DC Soft Start?


Thread Starter

Jussy Rusit

I'm working on a AC-DC unit that suffers from what we call "Load Shocks" and results in premature failure of the unit. The Power circuit is a simple rectified AC-DC with an RC circuit. An ideal condition would be a variable power supply, but the cost is prohibitive for this product.

Can anyone suggest a way I could slow down the inrush power, either voltage or current ( in Seconds) such that the load would not be subjected to Power shock?

I've looked into AC Soft start, but my application is not motor related.

thanks, - Jussy Rusit
Jussy.....hi i'm an electrician from South Africa and i haven't heard of a Dc soft start i just contacted our suppliers and they havn't heard of one either tell me is it inductive or resistive load ?
> Jussy.....hi i'm an electrician from South Africa and i haven't heard of a Dc soft start i just contacted our suppliers and they havn't heard of one either tell me is it inductive or resistive load ?

That's the scenario that I have, my supplier is pointing me to AC Soft start but my application is different, which is a resistive load BTW. The application is for temperature control. A 120VAC is rectified to DC. Switching is done by SSR's. The problem is, the on-off cycle is to harsh on the unit. I'm looking for a way to slow it down.

Ralph G. McDonald, P.E.

You can install a resistor (or resistor bank if required) in parallel with a bypass relay or contactor. Use a time delay relay to close the contactor after N seconds. Some of the older Square D VFDs used this type of system on startup as the DC bus was charging. You will have to size the resistor(s) based on current, load time and duty cycle. Size the contactor based on the load that it carries.

Email me and let me know what you use.


Curt Wuollet

Hi Jussy

What kind of power level are we talking about. Up to a point people use a thermistor for this. After that they use a resistor with relay. It's also possible to use a phase control, but in
rectifier circuits the peak currents during the small conduction angles tend to cause problems. A properly selected inductor can limit the di/dt also. The selection of one of the techniques
depends on the specific operating conditions. Basically the idea is to limit inrush current to reasonable values..

With a little more info I could probably help you more.



From the very limited info you have provided, I will assume you want to current limit a small device (I know-wrong!). Look at www.digikey.com for current inrush limiters. If those won't do the trick, give more specific data on your application (voltage? current? load? duty cycle?).


Willy Smith
Numatics Inc.
Costa Rica
First off, I would to thank everyone for the replies. This is my first time posting and I've already got pointed at the right direction. Again, do appreciate the feedback.

Just a recap: the application is for temperature control where there is a dedicated/custom microprocessor based controller which sends out 12VDC signal to a SSR. The SSR controls the 120VAC supply to the single phase rectifier and which is then filtered by a RC circuit. The load: a series of thermoelectric chips (http://www.thermoelectric.com/pg06.htm) which are resistive type. A circuit demand typically would be about 1000W.

What are my plans:
1. Have looked into the Crydom site and found the Series SST (AC Soft start,) Series PCV (Analog input power control)and the series RPC (Proportional controller). Since it is relatively easier to play with low voltage, I will be looking at the control signal and see if I could dampen it via (bank of resistors plus time delay - Ralph McDonalds idea ) or using an FET and an RC circuit. Thus, I could use the Crydom Series PCV and that should be it.

If not,
2. I will look into the thermistor, Inductor as an alternative.

3. Or figure another way via timers from www.ssac.com

With that, I think I will have my hands full for the next few weeks.

Once again, my thanks to all.

Jussy Rusit
Noah Precision Inc.
San Jose, CA
is there no way you can put anything on the AC side ? if it's only going through a rectifier would it make that much of a difference if you altered the AC side rather than the DC side ?

Colin Walker

A common method is to fit a series resistor to the AC supply that limits the inrush current. This causes the DC voltage across the filter capacitor to rise slowly. A relay with it's coil supplied by the filter capacitor picks up at close to full DC voltage and shorts out the AC input resistance using a normally open contact. Cost one resistor, one relay.
Hope this is of some help.
Regards Colin WALKER.
[email protected]

Curt Wuollet

Hi Jussy

Thermoelectric devices are kinda picky and susceptable to thermal fatigue when modulated at line frequencies so make sure whatever control means you use supplies clean DC or PWM in the khz range. Low ripple improves the efficiency and khz PWM cycles the devices on and off faster than their thermal time constant. Unfiltered, rectified DC will kill these even if you don't cycle them and the efficiency is not very high. I'm using thermoelectrics for cabinet cooling so I'd be interested in how you come out.



Brian Rode Engineered Handling Inc.

We have used a high wattage fixed DC switching power supply and ramped the power by pulseing a solid state relay as a speed ramp control.