Want to change old PS to custom SMPS but quantities not large

[BrianE22]

The controls for the motors for our winches have been using a big old, 60 hz. transformer, linear PS for over 15 years. It works fine. I'd like to switch over to an SMPS for the size and the universal input (about 85 VAC to 265 VAC). We use a microcontroller to turn on relays via NPN transistors. I'm looking at making everything 5 volts instead of the 24V/5V we currently use. The transistors, though, drop about .5 volts. I was hoping to use a ULN2003 to replace the individual transistors with a single chip. The ULN2003 (using Darlingtons) drops even more volts (about 1V for I=120 mA). So a 5 Volt PS won't quite cut it with the 5 volt relays I want to use.

I was hoping to find a vendor that could modify the AC/DC converter to deliver 6 volts instead of 5. I like the XP ECL15US05-P (20 W, 5 Volt). But for our quantities, about 400/year, they weren't interested. They suggested using the 9 volt model (and dropping the volts down a bit). That's a possibility.

I also tried Power Integrations. They have an Evaluation Board (Document RDR-839, Digi-Key PN 596-1530-ND) that I thought I could reproduce (with a bit different transformer and output components) to get my 6 volts. I was hoping they could send me some sample transformer cores and bobbins identical to what was used on the evaluation board. I haven't heard back from them since I mentioned we only use 400 pieces per year.

Any suggestions as to possible AC/DC SMPS vendors or another approach?

 

[itsmoked]

Gotta say, I can imagine a half-dozen problems switching from 24V to 5V can cause you. The current has to go up 5X for any 24V loads. Switching supplies often cause problems in "motor drive" systems. Almost always, unregulated linear supplies are specified in drive applications not because they're the cheapest but because they're more reliable and better able to cope with what motors demand.

I think you need to provide more details. What type motors and what are you switching from 24V to 5V? Or are the motors not driven by this 24/5V at all only the controls are. If this is the case how are the motor contactors being operated?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BrianE22]

I wasn't very clear - it's the controls I'd like to update. The motor is a universal. A microcontroller drives transistors that drive the contactors. The main reason I'd like to switch to a 5V (actually 6V) is because I'd like to use a super capacitor for our energy storage device for the electronics. Right now we us a large electrolytic and I like the space savings with the super cap.

I'm thinking all these small 15-20 W AC/DC converters are made in large quantities overseas. I don't think I'll find anyone willing to make them in relatively small quantities with a 6 volt output. I have a 9 volt XP Power board on order. I'll play with that.

 

[LionelHutz]

There are many products already marketed that use 5V logic and drive 5V relay coils so you're trying to fix a problem that doesn't exist. I suggest you look at the minimum pick-up voltage for the relay you are using. Also, a NPN transistor drops about 0.2V collector to emitter.

 

[BrianE22]

Good point LionelH - I had it in my mind that pickup was about 90% of rated. For the units I want to use it's actually 75%. Five volts should work fine.

 

[IRstuff]

Also, a NPN transistor drops about 0.2V collector to emitter.

Only if you're not using a crappy NPN:

Now, THIS is a knife

TTFN (ta ta for now)
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[LionelHutz]

I can't help it if someone doesn't know how to specify parts and uses junk.

 

[BrianE22]

Lionel -

Do you know of any 6 channel arrays (about 120 mA/Channel) with low Vce? It's not a killer to stay with the discreet transistors but an array would be nice.

 

[LionelHutz]

Not offhand.