match your description!

 

[Wizard564]

AC input from stator to AC term on rectifier. Other AC term on rectifier is connected to chassis. DC + output on rectifier is connected to LED light +. DC - output on rectifier is connected to - on LED lights. Please forgive my inexperience. Thanks very much!!

 

[AnalogKid]

Is the 4 V measurement with the load connected, or with no load? It might be that the alternator cannot supply the current you want.

15 A for LED lights is a lot. What are you lighting.

What is the value of the AC input when the output is 4 V?

ak

 

[Wizard564]

I think I resolved the voltage issue by repairing a bad chassis ground to the AC term on the rectifier. I am getting 14VAC input from the stator and 12.7VDC (No Load) or so at full RPM on the output side of the rectifier. I am still experiencing a flickering issue with the lights even on DC current. The lights operate at full intensity when at full RPM.
I am running about 130 Watts on my LED lights according to the specs on the packaging.

 

[gophert]

I am running about 130 Watts on my LED lights according to the specs on the packaging.

What? You expect a little lawnmower tractor alternator to put out 130W at idle? Good luck. We can solve design problems but we cannot change the laws of physics.

 

[Wizard564]

Seems to handle the load ok....just a flicker. The lights operate as intended at high RPM.

 

[Diver300]

You might try measuring the current. 130W seems a lot.

The capacitor is too small. I don't know what the frequency is, and you need to know the frequency to know what size of capacitor is best, but too big won't hurt. If you go to a really huge capacitor, the lights will take a noticeable time to hit full brightness when you start the engine, and will take time to fade out when you stop the engine, but you would have to spend a lot of money on capacitors for that to happen.

My rough guesses are that the alternator has 4 poles, and the idle speed of the engine is 1200 rpm. If those are right, the frequency is 40 Hz. With the full-wave rectifier, you have about 12.5 ms gaps between pulses from the rectifier, and the capacitor has to provide the current during those gaps. If you want to get 10 A with a voltage drop of 2 V during the gaps, then you need a capacitor of 62,500 uF

The capacitor size needed is proportional to the current, and inversely proportional to the number or poles, the engine speed and the voltage drop you can accept.

Make sure you get a capacitor that has a large enough voltage rating. I would say at least 25 V.

Something like this https://cpc.farnell.com/multicomp-pro/mclpr25v479m35x56/cap-47000-f-25v-alu-elec-snap/dp/CA08669

If you have a very small capacitor, such as the one you are using, the fast charging / discharging may overheat it an damage it. The one that I linked to is rated to have a ripple current of 9.2 A, and smaller ones will be lower. If you are using a 4700 uF capacitor, with a 10 A load, it will be running well above its rated ripple current, which will be around 2 A.

My suggestion is two of the 47,000 uF capacitors in parallel. They are only £5 each from CPC.

 

[rjenkinsgb]

Just to stress a point Gophert made earlier, in case it was not clear:

If one side of the AC input is grounded, the DC must be totally floating - no ground connections anywhere, direct or indirect.
(Or vice versa - for DC to be grounded, the AC input must be floating).

If you have grounds or any kind of interconnection at both the AC and DC sides of a simple bridge rec, it forms a short across one of the diodes and shorts the AC supply on one or the other half cycle.

That could be a cause of the flicker - or it could just be the idle speed is low enough to see the voltage variations..

 

[shortbus=]

Must be missing something here. Why not just wire to the battery? It must have a battery if it has an alternator in the flywheel of the motor.

 

[crutschow]

If one side of the AC input is grounded, the DC must be totally floating

Yes.
You cannot ground both the input and output of a bridge rectifier.
One or the other must float.

 

[Diver300]

Must be missing something here. Why not just wire to the battery? It must have a battery if it has an alternator in the flywheel of the motor.

Not necessarily. A pull-start lawnmower doesn't need a battery.

On another thread, I mentioned a motorbike that I had with a headlight running from AC directly from the alternator. Incandescent lights will work perfectly well on AC. One side of the lighting coil was connected to the frame. The battery and DC supply were tiny, and there was no starter-motor.

To connect LED lights to an arrangement like that you would have to have the DC part floating.

 

[shortbus=]

Not necessarily. A pull-start lawnmower doesn't need a battery.

They also don't have an alternator. They have an electronic ignition module try to take power from it and it won't run. That's why I said if the motor has an alternator it has a battery. Mopeds and such aren't the same as lawn mowers electrical wise.

 

[Diver300]

They also don't have an alternator. They have an electronic ignition module try to take power from it and it won't run. That's why I said if the motor has an alternator it has a battery. Mopeds and such aren't the same as lawn mowers electrical wise.

The last petrol mower that I owned had an ignition module and an alternator coil, both getting power from the same magnets of the flywheel. My mower happened to be electric start, but it had a pull-starter as well and would run without the battery. There was a version of the mower that didn't have the starter motor or the battery, but it did have some electronics to shut the engine off if the conditions weren't right (cutter running without the grass box in place etc) and those electronics were run from the alternator coil.

The manufacturers of a lawnmower engine will fit the electronic module to all engines, and a generator coil is likely to be an optional extra. The lawnmowers are made by others, who can use the generator coil however they want.

The OP could even have a mower that once had a battery and a starter, and he just wants to have the lights without the hassle of keeping a battery working.