Debugging Shunt regulator

[seanchan00]

Hi Guys,

I am testing homebrew shunt regulator by Chris Greacen. Reference site here:

http://www.homepower.com/magazine/downloads_homebrews.cfm

When I connect it on a battery not being charged should the output load voltage be the same as the battery? The second LED at output of pin 3 of the NE555 is no on so I know the shunt is supposedly off but the load connected still light up at only 13 volts reading. Is the IRFZ40 shorted here. Please help as my tower is coming on and I will be setting up the windmill soon.

(:>)

SeanChan.

[drdongle]

If by "output voltage" you mean the voltage across the 1-3 Ohm load resistor or lamps, no you should measure 0 volts across the load when the regulator is off. Make sure that the FET is connected correctly and that the 22V Zener is also connected correctly. These FETs usualy fail "open" rather than shorted

[Flux]

Make sure you have at least a small load connected before you measure volts.

Multimeters with a high resistance (10M ohm) will often show volts from leakage but this will go away with a small lamp or similar connected. If volts are present with a small load connected then you have a problem.

Flux

[seanchan00]

Thanks for input.

A 20 watt flourescent light goes on when the battery is connected and the voltage measured is same as the battery. Does this mean the FET has failed open? I can measure zero ohm resistance across the the source and the drain so naturally the circuit is open al the time. Have I got this correct and all I need to do is replace the IRFZ40?

SeanChan.

[Flux]

Your FET seems to have failed to short circuit, not open, otherwise you would get nothing on the dump leads.

Most likely you have exceeded the 20V max gate voltage during installing. They are static sensitive, not as bad as often claimed, but take reasonable precautions and connect the gate lead in circuit last.

Flux

[seanchan00]

Thanks Flux,

I was just reading how to test mosfets with VOM and realised my mistake. It has failed short. I will have to wait for a new FET to see. Please clarify your last few words.  "take reasonable precautions and connect the gate lead in circuit last." Does that mean  solder the gate last? I wonder if I failed the FET when I first connected with no load then try to measure voltage across load. Did you see my post on desulphaters. I am using 4 of them productively and they anr only Alastair Couper originals with no modifications.

SeanChan (:>)

[scottsAI]

SeanChan,

If everything looks OK, but you still measure volts across the load resistor, like Carpe Vigor, Dr.D said, they fail shorted. Measure the Gate voltage, verify hi or low:

Low, the gate is off, the FET should be off, the yellow LED should be off.

HI the gate is on,  the FET should be on, the Yellow LED should be on, fix if not. Then adjust the 1k pot to turn load off. This pot needs to be adjusted to turn on at the voltage your battery is fully charged.

Have fun,

Scott.

[seanchan00]

Thanks Scott.

I know the 1K pot determines at what voltage the losd opens. Which way does it go to increase or decrease voltage shunted?

SeanChan (:>)

[Flux]

Try to work away from static, dry rooms, carpets and nylon clothing are best avoided if at all possible with FETs . I find it best to handle the FET by its cooling tab, make contact with your circuit with one hand and contact the fet tab with the other, then solder in the drain and source, finally touch the gate lead on to the gate pin and solder it. I have killed them by soldering the gate first. Once they are in circuit there should be no problem.

I doubt that you damaged the FET without a load.

I am pleased that you have had good results with desulphators, I haven't been so lucky. At first I thought I had chosen batteries with other faults than sulphating, but I have since tried some that are definately sulphated and I have not managed to increase their capacity. I am more inclined to think they may be good for prevention rather than cure.

Flux

[seanchan00]

Admittedly I am not tech savvy enough to give you meaningful data on increased battery capacity before and after desulphation. Only anecdotal and my gut feeling. I have one pair golf club discarded trojan T-105s measuring 3 plus V and after desulphation on a float charge able to function as a back up to drive a fan or light a 20 watt(12V) light. It is now being desulphated again after 1 year of abuse by my employee in the farm and it can easily retain its charge of 13.5V after 2 to 3days left standing. I have just scrounged a discarded 70 AH car battery from a mechanic with good looking plates but definitely sulphated beyond normal use. I hope to use it today as a back up to my two batteries doing 24hr relay with that 20 watt light. After a week of desulphation on float the general whiteness has faded somewhat and I can now spot large crystal of white globs on otherwise lead color plates. Three days ago it could not start the 20 watt light. Lets see today. The light has a longish wire from battery, 30', and the battery has to be at least capable of holding 13.5 to 14 V to overcome this voltage drop due to the wire lenght.

By the way, how do I "Measure the Gate voltage, verify hi or low" as advised by Scott?

SeanChan.(:>)

[ghurd]

Dear SeanChan,

I admire your resolution in all these matters very much.

Malaysia would seem to be a very difficult area to pursue these endeavors.

