Power Supply - Voltage drop under load

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Power Supply - Voltage drop under load

Post by Warf135 » Thursday 4th Nov 2010, 3:17

This started out as a G-Comm 13.8V 12A Power Supply. When I got it all that remained were the Transformer, 6800uf Filter Caps, 2N3055's and Load resistors. I built this circuit:
NOTE: Although this circuit works, it is not good under load...Please do not build it. Read further down the topic to find a much better regulator circuit!
Image
NormsPSU.jpg
(63.18 KiB) Downloaded 322 times
BR1 = Bridge Rectifier
C4,C5 = 50v 6800uf Electrolytic
Q2 = LM317 Adjustable Voltage Regulator
D1,D2 = 1N4007
R1 = 240 Ohm
VR1,VR2 = 5k Trimmer
VR3 = 5k Pot
C3 = 35v 10uf Tantalum
C2 = 35v 4.7uf Tantalum
C1 = 0.1uf Ceramic
Q3,Q4,Q5 = 2N3055
R3,R4,R5 = 0.1 Ohm 10 Watt

I used the original Load resistors and 2N3055's and the power would drop by roughly 0.2 volts per amp drawn (13.8v became 13.2v under 3 amp load). Voltage at the base of the 2N3055's is stable (only dropping 0.01volts under a heavy load), so replaced the 2N3055's and the load resistors with new ones. Now it drops by about 0.1 volt per amp drawn (13.8v now becomes 13.5v under 3 amp load)

Can anyone see any reason for this voltage drop? or anything I can do to prevent it? If I were to pull 10 amps (theoretically) it would drop by 1 volt and thats not acceptable.
Last edited by Warf135 on Saturday 20th Nov 2010, 5:01, edited 1 time in total.
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Thursday 4th Nov 2010, 23:17

You need a better regulator circuit.

1) Regulators work by basically being a variable resistance in series with a load. That's what the '3055s are doing.

2) Emitter resistors are needed to ensure that the current is shared equally between the pass transtators.

3) Current across a resistance = heat + voltage drop.

To regulate effectively, the regulator element MUST be able to "see" all of the output resistance. In the '317, the sense point is at the output pin, and that's where it will regulate to. When you draw current, the fact you've got resistance will mean a voltage drop. That's a limitation of the circuit.

To "fix" it, you'll need a different regulator - one that takes it's "sense" input from the output terminal, and not halfway through the circuit!

One other bad point - '3055s commonly go short or leaky collector to emitter... this will feed your radio with 25 volts. Radio will fail to make a noise, and generate smoke instead.
You really should have an overvoltage protection circuit in there, especially if it's feeding expensive / not easily repaired equipment.
Do a google search for "crowbar circuit".


Hope this helps
Sue

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Re: Power Supply - Voltage drop under load

Post by lbcomms » Thursday 4th Nov 2010, 23:57

A better design (from the national semiconductor Analog devices master data book):

Regulator:
Image

Protection circuit:
Image

Happy soldering...

PS: If you are using this to power a transmitter, I'd add an LC filter between the top of the SCR and the output terminal (basically a heavy duty series inductor, bypassed to ground on both sides with 0.1uF capacitors) to stop RF getting into (and possibly confusing) the regulator IC.

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Friday 5th Nov 2010, 0:51

Hi Sue, Thanks for the detailed reply. I had planned to build an over volt circuit once I had got the voltage stable.

Noting what you said about my circuit taking its "sence" from the middle of the circuit and the design of the original circuit, which does not have the 2N3055's, I note that it is R1 in my circuit that is the "sence"... it sees the voltage at the output of the LM317 and adjusts the voltage to keep it constant... So a plan comes to mind (this may be a bad idea, and maybe my brain is doing overtime), But what if I was to disconnect R1 from the output of the LM317 and connect it to the load side of the load resistors (R3,R4,R5). Then it would be senceing the output and adjust the circuit accordingly??? -or maybe not!
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Friday 5th Nov 2010, 1:11

No, that won't work. The '317 uses a fixed voltage between ADJ and OUT (1.25V from memory, haven't used one in ages) and the 0.7V B-E voltage needed by the 3055s will muck this up big time.

