Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Testing Power Supplies

Status
Not open for further replies.

linguist

New Member
I want to be able to test high voltage power supplies & in particular the voltage sag of fullwave rectifier valves.

I need an adjustable dummy load for the output, I would need to go from part load then to full load to test things safely with no failures.

The DC output voltages from the power supplies will range between 200v to 500v & output current can be up to 250mA. I have a Variac to control the input voltage so I can power up slowly initialy.

I have looked at several Rheostats but these are expensive & the ones I have seen are rated at around 250v & I am not sure if they will take 500v even though there total power dissapation rating is more than I require.

Will a Rheostat rated at 250v 1A--250W take 500v 250mA--125W, it seems as though it would be ok but I thought I had better ask.
I was looking at the No-Taper Rheostat that will take maximum current through the full range.

Or better still is there a less expensive way to make a variable dummy load for what I need, I thought of switching between different high Watt resistors but there may be an arcing issue that I don't want.
Is there a way to set this up using heater elements or something maybe 240v light bulbs?

Cheers.
 
Last edited:
As I recall, the wattage rating for resistors no matter what the size is for the full length or the full size of the resistor. You can increase the wattage rating by submerging them in oil or using a fan to force air across the surface. Using light bulbs is not a good idea, as the cold resistance(the bulb not illuminated) is about 1/10 the hot resistance(bulb illuminated).
If the load testing is for a very short time the needed wattage can be less than the continous wattage load. It would be possibe to use to fixed values. One for part load and the other for full load. You could use put the normal load resistance on all the time and switch another resistor in parallel to make the full load current. Use a SCR to switch in the full load resistance, then turn the power supply off and the full load resistance would be disconnected.
 
Thanks for the reply k7elp60,

That's a good solution to the problem.

Like you said have one resistor as part load & then switch the parallel resistor on to create full load.

Just so I understand better.
So the SCR has to take the 500v max.
I have a (SEMIKRON SEMIPACK-1 SKKT 57/12 E) SCR, could I get someone to verify the Datasheet & make sure it is suitable for this application.
Is it suitable for DC switching, it has an AC symbol on pin 1 input.

EDIT;

I just had a look at the graphs on the datasheet, which I neglected to do before & it appears that it will switch DC as well as AC, is that correct or do I have it wrong.

Cheers
 
Last edited:
I have drawn a very basic SCR circuit in LTSpice to see if I can get it working so I can work out the circuit required for the above, is there a way to add working switches in the program.

Or how to simulate the switches somehow, I need two switches for the gaps in the circuit?

Circuit attached

Cheers
 

Attachments

  • SCR.gif
    SCR.gif
    3.2 KB · Views: 204
Last edited:
Thanks for the concern jessiehilty but the fact of the matter is I need to test these power supplies. I am building them so I need to test them.
I am 50yrs old & being cautious has got me this far, I understand about high voltages etc & how extreme care must be taken.

That is why I asked the question on this forum, basically asking for some advice to make sure that I am taking all necessary precautions.
Safety is the first & last obsticle.
There are plenty of people here who could help me out in no time & I hope they soon chime in as I have to do this anyway but I would rather use the expert help available on this forum.

Being cautious, is one thing, having enough common sense to ask for some help is a big step in the right direction I believe.

Not exactly sure how you can test what I want to with the thing unplugged?
Maybe I could run around with it in an electrical storm & wait for things to happen hey?

Just a joke!

I do need help & that's why I asked here, I wanted to try to design the circuit properly & then ask if it is suitable before taking the next step. I am not experienced with SCR's & do need help.

Cheers
 
I have worked the switches out in LTSPICE, now to the circuit.
Some advice would be greatly appreciated.

Cheers
 

Attachments

  • SCR.gif
    SCR.gif
    12.7 KB · Views: 220
Thanks for the reply k7elp60,

That's a good solution to the problem.

Like you said have one resistor as part load & then switch the parallel resistor on to create full load.

Just so I understand better.
So the SCR has to take the 500v max.
I have a (SEMIKRON SEMIPACK-1 SKKT 57/12 E) SCR, could I get someone to verify the Datasheet & make sure it is suitable for this application.
Is it suitable for DC switching, it has an AC symbol on pin 1 input.

EDIT;

I just had a look at the graphs on the datasheet, which I neglected to do before & it appears that it will switch DC as well as AC, is that correct or do I have it wrong.

Cheers
No you have it correct, the SCR will switch DC. One thing to remember is that on an SCR, what turns it off is when the voltage between the anode and the cathode falls to a low value it turns off. So on AC when the AC voltage is near 0 the SCR turns off. On DC removing the DC voltage will turn if off, or reducing the DC voltage to the turn off potential. A small voltage between the gate and cathode turns them on. Some SCR's have a gate current limit so a current limiting resistor is a good idea. Your first posted circuit is the best to use.
 
Last edited:
I have worked the switches out in LTSPICE, now to the circuit.
Some advice would be greatly appreciated.
Connect the R2 load resistor to ground, not to the SCR gate.

Connect S3 from the SCR gate to the output of S2 through a suitable resistor (to give the required gate trigger current for the SCR gate).

Then switching on S2 will apply load R2.

Momentarily closing S3 will trigger the SCR and add load RL.

Opening S2 will remove both loads and reset the SCR.
 
Thanks for the replies k7elp60 & crutschow,

I have a couple of questions.

Should I have some protection for the SCR as in a snubber diode etc?

The big SCR that I have (mentioned above) is largely overkill for this application but I have it & will use it. Looking at the internal diagram drawn on the SCR itself, this is just two SCR's in series, is that correct?

I have attached a picture of the internal connections & another of the SCR itself.

On the base of it, it has terminals 1 to 7, 4 & 6 are the low voltage Gate connections.

Now with the main terminals 1, 2 & 3, do I have to Bridge terminals 1 & 2 on the base.
Pin 3 is the output.

Cheers
 

Attachments

  • SCR-1.gif
    SCR-1.gif
    9.4 KB · Views: 216
  • SCR-2.gif
    SCR-2.gif
    36.6 KB · Views: 210
I was just thinking about this a bit more & instead of the SCR couldn't I just use a power Mosfet as a switch.
I have some rated at 600Vdss 6.8A, this would seem to be another way to do it.

What do you think?

Cheers
 
Last edited:
I was just thinking about this a bit more & instead of the SCR couldn't I just use a power Mosfet as a switch.
I have some rated at 600Vdss 6.8A, this would seem to be another way to do it.
The MOSFET would have a lower voltage drop, but that's not particularly important here. It's also less tolerant of overloads, but again, not likely a problem here. So I can't see a good reason to recommend one approach over the other.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top