Trying to convert 28VAC input to 120/240VAC or 9/36VDC....

[James9782]

I have a query from a customer who is looking to buy a 5W product from me which runs on 120/240VAC or 9/36VDC. The problem is, they are in an isolated location where they only have a 28VAC power supply. I have looked around the web and cannot find any obvious solutions, does anyone know of any solutions/products which will solve this issue?

Best regards,

James

 

[Bobster]

A couple of diodes.

 

[LeeH]

A bride rectifier the 28VAC TO DC.

That's a pretty odd supply voltage.

 

[Knobhead]

A bridge rectifier will give about 39VDC. If needs be use some form of voltage regulator.

 

[tazz]

If the 28vac is at 50hz then and bridge rectifier will output at 100hz .....As Tony stated will need regulating..
Suggest bridge rectifier first...output to LM7815 reg, this will give stable 15v dc out.
omit transformer from diagram

 

[chlee-penang]

If the 28VAC is from secondary of transformer with center tap, i.e. 14/0/14V, and ripple is not a problem to the lamp, I would suggest simpler solution by using 2 diodes to form a full wave rectifier, that will provide 0.9x RMS - 12.6 VDC.

Correct me if I am wrong.

 

[Besoeker]

I have a query from a customer who is looking to buy a 5W product from me which runs on 120/240VAC or 9/36VDC. The problem is, they are in an isolated location where they only have a 28VAC power supply. I have looked around the web and cannot find any obvious solutions, does anyone know of any solutions/products which will solve this issue?

Best regards,

James

I'm surprised that no one has suggested a transformer.
Seems to me it would be simplest and most obvious solution to get from 28Vac to 240Vac. Or 120Vac.

 

[tazz]

I'm surprised that no one has suggested a transformer.
Seems to me it would be simplest and most obvious solution to get from 28Vac to 240Vac. Or 120Vac.

Spot on...being ac you can use a 28/240 transformer in reverse...giving 240ac out.

 

[James9782]

Thank you gentlemen,

your replies have given me food for thought... Very grateful. For clarification, the appliance is going on-board a naval vessel.

Regards,

 

[telectrix]

but current would be high on the primary 28V side, approx. 4 x the 120V current.

 

[tazz]

Yes Tel......so if the unit is 5w at 28v then current is 0.18a .....@ 120v current will only be 0.04a.
everything equals out between primary & secondary (increase one...decreases the other)

 

[Besoeker]

but current would be high on the primary 28V side, approx. 4 x the 120V current.

The current on the 28V side is always going to be higher than on the 120V side for the same power. Regardless of conversion method used.
That's kinda how electricity works...........

 

[tazz]

It will always be equal.....and wattage is the key....primary voltage is higher lower current......secondary voltage lover, higher current...put this in equation of total power for both and you can see current is proportional V/IR ...P/V

 

[Besoeker]

It will always be equal.....and wattage is the key....primary voltage is higher lower current......secondary voltage lover, higher current...put this in equation of total power for both and you can see current is proportional V/IR ...P/V

More correctly, it's VA.

Neglecting losses, you get

V[SUB]1[/SUB]I[SUB]1[/SUB] = V[SUB]2[/SUB]I[SUB]2[/SUB]

 

[Knobhead]

but current would be high on the primary 28V side, approx. 4 x the 120V current.

Your 60W lamp at 230V = 0.261A

At 400,000V = 0.0000866

 

[Lucien Nunes]

As the supply is ELV and it is going shipboard, I would avoid stepping up to LV which might not be expected and might require additional safety precautions.

You could use a transformer to step down the AC by any convenient ratio, 2:1 would be fine, which when rectified and smoothed would give about 18V DC. If you were making a lot of these you could have a tiny autotransformer wound with a tap at say 75% and get 28V DC. I am assuming from the wide range of acceptable DC voltages that the supply does not need to be regulated.

The easiest and neatest off-the-shelf method that avoids going to LV and the one I'd probably use would be to rectify, smooth, convert voltage with a DC-DC converter such as:
Tracopower's THD 10-4815WIN
Buy Isolated DC-DC Converters 10 W Isolated DC-DC Converter / Vout 24 V dc, 416 mA 1.2 % 1 % TRACOPOWER THD 10-4815WIN online from RS for next day delivery.

Suggest bridge rectifier first...output to LM7815 reg, this will give stable 15v dc out.

The snag there is that the maximum input voltage of 78xx regs is 35V so nothing gained. You would have to use a higher rated part, however using any linear regulator, (Vsupply-Vout)*Iout will be dissipated as heat, so the highest output voltage will give the coolest running, especially if the input power of the load device is constant wrt changing supply voltage. The OP didn't say what it is but 9-36V is a standard 4:1 DC-DC converter input range so it might well be constant power. In that case:
For a 15V reg, Ploss=(38-15)*5/15=7.7W making the efficiency 39% and requiring a sizeable heatsink.
For a 30V reg, Ploss=(38-30)*5/30=1.3W making the efficiency 79% and hardly any heatsink needed.

But I'd use the DC-DC.

 

[James9782]

Thank you Lucien,

I have done some research on the comments you made and the dc-dc conversion looks like the most viable option. I will update all if there are any developments.