Keeping a Fan Spinning the same way when reversing current.

Thread Starter

Murrough

Joined Jul 7, 2019
3
Hello,

I am designing a small heater / cooler unit that users a Peltier Thermoelectric (TEC) device (12V, 60W). I've used the 'standard approach' of two - relay (SPDT) connections with the load connected across the common on each. What I want to do also is switch on the fans (12V 1.5W) * 2 when I turn on the TEC either to heat or to cool (based on which way round the current flows). However, I require the fans to spin in the same direction irrespective of which way round this power is being delivered to the TEC. I have been trying to draw a circuit with diodes and things for a couple of hours and I'm either missing something, or it's more advanced than I thought it would be. Has anyone got any pointers please.

Cheers

Murrough
 

jpanhalt

Joined Jan 18, 2008
11,088
Can you use DC fans and a full-wave rectifier? If not, tell us more about your fan motors. Not all AC motors are reversible.
 

Thread Starter

Murrough

Joined Jul 7, 2019
3
Can you use DC fans and a full-wave rectifier? If not, tell us more about your fan motors. Not all AC motors are reversible.
Everything is DC 12V powered, FANS are stand 2 wire 40mm things (current thinking for correct noise and airflow), there is no AC power available
 

jpanhalt

Joined Jan 18, 2008
11,088
If everything is DC, just use a bridge rectifier (single, 4-pin module) that will ensure the fans always get the same polarity, regardless of the input polarity.
 

Thread Starter

Murrough

Joined Jul 7, 2019
3
If everything is DC, just use a bridge rectifier (single, 4-pin module) that will ensure the fans always get the same polarity, regardless of the input polarity.
Thank you, that's what I had, but someone told me I could not use a Bridge on a DC source, so that kinda threw me as I thought you could.
 

crutschow

Joined Mar 14, 2008
28,204
"Someone" doesn't know what they are talking about.
You could use a 4-diode bridge with Schottky rectifiers for about a 1V voltage drop to the motors.
The fans are connected across the bridge output.

If you think a 1V drop is too much, you could use a bridge consisting of two N-MOSFETs and two P-MOSFETs (below) for just a few tenths of a volt drop (depending upon the MOSFETs selected).
Note that, in this circuit, the MOSFETs conduct in their reverse direction when ON (same as the forward direction of the substrate diode), and block in the forward direction when OFF (since MOSFETs conduct equally well in either direction when ON).
The MOSFETs can be just about any that have a low enough on-resistance that the I²R losses in each MOSFET are less than a watt due to the fans' current (to avoid the need for a heatsink for the MOSFETs).
(The simulation glitch in the fan voltage and current below around 3V, V(F1,F2), is due to the MOSFETs turning off, with the current then being carried by the MOSFET substrate diodes below that voltage.)

upload_2019-7-7_19-30-20.png
 
Last edited:

Techohead

Joined Nov 23, 2020
11
Hello,

I am designing a small heater / cooler unit that users a Peltier Thermoelectric (TEC) device (12V, 60W). I've used the 'standard approach' of two - relay (SPDT) connections with the load connected across the common on each. What I want to do also is switch on the fans (12V 1.5W) * 2 when I turn on the TEC either to heat or to cool (based on which way round the current flows). However, I require the fans to spin in the same direction irrespective of which way round this power is being delivered to the TEC. I have been trying to draw a circuit with diodes and things for a couple of hours and I'm either missing something, or it's more advanced than I thought it would be. Has anyone got any pointers please.

Cheers

Murrough
For better performance, I would suggest making up a bridge rectifier from Schottkey diodes rated at about 20V. They have about half the voltage drop of standard diodes. The best way would be to bring the fan wires out & connect them before the reversing relays.
 
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