Your continued success on so many difficult projects is an inspiration.

Good luck,

Best regards,

G-

[inode buddha]

Or what I do is to short all the leads together with a locking pliers or a hemostat until the FET is fully installed. Most TV/radio repair guys simply wrap it all up in some scrap wire until its all soldered in. If all of the potentials are equal, then the juice will not flow enough to damage it.

[scottsAI]

SeanChan,

On the schematic, as you adjust the pot up, the voltage set point Lowers!!

The voltage ratio is reduced.

Vout = Vin (3.0+1)(3.0+1+3.3) (turned all the way up)

Thus, the voltage it turns on the load decreases.

Adjust the pot down, the ratio is increased

Vout = Vin (3.0)(3.0+1+3.3) (turned all the way down)

Thus, the voltage it turns on the load increases.

Have fun,

Scott.

[seanchan00]

 Still having trouble debugging Shunt Regulator.

Hi Guys,

I carefully followed advice from all of your inputs and replaced the FET. Still the same so I rang up an engineer for help. It seems I had interpreted the schematics wrong at the FET area. I connected both the Gate and Drain to the negative thick wire. After correcting it I saw the amber lED come on together with the load then the amber LED flicker out but the load is still conducting. Adjusting the 1 K pot did not make a difference. Have I burned the FET again? Measurement with the ohm meter show conduction between drain and source even with circuit off the battery.

SeanChan (:<)

Why open a fresh story on this?

Please continue it in the original story because spreading it out all over the board makes it impossible to track what you are doing.

COmments disabled to encourage you to follow through on the original.

Editor

Where's the Bug? | 1 comment (1 topical, 0 editorial)

Re: Where's the Bug? (3.00 / 0) (#1)

by Flux on Sun Sep 25th, 2005 at 02:40:21 AM MST

(User Info)

Yes I think you have killed the fet again by connecting it wrongly. If the led is switching on and off it is likely that the rest of the circuit is working.

I am not quite sure what you did with the fet, but they don't like wrong connections.

If you sit the fet on the bench with its plastic side up and the leads towards you, gate is on the left, source is on the right and the drain is in the middle.

Drain goes to your dump load, source goes to the circuit ground and the gate goes to the 100 ohm resistor to pin 3 of 555 and also to your led.

If you are using something other than the IRFZ40 then it must be a N channel fet.

Flux

So Sorry Ed. Please Delete that post. Thanks Flux and again you seem to have found my trouble here. I am a medical professional and all electronic stuffs are just beginning to make sense for me. What is assumed to be known basic electronic stuffs I still sometimes understand fully wrong. I just found a detailed Data page for IRFZ40 and the pictorial round schematic for the mosfet shows I have got the drain and source reversed. My drain is connected to ground at present and the source to load which was what I thought the schematic represents until now. Right now I am using IRFZ44 which the shop tells me it will be almost the same except it is 60 V and 35 Amp instead of 50 V and 50 amp.

SeanChan (:>)

[deloiter]

Here's a circuit board layout for Chris Greacen's Shunt Regulator in case you are interested. I have not tested it and I'm going fishing for a week...priorities. The main issue will be getting the trace image to size correctly for the IC pins when printed out. IC sockets are a good idea. The layout is sized for 1/4 watt resistors, small 100v ceramic caps and the 1K pot is a ceramic top-adjust type. Of course you could panel-mount the pot, switch and the LEDs. Good luck.  

[Flux]

If you swop the drain and source it will conduct through the internal diode that is in parallel with the fet.

I am not sure why you killed it in this case but it is always difficult to remove them from a circuit without damage.

I was surprised to find that the IR data sheet did not give the connections, it gave all the pin dimensions but not which was which.

IRFZ44 is N channel and should be ok.

Flux

[seanchan00]

Flux,

My FET is not shorted. The first one I removed was tested out of circuit as follows and shows it OK. I had to forcibly cut it off leaving short stumps on the FET.

Mosfet test :A second simple test can be done with a meter and a 9 V battery. First, short the gate to the source to discharge any stored charge there. Then put your meter on ohms and connect it across the drain - source. It should measure as an open. Briefly connect the 9 V across the gate (+) to source (-) , again, NMOS polarities, and the meter resistance should fall to a very low resistance, on the order of an ohm or less. Removing the battery will not change the reading, because in a good FET, Ciss will remain charged for a long time and keep the FET on. Most FETs come on at Vgs=2 volts or so.

So here I can iunderstand I have the gate and source is on (-) and (+) so it is probably turned on to allow closed circuit mimicking a short. Have I got it right? Will be turning the FET around and keep you Guys informed later.

SeamChan (:>)

[Flux]

I hope that it will work when you turn the fet round. I couldn't see any good reason why you should have killed it.

