Basic question on amps & fuses (Gyraf Pultec)

Hi Guys,
Ive got a few questions regarding amp draw on my DIY dual Pultec. Just so you know, I have worked on several projects that run off mains, but never one with high voltages on the circuit. The pultec project is not hugely well documented like some other popular DIY builds, and with no BOM to work from, Im having to really think about amp draw for the first time.

The fuse on the PSU / Make up gain PCB is marked as 4AT. I guess this means 4 amp. Im a little unsure on this as the schem seems to say something different.

http://www.sowter.co.uk/schematics/gyrafeq.htm

First, of course, I need to validate that the above assumption of 4a is correct, otherwise the following calculations will be invalid. However, assuming the PSU does require a 4a fuse, does the following hold true?

1) I want to take the power from the mains to a switch before the torroid. As Im building a dual unit, this means I need to take live & neutral ‘twice’, so that after the switch I can go to 2 torroids. The cable I have is only rated to max of 6 amps. So am I best running 2 sets of cable in parallel (one for each torroid, assuming draw of 4amps each?).
2) I have a chunky on off toggle switch which is rated at 10 amps @240v, so I assume this is fine?
3) When I choose a fuse for my mains socket, I guess I need to simply double the draw of each PSU circuit. Given that Ive also got a little power lamp to run, will an 8amp fuse be sufficient (lmap is 24v, but dont have any other info)?

I know these are high voltages and potentially lethal circuits, which is why I don’t want to rely on a single assumption, so appreciate any thoughts. Im know some things about DIY electronics, but certainly don’t have the overall grounding in the subject matter to have much confidence in my logic!

Cheers
Jake

snipsnip said:

The fuse on the PSU / Make up gain PCB is marked as 4AT. I guess this means 4 amp.

Click to expand...

That means the fuse is a slow blow type rated at 4 amps. A slow blow fuse can withstand for a short duration a significantly larger current than its nominal rating. Necessary on most electronic equipment, in particular tube gear that draws at elevated current during the few seconds the heaters take to reach their temperature. The fuse is likely to be rated at 1.5-2x the steady-state current. The consumption of the unit is likely to be around 100W; even with a poor power factor, the actual steady-state current is probably ca. 2 amps at 115V

1) I want to take the power from the mains to a switch before the torroid. As Im building a dual unit, this means I need to take live & neutral ‘twice’, so that after the switch I can go to 2 torroids. The cable I have is only rated to max of 6 amps. So am I best running 2 sets of cable in parallel (one for each torroid, assuming draw of 4amps each?).

Click to expand...

Your cable is safe for use at 6amp steady-state for an indefinite duration, but can withstand many times that for a shorter duration. You should be ok to use only one.

2) I have a chunky on off toggle switch which is rated at 10 amps @240v, so I assume this is fine?

Click to expand...

Yes

3) When I choose a fuse for my mains socket, I guess I need to simply double the draw of each PSU circuit. Given that Ive also got a little power lamp to run, will an 8amp fuse be sufficient (lmap is 24v, but dont have any other info)?

Click to expand...

You should run the lamp from a low-volt winding. Running a 24v lamp from mains voltage implies either a large value high-voltage non-polarised cap (expensive) or a power resistor diisipating a lot of heat. Alternatively you could use a neon pilot light, which exist in mains voltage type

P = U X I (power = voltage X amperage)

Didn't see your PSU / Make up gain PCB, but are you sure that you didn't missed a little dot just before the "4"?
This would mean: 0.4 Amps = 400mA, and match the other datas we can get from the schematic.

On the schematic you posted, I can see a primary fuse marked: 315 mAT (T stands for: Temporized, or Slow Blow, as Abbey said)
The schematic also say that transformers are 220V/12 or 15 V- CA 1.5A
That means that, worst case scenario, the transformer is able to deliver on it's secondary, 15V at 1.5 Amp, which gives us 15 X 1.5= 22 VA or Watts.
But the fuse is on the primary side where 22VA means : 22VA / 220 (volts) = 0.1A or 100mA
So we should have a 100mA current on the primary side. Add some safety margin, rate it a 2X the steady state current, and choose the next upper value available. Here is your 315 or 400mA fuse.
Note: If you live in a 110V country, you will need 200 mA on the primary side, so a 315 mA fuse may be a bit short. Use a 500mAT if you want to keep some safety margin.

1) Your cable is probably rated for 6 Amps at 250V which means 250 X 6 = 1500 Watts, since you plan to use 2 X 22 VA = 44, your cable is totally safe

2) Your switch is rated 10 Amps at 240V. Same thing, 10 X 240 = 2400 Watts = safe.

3) What Abbey said. Use a 12V bulb from the first secondaty, or a 6.3V bulb from the DC heater rail, or a neon bulb straight from the mains switch.

Axel

Thank you both for your replies. I guess from looking at the schematic, yes, it may be .4AT! Ive soldered on the fuse holders now so cant easily check.

I think this resolves my questions for now.

Thanks again
jake

These are two different fuses. The 4A/T one on pcb, not shown on schematic, is secondary side between step-down and step-up transformer, as a second safetybelt in case this 2nd.transformer fails. You want both fuses.

Make sense...
My assumptions were only from the schematic since I haven't seen the pcb.

So Snipsnip needs:
-1 external 315mAT for 220V operation(or 500mA for 110V operation) fuse on the live wire between the IEC socket and AC mains switch
-and two 4AT fuses, one on each PCBs.

Axel

mad.ax said:

So Snipsnip needs:
-1 external 315mAT for 220V operation(or 500mA for 110V operation) fuse on the live wire between the IEC socket and AC mains switch
-and two 4AT fuses, one on each PCBs.

Axel

Click to expand...

Do I need to up the external fuse to approx 630mAT if im running 2 units?

Harpo - thanks for the additional info!

jake

What is your AC wall voltage?

As said worst case power consumption is 22 Watts per unit, so 44 watt for a pair, which means 200 mA under 220V. A 315 mAT should work fine.
Now if the heaters draw a lot of current at start-up, the fuse may blow. In this case, a bigger fuse will endure the current, but will offer less protection.

The purpose of type T (slow blow) is precisely to endure the inrush current at start-up

Axel

Snipsnip, including your location in your profile might help to better help you.
Fuse rating is dependant on transformer type and VA rating, you intend to use for this project. The manufacturer might have this number listed in your transformers datasheet.
Toroidals  have a higher inrush current for core magnetizing and when powering up, the empty caps after the rectifier nearly represent a short for some AC cycles, drawing all current this transformer can deliver, so this fuse rating is AC-mains and VA dependant and build type of transformer will decide, which time-current characteristic is required.
If the fuse rating is not listed by the manufacturer, dividing VA rating by mains AC voltage and round up to the next availiable fuse value for usual is a safe enough bet. This round up procedure might also cover the ideal VA*SQR(2) term, as there is no perfect transformer and your AC mains most probably isn't pure sine. YMMV.

Thanks Guys,

This is all very new to me, as all my previous build had fuses in BOM's, so I really appreciate the help!

Harpo, Ive updated my location as suggested, and FYI these are the transformers I am using.

http://search.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go &lang=en&site=uk&keywords=TE62063-ND