You need a regulator that is designed to be used with pass transistors - the 317 is NOT designed to do this, it has been made as a standalone regulator.

Keep the 317 for another project where you only need 1.5 amps. Buy a '723 IC (they cost about a dollar here in Oz) and a smaller pass transistor (a common TIP31x works great) and build the circuit shown.
You can re-use your 2N3055s and their big emitter resistors.

Total cost would be under $5, or under $10 if you include the overvoltage protection.

Sue

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Saturday 6th Nov 2010, 0:44

Ok, LM723 and TIP31 ordered... lets hope I build it properly and it works right!

Is there any alternatives to the SCR and Zenner diode on the Over-Volt protection circuit? I'm having trouble locating 2N6399 and 1N4744A. What about if I put 3 x 5.1v Zenners in series...would that work instead of the 1N4744A?
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Sunday 7th Nov 2010, 2:45

What about if I put 3 x 5.1v Zenners in series...would that work instead of the 1N4744A?
That will work OK, as long as you connect them correctly (all the same way, i.e. AK-AK-AK).

The SCR is not critical either - anything that can handle 10 to 15 amps for a few milliseconds (the time it takes for the fuse to blow) will be OK.
You shouldn't have any trouble getting them from the likes of RS components, Farnell, or most other "real" suppliers (i.e. not Tandy/Radioshack or your local TV / Hifi shop)

In the UK: Farnell part codes are as follows: 1459025 (cost = 146p) for the SCR, and 1612368 (cost = 4p) for the zener. It'll cost you more for postage :lol:

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Sunday 7th Nov 2010, 7:42

Yes, Postage costs are the stupid thing here I found the diode on a site yesterday..the diode was 30p, but they wanted £6.99 for the postage... its not so bad if you are buying lots of items I suppose, but for single items its a killer...

I have now found and ordered the diode from China. For 10 diodes + postage it came to less than £2, which is a lot less than I could buy it in this country with postage :banghead:
I have also found an SCR for a reasonable price too. Its a 2N6395 which is exactly the same as the 2N6399, but is only rated at 100v rather than 800v for the 6399. So all should be good, Hopefully!
Norm

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Tuesday 9th Nov 2010, 4:36

Hi Sue, I'm getting close to building the circuit now and have been studying the schematic for a while now. There are a couple of things I cant work out about the circuit:

I cant work out what the 270 ohm resistor and the .15 ohm resistors do? It seems to me that the 270 ohm with the .15 ohm's are basically parralleling the 680 ohm (current limiting) resistor?
and with the 2N3055's each having their own load resistor (.1 ohm 10 watt) are the .15 ohm resistors needed? I'm only thinking that the less resistance in the output path is better???

I've worked out what all the other components do but cant figure out what these three do.
reg1.jpg
pic of regulator circuit
reg1.jpg (20.51 KiB) Viewed 16547 times
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Tuesday 9th Nov 2010, 23:56


***** Download the full datasheet from the National website if you haven't done so already. *****
I cant work out what the 270 ohm resistor and the .15 ohm resistors do? It seems to me that the 270 ohm with the .15 ohm's are basically parralleling the 680 ohm (current limiting) resistor?
These form an over-current sensing circuit - as current is drawn from the circuit, a voltage appears across the two 0.15 ohm resistors, directly proportional to the current drawn.
This voltage us divided by the 270 / 680 ohm resistors, and then used to reduce the drive to the pass resistors if the output exceeds a certain value (about 12 amps with the values shown).
This in turn protects the supply from accidental overloads such as short circuit wiring or trying to run a 500 watt amp from it :)
I'm only thinking that the less resistance in the output path is better???
The only places resistance is critical:

1) Between the bottom of the 0.15 ohm / top of the 2.7K resistors, and the output terminal.
The IC cannot sense voltage drop there, so this needs to be a thick, heavy wire.

2) Between the negative of the main filter cap and the negative output terminal.
Again, the IC cannot sense voltage drop there, so this also needs to be a thick, heavy wire.