The reason why you lost control is that a fet has an inherent diode across it as a by product of the manufacturing process and with the leads reversed this diode was supplying your load. There may have been a very small change in the volt drop as the fet switched in parallel with the diode but you wouldn't have noticed it and the control would have been backwards as the gate is referenced to the source.

Hope it all works out.

Flux

[seanchan00]

Thanks Flux, and all you guys for your kind inputs.

I have tested on the battery and the load is off with the occasional flicker of the amber light for a fraction of a seccond.which I hope is the LM 723 working correctly so that only a sustained overvoltage trigger the NE 555 as described by Chris. Will be testing on a charging battery soon and then set the Voltage for shunting.

Still have to figure out how to measure that parameter.

Both my FET are OK. Will try to use the FET I cut out leaving only short stumps in future circuits. Cost a dollar "US" each. (:>)

SeanChan.

[Nando]

SeanChan:

Let's try the test procedure for this ckt.

Are you saying that the ballast (SHUNT) load has an indicator light across it ?.

If so, check to see if the Diode 22 V 5W is warm and connected wrongly ( backwards) the band should go to the Source and the ballast connection point, the opposite lead to ground.

Then if right, take a WIRE from the ground and the GATE of the MosFET to make sure that the GATE is at Zero volts, then measure the Source of the MosFET, it should read the Battery voltage,( Junction of MosFET source and Shunt load).

Measure the voltage from Source to ground, it should be Batt voltage if around 1 to 2 volts, then you may have the MosFET wired wrong, placing the Source to ground and the Drain to Bat voltage, turning the intrinsic MosFET diode on.

The MosFET PIN out for a TO220 case is, when facing the MosFET, left pin is GATE, Center PIN is Source and Right Pin is Drain.

Also Center PIN should connect to the Heat sink back metal if not isolated.

Make sure that the PIN3 NE555 LED diode is properly connected, it has a very small flatten edge near of the wires -- that is the Cathode and should be going to Ground ( Negative side of Battery)

Measure PIN 3 NE555 -- it should be below 0.3 volts

Measure PIN 2 NE555 -- It should be higher than 2/3 supply voltage ( bringing this pin momentarily below 1/3 Battery voltage the output ( PIN 3 NE555 ) should go high for about 1 second.

And if you keep PIN 2 shorted to ground the MOsFET should turn ON ( PIN 3 ON) and continues ON for 1 second after removing the PIN 2 short to ground.

DO NOT Measure PIN 6 & 7, BECAUSE the ckt has a 10 Meg resistor source and may bring the voltage down below the 2/3 bat voltage.

Regards

Nando

[seanchan00]

Thanks Nando, esp for the test procedure.

I had the ballast/shunt load on because I wrongly interpreted the schematic and reversed the source and drain connection. That has been corrected and now only the green led is on indicating the regulator seems to be working. The amber led flickers occasionally for a fraction of a sec. I am testing the set now and charging my battery from a constant 12V charger. The battery is now reading near 15 volts this morning after 12 hrs charging but despite playing with the 1 K pot to lower the voltage shunted the load just won't light up nor the amber led. How do I check what and/or where's the new bug? Is there a Bug? will the shunt work if the incoming(charger) voltage is not more than 12 V?

Will your test procedure pick out this problem?

Please help(:<)

SeanChan.

[seanchan00]

Hi Nando,

Just performed your test procedure and got the following results.

Diode 22 V 5W = OK.

Ground to FET gate zero V correct.

Ground to Source = Batt Voltage correct.

PIN 3 NE555 = 0.3 V correct.

PIN 2 NE555 =  0.2 V, not two thirds Batt Voltage.

PIN 2 shorted to ground mosfet didn't turn on the load!

Ground to before 10 k resistor between collector of 2n2222A and PIN 2 NE555 = 2/3 batt V.

Between 10 K resistor and PIN 2 NE555 0.2 V.

Suprisingly also almost zero V at pin of collector. Is this possible?

Where the bug now?

SeanChan.

[seanchan00]

Hi Guys,

Finally debugged the Shunt Regulator. It was the NE 555 as Flux indicated. Swapped the NE 555 from the desulphater and when I tested the desulphater for 45 milliamps to confirm everything is ok it blew the multimeter fuse. Lucky I did'nt connect it just touch the probe to the battery, saw the needle fly past 250 milliamp and immediately let go. The regulator load now comes on when I shorted Pin 2 to ground as in Nando's test procedure and remains on for a second after.

Now I am wondering how to calibrate so I accurately shunt at 15 Volt during charging. Is waiting for the load to come on then take the voltage of the battery, then adjust the 1 K pot by trial and error the only way? Seems very laborious. Any short cuts?

I will be making the next shunt regulator next.

SeanChan (:>>).