powering a DC motor with plug in transformer...

[oslin76]

Ok, this is weird but I can't figure out the problem.  I have a inflatable mattress that is powered by four D batteries (6 Volt ) and I want to use a plug in type transformer (output 7.2 Volt DC,  500mA) instead of the batteries.  But the motor will not turn... I tried a variety of different size transfo with no luck.  Any ideas anyone???

[grifftech]

use a 2.1 amp USB charger

[oslin76]

ok I will try that

[james_s]

If it runs off D batteries it could draw several Amps. I would start by measuring the current drawn by the motor when running from batteries. You might need 5 Amps or more.

[jm_araujo]

Also it's probably a brushed motor, which needs a high inrush current to start rotating.

[Zero999]

Also it's probably a brushed motor, which needs a high inrush current to start rotating.

Yes, the inrush current could be tens of Amps, which will cause a switched mode power supply, such as that used in USB charger, to shut down. You may need a soft start circuit to get the motor moving slowly before it can be run at full speed.

[james_s]

Let's not get overly complicated here. A beefy iron transformer type AC adapter will tolerate the high inrush just fine, but it may be a prohibitively large adapter to be practical.

I have a similar mattress inflater in which I use NiMH AA cells in adapters and that works fine. I have another one that takes 12V, I normally use a 3 cell LiPo pack on that.

[Zero999]

An old low frequency transformer mains adaptor would do fine but as you've said it may be too big to be practical and they're getting rarer nowadays and are banned in some countries because they're to inefficient.

A soft start needn't be complicated. A MOSFET, resistor and capacitor are all that's required. The attached example uses the Miller effect to make C1 look much larger, when the MOSFET is turning on. The simulation shows it will take about 3s for 90% of the supply voltage to appear across RL. If this is too slow, use a lower value capacitor or resistor.

Note that it isn't a logic level MOSFET (I chose it because it will be slow) but should be fine for 10 or so Amps when the battery voltage is 6V. Check the MOSFET will pass enough current when given a gate voltage of 6V.



I've used a similar circuit before to power a motor off a switched mode power supply and it worked quite well.