Need help from the electronic gurus

[skotl]

Hopefuldave, could you dumb it down a bit more for me, so if my tranny could make 15-20A continuous at 50V would this rectifier do the job if I added a large heat sink and fan or do a need a bigger rectifier to start off with as well as a fan and heat sink? Thanks.

If you forget the power factor for a minute then you're looking at 1KVA (20A * 50V).
In an earlier post you said that the rectifier is rated at "1000V and 75A capacity" - I'm just a little worried about the word "and" - as that could mean "or" or, literally, "and".
1000V * 75A = 75KVA, which is way, way, way more than you need. If the spec is 1000V * 75A.
If it is 1000V or 75A then that's a different matter...

Do you have a product code for the rectifier?

 

[Ed.]

Since I started this post I have learnt that it appears that it is 1000V "or" 75A anyway it is a FSB7510A, it's in the pic connected to the transformer in post 1. I took it at face value without think too much about it when I bought it, but it definitely won't do 75Kw.

 

[Memmeddu]

Does someone think that this transformer might be too small for a 2600W load?
39x20 is roughly 800W

 

[Rig Pig]

Alternatively a nice power supply for a beefy gainclone amp!
And buy a cheap switch mode power supply off eBay for your heater.

 

[Hopefuldave]

Hopefuldave, could you dumb it down a bit more for me, so if my tranny could make 15-20A continuous at 50V would this rectifier do the job if I added a large heat sink and fan or do a need a bigger rectifier to start off with as well as a fan and heat sink? Thanks.

It'll do the job if the spec' you've quoted is right - where did you find the info on it? It already has a heatsink built in, I assume the UPS maker sized it for the current / load the transformer could supply (they wouldn't spend extra and they wouldn't want it popping in use!) but it's likely it was fan cooled as a UPS won't have a lot of free air inside? A bigger heatsink certainly wouldn't hurt, though.

**--BUT--**

Something to be wary of is the initial inrush current if you're putting capacitors across the DC from the rectifier, the UPS makers will have built in a soft-start circuit to limit it to what the transformer and rectifier can stand - at switch-on the capacitors will have zero volts across them and look like a dead short to the transformer and rectifier, this is when poorly-designed power supplies go bang (the rectifiers melt down and turn into a short circuit) and the capacitors do an impression of a chaff launcher. The soft-start can be as simple as a hefty wire-wound resistor in series with the DC out from the rectifier that has a hefty relay to short it once up to running voltage - how the relay coil's fed can be quite crude (e.g. just a time delay), but a good designer will have put in a circuit to monitor the capacitor voltage and switch in the relay when up around 80 - 90% of the working voltage. At high power / current levels PSU design gets a bit complicated

Dave H. (the other one)

 

[Hopefuldave]

Does someone think that this transformer might be too small for a 2600W load?
39x20 is roughly 800W

I wonder that too - how hefty is the transformer? applying the Rule of Toe how many would it break if dropped on them? That usually gives a good clue
The 9 KVA max. transformer in my rotary converter takes Best Boy and me to lift (just, not carrying it far), or the baby forklift...

Dave H. (the other one)

 

[anjum]

have you still got the inverter/ ups case? The label on there should give you a good idea of the power rating of the tranny.

 

[skotl]

Since I started this post I have learnt that it appears that it is 1000V "or" 75A anyway it is a FSB7510A, it's in the pic connected to the transformer in post 1. I took it at face value without think too much about it when I bought it, but it definitely won't do 75Kw.

F is Fast Recovery Bridge Rectifier
SB is the package type
75 is Amps
10 is 1000 Volts


...but I don't believe it's 75kVA, just like you don't

 

[voipio]

Datasheets for the bridge rectifier are like hens teeth! So, guessing here, 1000V will probably be the Vrrm (Maximum Repeating Reverse Voltage) rating, and 75A the maximum average forward current. In a typical circuit, a bridge rectifier, when operating, will charge the reservoir capapcitor up to the peak of the incoming voltage, so for 240V AC input, the capacitor will charge to around 340V. On the next half cycle, the two diodes that were previously conducting, will now have the capacitor voltage plus the peak of the next half cycle across them so, for mains, this would mean there would be 680V across them. 1000V would then seem to be a reasonable rating for duty at up to 240V AC input, giving a 50% margin and enything less e.g. 39V AC input would be fine.

As for the transformer power rating, I have a 3000VA autotransformer which is rated at 3kVA for 25 minutes in every hour, and it weighs 20.7kg. Autotransformers tend to be around 20% lighter than a separate primary and secondary-type transformer, for a given rating. At a guess, the UPS the original transformer came from, only used the transformer to charge the batteries, and not to supply the rated UPS capacity when mains power is present.

 

[anjum]

F is Fast Recovery Bridge Rectifier
SB is the package type
75 is Amps
10 is 1000 Volts


...but I don't believe it's 75kVA, just like you don't

The power rating or dissipation is the volts DROPPED across the device, for a diode it's typically 0.7v or less for a schottky and it's AC not DC so you can factor that in as well to lower the dissipation. So the dissipation and the size of the heatsink will be a lot less!!
The power dissipated by the potential load that the bridge can support depends on the manufacturers honesty!