Voltage drop elsewhere - even several volts - can be ignored. It will be sensed at the top of the 2.7K resistor and pin 10 will increase its output to exactly compensate.
You just need to make sure that the low ohm resistances can handle the power (V squared over R, etc) and that you have enough voltage to spare at pins 11+12 and the collectors.

Cheers

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Wednesday 10th Nov 2010, 2:10

This voltage us divided by the 270 / 680 ohm resistors, and then used to reduce the drive to the pass resistors if the output exceeds a certain value (about 12 amps with the values shown).
This in turn protects the supply from accidental overloads such as short circuit wiring or trying to run a 500 watt amp from it
I dont wanna teach monkeys how to eat bananas, but surely a 12A fuse would offer the same protection? :idea: - My power supply design (although flawed by the wrong regulator circuit) uses a 12 amp fuse connected to the +side of the Bridge Rectifier which should blow if the current draw is too high (at least, that was the idea :lol: )

I have the national datasheet for the LM723 (June 1999 edition), it shows some circuits, but none with the 2 x .15 ohm resistors in and thats what confused me and so I needed to double check

Thanks for your info...Hopefully I can get building now (when the final components arrive) and should not need to bother you with anymore silly questions snooze
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Wednesday 10th Nov 2010, 23:38

I dont wanna teach monkeys how to eat bananas, but surely a 12A fuse would offer the same protection?
NO, on four counts:

1) A fuse takes time to blow - with a short (severe and catastrophic overload), anywhere from 5 to 50 milliseconds, depending on ther type of fuse and the severity of the overload. That big filter capacitor can deliver hundreds of amps in that time - this can easily destroy pass transistors if the short was quick enough, such as dropping a screwdriver across the output terminals. When 3055s blow this way, they go short E-C. The user replaces thefuse after clearing the short... hello 25 volts :-)

2) A fuse takes a LONG time to blow with mild overloads - up to 30 minutes for a 150% fault. This can result in things like burned out transformer windings.


3) A fuse has to be replaced if it blows. Current limiting will just limit to safe levels, and when the overload is cleared the power supply starts working again. No replacement parts needed.

4) With high currents, any slight resistance will generate heat. This reduces reliability where a fuseholder is used, due to fuseholder-to-fuse resistance. Modern surface-mount electronics solder the fuse to the PCB to overcome this (they look like oversized ceramic capacitors). A fuse should only be used to protect against "internal" faults in equipment, and NOT as a protection against accidents.

The only downside to current limiting is that it can generate a LOT of heat in the pass transistors under sustained gross overloads (for example, insulation breakdown in wiring at an unattended radio site). A well designed supply will have a thermistor on the heatsink that holds the pass transistors (or chopper transistors in a switchmode design). Have a look inside a PC power supply next time you get the chance to pull one apart - the thermistor is the component with thin wires, wrapped in black heatshrink, attached to the heatsink. The thermistor is connected in such a way as to shut the regulator down if the heatsink gets too hot.

Hope this helps
Sue

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Thursday 11th Nov 2010, 0:06

Hi Sue, Thanks again for your detailed info... I'm certainly learning a lot while building this power supply.

I am planning to make this power supply the best it can be, so after I've built the new regulator and the over-volt circuit, I will look into some kinda thermal shutdown circuit to add to it.

Also I'll be looking into filtering the mains input to the transformer too. (I know to be very careful with mains electricity as it can kill me) I have read something somewhere about doing it... I'll have to look it up again.

This power supply is taking me some time to get right, and at the end I could have probably bought a good second hand one for the price its cost me, but the most invaluable thing and something that I cant buy is learning, so I thank you for being paitent with me.
Norm

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Re: Power Supply - Voltage drop under load

Post by lbcomms » Thursday 11th Nov 2010, 0:30

Also I'll be looking into filtering the mains input to the transformer too. (I know to be very careful with mains electricity as it can kill me) I have read something somewhere about doing it
With that design, don't bother. It's only needed for switchmode (chopper) types - with a linear type such as yours, it won't help anything much.
Switchmodes add all sorts of noise and distortion to the mains, that's why they are almost always filtered.

