choosing a bleed resistor

[Doddy]

Or use a NTC thermistor.

BTW, that design - the resistor wattage values are wrong - if, as claimed, it's intended to limit the current to 5A then the resistors should be rated at 250W (P=I^2 x R). I'd also caution that C1 is specified at the limits of its operation - failure of that would result in the explosive failure of C2 C2 and likely destruction of R1 and RE1.

Treat with care.

Oh, you dissipate about 6W in that capacitor and relay.

And A-Camera, electric shock isn't the only hazard of a charged capacitor. The biggest hazard at this potential are burns and physical injury resulting from a spontaneous reaction to a sudden discharge. You mention batteries - of course you don't use a bleed resistor on batteries - that's a divisive argument. But the first instruction in any vehicle maintenance manual is to remove the -V lead before any work. Why?

[Neale]

Why build a second power supply just to run the relay? How about driving a 24V relay from the existing power supply output via a suitable zener? With a zener chosen to drop enough volts to give nominal relay voltage when PSU is at working voltage, the relay will operate a little earlier than that, but by then the initial surge will be over. Even simpler, use a series resistor to give nominal relay voltage at PSU nominal output. I'm not convinced that surge protection is needed at all (my system works fine without but I chose my input MCB carefully so nothing trips, and that's the only reason why you need to bother) but if it is, make it as simple as possible. I've never used the NTC approach, partly because it looks as if you have to choose carefully to match PSU current draw, which is pretty variable driving a CNC machine.

[cropwell]

I would like to know what the design input voltage is to this soft tart circuit in Elektuur. We are assuming 50v as that is what we need, but it has 40v caps and a 24v relay.

[Doddy]

Nope, it's 220-240 on the primary of the toroidal, something that should be borne in mind for our US readers.

It uses a potential divider across the C1 capacitor and the relay coil. It works as-is, but is fraught with problems when people start substituting components.

[cropwell]

Sorry Doddy, I was trying to delete my post as I realised it was mains input. Just had a senior moment !

[the great waldo]

Nope, it's 220-240 on the primary of the toroidal, something that should be borne in mind for our US readers.

It uses a potential divider across the C1 capacitor and the relay coil. It works as-is, but is fraught with problems when people start substituting components.

Hi Doddy
I'm using a 500va 2x18volt toroidial transformer with 3 or 4 4700uf 100 v caps. 35 amp rectifier in my power supply, would the elektor circuit be ok with that power supply ? or should I hunt around for another soft start circuit ? Thanks in advance.
cheers

Andrew

[Clive S]

Hi Doddy
I'm using a 500va 2x18volt toroidial transformer with 3 or 4 4700uf 100 v caps. 35 amp rectifier in my power supply, would the elektor circuit be ok with that power supply ? or should I hunt around for another soft start circuit ? Thanks in advance.
cheers

Andrew

Personally I don't think you need one. I have a 625vA one that runs without any trouble and 3 caps is ample.

[the great waldo]

Hi Clive
I'll put one I found on ebay https://www.ebay.de/itm/131927219743 in just not to strain my wiring. It's an old house in the country here in Austria with a new distribution box wired up about 15 years ago which happens to be in the workshop as well so cable runs are not too long. A little peace of mind is worth the added expense for me.
Cheers

Andrew

[driftspin]

Personally I don't think you need one. I have a 625vA one that runs without any trouble and 3 caps is ample.

Hi Clive S,

You helped me with the parts list for the diy psu i have.

500va 2x25v.


No problems here either on 16amp B type CB.


Andrew, do you have issues on the mains power side?



In rush limiting is ok but not really needed when mains side is no problem.




Grtz Bert.



Verstuurd vanaf mijn SM-A320FL met Tapatalk

[A_Camera]

Hi Doddy
I'm using a 500va 2x18volt toroidial transformer with 3 or 4 4700uf 100 v caps. 35 amp rectifier in my power supply, would the elektor circuit be ok with that power supply ? or should I hunt around for another soft start circuit ? Thanks in advance.
cheers

Andrew

If I were you I'd buy the one from Conrad, or something similar ready made. In my opinion it is not worth the effort and is safer and guaranteed to work. The reason I linked to that circuit is because you asked for an example. Anyway, if you decide to build it then in your case I think it needs some modification, so I'd modify the following:

1. The four resistors, as Doddy mentioned already. It is better to use more powerful type, even if the 5W may work for such application because of the short time current if flowing trough. In fact, I think a single resistor would be better, something like this:

https://www.conrad.at/de/ate-electro...st-420251.html

That one is 47 Ohm, so the current will be limited a little more, to 4.9A Instead of 5.75A with the 40 Ohm, but that doesn't matter.

2. The relay in the example is 8A, I'd take a more powerful one, for example this one:

https://www.conrad.at/de/printrelais...st-502934.html

3. The fuse should be 10A slow type.

4. The art work in my opinion should have more powerful tracks, especially on the K1, K1 and the relay part. No need to waste copper by removing more than what's necessary to provide isolation.

5. Resistors R1, R2 and R3 should be metal film, 0.6W or 1W type. Even if the current trough is low, it is good practice to use better resistors than coal film type.

https://www.conrad.at/de/metallschic...t-1417688.html

https://www.conrad.at/de/metallschic...t-1417626.html