 

[voipio]

Bear in mind, for a bridge, there are 2 x diode forward voltage drops, which could be 1V per diode close to the rated current, so could be up to 2V total. A realistic maximum design forward current might be 50A for this device, giving it 50% margin. From a mains input direct to the bridge, 50A at 240V is 12kW, with the bridge rectifier dissipating around 100W in the process.

 

[Ed.]

Looking further into this from all the replies, It looks like the rectifier amps and the voltage ratings are not related in the way that I thought. When I saw those two values I automatically assumed that the values were connected as in V x I but it appears that the voltage is a forward breakthrough voltage and not related to the amp value that the rectifier can handle, so at this stage I have no idea what amps that rectifier will handle at the voltage of the transformer. I suppose that if the "Magic smoke" escapes, that will mean I need a bigger rectifier!

Not even sure of the output of the transformer either. It may not put out as many amps as I thought that it would. "Hopefuldave" said it may put out 20A, but even if it just puts out 10A it will still do the job as I just want it to top up the batteries a bit whilst the heater is running, so it does not need to supply all of the heaters requirement provided I use the batteries. For the use I want to use it for it is which is mainly to heat up bars of metal for hardening and so it it only be on for about 5 minutes at a time, the 3-4 batteries will cope with that drain own their own anyway.

I have ordered up a amp meter so when it arrives I will just hook it up to the 4 batteries and the tranny and see what amps are produced. Probably not the most reliable way of testing but might give a rough idea. As I said before I have very little experience in this sort of stuff so just "winging it most of the time".

Hopefuldave, As for the Rule of Toe, I would not want to drop this one on my foot from a bench as it weighs 10Kg so it would probably break most of them.

I was comparing this tranny to one I pulled out of a 600W microwave oven which was much smaller and I converted that one to a battery charger, and the most I saw out of that one was 19A when I connected it a fairly flat 130A battery, but that one was at 13V output when first connected to the battery and not the 54V unloaded that this one is putting out with a capacitor. Which works out at 247W, maybe with a heavier load it might have got a bit higher and it drew about 2.5A on the primary coil at 240V which is right for about 600W.

When I first started this thread I was mainly concerned about charging the batteries with the 100Hz 39V (no capacitor) which apparently isn't an issue so might do without the capacitor and use 3 batteries, or use a capacitor and run 4 batteries and just tap off on 3 of them.

The only other transformer I played around with was a 140A arc welder that I converted to a spot welder, and that one I maxed out on the primary coil at 16A so that one used 3600W, no idea what the output amps are though but I will assume it is a reasonable amount.

 

[Hopefuldave]

As for the transformer power rating, I have a 3000VA autotransformer which is rated at 3kVA for 25 minutes in every hour, and it weighs 20.7kg. Autotransformers tend to be around 20% lighter than a separate primary and secondary-type transformer, for a given rating. At a guess, the UPS the original transformer came from, only used the transformer to charge the batteries, and not to supply the rated UPS capacity when mains power is present.

That's pretty consistent with my 9 kVA continuous (oil cooled) isolating transformer, the core once pulled out of the tank is around 90-100 kg.

Dave H. (the other one)

 

[Hopefuldave]

Hopefuldave, As for the Rule of Toe, I would not want to drop this one on my foot from a bench as it weighs 10Kg so it would probably break most of them.

Probably close to a kilowatt in intermittent duty then, I usually base guesses on 100 watts per toe...

Dave H. (the other one)

 

[Memmeddu]

Probably close to a kilowatt in intermittent duty then, I usually base guesses on 100 watts per toe...

Dave H. (the other one)

Generally microwave oven transformers are around 6kg
I believe this one could be 1200W but designed to have little bit of error percentage so 800W
Back to your oil cooled transformer I believe it's very difficult to move .
I have an air cooled buzz box with very poor duty cycle and is about 34kg without the leads and it's probably a 4kw transformer

 

[Hopefuldave]

Generally microwave oven transformers are around 6kg
I believe this one could be 1200W but designed to have little bit of error percentage so 800W
Back to your oil cooled transformer I believe it's very difficult to move .
I have an air cooled buzz box with very poor duty cycle and is about 34kg without the leads and it's probably a 4kw transformer

Difficult enough to move that I've twice wrecked my back for a week or two... And it's on wheels, it's getting it over any steps and ledges that hurts!

Last time I went to move it, Best Boy told me to get the forky thing, if only it were that easy...

Dave H. (the other one)

 

[Memmeddu]

Difficult enough to move that I've twice wrecked my back for a week or two... And it's on wheels, it's getting it over any steps and ledges that hurts!

Last time I went to move it, Best Boy told me to get the forky thing, if only it were that easy...

Dave H. (the other one)

It has a big advantage
If someone steal it you will reckon the thief very easily ,he will be at the hospital for his back

 

[skotl]

...I suppose that if the "Magic smoke" escapes, that will mean I need a bigger rectifier! ...

Just bear in mind that diodes are as likely to fail short circuit as they are open circuit, which your transformer and everything else upstream of it aren't going to enjoy.

To be honest, if it was me I'd buy another rectifier that I was sure of the specs or even four heat sink-attached diodes and make my own.