Input filter design is NOT a simple task, believe it or not there are computer CAD programs for designing them (just the filter, not the whole supply).
Have a look at :

http://switchmodetec.com/tech_articles/ ... sign_1.pdf

Hope you like algebra and big equations :-)

Sue

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Thursday 11th Nov 2010, 7:22

Very interesting... A lot of Linear power supplies I've seen have a large capacitor across the mains switch, which I assumed was a filter :? This is what I intended to fit to mine

Also lots of them have a fuse on the mains side of the transformer. I had thought about putting one in mine, but its already got a fuse in the mains plug, so is it really needed to have another close to the transformer?
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Friday 12th Nov 2010, 21:25

lots of them have a fuse on the mains side of the transformer
It depends on several factors. What size fuse is in the mains plug ? I've never seen that myself except for some really old UK imported equipment. Here in Oz, only a licensed electrician can put a plug on an appliance, and new appliances must be fitted with a molded (non removable) mains plug before they can be sold.

10A @ 14V = (10 x 14) = 140 watts. 140 watts divided by 240V = 0.6 amps primary current. There will be an inrush (due to the main filter capacitor charging up when it's first turned on) so use a "slow blow" or delay type, no more than 1.5 amps.

In addition, if there is not one already fitted inside the transformer, I'd add a thermal fuse in series with the active, and attached to the actual transformer. That way, if you get a "shorted winding" fault in the transformer, the power will be cut off before it catches fire. Transformers sold here in the last 10 to 15 years have these fitted internally, don't know anything about the UK regulations except they are a lot less stringent than the rules over here.

The cap across the mains switch was (over here anyway) used to filter out the control signals placed on the mains to control "off peak" electric hot water units. These were phased out over here in the early 90's, don't know if you use (or have ever used) the same system over there.

Cheers

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Saturday 13th Nov 2010, 4:44

The fuse in the plug will be 3 amps minimum (either 3, 5 or 13 amps are the standard ratings for mains fuses in plugs over here), so I think I will put a smaller fuse in line with the transformer and I will also look into the thermal fuse you mention. the mains lead at the moment goes straight into the transformer (through a switch)... there is evidence that the previous owner has re-soldered the connections to the transformer and also evidence of some old glue on the transformer which may have been holding a thermal fuse at some point. with that and the fact that the original regulator circuit were removed would probably indicate that the power supply was overloaded at some point.

Thankfully the transformer is not damaged, and its working ok - ie. not getting hot and the secondary is putting out the correct voltage.

Over here is the same as with you in that new products must have moulded plug fitted, but you dont have to have a qualified electrcian to change a plug if required. All second hand electrical equipment sold is supposed to have an "electrical safety test" otherwise known as a P.A.T. or "Portable Applience Test" (including a 1000v insulation test) to check that the item is safe to plug in before its sold. Items sold on ebay and in car boot sales/markets usually are not tested as thats classed as a "private sale" therefor its not required. Some sellers will write their own sticker and put on the device which gets me very angry to think that they would put someones life at risk just to make a few quid. I have reported a few sellers when I've seen items that clearly fail on visual inspection alone (ie. outer insulation damaged on mains lead or whatever)

We do have what they call "Economy 7" over here, thats when between midnight and 7am the electricity switches to a cheaper rate for charging night storage heaters ect. I'm not sure, but I think that its controlled by the electricity meter on a timer... certainly the meter makes a loud click around midnight every night.

I'm just waiting on the diode for the over voltage circuit to arrive (could take a while, coming from china!) then I'll be building this supply :D


Edit: I've been looking at thermal fuses to put on to the transformer... they range anywhere from 100 to 150c when used in transformers from what i can tell..I have found one at 109c. do you think this will be suitable?
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Tuesday 16th Nov 2010, 0:01

Hi,
Delayed reply (I've been up in Brisbane for a few days with no internet)
the mains lead at the moment goes straight into the transformer (through a switch)
I'm assuming here the supply has a metal case , connected to mains earth...
We do have what they call "Economy 7" over here, thats when between midnight and 7am the electricity switches to a cheaper rate for charging night storage heaters
It's similar over here, but the times vary slightly depending on the suburb, presumably to even out the total load on the system.
A timer next to the meter switches on a second circuit with its own meter, connected to a hot water heater. The second circuit has a much lower rate - about 1/3 of the "peak" or normal rate.

It's a fairly common (though totally illegal) mod to wire a regular power outlet to the second circuit, and use it to run high powered appliances such as tumble dryers from them in the middle of the night and save some $.
I've even seen several examples where people wire up "special" points that changeover with a relay (i.e. make nights= low rate, days = normal rate) and use them to run "24/7" power hogs like fridges and freezers.
Mostly ex-uni acquaintances who studied electrical engineering :D
I have found one at 109c. do you think this will be suitable?
Yes, it's not critical at all. Normal operation would be under 50 deg c, a fire wouldn't start until well over 100, so anywhere from 80 to 120 deg c will be OK.

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Tuesday 16th Nov 2010, 0:55

I'm assuming here the supply has a metal case , connected to mains earth...
Yes.
It's a fairly common (though totally illegal) mod to wire a regular power outlet to the second circuit
on our economy 7 system it puts all of the electricty on the cheap rate at night, so the normal power outlets are cheap too, so no "mods" needed :D (just a timer to start the washing machine after midnight :) ) the heaters are on a different circuit so they are only powered at night.
so anywhere from 80 to 120 deg c will be OK.
Cool, I'll fit that in asap!
Norm

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Saturday 20th Nov 2010, 4:25

Regulator and Crowbar circuit built and power supply working well :dance: only dropping by 0.04 volts maximum under heavy load :D

I have one slight issue though; if a load is connected whilst the power supply is turned on the fuse blows... if the crowbar circuit is disconnected this does not happen (the fuse is inline at all times). if the power supply is turned off and the load connected then there is no problem at all, increase/decrease in load (eg. Tx/Rx) is fine. It seems that the crowbar circuit is spiking if a load is connected while the supply is on. Is there anything I can do to stop this from happening? (most older radios turn the power on and off by cutting the power line, which would probably cause the fuse to blow too -I have not tested for that though)

Here is the circuit I built:
nreg.jpg
(43.9 KiB) Downloaded 277 times

(*cannot get pic to display on forum*)
I have 3 2N3055's and load resistors instead of 2 (forgot to add the extra one to the diagram)

EDIT: I have been doing some research into this and have noted that some crowbar circuits have an extra capacitor across A-K on the SCR to reduce false triggering. Some use 100n ceramic capacitor and others I've seen use 47uf 25v electrolytic... but which is best? or maybe I'll just add both! :wink:
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Monday 22nd Nov 2010, 1:03

if a load is connected whilst the power supply is turned on the fuse blows... if the crowbar circuit is disconnected this does not happen
There are two possibilities - both related to charging time for the capacitors. These need time to settle when a circuit is powered on.

1) The regulator is letting through a brief spike of high voltage when you first turn it on, and the crowbar is actually doing its job. The spike is being decoupled (filtered out) in the radio be the DC input choke + capacitor combo and therefore the radio doesn't get damaged. A different radio with lesser DC filtering might not be so lucky.

2) The crowbar itself (specifically the 0.1uF cap) might need a few milliseconds to settle, and is falsely triggering.
and have noted that some crowbar circuits have an extra capacitor across A-K on the SCR to reduce false triggering
Yes, I've seen that, mainly on supplies that are powering inductive loads such as solenoids. These loads can generate a "back emf" voltage (google it, including the quote marks, if you don't know what it means) that can cause false crowbar triggering.
but which is best? or maybe I'll just add both
Experiment by all means, but it probably won't help you.

It's generally considered bad to connect a load before a supply has settled down. PC power supplies, for example, have a "power good" wire that changes state only when the supply has settled. Until this line changes state, the PC will NOT attempt to boot up. All modern electronics works this way - you'll notice that your mobile phone, DVD player, computer monitor, etc have a delay when they are first turned on to when they are fully functional.

A start up delay is easily implemented. You'll need a heavy duty relay (those 1" cube car headlight types are great) , a 10K resistor, a 1K resistor, a 10 volt (or higher) 4700uF cap, a 1A NPN darlington like a BD681 (lots to choose from), and a diode like a 1N4004.

The normally-open relay contacts go in series with the output terminal of the supply - this must be the LAST thing before it connects to the load.
The 25V goes to the 10K, then a 1K, then the base of the darlington. Collector to -ve of coil, and +ve output terminal goes to the other side of the coil.
Emitter of darlington goes to ground.
Diode connects to the darlington - anode to emitter, cathode (the marked end) to collector.
The capacitor connects between ground and the 10K / 1K resistor join point.

What does this do? The cap delays the buildup of voltage at the 1K and therefore prevents the relay from turning on (and connecting the load) until the supply has had time to settle.
The values shown should delay it for a second or so, which should be plenty. To change the time, alter the 10K resistor or the capacitor values.

If you buy a modern supply, you'll find this already done for you (although with MOSFETs instead of relays).

Cheers

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Re: Power Supply - Voltage drop under load

Post by Warf135 » Monday 22nd Nov 2010, 10:33

Thanks for interesting reply. Sorry, But I seem to have given the impression of the wrong problem.. the problem I have is that I turn the power supply on and all is fine... I can leave it for maybe 5 mins so its definately settled... then I connect the load and thats what blows the fuse...but if the load is connected when the power supply is turned off, and then I turn on the supply its fine.

This is why I'm thinking that adding the extra cap to the crowbar may help... Certainly adding the electrolytic cap is supposed to slow down the action of the crowbar slightly and this may be enough to overcome the spike when the load is connected... I'll experiment in the next few days and see what happens. (good job I bought 10 fuses! :lol:)

I may still build the delayed startup circuit that you describe as that sounds interesting.

EDIT: adding the 47uf 25v electrolytic capacitor has somewhat fixed the issue... its still very sensitive, but I can now connect a load when the power supply is on without it blowing, however if i disconnect and re connect the load quickly the fuse still blows... I'm thinking of maybe replacing the fuse with some form of re-settable trip, I have a 5 amp one here but thats no good... i hope they make 12 amp ones!! (to keep replacing fuses could work out expensive :roll: )
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Re: Power Supply - Voltage drop under load

Post by Warf135 » Tuesday 23rd Nov 2010, 7:01

Just ordered this 12a panel mount circuit breaker... should do the job nicely I hope!
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... K:MEWNX:IT
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Re: Power Supply - Voltage drop under load

Post by lbcomms » Friday 26th Nov 2010, 0:44

then I connect the load and thats what blows the fuse
You might have to reduce the value of a capacitor somewhere.

The regulator will always try to maintain 13.8V at the output. If a sudden load is connected, it may try and overdo it, causing a spike a few milliseconds layer and tripping the SCR.

To see if this is what is happening:

Connect a peak hold circuit to the output terminals. From the +, a diode in series to the + side of a 100uF capacitor. The - side of the cap to the - output terminal. Add a 10M resistor across the cap.
Disconnect the crowbar, and place a multimeter on the volts range across the 100uF cap.

Now connect your load.
If the meter reads over 13.8V, then the cap scenario is correct (the regulator is spiking).
If the meter reads under 18.8V then the crowbar circuit itself may be the culprit.
PS: Just i case it is spiking, don't use an expensive radio as the load!

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Warf135
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Re: Power Supply - Voltage drop under load

Post by Warf135 » Friday 26th Nov 2010, 5:16

Thanks for the info... I have tested it with the circuit you suggest and the voltage is absoultely stable and does not change at all when load is connected/disconnected. At first, I thought it may be my digital meter smoothing out any spike, so I re-tested with analogue meter with the same results, the needle never even twitched when load was connected.

(voltage at PSU output 13.8v, voltage at test points on peak hold circuit 13.2v - approx 0.7v taken by the diode)

So thats a good thing really, meaning that my regulator circuit is working very well, but then why is the Crowbar triggering??? :? Certainly adding a 47uf 50v cap accross A-K on the SCR has improved it, but its still very sensitive...
Norm

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